Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 Example 15.18 Explain why ring-deactivating groups such as !COOH are 3- and 5- directing. Solution The Kekulé structure for benzoic acid is shown below. C O RR OH The electron-withdrawing effect of the !COOH group causes the benzene ring to acquire a positive charge that is delocalised over carbon-2, carbon-4 and carbon-6 as shown below. fi fi L + + C RRRR O RR OH fi L C OH – O L + CRR O OH – fi C O OH – As a result, the electron density at C-3 and C-5 is higher than those on C-2, C-4 and C-6. This makes C-3 and C-5 more susceptible to attack by electrophiles. C – O H + + + RR Oxidation of Benzene 1 Unlike alkenes, benzene is very stable towards oxidising agents such as potassium manganate(VII) and potassium dichromate(VI). 2 This resistance to oxidation is due to the stability of the benzene molecule containing the ring of delocalised p electrons. A lot of energy is required to rupture the 'ring'. 3 However, like all hydrocarbons, benzene burns in air with a sooty smoky flame, due to its high carbon content. 2C6H6 + 15O2 9: 12CO2 + 6H2O 92
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 Reactions of Alkylbenzene 1 Alkylbenzenes have the general formula of R 2 Examples are: CH3 CH3!CH!CH3 CH3 CH3 Methylbenzene (Toluene) 2-phenylpropane 1,2-dimethylbenzene 3 The reactions of alkylbenzenes (for example, toluene) can be classified into two types: (a) Reactions involving the benzene ring (b) Reactions involving the alkyl side chain Reactions Involving the Benzene Ring 1 Like benzene, alkylbenzenes undergo electrophilic substitution. 2 Since alkyl groups are ring-activating groups, alkylbenzenes are generally more reactive than benzene towards electrophiles. 3 The incoming electrophile will be directed to the 2- or/and 4- position in the ring. R R + E+ + H+ E R E + H+ 1,2-dimethylbenzene is also known as o-xylene. 2-phenylpropane is also known as cumene. Info Chem Alkyl benzene is more reactive than benzene towards electrophilic substitution. The structures of the intermediates are: R + E H and R E + H 93
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 Nitration 1 Methylbenzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid at 30 °C to produce 2-nitromethylbenzene (2-nitrotoluene) and 4-nitromethylbenzene (4-nitrotoluene). CH3 CH3 + HNO3 + H2O NO2 CH3 NO2 + H2O 2 At elevated temperature, dinitrotoluene and trinitrotoluene (2,4,6-trinitromethylbenzene with common name: T.N.T.) is produced. CH3 CH3 NO2 NO2 NO2 NO2 NO2 2,4-dinitrolethylbenzene 2,4,6-trinitromethylbenzene 3 The explosion of TNT is represented by the equation: 4C7H5(NO2)3(s) + 21O2(g) !: 28CO2(g) + 6N2(g) + 10H2O(g) The reaction is highly exothermic and the rapid build up of a large volume of gas and heat produces the destructive effect of TNT. 2010/P1/Q31 Quick Check 15.12 1 Using balanced equations, describe the mechanism that leads to the production of 4-nitrotoluene from toluene. 94
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 Halogenation 1 Methylbenzene reacts with chlorine (or bromine) in the presence of anhydrous aluminium chloride (or iron powder), in the dark, to produce a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene. CH3 CH3 + Cl2 + HCl Cl CH3 Cl + HCl 2 Further chlorination produces 2,4-dichloromethylbenzene and 2,4,6-trichloromethylbenzene. Other Electrophilic Substitution Reactions Other important electrophilic substitution reactions involving methylbenzene are summarised below. Take note that the 4- isomers are produced together with the 2- isomers. Alkylation: CH3 CH3 AlCl3 CH3 + CH3I !!: and CH3 CH3 + HI Acylation: CH3 CH3 COCH3 + CH3!C!Cl !!: and CH3 COCH3 R + HCl O AlCl3 2010/P1/Q30 2016/P3/Q16(c)(ii), (d) INFO Electrophilic Substitution 95
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 Sulphonation: CH3 SO3H + SO3 !: and CH3 CH3 SO3H Reactions Involving the Side Chain Halogenation 1 When chlorine gas is bubbled through methylbenzene boiling under reflux in the presence of ultraviolet light, substitution to the side chain occurs and (chloromethyl)benzene is produced. White fumes of hydrogen chloride are evolved. CH3 CH2Cl + Cl2 !!: + HCl 2 In the excess of chlorine, polysubstitution occurs to produce (dichloromethyl)benzene and (trichloromethyl)benzene: CHCl2 CCl3 3 The substitution follows the free radical mechanism. Info Chem This involves free radical substitution. Quick Check 15.13 Using balanced equations, illustrate the mechanism for the reaction between methylbenzene and chlorine to produce (chloromethyl)benzene. Oxidation 1 When boiled under reflux with acidified potassium manganate(VII) or acidified potassium dichromate(VI), the alkyl groups (irrespective of its size or complexity) that are bonded to the benzene ring are oxidised to carboxylic acid, !COOH and benzoic acid is produced. 2008/P1/Q48 2008/P1/Q31 2011/P2/Q4(b) 2014/P3/Q7 2017/P3/Q18(a) 2013/P3/Q3 2016/P3/Q16(c)(i) 96
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 R COOH Acidified KMnO4, reflux !!!!!!!!!: 2 The excess carbon atoms and hydrogen atoms on the alkyl group are oxidised to carbon dioxide and water respectively. This is a simple way to differentiate between alkyl benzene and alkanes or cycloalkanes. 3 This is a convenient synthesis route to produce benzoic acid. 4 For example: CH3 COOH + 3[O] !!: + H2O CH3—CH—CH3 COOH + 9[O] !!: + 2CO2 + 3H2O 5 Similarly: CH3 COOH CH3 COOH + 6[O] !!: + 2H2O Benzene-1,2-dicarboxylic acid 6 However, if the carbon atom that is bonded directly to the benzene ring (the benzylic carbon) has no hydrogen attached to it (called benzylic hydrogen), then it is not oxidisable. For example: CH3 CH3 C CH3 KMnO4/H+/Heat No reaction Benzylic carbon !!!!!!: Alkyl benzenes decolourise acidified KMnO4. The final product of oxidation is benzoic acid. Info Chem CO2 is liberated. 97
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 Quick Check 15.14 1 Write balanced equations for these reactions. (a) 1-phenylhexane with hot acidified KMnO4 (b) 4-ethyl-methylbenzene with hot acidified K2Cr2O7 2 Suggest how you would differentiate between the following pairs of compounds using simple chemical tests. (a) Benzene and methylbenzene (b) Benzene and cyclohexene (c) Methylbenzene and ethylbenzene (d) Ethylbenzene and phenylethene 3 Starting with benzene, show schematically how you would convert it into (a) 4-nitrobenzoic acid, HOOC! !NO2 (b) 3-nitrobenzoic acid COOH NO2 (c) CH3!!CH C!!CH3 & ∫ CH3 O Uses of Arenes 1 One of the most important uses of arenes is as non-polar solvents. 2 However, precautions have to be taken when handling arenes because most arenes, especially the polyaromatic hydrocarbons (PAHs) are carcinogens (cancer causing agents). 3 Examples of PAHs are: Acenaphthene Chrysene Fluoranthene 4 Benzene and methylbenzene are added to petrol to enhance the efficiency of petrol. 98
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 SUMMARY SUMMARY 15.1 1 Alkanes are saturated hydrocarbons with the general formula of CnH2n+2 (where n > 1). 2 Cycloalkanes are saturated hydrocarbons with the general formula of CnH2n (where n > 3). 3 Alkanes are used mainly as fuels. 4 The main source of alkanes is petroleum or crude oil. 5 The different components in petroleum are separated via fractional distillation. 6 Cracking is the process where large hydrocarbon molecules are broken down into smaller molecules. 7 The melting point and boiling point of alkanes increase with increasing molecular mass. 8 All alkanes are insoluble in water. 9 Alkanes are chemically inert because the molecule is non-polar. 10 Alkanes undergo free radical substitution. 15.2 1 Alkenes are unsaturated hydrocarbons with the general formula of CnH2n (n > 2). 2 Cycloalkenes are unsaturated hydrocarbons with the general formula of CnH2n–2 (n > 3). 3 The main reaction of alkenes is electrophilic addition. 4 The presence of CRC bond(s) is indicated by: • decolourisation of acidified potassium manganate(VII), • decolourisation of bromine in tetrachloromethane. Reagent Product Remarks H2, Ni and heat & & !C!C! & & H H Electrophilic addition (Hydrogenation) X2, in CCl4 & & !C!C! & & X X Electrophilic addition Decolourisation of bromine HX, room temperature & & !C!C! & & H X Electrophilic addition H2O(g), H3PO4 catalyst, heat & & !C!C! & & H OH Electrophilic addition (Hydration) HOXr(aq), room conditions & & !C!C! & & X OH Electrophilic addition Fuming H2SO4, room temperature & & !C!C! & & H OSO3H Electrophilic addition Boiling product with water produces alcohol Cold, dilute acidified KMnO4 & & !C!C! & & OH OH Mild oxidation Hot, concentrated acidified KMnO4 Acids or ketones Vigorous oxidation (Cleavage of molecule occurs) O2(g), Ag(s) catalyst, heat & & !C!C! O Catalytic oxidation Boiling the product with water produces diol Ozone, warm with dilute H2SO4 and Zn Carbonyl compounds Oxidation (cleavage of C=C bond) 99
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 15.3 1 Arenes are unsaturated aromatic hydrocarbons with the general formula of CnH2n–6 (where n > 6). 2 The first member of arenes is benzene with the formula of C6H6. 3 The main reaction of benzene is electrophilic substitution. 4 Most substitution of benzene requires the presence of catalyst. 5 Alkyl benzene can undergo electrophilic substitution as well as free radical substitution to the alkyl side chains. 6 The alkyl groups are 2 and 4 directing. Reactions of Benzene Reagent Product Remarks Concentrated HNO3 and H2SO4, 55 °C NO2 Electrophilic substitution (Nitration) A yellowish oil Cl2, anhydrous AlCl3 or Fe, room temperature Cl Electrophilic substitution White fumes of HCl are released RCl, anhydrous AlCl3 or Fe, room temperature R Electrophilic substitution (Friedel-Craft alkylation) RCOCl, anhydrous AlCl3, room temperature O ∫ C!R Electrophilic substitution (Friedel-Craft acylation) Fuming H2SO4, heat SO3H Electrophilic substitution (Sulphonation) STPM PRACTICE 15 Objective Questions 1 Which of the following is a termination step in the photochemical chlorination of methane? A . CH3 + . CH3 !!: C2H6 B CH4 + . Cl !!: . CH3 + HCl C Cl2 !!: 2 . Cl D CH4 !!: C + 2H2 2 Which statement is true about compound 2-methyl-1,3-butadiene? A It exhibits geometrical isomerism. B It is a monomer of natural rubber. C It undergoes electrophilic addition. D It has a molecular formula of C5H10. 3 Which of the following regarding alkanes is not true? A They are all insoluble in water. B They are non-polar molecules. C They do not exhibit stereoisomerism. D The number of isomers increases with the number of carbon atoms in the molecules. 4 Larger hydrocarbon : Smaller hydrocarbon molecules molecules The above process is known as A Distillation C Elimination B Decomposition D Cracking 100
