142 Chemistry Term 3 STPM Chapter 16 Haloalkanes 16 (ii) Write a balanced equation for the manufacture of Freon-12 from the above reaction. (iii) Freon-12 is used extensively as refrigerant, cleaning agent, sterilisation, as propellant in aerosol cans and as cleaning agent. State the properties of Freon-12 that enable it suitable for the uses mentioned. (b) Ozone, O3, is constantly produce and destroy in the stratosphere. Ozone is responsibe for shielding the Earth from harmful ultra-violet radiation from the Sun. Another important function of ozone in the stratosphere is keeping the Earth warm. (i) Describe how ozone is produced and destroyed in the stratosphere and how it helps in preventing ultra-violet radiation from reaching the Earth. (ii) Explain how does Freon (such as dichlorodifluoromethane) destroy the ozone in the stratosphere. (iii) Explain briefly how ozone in the stratosphere keeps the Earth warm. 12 Compound X has the molecular formula of C4H9Cl. (a) Draw all possible structural isomers of X. (b) Which of the isomers in (a) is optically active? Explain your answer. (c) One of the isomers in (a) reacts with hot, ethanolic potassium hydroxide to produce three isomers of unsaturated hydrocarbons. (i) Identify the isomer. (ii) Draw the structures of the three isomers of the unsaturated hydrocarbons. 13 2-methyl-2-iodopropane reacts with warm aqueous sodium hydroxide to produce 2-methyl-2-propanol. (a) Name and describe the mechanism of the reaction. (b) Draw the reaction-energy profile for the reaction. (c) What would be the effect on the rate of reaction if 2-methyl-2-iodopropane is replaced with 2-methyl-2-chloropropane? Explain your answer.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 17 CHAPTER HYDROXY COMPOUNDS 17 Concept Map Hydroxy Compounds Comparison between Phenol and Ethanol Uses of Alcohols and Phenols Introduction to Hydroxy Compounds • Nomenclature • Structural and optical isomerism in alcohols • Physical properties of hydroxy compounds Phenols • Relative acidity of water, phenol and ethanol • Reactions of phenols • Tests for phenols [bromine water, aqueous iron(III) chloride] • Preparation of phenol (cumene process) Alcohols • Reactions of alcohols • Reactivity of primary, secondary and tertiary alcohols to form haloalkanes • Synthesis and manufacture of ethanol • Preparation of ethanol by fermentation Learning earning Outcomes Students should be able to: Introduction to hydroxy compounds • write the general formula for hydroxy compounds; • name hydroxy compounds according to the IUPAC nomenclature; • describe structural and optical isomerism in hydroxy compounds; • state the physical properties of hydroxy compounds. Alcohols • classify alcohols into primary, secondary and tertiary alcohols; • classify the reactions of alcohols whereby the RO−H bond is broken: formation of an alkoxide with sodium, esterification, acylation, oxidation to carbonyl compounds and carboxylic acids; • classify the reactions of alcohols whereby the R−OH is broken and −OH is replaced by other groups: formation of haloalkanes and dehydration to alkenes and ethers; • explain the relative reactivity of primary, secondary and tertiary alcohols as exemplified by the reaction rate of the alcohols to give haloalkanes, and the reaction products of KMnO4/K2Cr2O7 oxidation in the presence of sulphuric acid; • explain the reaction of alcohols with the structure CH3CH(OH)9 with alkaline aqueous solution of iodine to form triiodomethane; • describe the laboratory and industrial preparation of alcohol as exemplified by ethanol from the hydration of ethane; • describe the synthesis of ethanol by fermentation process; • state the uses of alcohols as antiseptic, solvent and fuel. Phenols • explain the relative acidity of water, phenol and ethanol with particular reference to the inductive and resonance effects; • describe the reactions of phenol with sodium hydroxide, sodium, acyl chlorides and electrophilic substitution in the benzene ring; • describe the use of bromine water and aqueous iron(III) chloride as tests for phenol; • describe the cumene process in the manufacture of phenol; • explain the use of phenol in the manufacture of cyclohexanol, and hence, nylon-6,6.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 144 17 17.1 Introduction to Hydroxy Compounds 1 Alcohols are a group of oxygen-containing organic compounds, having the hydroxyl functional group, !OH, attached to a carbon atom. 2 Aliphatic alcohols are derivatives of alkanes where one or more hydrogen atoms are replaced by the !OH group. 3 Monohydric aliphatic alcohols are compounds that have only one !OH group in their structures. They have the general formula of CnH2n+1OH or CnH2n+2O. 4 The most common and the most important alcohol in our life is ethanol, C2H5OH, which is used as a fuel additive, in alcoholic drinks and as solvents. 5 Polyhydric aliphatic alcohols have more than one !OH groups in their structure. Those with two !OH groups per molecule are called diols, while those with three !OH groups are called triols. For example: CH2!CH2 CH2!CH!CH2 & & & & & OH OH OH OH OH Ethane-1,2-diol Propan-1,2,3-triol 6 Phenols are a class of compounds where the !OH group is attached directly to a carbon atom in the benzene ring. Examples of phenols are: OH OH OH & & & & & CH3 OH Phenol 4-methylphenol 1-4-benzenediol (or p-Cresol) (or hydroquinone) 7 However, the following compound: CH2OH is not a phenol because the !OH group is not attached directly to the benzene ring. It is an aryl alcohol or the phenyl derivative of an aliphatic alcohol. The phenols
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 145 17 Nomenclature Aliphatic Alcohols 1 In the IUPAC system, the longest chain of carbon atoms containing the !OH group is selected as the parent chain. 2 The parent chain is numbered starting from the end closer to the carbon containing the !OH group. 3 Substitute the suffix -ane of the corresponding alkane with -anol. 4 Note that the location of the !OH group takes precedence over alkyl or halogen groups in the chain. 5 For example (the names in parantheses are the common names): CH3OH Methanol (Methyl alcohol) CH3CH2OH Ethanol (Ethyl alcohol) CH3CH2CH2OH 1-propanol (Propyl alcohol) CH3CHCH3 & OH 2-propanol (Isopropyl alcohol) CH3CH2CH2CH2OH 1-butanol (Butyl alcohol) CH3CH2CHCH3 & OH 2-butanol (sec-butyl alcohol) CH3!CH!CH2OH & CH3 2-methyl-1-propanol (Isobutyl alcohol) CH3 & CH3!C!CH3 & OH 2-methyl-2-propanol (tert-butyl alcohol) OH Cyclohexanol – CH3 OH 4-methylcyclohexanol – 6 For dihydric alcohols (compounds having two !OH groups in their structures), they are named as diols. The common name for dihydric alcohols is glycol. CH2 ! CH2 & & Ethane-1,2-diol (Ethylene glycol) OH OH CH3 ! CH ! CH2 & & 1,2-propanediol (Propylene glycol) OH OH 7 Aliphatic alcohols that have a double bond in their structures are named as enol. The chain is numbered in such a way that the lowest number is assigned to the !OH group. Info Chem For cyclic alcohols, the !OH group is always presumed to be at carbon-1. Hence, it is not necessary to name the last alcohol in the table as 4-methyl-1-cyclohexanol.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 146 17 CH2RCH!OH Ethenol CH3CHRCH!OH 1-propenol CH2RCHCH2CH2CH2CH2OH 5-hexen-1-ol and not 1-hexen-5-ol Aryl Alcohols 1 Aryl alcohols are named as phenyl derivatives of aliphatic alcohols. 