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 5 Which of the following isomers is expected to have the highest boiling point? A CH3CH2CH2CH2CH2CH3 B CH3CHCH2CH2CH3 & CH3 C CH3CH2CHCH2CH3 & CH3 D CH3 & CH3CHCH2CH3 & CH3 6 Which of the following is not true about the reaction between cyclohexane and chlorine in the presence of ultra-violet light? A The reaction involves free radical substitution. B The number of propagation steps involved in the formation of C6Cl12 is 12. C In order to produce monochlorosubstituted cyclohexane, cyclohexane has to be present in large excess. D The first step in the reaction involves the formation of chlorine atoms from chlorine molecules. 7 Which reagent can be used to distinguish between methylcyclohexane and methylbenzene (toluene)? A K2Cr2O7/H+ B Br2 in CCl4 C Cl2/Ultra-violet light D 2,4-Dinitrophenylhydrazine 8 Compound X reacts with oxygen according to the following equation. O2, Heat X 999: 6CO2 + 7H2O X could be I hexane II 2,2-dimethylbutane III hexene A I B II C I and II D I, II and III 9 Which of the following reagents, under suitable conditions, will react with 2,3,4-trimethylnonane? I Zeolite II Oxygen III Ethanolic sodium hydroxide A I C I and II B II D II and III 10 An organic compound, X, undergoes oxidation and addition with acidified potassium dichromate solution. What is X? A Benzene C Cyclohexane B Toluene D 2-Butene 11 Which of the following statements are true regarding alkanes? I They are saturated hydrocarbons. II They are chemically inert. III They are a source for energy. IV Combustion of alkanes is an exothermic process. A I and II C II, III and IV B I, II and III D I, II, III and IV 12 The empirical formula of a solid unsaturated hydrocarbon, X, is CH2. X could be A C2H4 C C6H12 B C2H2 D C20H40 13 Which is a termination step in the free radical reaction between chlorine and methane? A CH4 + Cl. 9: CH3Cl + H. B . CH2Cl + . CH2Cl 9:CH2ClCH2Cl C Cl2 9: 2Cl. D H. + Cl. 9: HCl 14 Which of the following is formed when a mixture of ethene gas and oxygen gas is passed over heated nickel catalyst and the product hydrolysed? A CH2!CH2 C CH2!CH2 & & O OH OH B CO2 and H2O D HOOC!COOH 15 What is the major product formed when propene reacts with bromine water? A CH2BrCHRCH2 B CH2BrCHBrCH2Br C CH3CH(OH)CH2Br D CH3CHBrCH2OH 101
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 16 Cyclohexane and cyclohexene can be differentiated using the following except A bromine dissolved in tetrachloromethane B chlorine in the presence of strong sunlight C cold, dilute potassium manganate(VII) D phosphorus(V) chloride 17 What is the reagent(s) for the following conversion? CH3CHRCH2 !!: CH3CH(OH)CH2Br A Bromine in tetrachloromethane B Concentrated hydrobromic acid in the presence of copper(I) bromide as catalyst C Bromine in water D Hydrogen bromide gas under pressure 18 Chloroethene, CH2RCHCl is the monomer for polyvinyl chloride (PVC). With which of the following does chloroethene react to give a non-chiral compound? A HBr B Br2 C H2O D H2 19 Combustion of hydrocarbon X in excess of oxygen produces 1.76 g carbon dioxide and 0.72 g water. At s.t.p. X is a gas with density 2.50 g dm–3. What could be the molecular formula of X? A C3H6 B C4H8 C C4H10 D C5H10 20 Toluene reacts with chlorine to produce chloromethylbenzene. Which statement is true of the reaction? A The reaction requires ultra-violet light. B The first step in the reaction involves the formation of Cl+ electrophile. C The reaction is electrophilic substitution. D The reaction also produces 3-chlorotoluene. 21 Hydrolysis of an alkene by steam produces an alcohol. What type of reaction mechanism is involved in this reaction? A Nucleophilic addition B Electrophilic addition C Elimination D Condensation 22 One mole of compound X reacts with 2 moles of hydrogen. What could X be? I CH2=CHCH2CH=CH2 II CH3CH2CCH III CH = CH2 A II C I and III B I and II D I, II and III 23 A hydrocarbon with three of its carbon atoms labelled p, q and r is shown below: CH CH2 p q r Which of the following combination is correct regarding the type of hybridisation of the three carbon atoms? p q r A sp2 sp2 sp B sp3 sp2 sp2 C sp3 sp2 sp D sp sp3 sp3 24 When 0.10 mol of A is burned in excess oxygen, 7.6 g of carbon dioxide is produced. 0.10 mol of A reacts with 4.48 dm3 of hydrogen under s.t.p. A could be I CH2=CHCH=CH2 II CH2=C=CHCH3 III CH3CH=CHCH3 A I C II and III B I and II D I, II and III 25 Propene reacts with hydrogen halide acid, HX, according to the equation: CH3CH=CH2 + HX 9: CH3CHCH3 & X Which of the following statements are correct regarding the reaction? I The addition follows Markovnikov's rule. II The intermediate is a carbonium ion. III The rate of reaction increases in the order of HCl < HBr < HI. A I and II C II and III B I and III D I, II and III 102
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 26 Which of the following compounds is the product obtained when ethene reacts with ozone followed by hydrolysis? A CH2OH—CH2OH B HCHO C CH3CH2OH D H—C—C—H ' ' O O 27 The chlorination of toluene gives two different products according to the reaction conditions. CH3 + Cl2 : CH2Cl + HCl (I) CH3 + Cl2 : Cl CH3 + HCl (II) Which of the following is not true regarding the above reactions? A Both are substitution reactions. B The rate of reaction of reaction (I) increases in the presence of aluminium chloride. C Reaction (II) involves electrophile. D The product of reaction (II) consists of a mixture of structural isomers. 28 Cyclohexene and benzene are both unsaturated hydrocarbons. Which mechanisms are correct about the reaction between cyclohexene and benzene with bromine? Cyclohexene Benzene A Free radical substitution Electrophilic addition B Free radical addition Electrophilic substitution C Electrophilic addition Nucleophilic substitution D Electrophilic addition Electrophilic substitution 29 Benzene and ethylbenzene can be differentiated using A concentrated nitric acid B bromine in tetrachloromethane C acidified potassium manganate(VII) D bromine water 30 Methylbenzene reacts with methyl chloride to form 1,2-dimethylbenzene. + CH3Cl + HCl CH3 CH3 CH3 !: Which of the following statements regarding the above reaction is incorrect? A A catalyst is needed for the reaction to occur. B The attacking species is CH3 – . C It is a substitution reaction. D The reaction also produces 1,4-dimethylbenzene. 31 Which of the following is a direct consequence of the delocalisation of electrons in the benzene molecule? A Benzene is insoluble in water. B Benzene is a non-electrolyte. C Benzene does not decolourise bromine in tetrachloromethane. D Benzene burns with a smoky flame. 32 Which of the following substituents in a benzene ring is meta-direction? A —F B — OH C — COOH D — NH2 33 Which compound(s) will react with benzene, under suitable conditions, to produce 2-phenylbutane? I 1-butene II 2-butene III 2-chlorobutane A II C I and II B III D I, II and III 34 Which of the following is not true of the reaction between benzene and a mixture of concentrated nitric acid and concentrated sulphuric acid at 55 °C? A Nitric acid acts as a Bronsted-Lowry base. B The mechanism involves nucleophilic substitution. C The attacking species is the NO2 + ion. D The intermediate is a carbonium ion. 103
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 35 Consider the two reactions below: CH3 + Cl2 CH3 Cl + HCl AlCl3 Reaction I: Reaction II: + Cl2 Cl Cl Which mechanisms correspond correctly for both the reactions? Reaction I Reaction II A Electrophilic substitution Electrophilic addition B Nucleophilic substitution Nucleophilic addition C Electrophilic substitution Free radical addition D Electrophilic addition Free radical addition 36 Consider the reaction scheme below: I Cl X NO2 II Which statements are correct for the reactions? I X is nitrobenzene. II Both steps I and II involve electrophilic substitution. III Step II requires a Lewis acid as catalyst. A I and II C II and III B I and III D I, II and III 37 Which of the following functional groups is electron-donating when attached to a benzene ring? A —OH C —COCl B —COOCH3 D —NO2 38 Aluminium chloride catalyses the reaction between benzene and chloroethane to form ethylbenzene according to the equation: C6H6 + C2H5Cl 9: C6H5C2H5 + HCl What is the first step occurring in the reaction? A AlCl3 + C6H6 9: HAlCl3 – + C6H5 + B AlCl3 + C6H5 9: C6H5AlCl3 C AlCl3 + C2H5Cl 9: AlCl4 – + C2H5 + D AlCl3 + C2H5Cl 9: C2H5AlCl3 – + Cl+ 39 Which of the following compounds will react with benzene to form cumene? A CH3COCl B (CH3)3CBr C C6H5I D CH3CH(Cl)CH3 Structured and Essay Questions 1 Crude oil is a mixture of hydrocarbons. (a) Name the process used to separate the various components in crude oil. (b) One of the components obtained from (a) is decane, C10H22. The catalytic cracking of decane produces propene and compound X. (i) Name the catalyst used in catalytic cracking. (ii) Give one advantage of catalytic cracking compared to thermocracking. (iii) Deduce the molecular formula of X. (c) X can be converted into toluene by a process called reforming. Write a balanced equation for the reforming process. (d) Using balanced equations, show how toluene can be converted to (i) 2-phenylethanamine (ii) benzylethanoate 104