2 For example: (a) CH2OH & Phenylmethanol (The phenyl derivative of methanol) 2 1 (b) CH2CH2OH 2-phenylethanol & (The phenyl group is at C-2) 2 1 (c) CH3CHOH 1-phenylethanol (The phenyl group is at C-1) 3 2 1 (d) CH3!CH!CH2OH 2-phenyl-1-propanol (e) OH 1 &2 3 CH3! C ! CH3 2-phenyl-2-propanol Enols Info Chem • Phenylmethanol is also called benzyl alcohol • CH2 is called the benzyl group.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 147 17 Phenols 1 Phenols are compounds where the !OH group is attached directly to the benzene ring. 2 The names of some common phenols are as follows. OH OH CH3 Phenol 2-methylphenol (o-Cresol) OH HO OH CH2CH3 3-ethylphenol Benzene-1,4-diol 3 However, in certain case, the !OH group that is attached to the benzene ring is named as a hydroxy substituent. For example, HO COOH is called 4-hydroxybenzoic acid and HO NH2 is called 4-hydroxyphenylamine. Structural and Optical Isomerism in Alcohols 1 Alcohols with three or more carbons exhibit structural isomerism. For example, C3H8O: CH3CH2CH2OH 1-propanol CH3—CH—CH3 2-propanol & OH 2 Note that alcohols are also isomeric with ethers, R—O—R. The above two structures are isomeric with: CH3—CH2—O—CH3 Methylethyl ether Info Chem OH CH3 4-methylphenol is known as p-Cresol.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 148 17 3 Alcohols with four or more carbon atoms exhibit optical isomerism. For example, 2-butanol. C* OH CH3 C2H5 H C* HO C2H5 CH3 H Physical Properties of Hydroxy Compounds Boiling Point 1 Due to the presence of the very polar !OH group, alcohols are polar compounds. Hδ+ δ– O C 2 Alcohols can form intermolecular hydrogen bonds with one another. O H H H H R O R O O R R 3 As a result, alcohols have higher melting/boiling points than their corresponding alkanes or haloalkanes. Alcohol Structure Boiling point/°C Methanol CH3OH 65 Ethanol CH3CH2OH 78 1-propanol CH3CH2CH2OH 97 1-butanol CH3CH2CH2CH2OH 117 4 The boiling point of the alcohols increases with increasing molecular mass due to the increase in the size of, and the total number of electrons in the molecule that increases the strength of the van der Waals forces. The O!H bond is highly polarised. Hydrogen bonding increases the boiling point of the alcohols. 2015/P3/Q3
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 149 17 5 Relatively, polyhydric alcohols have higher boiling points than monohydric alcohols of comparable molecular mass, because they can form more extensive hydrogen bonds. Alcohol Structure Molecular mass Boiling point/°C 1-pentanol CH3CH2CH2CH2CH2OH 88 138 1,4-butanediol CH2CH2CH2CH2 & & OH OH 90 230 6 Branched-chain isomers have lower boiling points than their straight-chain counterparts. Compound Structure Boiling point/°C Butane-1-ol CH3CH2CH2CH2OH 117 Butane-2-ol CH3CH2CHCH3 & OH 100 2-methyl-propane-2-ol CH3 & CH3!C!CH3 & OH 84 This is because, the greater the branching, the smaller is the molecular volume. This decreases the strength of the intermolecular van der Waals forces. Example 17.1 The boiling points of ethanol, CH3CH2OH and dimethyl ether, CH3OCH3 are 78 °C and –24 °C respectively. Explain the difference in terms of intermolecular force. Solution In ethanol molecule, there is a hydrogen atom bonded to the oxygen atom in the !OH group. As a result, ethanol can form intermolecular hydrogen bonding with one another. On the other hand, dimethyl ether cannot form intermolecular hydrogen bonding because there is no hydrogen atom bonded directly to the oxygen atom. The intermolecular force between dimethyl ether molecules is the weak van der Waals force. Quick Check 17.1 1 Arrange the following compounds in the order of increasing boiling points. Explain your answer. CH3CH2CH2CH2CH2CH3, HOCH2CH2CH2CH2OH, CH3CH2CH2CH2CH2OH 2 Explain why the boiling point of methanol (relative molecular mass = 32) is higher than that of ethane (relative molecular mass = 30). 3 The boiling point of phenol is 182 °C, but the boiling point of methylbenzene is 110.5 °C. Explain the difference. Exam Tips Polyhydric alcohols can form more intermolecular hydrogen bonds. Butane-1-ol: Butane-2-ol: 2-methyl-propane-2-ol: OH OH OH
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 150 17 Solubility in Water 1 The very polar !OH group can form hydrogen bonding with water. As a result, they are more soluble in water than alkanes, alkenes or haloalkanes of comparable molecular mass. O H H H H H O H O O R H 2 However, their solubility decreases with the increase in the size of the non-polar hydrophobic hydrocarbon group. 3 The table below lists the solubility of some hydroxy compounds at 298 K. Alcohol Structure Solubility (g/100 g water) Methanol CH3OH Infinity Ethanol CH3CH2OH Infinity 1-propanol CH3CH2CH2OH Infinity 1-butanol CH3CH2CH2CH2OH 8.0 1-pentanol CH3CH2CH2CH2CH2OH 2.3 4 Due to the presence of a large hydrophobic benzene ring. Phenol is insolube in water. R 99 OH c c Hydrophobic Hydrophilic Quick Check 17.2 1 Explain why ethanol is miscible with water whereas hexanol is not. 2 Arrange the following three compounds in the order of increasing solubilty in water. Explain your answer. 1-propanol; 1,2-propanediol; 1,2,3-propanetriol 3 1-butanol and 1,2-propanediol have similar molecular mass. Which is more soluble in water? Explain your answer. 17.2 Alcohols Classification 1 Alcohols can be classified as primary, secondary or tertiary depending on the number of alkyl groups that are attached to the oxygen-carrying carbon atoms.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 151 17 2 In primary alcohols, the !OH group is attached to a primary carbon atom and has the general structure as shown below. (where R = H, alkyl or aryl) The functional group is !CH2OH H H R C OH 3 In secondary alcohols, the !OH group is attached to a secondary carbon atom. (where R = alkyl or aryl) & The functional group is !CH!OH R H R C OH 4 In tertiary alcohols, the !OH group is attached to a tertiary carbon atom. (where R = alkyl or aryl) & The functional group is !C!OH & R R R C OH Note: There are no quaternary alcohols. Quick Check 17.3 1 Classify the following hydroxy compounds as primary, secondary or tertiary alcohols. (a) Methanol (d) 2,4-dimethylphenol (b) Cyclohexanol (e) 3-penten-2-ol (c) 1-methylcyclohexanol Reactions of Alcohols 1 The functional group of alcohols is hydroxyl group. 2 This group consists of an oxygen atom that is covalently bonded to a hydrogen atom (O!H) which in turn is bonded to a carbon atom by a carbon-oxygen bond (C!O). & ! C ! O ! H & 2008/P1/Q28 2008/P2/Q9 2009/P2/Q10(a) 2017/P3/Q17 2009/P2/Q4(a) – (c)
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 152 17 3 There are two ways in which the !C!O!H bond can break during reactions. (a) Reactions involving the breaking of the O!H bond & ! C ! O H & (b) Reactions involving the breaking of the C!O bond & ! C O ! H & Reactions Involving the Breaking of the O!H Bond As Weak Acids 1 In aqueous solutions, alcohols undergo partial dissociation to produce H3O+ ions. ROH + H2O L RO– + H3O+ For example: CH3CH2OH + H2O L CH3CH2O– + H3O+ Ethanol Ethoxide ion 2 Alcohols are weak acids as indicated by their Ka values. Alcohol Structure Ka/mol dm–3 pKa Methanol CH3OH 3.16 × 10–16 15.5 Ethanol CH3CH2OH 1.00 × 10–16 16.0 1-propanol CH3CH2CH2OH 1.00 × 10–17 17.0 (Note: The higher the value of Ka, the stronger the acid. pKa = –log Ka) Example 17.2 The Ka values for 1-propanol and 2-methyl-2-propanol are 1.0 × 10–17 and 1.0 × 10–18 mol dm–3 respectively. (a) Write an equation for the dissociation of 1-propanol. (b) Write an expression for the Ka of 1-propanol. (c) Explain the difference in their Ka values. Solution (a) CH3CH2CH2OH + H2O L CH3CH2CH2O– + H3O+ [CH3CH2CH2O– ][H3O+] (b) Ka = [CH3CH2CH2OH] (c) The Ka values show that 2-methyl-2-propanol is a weaker acid than 1-propanol. This is because there are three electronreleasing alkyl groups that strengthen the O!H compared to only one alkyl group in 1-propanol. Info Chem The smaller the pKa value, the stronger the acid. Info Chem Reactions involving the breaking of the O–H bond are: • As weak acid • Esterification • Acylation • Oxidation
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 153 17 CH3CH2CH2 !: O!!H CH3 s p CH3 !: C !: O!H q s CH3 As a result, the O!H bond in 2-methyl-2-propanol is relatively stronger than that in 1-propanol and is more difficult to break. Quick Check 17.4 1 Arrange the following compounds in the order of increasing acid strength. CH3CHCH3 CH2OH OH OH CH3OH 2 The Ka values of ethanol and 2-fluoroethanol are 1.0 × 10–16 and 1.0 × 10–12 mol dm–3 respectively. (a) Among these two alcohols, which is a stronger acid? (b) Explain the difference in the acid strength of the two compounds. Reaction with Sodium 1 Alcohols react with electropositive metals such as sodium at room temperature to produce hydrogen gas. 2R!OH + 2Na !: 2R!O– Na+ + H2 2 For example: 2CH3CH2OH + 2Na !: 2CH3CH2O– Na+ + H2 Sodium ethoxide 3 A mixture of ethanol and sodium is sometimes used as a reducing agent in organic reactions. 4 The alkoxides (being salts of weak acids) are hydrolysed back to alcohols by dilute mineral acids or aqueous carbon dioxide (carbonic acid). R!O– Na+ + H+ !: R!OH + Na+ R!O– Na+ + CO2 + H2O !: R ! OH + HCO3 – + Na+ For example: CH3CH2O– Na+ + H+ !: CH3CH2OH + Na+ 2016/P3/Q7
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 154 17 Quick Check 17.5 1 Calculate the volume of hydrogen gas released (measured at s.t.p.) when 23 g of ethane-1,2-diol reacts with excess sodium metal. 2 30.0 g of a monohydric alcohol reacts with excess sodium metal to produce 6.0 dm3 of hydrogen measured at room conditions. (a) Determine the molecular formula of the alcohol. (b) Draw all possible isomers of the alcohol. 3 Write balanced equations for the following reactions (if any). (a) Methanol and chlorine in the presence of ultraviolet radiation (b) Ethane-1,2-diol with sodium metal (c) 1,2,3-propanetriol with sodium metal (d) 2-methyl-2-propanol with sodium metal (e) 1-cyclohexenol with bromine in tetrachloromethane 4 Suggest how you would differentiate between 1-cyclohexenol and 1-cyclohexanol. Esterification 1 When an alcohol is boiled under reflux with a carboxylic acid in the presence of a little concentrated sulphuric acid, an ester is produced. R ! C ! OH + R9 ! O ! H L R ! C ! O ! R9 + H2O ∫ ∫ O O This is known as esterification. 2 Note how the H2O molecule is eliminated in the reaction. 3 Esterification is a reversible reaction. It is extremely slow at room temperature. Concentrated sulphuric acid is added as a catalyst. 4 For example: CH3COOH + CH3CH2OH L CH3COOCH2CH3 + H2O Ethyl ethanoate 5 Esterification involving phenol is discussed in the next section. Reaction with Acyl Chlorides 1 Acyl chlorides (or acid chlorides) are organic compounds with the following structure. R!C!Cl ∫ O They are obtained from carboxylic acids by replacing the !OH group with chlorine atom. 2 Alcohols react with acyl chlorides to form esters. R!C!Cl + H! OR9 !: R ! C ! OR9 + HCl ∫ ∫ O O Acyl chlorides are very reactive. No catalyst or heat is required. 2011/P1/Q33 Concentrated H2SO4 acts as a catalyst. Info Chem Concentrated H2SO4 acts as a catalyst as well as a drying agent. 2009/P1/Q36 The reaction is irreversible.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 155 17 3 For example: CH3COCl + CH3CH2OH !: CH3COOCH2CH3 + HCl Ethanoyl chloride Quick Check 17.6 1 Draw the structures of the esters produced in the following reactions. (a) Benzoic acid with cyclohexanol (b) 1,4-butanedioic acid (HOOCCH2CH2COOH) with methanol (c) Oxalic acid (HOOC!COOH) with phenylmethanol (d) Benzoic acid with 2-propanol 2 Name the carboxylic acids and alcohols from which each of the following esters is derived. (a) HCOOCH3 (c) CH3OOCCH2CH2COOCH3 (b) COOCH3 (d) COOCH2CHRCH2 3 Consider the following reaction involving isotopically labelled alcohol. CH2COOH + CH3CH2 18OH L Complete the equation and show clearly where in the products the isotope 18O is found. Oxidation of Alcohols 1 Alcohols can be oxidised to their corresponding carbonyl compounds. H Oxidisable hydrogen atom & ! C ! + [O] !: !C! + H2O & ∫ O ! H O Carbonyl group The type of carbonyl products formed depends on the class of alcohol used. 2 During the reaction, two hydrogen atoms are removed from the alcohol. One from the !OH group and the other from the oxygen-carrying carbon atom. 3 The common oxidising agents used are acidified potassium dichromate(VI) or acidified potassium permanganate(VII) together with heat. 2008/P1/Q34, 49 2010/P1/Q34 2012/P1/Q33 2014/P3/Q9 2010/P2/Q10(a)
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 156 17 4 During oxidation, the orange colour of potassium dichromate(VI) changes to green, while the purple colour of potassium permanganate(VII) is decolourised. Oxidation of Primary Alcohols 1 Primary alcohols are oxidised to aldehydes which can be further oxidised to carboxylic acid depending on the experimental conditions. H & [O] R ! C ! H + [O] !: R ! C ! H + H2O !: R! C ! OH & ∫ ∫ O ! H O O Aldehyde Carboxylic acid Oxidisable hydrogen atom 2 For example, when ethanol is heated under reflux with acidified potassium dichromate(VI), it produces ethanal (an aldehyde) which is further oxidised to ethanoic acid. CH3CH2OH + [O] !: CH3CHO + H2O Ethanal [O] CH3COOH Ethanoic acid (Acetic acid) The overall reaction is CH3CH2OH + 2[O] !: CH3COOH + H2O 3 Other examples are: CH2OH + 2[O] !: COOH + H2O Phenylmethanol Benzoic acid CH3 CH3 & & CH3 !C ! CH2OH + 2[O] !: CH3 ! C ! COOH + H2O & & CH3 CH3 2,2-dimethyl-1-propanol 2,2-dimethylpropanoic acid 4 In order to oxidise the primary alcohol to aldehyde only, the oxidation is carried out by heating the mixture of alcohol and acidified potassium dichromate(VI) at a temperature lower than the boiling point of the alcohol but slightly higher than the boiling point of the aldehyde, so that the aldehyde produced is distilled out as soon as it is formed. xxxxxxxxxxxxxx Water out Water in Mixture of ethanol and acidified potassium dichromate(VI) Heat
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 157 17 5 For example, in the oxidation of ethanol (b.p. = 78 °C) to ethanal (b.p. = 21 °C), ethanol is mixed with acidified potassium dichromate(VI) and the mixture warmed to about 30 °C, so that the ethanal produced is distilled over before it can be further oxidised to ethanoic acid (b.p. = 118 °C). 30 °C CH3CH2OH + [O] !!: CH3 ! C ! H + H2O ∫ O Ethanal Oxidation of Secondary Alcohols 1 Secondary alcohols are oxidised to ketones when heated under reflux with either acidified potassium manganate(VII) or acidified potassium dichromate(VI). 2 Ketones are extremely difficult to be oxidised further to carboxylic acids. H & R ! C ! R + [O] !: R ! C ! R + H2O & ∫ O ! H O Ketone 3 For example: CH3 ! CH ! CH3 + [O] !Δ : CH3 ! C ! CH3 + H2O & ∫ OH O 2-propanol Propanone OH + [O] !Δ : RO + H2O Cyclohexanol Cyclohexanone Oxidation of Tertiary Alcohols 1 Tertiary alcohols and phenols do not have any hydrogen atom attached directly to the carbon atom carrying the !OH group. They are resistant to oxidation. R & R ! C ! R + [O] !Δ : No reaction & OH 2 Moreover, the carbon atom attached to the !OH group is bonded to three alkyl groups and therefore cannot form a carbon-oxygen double bond without breaking strong carboncarbon bond. Acidified potassium dichromate(VI) Water Water Ethanal Ethanol Heat No oxidisable hydrogen atoms in tertiary alcohols.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 158 17 Example 17.3 How would you differentiate between 1-butanol and 2-methyl-2-propanol? Solution 1-butanol is a primary alcohol, and 2-methyl-2-propanol is a tertiary alcohol. CH3CH2CH2CH2OH 1-butanol CH3 & CH3 ! C ! CH3 2-methyl-2-propanol & OH Warm both the alcohols separately with acidified potassium dichromate(VI). 1-butanol: Orange colour of K2Cr2O7 changes to green. 2-methyl-2-propanol: No reaction. CH3CH2CH2OH + 2[O] !: CH3CH2CH2COOH + H2O Quick Check 17.7 1 Draw the structures of the products formed (if any) when the following compounds are heated with acidified potassium manganate(VII). (a) CH3OH (d) OH (b) CH3 & CH3 ! C ! CH3 & OH (c) CH2OH (e) OH CH3 & 2017/P3/Q19 Exam Tips Exam Tips Dehydrogenation of alcohols. When an alcohol is heated with copper, a carbonyl compound is produced. H ǀ —C— !: —C— + H2 ǀ ‖ OH O Cu Δ