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 2 Propane reacts with chlorine to produce 2-chloropropane. CH3CH2CH3 + Cl2 !!: CH3CHCH3 + HCl & Cl (a) State the condition necessary for the reaction. (b) Using balanced equations, describe the mechanism of the reaction. (c) Explain why 2-chloropropane is formed instead of 1-chloropropane. (d) Explain the formation of 2,3-dimethylbutane as a side product in the above reaction. 3 When 3,3-dimethylheptane undergoes catalytic cracking, a possible reaction is as follows: CH3 & CH3CH2CCH2CH2CH2CH3 !: P + C4H10 & CH3 (a) Name a catalyst for the reaction. (b) P decolourises bromine in tetrachloromethane and does not exhibit geometrical isomerism. Ozonolysis of P produces ethanal and propanone. Draw the structure of P and write a balanced equation for the ozonolysis. 4 10.0 cm3 of a gaseous hydrocarbon, A, was exploded with 100.0 cm3 of oxygen (an excess). The total volume after explosion was 75.0 cm3 . When the resulting mixture is shaken with aqueous sodium hydroxide, there was a decrease in volume of 25.0 cm3 . [All volumes are measured at room conditions.] (a) Explain why there was a decrease in volume when the mixture was shaken with aqueous sodium hydroxide. Illustrate your answer with a balanced equation. (b) Why was the hydrocarbon mixed with excess of oxygen before the reaction? (c) Determine the empirical formula of A. (d) Determine the molecular formula of A given its molar mass is 70.0 g mol–1 . (e) Draw all possible structural isomers for A. (f) Which of the isomer in (e) on vigorous oxidation yields butanone as the only organic product? Write a balanced equation for the reaction. 5 The following scheme shows some reactions starting from propene. MnO4 –, reflux MnO4 –, cold C C3H6 B Concentrated H2SO4 H2O, heat D E (a) Can propene exhibit geometrical isomerism? Explain your answer. (b) Propene can be obtained from petroleum fractions via the cracking process. Write a balanced equation to show how propene might be produced by the cracking of C12H26. (c) Identify compounds B, C and D. Write equations for the reactions taking place. (d) Propene can be polymerised to poly(propene). Draw a repeating unit of poly(propene). 105
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 6 (a) What do you understand by ‘cis-trans isomerism’? (b) 0.84 g of a gaseous hydrocarbon X occupies a volume of 360.0 cm3 . In a separate experiment, 0.84 g of X reacts completely with 338 cm3 of hydrogen. All volumes are measured at room conditions. X reacts with bromine in tetrachloromethane to produce Y. When X is treated with cold dilute potassium manganate(VII), Z is formed. Both Y and Z have two chiral carbons in their structures. (i) Determine the molecular formula of X. (ii) Determine the structures of X, Y and Z. (c) X exhibits geometrical isomerism. Draw the structures of the two geometrical isomers. 7 Show how you would carry out the following conversions. (a) Propene to 1,2-propandiol (b) 2-butene to ethanoic acid 8 (a) Propene, C3H6, reacts with hydrogen bromide to produce two isomeric products. (i) Draw the structures of the two isomeric products. (ii) Which of the two is the major product? Explain why it is so. (b) Propene reacts with bromine water to produce a compound which is optically active. (i) What do you understand by the term ‘optical isomers’? (ii) Draw the structures of the two isomers. (iii) In what way do the two isomers differ from one another? 9 A gaseous hydrocarbon has the following composition by mass: C, 85.7%; H, 14.3% A 0.25 g sample of the hydrocarbon has a volume of 100 cm3 at s.t.p. When the hydrocarbon is bubbled through an acidified solution of potassium manganate(VII) at room temperature, the purple colour of potassium manganate(VII) is decolourised. (a) Determine the empirical formula and the molecular formula of the hydrocarbon. (b) Draw all possible structural formulae (including stereoisomers, if any) of the hydrocarbon. (c) The hydrocarbon has another structural isomer which does not decolourise acidified potassium manganate. Draw the structure and give the name of the isomer. 10 When ethene is bubbled through hydrogen iodide in tetrachloromethane, only one organic product is formed. But when ethene is bubbled through hydrogen iodide in aqueous sodium chloride, three different organic products are formed. However, if the mixture is boiled under reflux, only one organic product remains. Explain the observations. 11 (a) 1,2-Dimethylcyclopentane (also known as neopentane) is an example of cycloalkane. (i) Draw the structural formula of neopentane and state the type of isomerism. (ii) Suggest a chemical test how you would distinguish neopentane from pentene. (b) Cumene or 2-phenylpropane is an example of arenes. (i) Cumene reacts with chlorine in the presence of ultra-violet light to produce 2-chloro-2- phenylpropane. Draw the mechanism for the reaction and name the type of reaction. (ii) Draw the structure of another possible monosubstituted product formed in the reaction. Explain why this is a minor product. 12 Ethene reacts with bromine via electrophilic addition mechanism. (a) What is meant by electrophile? (b) Write a balanced equation for the reaction taking place. (c) Describe the mechanism involved. 106
Chemistry Term 3 STPM Chapter 15 Hydrocarbons 15 13 Identify the following compounds from the informations given. (a) A, C6H12, does not decolourise acidified potassium manganate(VII). (b) B, C7H8, on refluxing with a concentrated solution of acidified potassium manganate produces a dicarboxylic acid. (c) C, C4H8, reacts with hydrogen bromide to produce a compound that is optically active. (d) D, C5H10, on vigorous oxidation produces ethanoic acid and propanone. 14 Show how you would carry out the following conversions. (a) Benzene to benzoic acid (b) Benzene to 3-chlorobenzoic acid (c) Benzene to 4-bromobenzoic acid 15 A hydrocarbon, A has the following composition by mass: C, 93.3%; H, 6.7% When 18.0 g of A is vaporised, the vapour occupies a volume of 2.4 dm3 at room conditions. (a) Determine the empirical formula of A. (b) Calculate the relative molecular mass of A, and hence determine its molecular formula. (c) A has two structural isomers B and C. Each contains two benzene rings in their structures. On vigorous oxidation, B produces benzoic acid as the only organic product, while C produces diphenylketone as its only organic product. (i) Determine the structures of B and C. (ii) Write equations for the reactions occurring in (c). (d) Both B and C reacts with hydrogen bromide. Which isomer, B or C, would produce a product that is optically active? 16 Photochemical smog is a secondary pollutant formed when nitrogen oxides and volatile hydrocarbon, from car exhaust, react in the atmosphere in the presence of sunlight. (a) Explain briefly how nitrogen oxides and volatile hydrocarbons are found in car exhaust fumes. (b) One way to reduce the amount of pollutants in car exhaust fumes is to install catalytic converter in the vehicle. (i) Name two of the catalysts used. (ii) Write two equations of reactions involved in the catalytic converter. (c) Suggest two other ways to decrease air pollution from burning of petrol in car engines. 17 Using methane, ethene and benzene as example, describe the following reaction mechanisms. (a) Free radical substitution (b) Electrophilic addition (c) Electrophilic substitution 107
Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 CHAPTER HALOALKANES 16 Haloalkanes Classification • Primary • Secondary • Tertiary Nomenclature Physical Properties of Haloalkanes • Melting and boiling points • Solubility in water and density Organometallic Compounds • Reactions involving Grignard reagent Reactivity of Chlorobenzene and Chloroalkanes in Hydrolysis Reactions Structural and Optical Isomerism in Haloalkanes Elimination reaction Nucleophilic Substitution of Haloalkanes • Mechanisms of nucleophilic substitution (SN1 and SN2) Learning earning Outcomes Students should be able to: • write the general formula of haloalkanes; • name haloalkanes according to the IUPAC nomenclature; • describe the structural and optical isomerism in haloalkanes; • state the physical properties of haloalkanes; • describe the substitution reactions of haloalkanes as exemplified by the following reactions of bromoethane: hydrolysis, formation of nitriles and formation of primary amines; • describe the elimination reactions of haloalkanes; • describe the mechanism of nucleophilic substitution in haloalkanes (SN1 and SN2); • explain the relative reactivity of primary, secondary and tertiary haloalkanes; • compare the reactivity of chlorobenzene and chloroalkanes in hydrolysis reactions; • explain the use of haloalkanes in the synthesis of organomagnesium compounds (Grignard reagents), and their use in reactions with carbonyl compounds; • describe the uses of fluoroalkanes and chlorofluoroalkanes as inert substances for aerosol propellants, coolants and fire extinguishers; • state the use of chloroalkanes as insecticide such as DDT; • describe the effect of chlorofluoroalkanes in the depletion of ozone layer, and explain its mechanism. Concept Map Uses and Effects of Haloalkanes • DDT • CFC
109 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 16.1 Classification 1 Haloalkanes are organic compounds in which one or more halogen atoms are covalently bonded to saturated carbon atoms. 2 Haloalkanes are sometimes called alkyl halides and have the general formula of CnH2n + 1X, where X is a halogen atom (F, Cl, Br or I). 3 Haloalkenes (sometimes called vinylic halides) are compounds in which the halogen atom is bonded to an unsaturated carbon atom. 4 Haloarenes (or aryl halides) are compounds where the halogen atom is bonded directly to a benzene ring. 5 There are not many naturally occurring haloalkanes, but they are among the most important groups of synthetic compounds. Examples of haloalkanes include plastics, teflon, solvents and a number of anesthetics. 6 Haloalkanes are also important intermediates in organic synthesis. 7 Haloalkanes are classified as primary (1°), secondary (2°) and tertiary (3°) depending on the number of alkyl groups that are attached to the carbon atom carrying the halogen atom. 8 Primary haloalkanes contain !CH2X group. CH H H X R C H H X 9 Secondary haloalkanes contain !CHX group. R C R H X 10 Tertiary haloalkanes contain !CX group. R C R R X & !C!X & C C X R X Exam Tips Exam Tips Primary: Have at least 2 hidrogen atoms bonded to the halogen-carrying carbon. Secondary: Have 1 hydrogen atom bonded to the halogencarrying carbon. Tertiary: No hydrogen atom bonded to the halogencarrying carbon. Info Chem There are no quaternary haloalkane.