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 159 17 Reactions Involving the Breaking of the C!O Bond In this class of reaction, the carbon-oxygen bond is broken and the !OH group is replaced by some other atoms/groups through nucleophilic substitution. Reaction with Halogen Acids 1 Alcohols react with halogen acids to form the corresponding haloalkanes. R ! OH + H ! X !: R ! X + H2O 2 The mechanism is as follows: (a) Protonation of an alcohol by a halogen acid. H!X !: H+ + X– R ! .. O !H + H+ !: R ! O+!H & H (b) Elimination of H2O to produce a carbonium ion. R!O+!H L R+ + H2O & H (c) Reaction of the carbonium ion with a nucleophile (X– ) to produce haloalkanes. R+ + X– !: R ! X 3 As the intermediate involves a carbonium ion, the reactivity of alcohols towards halogen acids increases in the order: Primary alcohol < secondary alcohol < tertiary alcohol 4 This is due to the stability of the carbonium ions formed. Tertiary > secondary > primary 5 The reactivity of halogen acids is HI > HBr > HCl This is because H!I bond is the weakest followed by H!Br and finally H!Cl. With Hydrochloric Acid 1 Tertiary alcohols react very rapidly with hydrochloric acid. Mixing a tertiary alcohol with concentrated hydrochloric acid at room temperature produces chloroalkanes almost instantly. 2009/P1/Q50 Info Chem Reactions involving the breaking of C–OH bond are: • With halogen acids • With PCl5 or SOCl2 • Dehydration Exam Tips Exam Tips This is an example of nucleophilic substitution. Exam Tips Exam Tips Carbonium ion can also decompose to produce an alkene. ǀ ǀ —C—C— : C ‖C + H+ ǀ + H
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 160 17 For example: CH3 CH3 & & CH3 !C!CH3 + HCl !: CH3 !C!CH3 + H2O & & OH Cl 2-methyl-2-propanol 2-methyl-2-chloropropane 2 Under the same conditions, primary alcohols and secondary alcohols react slowly. 3 Primary alcohols require heating with concentrated hydrochloric acid in the presence of zinc chloride. CH3CH2CH2CH2OH + HCl(aq) !: CH3CH2CH2CH2Cl + H2O Δ 4 Alternatively, dry hydrogen chloride gas is bubbled through the primary alcohol boiling under reflux with zinc chloride as a catalyst. CH3CH2CH2CH2OH + HCl(g) !: CH3CH2CH2CH2Cl + H2O Δ 5 The reaction of alcohols with a mixture of concentrated hydrochloric acid and zinc chloride is used to differentiate between primary, secondary and tertiary alcohols. This is known as the Lucas test, and the mixture is called the Lucas reagent. 6 The Lucas test is carried out by shaking the alcohol with Lucas reagent, and the time taken for the solution to turn cloudy (due to the formation of insoluble chloroalkane) is noted. Alcohol Time taken for solution to turn cloudy Tertiary Almost immediately Secondary After a couple of minutes Primary More than 10 minutes With Hydrobromic Acid 1 Hydrobromic acid is generated in situ by the reaction between concentrated sulphuric acid and potassium bromide. H2SO4 + KBr !: KHSO4 + HBr Δ 2 When an alcohol is refluxed with a mixture of concentrated sulphuric acid and potassium bromide, a bromoalkane is produced. R ! OH + H ! Br !: R!Br + H2O Δ Or ROH + H2SO4 + KBr !: RBr + KHSO4 + H2O Δ The water-insoluble chlorocompound separates out from the aqueous layer as a white precipitate. The Lucas test Info Chem Some of the HBr will be oxidised to Br2 by concentrated sulphuric acid. 2HBr + H2SO4 !: Br2 + SO2 + 2H2O VIDEO The Lucas Test
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 161 17 3 For example: CH3!CH!CH3 + HBr !Δ : CH3!CH!CH3 + H2O & & OH Br OH + HBr !: Br + H2O Bromocyclohexane Δ With Hydroiodic Acid 1 Hydroiodic acid is produced by the reaction between concentrated phosphoric acid and potassium iodide. H3PO4 + KI !: KH2PO4 + HI Δ 2 Concentrated sulphuric acid is not used because it will oxidise the hydroiodic acid to iodine. No such oxidation occurs with phosphoric acid, which is non-oxidising. H2SO4 + KI !: KHSO4 + HI Δ 2HI + H2SO4 !: I2 + SO2 + 2H2O Δ 3 The equation of the reaction is R ! OH + H ! I !: R!I + H2O Reaction with Phosphorus(V) Chloride 1 All alcohols (with the exception of phenols) react with phosphorus(V) chloride releasing dense white fumes of hydrogen chloride. R ! OH + PCl5 !: R ! Cl + POCl3 + HCl(g) For example: CH3CH2OH + PCl5 !: CH3CH2Cl + POCl3 + HCl CH2OH + PCl5 !: CH2Cl + POCl3 + HCl 2 The alcohols must be free from water, because water also reacts with PCl5 with the same result. 3 This serves as a test for the !OH group (not necessarily alcohols) in a molecule. Ethanoic acid also gives the same observation due to the presence of the !OH group in its structure. CH3 ! C ! OH + PCl5 !: CH3 ! C ! Cl + POCl3 + HCl ∫ ∫ O O A test for the !OH group Another reagent used is thionyl chloride: R–OH + SOCl2 9: RCl + HCl + SO2
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 162 17 Example 17.4 Show how you would differentiate between the following pairs of compounds. (a) Cyclohexanol and phenol (b) 4-methylphenol and phenylmethanol (c) Cyclohexenol and cyclohexene Solution (a) Cyclohexanol reacts with PCl5 and releases dense white fumes, but phenol does not. OH + PCl5 !: Cl + POCl3 + HCl (b) Phenylmethanol reacts with PCl5 and releases white fumes of HCl, but 4-methylphenol does not. CH2OH + PCl5 CH2Cl + POCl3 + HCl !!: (c) Cyclohexenol reacts with PCl5 and releases white fumes of HCl, but cyclohexene does not. + PCl5 !: OH + POCl3 + HCl Cl Dehydration 1 Elimination of water (dehydration) from an alcohol results in the formation of an alkene. C RR C H C + H2O OH C !: Note that the H2O is eliminated from adjacent carbon atoms. 2 Dehydration can be carried out by either one of the two ways described below. (a) The alcohol is heated with excess of concentrated sulphuric acid at about 180 °C. (b) The vapour of the alcohol is passed over heated aluminium oxide. This is known as vapour phase dehydration. Liquid phase dehydration 2011/P1/Q34 2016/P3/Q20(a)(i) 2013/P3/Q10 2017/P3/Q8, Q19
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 163 17 3 Examples are: CH3CH2OH !: CH2RCH2 + H2O !: OH H + H2O 4 Certain alcohols can form more than one alkene upon dehydration. In such cases, the major product is determined by Zaitsev rule, which states that in an elimination reaction which results in the formation of isomeric alkenes, the major product is the alkene that has the greatest number of alkyl groups attached to the unsaturated carbon atoms. 5 For example, dehydration of 2-butanol would produce 2-butene as the major product and 1-butene as the minor product. –H2O CH3CH2CHRCH2 1-butene (minor product) CH3CH2CHCH3 & –H2O OH CH3CHRCHCH3 2-butene (major product) 6 However, if the dehydration process is carried out in excess of the alcohol and at lower temperature (≈140 °C), an ether will be produced. R ! OH + H ! OR !: R ! O ! R + H2O 7 For example: 2CH3CH2OH !: CH3CH2OCH2CH3 + H2O Diethyl ether 2 OH 9: O + H2O Dicyclohexyl ether 8 Ethers are relatively unreactive. They are often used as non-polar solvents. Vapour phase dehydration Zaitsev rule Formation of ether
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 164 17 Quick Check 17.8 1 Draw the structures of all possible alkenes formed (if any) when the following alcohols are subjected to dehydration with excess cocentrated sulphuric acid. Where isomeric alkenes are possible, predict which alkene is the major product. (a) CH3OH (e) 1-phenylethanol (b) 2-propanol (f) 4-methylphenol (c) 2-methyl-2-propanol (g) 2-methylcyclohexanol (d) Phenylmethanol (h) 1,4-butanediol Reactivity of Primary, Secondary and Tertiary Alcohols to Form Haloalkanes 1 Alcohols react with concentrated hydrohalide acids to form haloalkanes. For example, R!OH + HCl 9: R!Cl + H2O 2 The reaction involves nucleophilic substitution and takes place via the following steps. Protonation: R! .. O!H + H+ 9: R—O+!H & H Elimination of water: R!O+!H 9: R+ + H2O & H Attack by the nucleophile: R+ + X– 9: R!X 3 The reaction proceeds via a carbonium ion intermediate. Since the relative stability of the carbonium ion is in the order: tertiary > secondary > primary, the reactivity of the alcohols towards nucleophilic substitution is also in the order: tertiary > secondary > primary. Triiodomethane Test 1 Triiodomethane (common name: iodoform), CHI3, is a yellow solid that is insoluble in water. 2 All alcohols with the following structure in their molecule: H & CH3 ! C ! R (where R = H, alkyl or aryl group) & OH The chlorine counterpart of iodoform is chloroform, CHCl3. 2014/P3/Q9 2016/P3/Q18(d), 20(a)(ii)
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 165 17 will give a yellow precipitate of triiodomethane when warmed with a solution of iodine in aqueous sodium hydroxide (alkaline iodine). 3 The equation of the reaction is H & CH3!C!R + 4I2 + 6NaOH !Δ : CHI3 + RCOO– Na+ + 5NaI + 5H2O & OH 4 For example: H & CH3!C!CH2CH3 + 4I2 + 6NaOH !!Δ !: & OH CHI3 + CH3CH2COO– Na+ + 5NaI + 5H2O 5 Alcohols that do not have the above structure will not give a yellow precipitate with alkaline iodine. 6 The test is usually carried out as follows: (a) Dilute aqueous sodium hydroxide is added drop by drop to aqueous iodine until the solution just turns from yellow to colourless. (b) The alcohol is then added to the colourless solution, and the mixture is warmed. (c) Appearance of a yellow precipitate indicates a positive test. Quick Check 17.9 1 State whether the following alcohols will give a positive or negative iodoform test. (a) Methanol (e) 2-methyl-2-propanol (b) Ethanol (f) 4-methylphenol (c) 1-propanol (g) Cyclohexanol (d) 2-propanol 2 Draw all the structural isomers for an alcohol with the molecular formula of C4H10O. State the isomers which will react with alkaline iodine to give a yellow precipitate. Synthesis and Manufacture of Ethanol Hydration of Ethene 1 A mixture of ethene gas and steam is passed over phosphoric acid, which is coated on the surface of silicon dioxide, kept at 300 °C and under a pressure of 60 – 70 atm, where steam and ethene combine to form ethanol. CH2=CH2(g) + H2O(g) 9: CH3CH2OH(l) OH ! RCOO– CH3 ! CH!R CHI3 Exam Tips Exam Tips Other catalysts are zeolite and aluminium oxide.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 166 17 2 However, only about 5% of the ethene is converted into ethanol at each pass. 3 By removing the ethanol produced and recycling the ethene, an overall yield of 95% is obtained. Preparation of Ethanol by Fermentation 1 Ethanol, one of the most common and most important alcohol, can also be obtained by the fermentation of aqueous glucose using yeast especially in countries where there are abundance of sugar cane and other types of grains. 2 In the early 1940s, almost 75% of the total world production of ethanol is through the fermentation process. 3 However, nowadays, only about 10% of the ethanol is still produced through fermentation. The rest are from the hydration of ethene (obtained from petroleum). 4 Yeast contains enzyme zymase which is responsible for the fermentation process. 5 Fermentation is carried out under anaerobic condition (without oxygen) at a temperature of about 35 °C, to produce an aqueous solution of ethanol and carbon dioxide. Zymase, 35 °C C6H12O6(aq) !!!!!: 2CH3CH2OH(aq) + 2CO2(g) 6 Air must be excluded from the process. Otherwise it will oxidise the ethanol to ethanoic acid (this is called souring of alcohol). 2CH3CH2OH + 3O2 !: 2CH3COOH + 4H2O 7 It is not possible to produce a solution with ethanol content of more than 15%. This is because beyond this concentration, the enzyme in yeast will be destroyed. 8 Fractional distillation of the aqueous solution produces an azeotropic mixture containing 95% ethanol and 5% water. 9 Pure ethanol (known as absolute ethanol) can be obtained by distilling the azeotrope with anhydrous phosphorus(V) oxide, P4O10. 17.3 Phenols Relative Acidity of Water, Phenol and Ethanol 1 The acid dissociation constants, Ka, for phenol, water and ethanol are given in the table below: Compound Phenol Water Ethanol Ka/mol dm–3 1.3 × 10–10 1.8 × 10–16 1.0 × 10–16 (Note: The higher the value of Ka, the stronger the acid.) Ethanol content of >15% kills the enzyme. Info Chem An azeotropic mixture is one whose composition does not change through fractional distillation. 2015/P3/Q9 Exam Tips Exam Tips • 35°C is about body temperature where the enzyme works most efficiently. • Higher temperature would ʻkillʼ the enzyme.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 167 17 2 Phenol, water and ethanol dissociate according to the following equations: ——O——H ——O– + H+ H——O——H H——O– + H+ C2H5—O—H C2H5—O– + H+ 3 From the Ka values, the acid strength is: ——OH> H2O > C2H5OH The relative strength can be explained using either the inductive effect or the resonance effect. Inductive Effect 1 The acid strength of the three compounds depends on the strength of the O—H bond. 2 The phenyl group is an electron-withdrawing group (it exhibits negative inductive effect) that weakens the O—H bond. 3 On the other hand, the ethyl group is an electron-donating group (it exhibits positive inductive effect) that strengthens the O—H bond. 4 As a result, the O—H bond in phenol is the weakest following by water and ethanol which has the strongest O—H bond. 5 This accounts for the trend of the acid strength. Resonance Effect 1 The degree of dissociation of the hydroxy compounds depends on the stability of the anions formed. The more stable the anion, the easier for it to form and the higher the degree of dissociation. 2 The negative charge on the phenoxide ion can be delocalised into the benzene ring, thereby stabilising it and making it easier to form. O– O– 3 On the other hand, the electron-releasing ethyl group increases the electron density of the ethoxide ion thereby destabilising it, making it more difficult to form. C2H5:O– ;O9H C2H5:O9H