110 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 16.2 Nomenclature 1 Haloalkanes are named by adding prefix fluoro, chloro, bromo or iodo to the name of the corresponding alkanes. 2 The parent chain is numbered from the end which gives the first substituent encounter, whether it is a halogen atom or an alkyl group, whichever the lowest number. 3 The common name of some haloalkanes is alkyl halides. 4 Examples are: CH3Cl Chloromethane (Methyl chloride) CH3CH2I Iodoethane (Ethyl iodide) CH3CH2CHCH3 & 2-bromobutane (sec-butyl bromide) Br CH3CHCHCH2CH2CH3 & & 2-iodo-3-methylhexane I CH3 Bromocyclohexane Br 5 There are also polyhaloalkanes where more than one hydrogen atoms are replaced by halogen atoms. Examples are: CH2Cl2 Dichloromethane (Methylene chloride) CHCl3 Trichloromethane (Chloroform) CCl4 Tetrachloromethane (Carbon tetrachloride) 6 When all the hydrogen atoms in the alkane molecules are replaced by halogens, they are named as perhaloalkanes. C2F6 : 1,1,1,2,2,2-hexafluoroethane (Perfluoroethane) C3Cl8 : 1,1,1,2,2,3,3,3-octachloropropane (Perchloropropane) 7 Haloalkenes are named and numbered in such a way that the double bond has the lowest number in the parent chain. CH2RCHCl Chloroethene (Vinyl chloride) CH2RCHCH2Cl 3-chloropropene (Allyl chloride) CH2RCHCH2CHCH3 & 4-bromo-1-pentene and not Br 2-bromo-4-pentene 4-chlorocyclohexene and not 1-chloro-3-cyclohexene Cl Cl2CRCCl2 1,1,2,2-tetrachloroethene (Perchloroethylene)
111 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 8 Examples of haloarenes or aryl halides are: Cl I I Chlorobenzene 1,3-diiodobenzene CH3 Cl Br F 2-chloromethylbenzene 1-bromo-4-fluorobenzene (or 2-chlorotoluene) 9 Note that the following compound is not a haloarene. CH2Cl It is a phenyl-substituted haloalkane. It is named (chloromethyl) benzene. 10 Similarly, the following compound: Br CH3!C!CH3 is not a haloarene. It is named 2-phenyl-2-bromopropane. Quick Check 16.1 1 Classify the following haloalkanes as primary, secondary or tertiary. Which of these compounds are optically active? (a) 2-bromo-2-methylbutane (f) 1-methyl-1-bromocyclohexane (b) Chlorobenzene (g) 3-bromopropane (c) Iodocyclohexane (h) (Iodomethyl)benzene (d) Vinyl chloride (i) 2-chloro-1,3-butadiene (e) 2,2-dimethyl-1-fluoropropane Exam Tips Exam Tips The halogen substituents are named in alphabetical order. bromo, chloro, fluoro and iodo.
112 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 16.3 Structural and Optical Isomerism in Haloalkanes Structural Isomerism 1 Monohaloalkanes with two carbon atoms or less do not exhibit structural isomerism. 2 Structural isomerism is exhibited by monohaloalkanes with three or more carbon atoms. 3 For example, there are two structural isomers for chloropropane, C3H7Cl: CH3CH2CH2Cl CH3—CH—CH3 & Cl 1-chloropropane 2-chloropropane Optical Isomerism 1 Monohaloalkanes with four or more carbon atoms can exhibit optical isomerism. 2 One example is 2-chlorobutane. C* Cl CH3 C2H5 H C* Cl C2H5 CH3 H 16.4 Physical Properties of Haloalkanes General 1 Due to the difference in the electronegativity between carbon and halogens, the carbon-halogen bond is polarised as shown below. δ+C!!Xδ– 2 As a result, the intermolecular forces between haloalkane molecules are stronger than their corresponding alkanes or alkenes which are non-polar. 3 The strength of the van der Waals force between haloalkane molecules with the same number of carbon atoms increases in the order: RF < RCl < RBr < RI Carbon-halogen bond is polarised.
113 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 4 The extent of polarisation depends on how tightly the electrons in the molecule are held by the nucleus. The further the electrons away from the nucleus, the higher is the polarisibility. 5 The size of halogen atoms increases from fluorine to iodine which causes a corresponding increase in the extent of polarisation. As a result, the intermolecular van de Waals forces become stronger. Melting and Boiling Points 1 Due to their small sizes, chloromethane (CH3Cl), chloroethane (C2H5Cl) and bromomethane (CH3Br) are gases. 2 Higher haloalkanes are either liquids or solids. 3 The melting and boiling points of some straight-chain haloalkanes are given in the table below. Name Formula Physical state Melting point/°C Boiling point/°C Fluoromethane CH3F Gas –142 –78 Chloromethane CH3Cl Gas –97 –24 Bromomethane CH3Br Gas –94 4 Iodomethane CH3I Liquid –67 42 Chloroethane CH3CH2Cl Gas –136 12 Bromoethane CH3CH2Br Liquid –118 39 Iodoethane CH3CH2I Liquid –108 72 4 For isomeric haloalkane, the greater the degree of branching, the smaller is the molecular volume, the lower the melting point and boiling point. CH3 & CH3CH2CH2CH2Br CH3!C!CH3 Boiling point = 100 °C & Br Boiling point = 72 °C Solubility in Water and Density 1 Although haloalkane molecules are polar, they cannot form hydrogen bonds with water. They are all insoluble in water but dissolve readily in non-polar organic solvents. 2 All monochloroalkanes are less dense than water. However, bromoalkanes and iodoalkanes are denser than water. Haloalkane Formula Density/g cm–3 Chloromethane CH3Cl 0.916 (at s.t.p.) Chloroethane CH3CH2Cl 0.898 (at s.t.p.) 1-chloropropane CH3CH2CH2Cl 0.891 (at s.t.p.) Bromomethane CH3Br 1.676 Bromoethane CH3CH2Br 1.461 Iodomethane CH3I 2.279 Iodoethane CH3CH2I 1.936 Br Br All haloalkanes are insoluble in water.
114 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Quick Check 16.2 1 Which of the two, 1-chloropropane or pentane will have a higher boiling point? Explain your answer. 2 Explain why 1-iodobutane is immiscible with water. 16.5 Nucleophilic Substitution of Haloalkanes 1 Generally, haloalkanes are more reactive due to the polarisation of the carbon-halogen bond. Cδ+!!Xδ– 2 As a result, the electron-deficient carbon atom carrying the halogen is readily attacked by nucleophiles. 3 Being saturated, the main reaction of haloalkanes is nucleophilic substitution. & & !Cδ+!Xδ– + :Nu– !!: !C!Nu + X– & & 4 The reactivity of haloalkanes towards nucleophiles depends on two factors: (a) The magnitude of the partial positive charge on the electrondeficient carbon atom (b) The strength of the carbon-halogen bond 5 However, experiments show that the bond strength factor outweighs the bond polarity. That is, bond strength is the more important factor that affects the rate of substitution. 6 As such, for a similar alkyl group, R, the reactivity increases in the order: R!F < R!Cl < R!Br < R!I This is due to the decrease in the strength of the carbon-halogen bond when the size of the halogen atom increases, even though the C!F bond is the most polarised and the C!I bond the least. Bond C!F C!Cl C!Br C!I Bond energy/kJ mol–1 467 346 290 228 7 Other than nucleophilic substitution, haloalkanes also undergo elimination reaction under suitable conditions. Info Chem Halogens are more electronegative than carbon. Info Chem The C―X bond gets weaker as the size of the halogen atom gets larger. 2008/P2/Q10(b) 2010/P2/Q9(b) 2013/P3/Q7, Q18 2011/P2/Q9(a) 2015/P3/Q19(b) 2016/P3/Q5, Q6, Q19(a)(b) 2012/P1/Q32