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 168 17 Acidity of Substituted Phenol 1 The presence of electron-withdrawing group in the benzene ring further increases the acidity of phenol, whereas the presence of electron-releasing group decreases the acidity of phenol. OH OH OH CH3 NO2 Ka = 6.6 × 10–11 Ka = 1.3 × 10–10 Ka = 7.1 × 10–8 2 The presence of three electron-withdrawing groups in 2,4,6-trinitrophenol makes it a stronger acid than phosphoric acid. OH NO2 Ka = 4.2 × 10–1 Ka = 1.2 × 10–2 NO2 H3PO4 O2N Reactions of Phenols With Sodium and Sodium Hydroxide 1 Like alcohols, phenols react with sodium metal to release hydrogen gas. OH 2 + 2Na O– Na+ 9: 2 + H2 2 Being a stronger acid than alcohols, phenols form salt with aqueous sodium hydroxide, but alcohols do not form salts with sodium hydroxide. OH + NaOH O– Na+ 9: + H2 O Info Chem Phenols do not react with sodium carbonate or sodium hydrogencarbonate. Info Chem • NO2 is electron – withdrawing • CH3 is electron – releasing 2016/P3/Q8 2017/P3/Q9
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 169 17 3 Sodium phenoxide is decomposed back to phenol by the action of dilute mineral acid or aqueous carbon dioxide. OH + H+ O– Na+ + Na 9: + OH + H2O + CO2 O– Na+ 9: + NaHCO3 With Acyl Chloride 1 In the reaction with acyl chloride, phenol is first dissolved in aqueous sodium hydroxide to convert it into more reactive sodium phenoxide. 2 The reaction produces an ester. No heat or catalyst is required. —COCl + —O– Na+ : —COO— + NaCl Electrophilic Substitution 1 The benzene ring in phenol can undergo electrophilic substitution. 2 As the —OH group is a ring-activating group, phenol is more reactive than benzene itself in the substitution reaction at the 2nd and 4th positions. 3 An example is the reaction between phenol and nitric acid. Example 17.5 Explain why phenol is soluble in aqueous sodium hydroxide but cyclohexanol is not. Solution Dissolution of phenol in aqueous sodium hydroxide can be viewed as a displacement reaction. Phenol, being a stronger acid than water displaces !OH from NaOH as H2O. OH O– Na+ + NaOH !: + H2O Stronger acid (pKa = 9.9) Weaker acid (pKa = 15.7) Info Chem Phenyl benzoate is a white solid. Info Chem The lower the pKa value, the stronger the acid.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 170 17 On the other hand, cyclohexanol is a weaker acid than water. As a result, it cannot displace !OH from NaOH as H2O and hence is insoluble in sodium hydroxide. OH + NaOH !: O– Na+ Stronger acid Weaker acid (pKa = 16.0) + H2O (pKa = 15.7) With Nitric Acid 1 Phenol reacts with dilute nitric acid to produce a mixture of 2-nitrophenol and 4-nitrophenol. OH + HNO3 !: and OH NO2 + H2O OH NO2 No sulphuric acid is required. 2 Compare this with the nitration of benzene, which requires a mixture of concentrated sulphuric acid and concentrated nitric acid. 3 With concentrated nitric acid, phenol reacts to form 2,4,6-trinitrophenol (common name: picric acid). OH + 3HNO3 !: + 3H2O OH NO2 NO2 O2N Test for Phenols Reaction with Bromine Water 1 When bromine water is added to an aqueous solution of phenol at room temperature, the bromine is decolurised and a white precipitate of 2,4,6-tribromophenol is formed. No concentrated sulphuric acid is required. Info Chem –OH is a ring-activating group that increases the electron density in the benzene ring making it more supcetable to attack by electrophiles. Picric acid is a yellow-crystalline solid used in the manufacture of explosive. 2008/P1/Q33 2010/P2/Q10(b)
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 171 17 OH OH + 3Br2 !: + 3HBr Br Br Br 2 Non-phenols do not give a white precipitate with bromine water. Hence, this is a simple way to differentiate between phenols and non-phenols. Quick Check 17.10 Which of the following will give a white precipitate with aqueous bromine at room temperature? (a) CH2OH (b) CH2OH OH (c) CH3 OH (d) H (e) OH C OH With Aqueous Iron(III) Chloride 1 Phenols react with aqueous iron(III) chloride to give a blue or purple solution. OH + Fe3+ !: Fe3+ + 6H+ O– 6 6 Or 6C6H5OH + Fe3+ !: [C6H5O– ]6 Fe3+ + 6H+ Test for phenols Test for phenols
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 172 17 2 This reaction can also be used to differentiate between phenols and non-phenols. Phenol in the Manufacture of Nylon-6,6 1 When a mixture of hydrogen gas and phenol vapour is passed over heated nickel, cyclohexanol is formed. OH + 3H2 !: OH Cyclohexanol is used in the manufacture of nylon. 2 The scheme below shows the steps involved in the manufacture of nylon-6,6 starting from phenol: OH OH H2/Ni Conc HNO3 99: 999: Heat HOOC(CH2)4COOH (1,6-hexadioic acid) 2NH3 H4NOOC(CH2)4COONH4 Heat, –H2O H2NOC(CH2)4CONH2 Heat, P4O10 (Dehydration) NC(CH2)4CN H2/Ni (Reduction) H2NCH2(CH2)4CH2NH2 (1,6-hexadiamine) Reaction between 1,6-hexadioic acid and 1,6-hexadiamine produces nylon-6,6. nHOOC(CH2)4COOH + nH2NCH2(CH2)4CH2NH2 –H2O 9[9C9(CH2)49C9N9(CH2)69N9]9n + (2n – 1)H2O ' ' & & O O H H Nylon-6,6 Nylon-6,6
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 173 17 Quick Check 17.11 Give the structure of the organic products formed in the following reactions. 1 3-methylphenol + bromine water 2 Sodium phenoxide + benzoyl chloride 3 2,4,6-trinitrophenol + sodium carbonate 4 4-methylphenol + chlorine in the presence of UV light. Preparation of Phenol (The Cumene Process) 1 In the cumene process, phenol is prepared from benzene which is obtained from petroleum. 2 Benzene vapour and propene gas are passed over concentrated phosphoric acid at 250 °C and 30 atm to produce cumene (or 2-phenylpropane). CH3!CH!CH3 + CH3CHRCH2 !: Cumene 3 Air is then bubbled through cumene at about 120 °C and a pressure of 4 atm, where cumene is oxidised to cumene hydroperoxide. CH3!C!CH3 O!OH CH3!CH!CH3 + O2 !!: Cumene hydroperoxide 4 When warmed with dilute sulphuric acid, cumene hydroperoxide decomposes to phenol and propanone. OH + CH3 ! C ! CH3 O CH3 ! C ! CH3 O ! OH H+ !: R 5 The side product, propanone (acetone) is an important organic solvent. Info Chem Propanone is a ketone. Info Chem Cumene is also known as 1-methylethylbenzene or isopropylbenzene or 2-phenyl propane. 2011/P2/Q9(b) 2012/P1/Q34
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 174 17 Uses of Alcohols and Phenols 1 Most simple alcohols such as methanol, ethanol and propanol are used as solvents. 2 Alcohols are also used as fuels for domestic purpose and for internal combustion engines. 3 Methanol is used to make methanal which in turn is a monomer for thermosetting plastics such as Bakelite. The structure of Bakelite is shown below: CH2 CH2 OH CH2 OH CH2 CH2 CH2 CH2 OH CH2 OH It is a co-polymer formed between phenol and methanal (formaldehyde). 4 Phenol is used to produce Dettol (2,4-dichloro-3,5- dimethylphenol), a strong antiseptic. Cl OH Cl CH3 CH3 5 Phenol is used in the manufacture of nylons and dyes. 6 The comparison between ethanol and phenol is shown in the table below. Reagent Ethanol Phenol Acid strength Weaker acid than water Stronger acid than water and ethanol Sodium metal H2 gas liberated H2 gas liberated NaOH No reaction Dissolves to forms sodium phenoxide CH3COOH Forms ester in the presence of concentrated H2SO4 No reaction CH3COCl Forms ester Forms ester in alkaline solution PCl5 White fumes No white fumes HX Forms R!X No reaction Dehydration Forms ethene No dehydration KMnO4/H+ Decolourises KMnO4 No reaction K2Cr2O7/H+ Orange to green No reaction I2 in NaOH Yellow precipitate No reaction Bromine water No reaction White precipitate FeCl3(aq) No reaction Blue/purple solution 2009/P1/Q33
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 175 17 SUMMARY SUMMARY 1 The functional group of alcohols is — OH group. 2 Phenols are compounds where the —OH group is directly attached to the benzene ring. 3 Alcohols can be classified as primary, secondary and tertiary alcohols. 4 Alcohols and phenols have higher boiling point than expected because they can form intermolecular hydrogen bonds with their own molecules. 5 Alcohols and phenols are weak acids. 6 Phenols are stronger acid than alcohols. 7 The test for the —OH group is by using phosphorus pentachloride or thionyl chloride. Steamy white fumes of HCl chloride is released. 8 Phenols give white precipitate with aqueous bromine or aqueous chlorine. 9 Tertiary alcohols cannot be oxidised. Reactions of Alcohols, ROH Reagent Product Remarks Sodium, room temperature RO–Na+ + H2 Acid-metal R9COOH, concentrated H2SO4, reflux R’COOR Esterification PCl5, room temperature RCl + POCl3 + HCl Nucleophilic substitution (White fumes liberated) SOCl2, room temperature RCl + SO2 + HCl Nucleophilic substitution (White fumes liberated) R9COCl, room temperature R9COOR Esterification Concentrated HX(aq) RX Nucleophilic substitution Excess concentrated H2SO4, 180 °C CRC Dehydration to alkenes Excess alcohol, concentrated H2SO4, 145 °C R!O!R Dehydration to ether Acidified KMnO4, warm (Primary alcohol) R!C!H ∫ O Oxidation to aldehydes. Strong heating would oxidise the aldehyde to carboxylic acid Acidified KMnO4, heat (Secondary alcohol) R!C!R ∫ O Oxidation to secondary alcohol Reactions of Phenol Reagent Product Remarks Sodium, room temperature O–Na+ Acid-metal reaction Hydrogen gas liberated Aqueous NaOH, room temperature O–Na+ Acid-base neutralisation Bromine water, room temperature OH Br Br Br Electrophilic substitution White precipitate formed Aqueous iron(III) chloride (C6H5O–)3Fe3+ Blue or purple colouration
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 176 17 Triiodomethane Test Any alcohol with the following group will react with an alkaline solution of iodine to give a yellow precipitate. H & CH3!C!R (where R = H, alkyl or aryl groups) & OH STPM PRACTICE 17 Objective Questions 1 Ethanol and 1-propanol can be differentiated using A phosphorus pentachloride B acidified potassium manganate(VII) C sodium carbonate D alkaline iodine 2 Glycerol (1,2,3-propanetriol) is more soluble in water than 1-propanol because A glycerol has a higher relative molecular mass B only glycerol can form hydrogen bonds with water molecules C glycerol can form more hydrogen bonds than 1-propanol D glycerol undergoes hydrolysis with water but 1-propanol does not 3 Which of the following reagents will react with phenol but not with cyclohexanol? A PCl5 C CH3COCl B CH3COOH D Cl2(aq) 4 Ethanol can be differentiated from other primary alcohols by using A phosphorus pentachloride B acidified potassium manganate(VII) C 2,4-dinitrophenylhydrazine D alkaline iodine 5 Which of the following properties of phenol is not correct? A It is completely miscible with water at temperature above 70 °C. B It reacts with aqueous bromine to give a yellow precipitate. C Phenol is soluble in dilute hydrochloric acid but not in aqueous sodium hydroxide. D Phen o l i s a s t ro n g e r a c i d t h a n cyclohexanol. 6 Which of the following reacts the fastest with Lucas reagent (zinc chloride in concentrated hydrochloric acid)? A 1-butanol B 2-butanol C 2-methyl-2-propanol D 2-methyl-1-propanol 7 What reagent can be used to distinguish between cyclohexanol and phenol? A KMnO4/H+ B Br2(aq) C NaOH(aq) D Na metal 8 Which of the following would not produce ethanol as one of its products? A Fermentation of glucose B Ethene with steam in the presence of phosphoric acid C Ethane with aqueous sodium hydroxide D Methyl magnesium chloride with methanal 9 What is the expected observation when phenol is added to freshly prepared aqueous iron(III) chloride? A A yellow precipitate B A violet-blue solution C White fumes of hydrogen chloride D A reddish-brown solution
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 177 17 10 Compound X has the following properties: • Gives a yellow precipitate with alkaline iodine • Gives white fumes with thionyl chloride • Changes the colour of acidified potassium dichromate from orange to green What could X be? I CH3OH II CH3CH2OH III CH3CH(OH)CH3 IV (CH3)3COH A I B I and II C II and III D II, III and IV 11 Compound Y has the structure below: C2H5 HO HO CH2OH Which statement is not true of Y? A It is optically active. B It reacts with sodium carbonate to liberate carbon dioxide. C It has two functional groups. D It can be dehydrated by concentrated sulphuric acid. 12 Which of the following products use phenol in its manufacturing? I Bakelite II Nylon III Antiseptic A I and II B I and III C II and III D I, II and III 13 Which are the intermediate species formed when ethanol reacts with concentrated hydrobromic acid to form bromoethane? I CH3CH2 + II CH3CH2O– III CH3CH2OH2 + A I and II C II and III B I and III D I, II and III 14 The structure of geraniol (a component of certain perfumes) is shown below: (CH3)2C=CHCH2CH2C=CHCH2OH & CH3 Which statement(s) is/are true of geraniol? I It is soluble in water. II It forms a yellow precipitate with alkaline iodine. III It decolourises acidified potassium manganate(VII). A I C I and II B III D II and III 15 Phenol, C6H5OH is a stronger acid than phenylmethanol, C6H5CH2OH because A phenol dissolves in aqueous sodium hydroxide but phenylmethanol does not B ph e n o l i s s o l u b l e i n w a te r b u t phenylmethanol does not C the O9H bond in phenol is stronger than that in phenylmethanol D the phenoxide ion is more stable than the phenylmethoxide ion 16 Compound Y has the structure below: CH3 & HOCH29C9CH2CH2OH & OH Which of the following statements are true of compound Y? I It is optically active. II It can be dehydrated to an alkene. III It decolourises acidified potassium manganate(VII). IV It is insoluble in water. A I and III C I, II and III B II and IV D I, II, III and IV 17 Ethanol is warmed gently with an acidified solution of potassium manganate(VII). Which statement is true of the reaction that occurs? A Potassium manganate(VII) acts as a reducing agent. B The colour of potassium manganate(VII) changes from purple to green. C Ethanoic acid is produced. D Effervescence occurs.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 178 17 Structured and Essay Questions 1 An organic compound, A has the following composition by mass: C, 64.9%; H, 13.5%; O, 21.6% 0.25 g of A when vaporised at 100 °C and 200 kPa occupies a volume of 52.4 cm3 . When A is warmed with acidified potassium manganate(VII), compound B (an aldehyde) is formed. On heating with excess of concentrated sulphuric acid, compound C which decolourises bromine in tetrachloromethane is produced. When C is reacted with hydrogen bromide, D is formed. When D is refluxed with aqueous sodium hydroxide, E, an isomer of A is produced. Acidified potassium manganate(VII) has no effect on E. Identify compounds A, B, C, D and E. 2 Identify the following compounds. (a) A, C8H10O, decolourises acidified potassium manganate and gives white fumes with phosphorus(V) chloride. A reacts with alkaline iodine to produce a yellow precipitate. (b) Hydrogenation of a compound B, C5H10, produces 2-methylbutane. B reacts with hydrogen bromide to produce an alkyl halide C. Boiling C with sodium hydroxide produces an alcohol D. Mild oxidation of D produces E. 3 Suggest how you would carry out the following conversions. State the reagents and conditions used. (a) 1-propanol to propanoic acid (b) 1-propanol to 2-methylpropanoic acid 4 Explain the following observations. (a) Phenol dissolves in aqueous sodium hydroxide but does not dissolve