115 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Hydrolysis 1 When a haloalkane is boiled under reflux with aqueous sodium hydroxide, the corresponding alcohol is produced. R!X + NaOH !!Δ !: R!OH + NaX The nucleophile is the OH– ion from NaOH. 2 Examples are: CH3CH2CH2I + NaOH !!: CH3CH2CH2OH + NaI Cl OH + NaOH !!: + NaCl Chlorocyclohexane Cyclohexanol 3 For a given alkyl group, the rate of hydrolysis increases in the order: R—Cl < R—Br < R—I 4 During hydrolysis, the carbon-halogen bond breaks. & & !C!X + OH– !!: !C!OH + X– & & 5 The carbon-halogen bond becomes progressively weaker from chlorine to iodine due to the increase in the size of halogen atoms. Bond C—Cl C—Br C—I Bond energy/kJ mol–1 +346 +290 +228 As a result, the reactivity increases in the order: R—Cl < R—Br < R—I Hydrolysis of Polysubstituted Haloalkanes 1 Boiling 1,2-dibromoethane with aqueous sodium hydroxide produces 1,2-ethanediol. BrCH2CH2Br + 2NaOH !: HO!CH2CH2!OH + 2NaBr 2 However, when 1,1-dibromoethane is boiled with aqueous sodium hydroxide, ethanal (an aldehyde) is produced. 3 The reaction can be thought of producing 1,1-ethanediol in the first step. Br OH & & CH3!C!H + 2NaOH !: CH3!C!H + 2NaBr & & Br OH Production of alcohols Exam Tips Exam Tips OH– acts as a nucleophile. Info Chem When two –OH groups are attached to the same carbon atom, the species is not stable. It will decompose via elimination of H2O. 2008/P1/Q34 2009/P1/Q30 2011/P1/Q31 2014/P3/Q8 2017/P3/Q18(a) 2010/P1/Q32
116 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 However, 1,1-ethanediol is unstable and decomposes to ethanal through the elimination of a water molecule. O!!H OH CH3!C!H !!: CH3!C!H + H2O O R The overall reaction is Br O & ∫ CH3!C!H + 2NaOH !: CH3!C!H + 2NaBr + H2O & Br 4 Hydrolysis of 1,1,1-tribromoethane produces ethanoic acid. Br OH & & CH3!C!Br + 3NaOH !: CH3!C!OH + 3NaBr & & Br OH O!!H OH CH3!C!OH !!: CH3!C!OH + H2O O R The overall reaction is Br O & ∫ CH3!C!Br + 3NaOH !: CH3!C!OH + 3NaBr + H2O & Br Example 16.1 How would you differentiate between 1-chlorobutane and 1-iodobutane? Solution Boil both the compounds separately with aqueous sodium hydroxide. Let the content cool to room temperature. Add in a slight excess of dilute nitric acid to neutralise left over sodium hydroxide. Then add a few drops of aqueous silver nitrate. The test tube containing 1-chlorobutane will give a white precipitate of silver chloride, while the one with 1-iodobutane will give a yellow precipitate of silver iodide. Production of carboxylic acid Exam Tips Exam Tips The test can also be performed using ethanolic silver nitrate (silver nitrate dissolved in a mixture of water and ethanol). R–Cl + Ag+ + H2O : ROH + H+ + AgCl R–I + Ag+ + H2O : ROH + H+ + Agl
117 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 1-chlorobutane: CH3CH2CH2CH2Cl + NaOH !: CH3CH2CH2CH2OH + NaCl NaCl + AgNO3 !: AgCl + NaNO3 1-iodobutane: CH3CH2CH2CH2I + NaOH !: CH3CH2CH2CH2OH + NaI NaI + AgNO3 !: AgI + NaNO3 [Note: As NaOH also gives a precipitate with silver nitrate. It must be removed before adding silver nitrate to test for halide ions.] Example 16.2 How would you carry out an experiment to compare the rate of hydrolysis of 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane? Solution 1-cholorobutane is a primary haloalkane, 2-chlorobutane is a secondary haloalkane and 2-chloro-2-methylpropane is a tertiary haloalkane. Warm the three compounds separately with aqueous ethanolic silver nitrate to precipitate the chlorine as silver chloride. R!Cl + H2O + Ag+ !: R ! OH + AgCl + H+ The time taken for the precipitate to form increases in this order: 2-chloro-2-methylpropane < 2-chlorobutane < 1-chlorobutane Hence, the rate of hydrolysis increases in the following order: 1-chlorobutane < 2-chlorobutane < 2-chloro-2-methylpropane Quick Check 16.3 Draw the structures of the alcohols produced when the following compounds are boiled with aqueous sodium hydroxide. 1 CH3!CH!CH3 3 CH3 Br & I 2 4 Cl Cl !!CH!!Br & & Br Info Chem Rate of hydrolysis: 3° > 2° > 1°
118 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Reaction with Ammonia 1 When a haloalkane is heated with concentrated ammonia in a sealed tube (to prevent the escape of ammonia), substitution takes place, and a primary amine is formed. R!X + 2NH3!!Δ !:R!NH2 + NH4X The nucleophile is the NH3 molecule. 2 For example: CH3CH2CH2I + 2NH3 !: CH3CH2CH2NH2 + NH4I 1-iodopropane Propylamine Cl NH2 + 2NH3 !: + NH4Cl Chlorocyclohexane Cyclohexylamine CH3!CH!CH3 + 2NH3 !: CH3!CH!CH3 + NH4I & & I NH2 2-iodopropane 2-aminopropane 3 However, the amine formed is a stronger nucleophile than ammonia and will attack the haloalkane molecules to produce secondary and tertiary amines, and quaternary ammonium salt. R!X + R!NH2 !: R!N!R + HX & H R!X + R!N!R !: R!N!R + HX & & H R R + & R!X + R!N!R !: R!N!R + X– & & R R 4 For example: CH3CH2CH2NH2 + CH3CH2CH2I !: CH3CH2CH2!N!CH2CH2CH3 + HI & H Dipropylamine Info Chem Reaction mechanism: H3N H H R !X ! H!N !+ !R !X– RNH2 + HX : δ+ δ– !: !:
119 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 (CH3CH2CH2)2NH + CH3CH2CH2I !: (CH3CH2CH2)3N + HI Tripropylamine (CH3CH2CH2)N + CH3CH2CH2I !: (CH3CH2CH2)4N+ I– Tetrapropylammonium iodide 5 In order to get a good yield of primary amine, ammonia must be in large excess. Reaction with Alcoholic Potassium Cyanide 1 When a haloalkane is heated with a solution of potassium cyanide in ethanol, the halogen atom is substituted by the cyanide group to form a nitrile. Alcohol, heat R!X + KCN !!!!!: R!C#N + KX The nucleophile is the cyanide ion, CN– . 2 For example: CH3CH2CH2Br + KCN !!: CH3CH2CH2CN + KBr Butanenitrile CH2Cl CH2CN & & + KCN !: + KCl (Chloromethyl)benzene Benzylnitrile 3 Note that the products have one extra carbon atom compared with the starting haloalkane. This is thus a convenient way to increase the chain length of an organic compound by one carbon. 4 The nitriles can be hydrolysed to carboxylic acids or the salt of carboxylic acids by boiling it with dilute mineral acids or bases respectively. CH3CH2CH2CN + 2H2O + H+ !Δ : CH3CH2CH2COOH + NH4 + Butanoic acid CH3CH2CH2CN + H2O + NaOH !Δ : CH3CH2CH2COO– Na+ + NH3 Sodium butanoate 5 The nitriles can be reduced to primary amines by lithium aluminium hydride in dry ether followed by acidic hydrolysis. R!C#N + 4[H] !: RCH2NH2 For example: CH2CN CH2CH2NH2 & & + 4[H] !: 2-phenylethanamine Use excess of NH3 to get a good yield of primary amine. Exam Tips Exam Tips This is a convenient way to increase the number of carbon atoms in an organic molecule by one. Hydrolysis of nitriles Reduction of nitriles 2008/P1/Q38 2009/P1/Q35
120 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Example 16.3 Give the reagents and conditions to show how you would perform the following conversion. CH3CH2OH !!: CH3CH2COOH Solution Anhydrous alcohol, warm CH3CH2OH + PCl5 !!!!!!!!: CH3CH2Cl + POCl3 + HCl Ethanol, heat CH3CH2Cl + KCN !!!!!!: CH3CH2CN + KCl Dilute sulphuric acid, heat CH3CH2CN + 2H2O + H+!!!!!!!: CH3CH2COOH + NH4 + 1 Draw the structures for compounds A to F. Ethanol Dilute H2SO4 (a) CH3CHCH3 + KCN !!!: A !!!!: B & Br (b) Ethanol Dilute H2SO4 + KCN !!!: C !!!!: D Cl Cl (c) Ethanol [H] !CH2CH2CH! + KCN !!!: E !!: F & I 2 Show how you would carry out the following conversion. (a) CH2COOH !!: (b) !!: CH3!C!COOH CH3 (c) CH2RCH2 !: HOOC!CH2CH2!COOH (d) Cl CH2NH2 & & & & !!: Quick Check 16.4 Info Chem The final product has one carbon atom more than the starting material.
121 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 16.6 Elimination Reaction 1 When a haloalkane is heated with ethanolic potassium hydroxide, elimination occurs and an alkene is formed. & & Ethanol, heat & & !C!C! + KOH !!!!!: CRC + KX + H2O & & & & H X 2 In this reaction, the OH– acts as a base and attacks the hydrogen atom that is bonded to the carbon atom adjacent to the one carrying the halogen. H & & HO– + !C!!C! !: + H2O + X– & & C C R X 3 Take note that elimination follows the Zaitsev rule which states that, in an elimination reaction, the major product is the alkene that has the greatest number of alkyl groups attached to the unsaturated carbon atoms. For example, elimination in 2-chlorobuthane will produce 2-butene as the major product and 1-butene as the minor product. CH3!CHRCH!CH3 (Major product) CH3CH2CHCH3 –HCl & Cl CH3!CH2!CHRCH2 (Minor product) Quick Check 16.5 1 Draw the structures of the major products formed (if any) when the following haloalkanes are subjected to elimination by ethanolic potassium hydroxide. (a) Iodoethane (e) 2-iodo-1-methylcyclohexane (b) 2-bromopropane (f) (Chloromethyl)benzene (c) 1-chlorobutane (g) 2-phenyl-1-iodoethane (d) Chlorocyclohexane 2 Show how you would carry out the following conversions. (a) Ethane to ethene (b) 1-butene to 2-butene (c) 2-methyl-1-chloropropane to 2-methyl-2-propanol (d) Bromocyclohexane to 1,2-dibromocyclohexane Exam Tips Exam Tips In this mechanism, all the three pairs of electrons (represented by three curved arrows) move simultaneously and there is no intermediate species. 2009/P1/Q30 2016/P3/Q19(c)
122 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 16.7 Mechanism of Nucleophilic Substitution 1 The general equation for nucleophilic substitution reaction is & & !Cδ+!Xδ– + :Nu– !: !C!Nu + X– & & Or: R!X + :Nu– !: R!Nu + X – 2 The substitution can take place via two different mechanisms: SN2 and SN1 (pronounced as S N two and S N one). 3 They are the short-form for nucleophilic substitution bimolecular and nucleophilic substitution unimolecular respectively. SN 2 Mechanism 1 In SN2 mechanism, the rate of reaction is directly proportional to the concentration of the haloalkane and the nucleophile. The rate equation is Rate = k[RX][Nu– ] 2 The reaction is first order with respect to RX and Nu– . That means the rate-determining step involves both species. 3 The proposed mechanism is: (a) The nucleophile attaches itself to the carbon atom carrying the halogen atom to form a five coordinated intermediate or an activated complex. & !C!X + Nu– !!: Nu– C X & & (b) In the activated complex, the carbon-halogen bond is in the process of breaking (lengthening), while the carbonnucleophile bond is in the process of forming (shortening). (c) Eventually, the C!Nu bond is completely formed and the C!X bond breaks completely to form the products. & & Nu– C X 9: !!C!!Nu + X– & & & & VIDEO Nucleophilic Substitution
123 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 4 The energy profile is as follows: Nu C X – Energy !C!X + Nu– !C!Nu + X– SN1 Mechanism 1 In SN1 mechanism, the rate of reaction is independent of the concentration of the nucleophile. The rate equation is Rate = k[RX] 2 The reaction is first order with respect to the haloalkane, but zero order with respect to the nucleophile. 3 This means that the rate-determining step of the reaction does not involve the nucleophile. 4 The proposed mechanism is (a) heterolytic fission of the carbon-halogen bond to produce a carbonium ion, & Slow & !C!X !!: !C+ + X – & & (b) the carbonium ion formed then combines with the nucleophile to give the product. & Fast & !C+ + Nu– !!: !C!Nu & & Exam Tips Exam Tips The energy profile for SN2 has one hump. This is an example of one-step reaction. However, the nucleophile must be present for the reaction to occur. This is a slow step as it involves bond breaking. This is a fast step as it involves the combination of two oppositely-charged ions. 2010/P1/Q49 2010/P2/Q9(a) 2017/P3/Q18(b)(ii)
124 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 5 The energy profile is: 16.8 Reactivity of Primary, Secondary and Tertiary Haloalkanes 1 Given a particular haloalkane, we can predict the mechanism that it follows. 2 For SN1 to take place, the carbonium ion formed in the intermediate has to be relatively stable. 3 The stability of a carbonium ion decreases in the order: Tertiary > secondary > primary H R R & p p R : C+ R : C+ R : C+ & & q H H R Primary Secondary Tertiary carbonium ion carbonium ion carbonium ion 4 Therefore, tertiary haloalkanes would favour SN1 mechanism. 5 The other factor to consider is steric hindrance. In SN2 mechanism, the nucleophile must approach the electron-deficient carbon atom for it to form a new bond with it. 6 The approach of the nucleophile towards a tertiary carbon atom is more difficult because of the presence of three bulky alkyl groups that shield the electron-deficient carbon. This is known as steric hindrance. Exam Tips The energy profile for SN1 has two humps. This is an example of a two-step reaction. Energy !C!X + Nu– !C+ + X– + Nu– !C!Nu + X– 2013/P3/Q16(a) 2017/P3/Q18(b)(i) The rate of nucleophilic substitution increases in the order: 1° < 2° < 3°
125 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 C R R R X Nu– Approach is blocked 7 However, for primary haloalkanes, only one R group shields the electron-deficient carbon atom. Hence, the approach of the nucleophile is not hindered. C R H H X Nu– 8 The preference mechanism for nucleophilic substitution is summarised below. CH3! Primary Secondary tertiary SN1 SN2 Example 16.4 Arrange the following haloalkanes in the order of increasing reactivity towards SN1 reaction. CH3 & CH3Cl, CH3CH2Cl, CH3CHCH3, CH3!C!CH3 & & Cl Cl Solution CH3 & CH3Cl < CH3CH2Cl < CH3CHCH3 < CH3!C!CH3 & & Cl Cl Primary Primary Secondary Tertiary Steric hindrance Primary haloalkanes favour SN2. Tertiary haloalkanes favour SN1.
126 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Quick Check 16.6 1 Predict the type of nucleophilic substitution mechanism of the following compounds. (a) Iodomethane (c) 1-methyl-1-bromocyclohexane (b) 2-methyl-2-chloropropane Example 16.5 Describe the mechanism for the hydrolysis of iodoethane to ethanol by aqueous sodium hydroxide. Solution Iodoethane is a primary haloalkane. Hence, it undergoes SN2 mechanism. The first step of the reaction involves the collision of an iodoethane molecule and the nucleophile OH– to form an intermediate (or transition state complex). C!!I Slow H H H H CH3 OH– CH3 !!: HO–!!C!!I In the intermediate species, the carbon-oxygen bond is in the process of forming, while the carbon-iodine bond is in the process of breaking. The reaction is completed when the carbon-oxygen bond is formed completely and the carbon-iodine bond breaks completely. H H CH3 Fast HO–!!C!!I !!: H CH3 OH H C + I– Example 16.6 Describe the mechanism for the hydrolysis of 2-chloro-2-methylpropane to 2-methyl-2-propanol. Solution 2-chloro-2-methylpropane is a tertiary haloalkane. Hence, it undergoes SN1 mechanism. The first step involves the heterolytic breakage of the C!Cl bond in the haloalkane to generate a carbonium ion intermediate. CH3 CH3 & Slow & CH3!C!CH3 !!: CH3!C!CH3 & + Cl
127 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 This is a slow step because it involves the breaking of a covalent bond. The carbonium ion formed reacts with the nucleophile, OH– , to form the alcohol. CH3 CH3 & Fast & CH3!C!CH3 + OH– !!: CH3!C!CH3 + & OH 16.9 Reactivity of Chlorobenzene and Chloroalkanes in Hydrolysis Reactions 1 Haloarenes are compounds in which the halogen atom is bonded directly to a carbon atom of the benzene ring. An example is chlorobenzene. Cl 2 When compared to haloalkanes, haloarenes are relatively unreactive towards nucleophilic substitution. 3 For example, chlorobenzene does not react with aqueous sodium hydroxide, ammonia, alcoholic potassium cyanide or alcoholic sodium hydroxide. 4 The lack of reactivity of chlorobenzene is because one of the p orbital on the chlorine atom is in the same plane as the p orbitals in the benzene ring. 5 The electrons in the p orbital of the chlorine can interact with the delocalised π electrons of the benzene ring. Cl Cl 6 This delocalisation increases the electron density of the carbonchlorine bond, which now has partial double bond character, and is more difficult to break. 7 The delocalisation also reduces the partial positive charge on the halogen-carrying carbon atom, making it less susceptible to attack by nucleophiles. 8 Furthermore, the high electron density of the benzene ring repels any approaching nucleophiles. CH2Cl is not a haloarene. Haloarenes are inert towards nucleophilic substitution. The carbon-chlorine bond has partial double bond character. Repulsion between approaching nucleophile and high electron density of the benzene ring 2008/P1/Q32 2010/P1/Q32 2013/P3/Q8 2014/P3/Q8
128 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Quick Check 16.7 Draw the structures of the products formed when chlorobenzene reacts with 1 1-iodopropane in the presence of anhydrous aluminium chloride, 2 ethanoyl chloride in the presence of anhydrous aluminium chloride, 3 chlorine in the presence of iron powder. 16.10 Organometallic Compounds Organometallic compounds are compounds containing carbon-metal bonds in their structure. One example is Grignard reagents. Grignard Reagents 1 Grignard reagents are organomagnesium compounds. Their molecules contain polar carbon-magnesium covalent bond. 2 Grignard reagent can be prepared by adding magnesium turnings to a solution of haloalkane in dry ether. 3 An exothermic reaction occurs and a cloudy solution containing the Grignard reagent is obtained. Dry ether R!X + Mg !!!!: R!MgX A little iodine is usually added as a catalyst. 4 The reaction has to be carried out in dry ether because Grignard reagents are highly reactive and will react with water. 5 Examples of Grignard reagents: Dry ether CH3CH2CH2CH2Br + Mg !!!!: CH3CH2CH2CH2MgBr Butylmagnesium bromide Cl MgCl + Mg !: Phenylmagnesium chloride 6 The carbon-magnesium bond in Grignard reagent is polarised as shown below. & !Cδ–!Mgδ+ + X– & 7 In reactions involving Grignard reagent, it acts as a carbanion, where the carbon atom carries a partial negative charge. It will attack centres that are electron-deficient. & 9Cd–9Mgd+ & Grignard reagents decompose in the presence of water. Grignard reagent acts as a carbanion supplier: RMgX : R– + Mg2+ + X– Exam Tips Exam Tips Reaction involving Grignard´s reaction must be in anhydrous condition, otherwise the reagent would decompse to produce a hydrocarbon: R!MgX + H!OH : RH + Mg(OH)X
129 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Example 16.7 Draw the structures of the Grignard reagents form when the following halohydrocarbons react with magnesium in dry ether. (a) 2-iodopropane (b) 2-methyl-2-bromobutane (c) Chlorobenzene (d) Chloroethene Solution (a) CH3!CH!CH3 (c) MgCl & MgI CH3 & (b) CH3!CH2!C!CH3 & MgBr (d) CH2RCH!MgCl Reactions Involving Grignard Reagents 1 Grignard reagents are useful intermediates in many organic synthesis. 2 The Grignard reagent is not isolated from the ethereal solution. Instead, the required reagent is added to the ethereal solution. With Methanal 1 When methanal (formaldehyde) is added to the Grignard reagent and warmed, a primary alcohol is produced. 2 Methanal is the first member of the aldehyde series with the structural formula of H!C!H ∫ or HCHO O The RO group is called a carbonyl group. 3 Due to the higher electronegativity of oxygen, the carbonyl group is polarised as shown below. H!Cδ+!H ∫ Oδ– 4 The carbanion of the Grignard reagent attacks the electrondeficient carbon atom of the carbonyl group to produce a magnesium alkoxide intermediate. The common name for methanal is formaldehyde. C R C INFO Grignard Reagents
130 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Rδ–!MgX + H!C R δ+!H !!: H!C!H O R O– [MgX]+ 5 This is then followed by warming with a dilute mineral acid which decomposes the magnesium alkoxide intermediate. H & R!C!O– [MgX]+ + HO!H !: RCH2OH + Mg(OH)X & H 6 For example: CH3CH2MgI + HCHO + H2O !: CH3CH2CH2OH + Mg(OH)I Ethylmagneisum iodide 1-propanol MgCl CH2OH + HCHO + H2O !: + Mg(OH)Cl Phenylmagnesium chloride Phenylmethanol (or benzyl alcohol) 7 Note that the alcohols produced have one carbon atom more than the Grignard reagent. With Aldehydes other than Methanal 1 Treatment of Grignard reagent with aldehydes (except methanal) followed by acidic hydrolysis gives a secondary alcohol. 2 Aldehydes (other than formaldehyde) have the general formula of R!C!H ∫ O 3 Their reactions with Grignard reagent can be summarised as Rfi!MgX + R!C R !H !!: R!C!H O Rfi O– [MgX]+ R!C!Rfi + H2O !!: R!C!Rfi + Mg(OH)X H OH H O– [MgX]+ Overall equation: RMgX + HCHO + H2O !: RCH2OH + Mg(OH)X O OH ' RMgX & H–C–H 99: H–C–H & R O OH ' RMgX & R'–C–H 99: R'–C–H & R
131 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 4 The overall equation is R9MgX + RCHO + H2O !: R9!CH!R + Mg(OH)X & OH 5 For example: CH3CH2MgI + CH3CHO + H2O !!: CH3CH2!CH!CH3 + Mg(OH)I & OH Ethylmagnesium iodide 2-butanol With Ketones 1 Ketones are carbonyl compounds with the general formula of R!C!R9 ∫ O (where R and R9 = Alkyl or aryl groups) 2 The simplest ketone is propanone (common name: acetone). CH3!C!CH3 ∫ O 3 Reaction of ketones on Grignard reagent produces tertiary alcohols. Rfi!MgX + R!C R !R !!: R!C!R O Rfi O– [MgX]+ R!C!R + H2O !!: R!C!R + Mg(OH)X Rfi OH Rfi O– [MgX]+ 4 The overall equation is R & R9MgX + RCOR + H2O !: R9!C!R + Mg(OH)X & OH 5 For example: CH3 & CH3MgI + CH3!C!CH3 + H2O : CH3!C!CH3 + Mg(OH)I ∫ & O OH Propanone (Acetone) 2-methyl-2-propanol O OH ' R' MgX & R–C–R 999: R–C–R & R' 2017/P3/Q7
132 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 6 Summary of the reactions of Grignard reagent: Reagent Product Methanal Primary alcohol Other aldehydes except methanal Secondary alcohol Ketone Tertiary alcohol Carbon dioxide Carboxylic acid Reaction with carbon dioxide: RMgX + CO2 + H2O 9: RCOOH + Mg(OH)X Quick Check 16.8 1 Give the formula of the products formed in each of the following reactions involving Grignard reagents. (a) CH3!CH!CH3 and methanal & MgCl (b) !CH2MgBr and formaldehyde (c) CH3 & !!C!!CH2MgBr and CH3CH2CH & ∫ CH3 O (d) CH3 & !!C!!CH2MgBr and !!C!!CH3 & ∫ CH3 O 2 An alcohol has the following structure: CH3 & & !!C!! & & OH Show how you would prepare this alcohol from a suitable Grignard reagent. 16.11 Uses of Haloalkanes 1 Haloalkanes are important intermediates in most organic synthesis. 2014/P3/Q18(b)(c)
133 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 2 Chlorofluorocarbons or CFCs are used as propellant in aerosol cans, in fire extinguishers and as refrigerants as well as cleaning agents. This is because CFCs are chemically inert, odourless, non-flammable and volatile. One example of CFC is dichlorodifluoromethane (which is also known as Freon-12). F & Cl!C!Cl & F 3 Haloalkanes are used in the manufacture of pesticides such as DDT and chlordane. 4 Haloalkanes are used as anesthetics. One example is 2-bromo-2-chloro-1,1,1-trifluoroethane (common name: halothane). F Br & & F!C!C!H & & F Cl 16.12 DDT 1 DDT (DichloroDiphenylTrichloroethane) has the following structure. It is used as an insecticide. Cl!! !!C!! H Cl !!Cl !! Cl!!C!!Cl !!!! 2 DDT is a white crystalline solid which is odourless and tasteless. It was first prepared in 1874. But it was not until the late 1930s its potential as an insecticide was recognised. 3 DDT is extremely poisonous against mosquitoes (that spread malaria), lice (that spread typhus) and houseflies, but it does not affect mammals and thus can be used safely around farms. 4 It is so effective in killing crop-destroying pests that crop yields in many areas of the world increased dramatically. DDT kills selectively. Info Chem C!F and C!Q bonds are strong and need a lot of energy to break.
134 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 5 However, DDT is stable, non-biodegradable and insoluble in water. It remains in the treated soil for decades. 6 When ingested by microorganisms, birds, fish or other small animals, it remains and accumulates in the fatty tissue of the organisms. 7 These small animals when ingested by other animals higher up in the food chain, including humans, over a long period of time will be fatal because of biomagnifications. 8 Furthermore, due to the indiscriminate use of DDT, many species of insects develop resistance to DDT. 9 Due to these problems, almost all nations on earth have now banned the use of DDT for agricultural purpose. However, it is still used in certain areas to control disease-spreading insects. 16.13 Effects of CFC on the Environment 1 Of all the haloalkanes, the chlorofluorocarbons manufactured under the trade name of Freons are the most widely known. 2 CFCs are non-toxic, chemically inert, odourless, tasteless and non-corrosive. 3 They are used primarily as solvents in cleaning agents, as refrigerants in refrigerating systems and also as propellant for aerosol sprays. 4 Some examples of CFC are: CCl3F Trichlorofluoromethane (Freon-11) CCl2F2 Dichlorodifluoromethane (Freon-12) CCl2FCCl2F 1,2,2-trichloro-1,2,2-trifluoroethane 5 Concern about the environmental effect of CFC arose in the 1970s when it was found that more than 5 × 105 kg of Freons per year was released into the atmosphere. 6 In 1974, two American scientists, Mario Molina and Sherwood Rowland demonstrated that CFC destroys the ozone layer in the stratosphere (10 – 50 km above the Earth surface). [Rowland, Mario and Paul Crutzen (a Ducth chemist) were awarded the Nobel Prize for Chemistry in 1995.] 7 Scientists divide our atmosphere into four levels based on the distance from the Earth surface, temperature variation and composition. Disadvantages of DDT Properties of CFC Uses of freon Destruction of ozone 2014/P3/Q17(c) INFO Freon and Health
135 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 km 500 80 50 10 0 Troposphere Stratosphere Mesosphere Thermosphere (Ion sphere) Distance from the surface of the Earth 8 80% of the gases in our atmosphere are found in the troposphere which extends to 10 km above the Earth surface. 9 Above the troposphere is the stratosphere which contains mainly oxygen, nitrogen and ozone. 10 Ozone, O3 is an allotrope of oxygen. It is formed in the stratosphere from the following reactions. (a) Oxygen molecules absorb ultraviolet radiation from the Sun and undergo homolytic fission to produce oxygen atoms. Ultraviolet radiation O2(g) !!!!!!!: 2O(g) Different levels of the atmosphere Info Chem Ozone is an oxidising agent.
136 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 (b) The oxygen atom (free radical) combines with an oxygen molecule to form ozone. O2(g) + O(g) !: O3(g) 11 However, the ozone which is formed undergoes decomposition, by absorbing ultraviolet radiation from the Sun, to reproduce oxygen molecules and oxygen atoms. O3(g) !: O2(g) + O(g) 12 Hence in the stratosphere, ozone is constantly produced and destroyed. In the process, it absorbs short-wavelength ultraviolet radiation from the Sun and prevents harmful rays from reaching the surface of the Earth which can induce skin cancer, causes genetic mutations, destroys aquatic life, crops and other forms of vegetation. 13 The formation and destruction of ozone by natural process is a dynamic equilibrium that maintains a constant concentration of ozone in the stratosphere. 14 However, it was found that CFC which is released in the atmosphere is also involved in the destruction of ozone. The dynamic equilibrium is disturbed. The rate of destruction of ozone now exceeds its formation leading to the depletion of ozone layer in the stratosphere. 15 As a result, more short-wavelength ultraviolet radiation from the Sun now reaches the Earth surface. 16 CFC, due to the strong carbon-halogen bonds is chemically inert. When released into the atmosphere, it drifts unchanged up to the stratosphere, where the molecules absorb ultraviolet radiation and undergo decomposition to produce chlorine atoms (free radicals). CCl2F2 !: • CClF2 + • Cl CFCl3 !: • CFCl2 + • Cl 17 The reactive chlorine atoms formed then undergo the following reactions. O3 + • Cl !: O2 + • ClO • ClO + O !: • Cl + O2 The overall reaction is the destruction of ozone. O3 + O !: 2O2 18 In the above reactions, the chlorine atom acts as a catalyst in the chain reaction. One chlorine atom can destroy up to about 100 000 O3 molecules before it is removed by other reactions. 19 Due to the environmental impact of CFC, countries around the world are in the process of limiting its usage or completely removing it. Overall equilibrium: 3O2(g) L 2O3(g) Info Chem Another function of ozone in the stratosphere is to keep the Earth warm by preventing the heat that is reflected from the Earth surface from being re-radiated into space. INFO Ozone Depletion
137 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 SUMMARY SUMMARY 20 Other suggestions include substituting CFC with something harmless. An example is the use of hydrofluorocarbons (HFC) or hydrocarbons which does not contain chlorine. One of the hydrofluorocarbons is 1,1,1,2-tetrafluoroethane. F F & & F!C!C!H & & F H Info Chem The C!H and C!F bonds are much stronger than the C!Cl bond. 1 The functional group of haloalkanes is halogen atom, X (X = Cl, Br, I). 2 Haloalkanes are polar molecules. 3 The main reactions are • nucleophilic substitution • elimination • formation of organometallic compounds 4 Organolithium compounds are more reactive than the corresponding organomagnesium compounds (Grignard reagent) because lithium is more electropositive than magnesium. 5 The reactivity towards nucleophilic substitution is • RI > RBr > RCl • 3° > 2° > 1° 6 Primary haloalkanes undergo SN2 mechanism. 7 Tertiary haloalkanes undergo SN1 mechanism. 8 Haloarenes are inert towards nucleophilic substitution. Reactions of Haloalkane, R!X Reagent Product Remarks NaOH(aq), boiling R!OH Nucleophilic substitution (Hydrolysis) NH3(l), heat in a sealed tube R!NH2 Nucleophilic substitution (Ammonia in excess) KCN(aq) in ethanol, boiling R!CN Nucleophilic substitution RCN can be reduced to RCH2NH2 RCN can be hydrolysed to RCOOH KOH in ethanol, heat C C R Elimination Mg(s) in ether, heat RMgX Grignard Reagent Reactions of Grignard Reagent, RMgX Reagent Remarks HCHO (methanal), heat RCH2OH R9CHO (other aldehydes), heat R!CH!R9 & OH R9!C!R0 (ketones), heat ∫ O R9 & R!C!R0 & OH CO2, heat RCOOH
138 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 STPM PRACTICE 16 Objective Questions 1 2-chloro-2-methylpropane and 1-chlorobutane are separately boiled with aqueous sodium hydroxide. Which of the following can be used to differentiate the products of the reactions? A Phosphorus pentachloride B Tollens reagent C Alkaline iodine D Acidified potassium manganate(VII) 2 Which of the following reagents does not react with both benzyl chloride and chlorobenzene under suitable conditions? A Magnesium in ether B Sodium metal C Bromine in the presence of iron(III) bromide D Potassium hydroxide 3 A reaction scheme is shown below. KCN, ethanol Dilute H2SO4 CH3CH2CH2I X Y What are the structures of X and Y? X Y A CH3CH2CH2I – K+ CH3CH2CH2SO3H B CH3CH2CH2CN CH3CH2CH2COOH C CH3CH2CH2CN CH3CH2CH2COO– K+ D CH3CHCH3 & CN CH3CHCH3 & COOH 4 Which of the following is not a property of ‘Freons’? A They are chemically inert. B They are easily liquefied by pressure. C They have a pungent smell. D They are volatile. 5 Freons (chlorofluorocarbon) are used as cleaning agents. Which of the bonds in the structure below has the lowest bond energy? H H & & A & B & C Cl !!C !!C !!F & & & & H H 6 When 1,2-diiodoethane is boiled under reflux with aqueous sodium hydroxide, ethane-1,2- diol is produced. What is the product formed when 1,1-diiodoethane is boiled under reflux with aqueous sodium hydroxide? A Ethane-1,1-diol B Ethanal C Ethanoic acid D No reaction 7 What is the pro duc t for me d w hen 1,1,1-trichloroethane is boiled under reflux with aqueous sodium hydroxide? A CH3COOH B CH3CH2ONa C CH3COONa D CH3COCH3 8 Which reaction is a nucleophilic substitution reaction? A Benzene and a mixture of concentrated nitric acid and concentrated sulphuric acid at 55 °C B Chloromethylbenzene and ethanolic potassium cyanide C Propanone and aqueous hydrogen cyanide in the presence of sodium cyanide D Chlorocyclohexene with acidified potassium manganate(VII) 9 Which compound reacts with methylmagnesium iodide to produce 3-methyl-2-butanol? A CH3CH2COCH3 B CH3COCH3 C CH3CH2CH2CHO D (CH3)3CH2OH 10 Which statement about the compound below is not correct? CH3CH2CH2CH2MgI A It contains the carbon-magnesium covalent bond. B It can be used to prepare alcohols. C It reacts vigorously with water. D It is a convinient source of carbonium ion. D
139 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 11 Which of the following would react with C6H5MgCl, followed by acidic hydrolysis to produce a tertiary alcohol? A Carbon dioxide B 2-Propanol C Methanal D 2-methyl-2-pentanol 12 Which of the following accounts for the inertness of chlorobenzene towards hydrolysis by sodium hydroxide? I Chlorobenzene is unsaturated. II Chlorine is more reactive than sodium hydroxide. III The partial double bond character of the C—Cl bond IV The stability of the benzene ring A I B III C II and IV D I, III and IV 13 Iodoethane is more reactive than chloroethane towards hydrolysis with sodium hydroxide. C2H5X + NaOH 9: C2H5OH + Na+ + X– (X = Cl or I) This is because A the C—I bond is more polar than the C—Cl bond B I – is larger than Cl– C the C—Cl bond is shorter than the C—I bond D chlorine is a stroger oxidising agent than iodine 14 Consider the following reaction: C4H9I + 2NH3 9: C4H9NH2 + NH4I Which of the following statements are true about the reaction? I Ammonia is in excess. II The reaction involves nucleophilic substitution. III The attacking species is NH2 – . A I and II B I and III C II and III D I, II and III 15 Tetrafluoromethane, CF4, is used widely as solvent. Which of the following statements explains why tetrafluoromethane is chemically inert? A The strength of the carbon-fluorine bond. B Fluorine is the most electronegative element. C The carbon-fluorine bond is non-polar. D It has a low boiling point. 16 Which of the following haloalkanes undergoes SN1 mechanism? A CH3CH2CH2CH2Cl B CH3—CH—CH2CH3 & Br C (CH3)3CBr D C6H5I 17 C2H5MgBr is an example of a Grignard reagent. Which statements are true of the compound? I It contains the carbon-magnesium covalent bond. II It can be prepared by the reaction between magnesium and bromoethane in dry ether. III It reacts with ketones to form tertiary alcohols. A I and II B I and III C II and III D I, II and III
140 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 Structured and Essay Questions 1 (a) Draw the structural formulae for chlorocyclohexane and chlorobenzene. (b) Device a simple experiment to compare the ease of hydrolysis of the two chlorocompounds above. State the expected observations and offer an explanation for the results. 2 Two isomeric compounds X and Y have molecular formula of C7H7Br. X undergoes hydrolysis when boiled with aqueous sodium hydroxide. Y, on the other hand has no reaction with sodium hydroxide under the same conditions. (a) Identify A and B and draw their structures. (b) Show schematically how you would convert benzene to X. 3 1-Chloro-1-methylcyclohexane reacts with aqueous sodium hydroxide to form P, and with an alcoholic solution of sodium hydroxide to form Q. (a) Name the type of reaction involved with (i) aqueous sodium hydroxide and, (ii) alcoholic sodium hydroxide. (b) Draw the structures of the products formed when 1-chloro-1-methylcyclohexane reacts with (i) aqueous sodium hydroxide and, (ii) alcoholic sodium hydroxide. (c) Using balanced equations, describe the reaction mechanism for the reaction between 1-chloro-1-methylcyclohexane and aqueous sodium hydroxide. 4 (a) Starting with ethanol, show how you would prepare the following compounds. (i) Bromoethane (ii) Propanoic acid (b) Bromoethane reacts with an alloy of sodium and lead to produce a compound having the following composition by mass: C, 29.7%; H, 6.2%; Pb, 64.1% (i) Determine the formula of the product. (ii) Draw the structural formula of the product. (iii) State one important use for the product in the automotive industry. 5 Isopropyl benzene (a.k.a. Cumene) reacts with chlorine in the presence of ultra-violet light to produce monosubstituted products, W and X. W is optically active but X is not. When W and X are treated separately with aqueous sodium hydroxide, Y and Z are formed respectively. (a) Determine the structural formulae of W, X, Y and Z and write equations for the reactions that take place. (b) Describe the mechanism for the production of W and X. Explain which of the two is the major product. (c) Draw a reaction scheme to show how you would convert W to X.
141 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 6 (a) Draw the structural formulae of aromatic compounds with the molecular formula of C7H7Cl. (b) Draw the structure of the products formed (if any) when the above isomers are boiled with aqueous sodium hydroxide. (c) One of the isomers can be converted into an acid with the molecular formula of C8H8O2. (i) Identify the isomer. (ii) Suggest a reaction scheme to show how the conversion can be carried out. 7 Draw the full structural formulae of all compounds having the molecular formula of C4H9Cl. (a) Which of the isomers give one product only when heated with ethanolic potassium hydroxide? (b) Which of the isomers is optically active? (c) Which isomer reacts with magnesium in dry ether followed by formaldehyde (methanal) to form 2,2-dimethyl-1-propanol? (d) Which isomer on hydrolysis with aqueous sodium hydroxide produces a tertiary alcohol? (e) Which isomers on hydrolysis with aqueous sodium hydroxide produce primary alcohols? 8 An optically active compound, A (Relative molecular mass = 168.5), has the following composition by mass: C: 71.2%; H: 7.72%; Cl: 21.08% A reacts with aqueous sodium hydroxide to form B via SN1 mechanism. A reacts with magnesium in dry ether followed by propanal to form C. (a) Deduce the molecular formula of A. (b) Deduce the structures of A, B and C and write balanced equations for all the reactions involved. (c) A undergoes elimination reaction with ethanolic potassium hydroxide to produce an alkene, D, that exhibits cis-trans isomerism. Deduce the structure of D and draw the structures of the isomers. 9 Show, by giving equations and conditions, for the following conversions. (a) 1-iodobutane to 2-iodobutane (b) 1-propanol to 2-propanol (c) Benzene to chlorobenzene (d) Benzene to (chloromethyl)benzene (e) 1-iodopropane to butanoic acid 10 (a) What do you understand by ‘Grignard reagent’? (b) Starting with 1-iodopropane, suggest how you would prepare a sample of Grignard reagent. (c) Write equations to illustrate the reactions of the Grignard reagent in (b) with (i) methanal (ii) propanone 11 (a) The chloro-fluoro derivatives of methane, ethane and propane are called Freon. An example is dichlorodifluoromethane (commercially known as Freon-12, freezing point = –29.8°C). It is prepared by the reaction between carbon tetrachloride and antimony trifluoride (SbF3) in the presence of antimony pentachloride (SbCl5) as catalyst. The only by-product is antimony trichloride. (i) Draw the structure of a molecule of Freon-12 indicating the shape and the type of hybridisation undergo by the carbon atom.