in aqueous sodium carbonate. (b) Cyclohexanol decolourises potassium manganate(VII), but phenol does not. (c) Phenol is a stronger acid than ethanol. (d) Phenol is soluble in aqueous sodium hydroxide but cyclohexanol does not. 5 Compound A, C4H10O is optically active. When A is heated with copper, B, C4H8O is produced. B does not react with an aqueous solution of ammonical silver nitrate. When B is warmed with an alkaline solution of iodine, a yellow precipitate is formed. Treatment of A with excess concentrated sulphuric acid produces two isomeric compounds, C and D with the molecular formula C4H8. Ozonolysis of C produces a single product E, C2H4O while ozonolysis of D produces F, C2H6O and G, CH2O. B reacts with aqueous hydrogen cyanide in the presence of a little sodium cyanide to produce H, C5H9N, which on boiling with dilute sulphuric acid gives I, C5H10O2. (a) Draw the structural formulae of compounds A, B, C, D, E, F, G, H and I. (b) Write the chemical equations for all the reactions involved. (c) Write the reaction mechanism that produces H. 6 The reaction scheme of a hydroxyl compound, A is shown below. I II C6H5CH=CH2 : A : C6H5CH(CH3)OOCC6H5 (a) Draw the structural formula of A. (b) State the reagents and conditions labelled (I) and (II) above. (c) A has an isomer, B which is optically inactive. Draw a possible structure for B and explain how both isomers can be distinguished using a simple chemical test.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 179 17 7 An organic compound A (relative molecular mass = 60) has the following composition by mass: C, 60.0%; H, 13.4%; O, 26.6% A reacts with phosphorus(V) chloride to release dense white fumes of hydrogen chloride. When the vapour of A is passed over heated aluminium oxide, a gas B, which decolourises bromine in tetrachloromethane is produced. When B is bubbled through concentrated sulphuric acid and the product diluted with water and warmed, compound C, which is an isomer of A is produced. A does not give a yellow precipitate with alkaline iodine. (a) Identify compounds A to C. (b) Both A and C are oxidised by hot, concentrated acidified potassium manganate. Write equations to represent the oxidation reactions. (c) Suggest a test to differentiate between A and C. 8 Four isomeric organic compounds A, B, C and D have the molecular formula of C4H10O. B, C and D react with phosphorus(V) chloride, but A does not. B and C are oxidised by acidified potassium manganate(VII), but D does not. B gives a positive test with alkaline iodine, but C gives a negative result. (a) Identify compounds A, B, C and D. (b) Which of the four compounds is optically active? (c) When the optically isomer is warmed with excess concentrated sulphuric acid, compound E, which exhibits geometrical isomerism is produced. Draw the structures of the geometrical isomers. 9 (a) Explain why ethanol is miscible with water but phenol is insoluble in water. (b) Explain why ethanol gives a yellow precipitate when warmed with an alkaline solution of iodine, but phenol does not. (c) When phenol is warmed with a mixture of concentrated nitric acid and concentrated sulphuric acid, monosubstitution occurs. (i) Draw the structures of the monosubstituted products. (ii) Name the mechanism involved. (d) In what way is phenol different from ethanol when phosphorus(V) chloride is added separately to both the compounds. 10 (a) Consider the reaction scheme below involving 2-propanol. CH3CHCH3 CH3CHRCH2 I II III OH CH3CHCH3 Cl CH3CCH3 O R For each of the reactions I, II and III, name the reagents and conditions used. (b) If 2-methyl-2-propanol is used instead of 2-propanol in the above reaction scheme, identify the organic products (if any) of reactions I, II and III.
Chemistry Term 3 STPM Chapter 17 Hydroxy Compounds 180 17 11 Compound X has the structure below: HO99 99CHCH3 & & OH (a) Compound X is optically active. Draw the structures for the two optical isomers of X and mark on your diagram the chiral carbon. (b) Predict whether X is soluble or insoluble in water. (c) Draw the structural formulae of the organic product(s) formed, if any, when X reacts with each of the following reagents. (i) Sodium hydroxide (ii) Sodium metal (iii) Warm acidified potassium dichromate (iv) Iodine in aqueous sodium hydroxide (v) Dilute nitric acid 12 Compound Y (Mr = 74 g mol–1) has the following composition by mass: C, 64.9%; H, 13.5%; O, 21.6% Y reacts with phosphorus(V) chloride with the release of white fumes. (a) Determine the molecular formula of Y. (b) Draw all possible structural isomers of Y. (c) One of the isomers of Y reacts with acidified potassium manganate(VII) to produce a ketone. Identify the isomer and write equation for the reaction. (d) Another isomer of Y does not react with acidified potassium manganate(VII). Identify the isomer and explain the observation. 13 Compound X, C4H10O, reacts with phosphorus pentachloride to release white fumes of hydrogen chloride, but it does not react with acidified solution of potassium dichromate. (a) Explain the above observations and draw the structural formula of X. (b) Suggest how you would prepare X starting from iodomethane.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 18 CHAPTER CARBONYL COMPOUNDS 18 Concept Map Carbonyl Compounds Carbohydrates • Monosaccharides Nomenclature Physical Properties • Aldehydes • Ketones Preparation of Carbonyl Compounds • Oxidation of alcohols • Friedel-Craft acylation • Ozonolysis of alkenes Reactions of Carbonyl Compounds • Nucleophilic addition • Oxidation • Reduction • Triiodomethane test Learning earning Outcomes Students should be able to: • write the general formula of carbonyl compounds: aliphatic and aromatic aldehydes and ketones; • name aliphatic and aromatic aldehydes and ketones according to the IUPAC nomenclature; • describe structural and optical isomerism in carbonyl compounds; • state the physical properties of aliphatic and aromatic aldehydes and ketones; • write the equations for the preparation of aldehydes and ketones; • explain the reduction reactions of aldehydes and ketones to primary and secondary alcohols respectively through catalytic hydrogenation reaction and with LiAlH4; • explain the use of 2,4-dinitrophenylhydrazine reagent as a simple test to detect the presence of >C=O group; • explain the mechanism of nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones; • explain the oxidation of aldehydes; • differentiate between aldehydes and ketones based on the results of simple tests as exemplified by Fehling’s solution and Tollens’ reagent; • explain the reactions of carbonyl compounds with the structure CH3−C=O with alkaline aqueous solution of iodine to give triiodomethane (iodoform test); • explain natural compounds such as glucose, sucrose and other carbohydrates which have the >C=O group; • explain the characteristics of glucose as a reducing sugar.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 182 18 18.1 Introduction 1 Carbonyl compounds, with the general formula of CnH2nO are a group of organic compounds with the carbonyl functional group in their structures. !C! ∫ O 2 Some common carbonyl compounds are propanone (used in nail varnish), formaldehyde (used in the preservation of biological specimens), vanillin (the vanilla flavour), cinnamaldehyde (found in the spice, kayu manis), progesterone (female hormone) and testosterone (male hormone). 3 The two main classes of carbonyl compounds are aldehydes and ketones. 4 In aldehydes, at least one of the atoms that is bonded to the carbonyl carbon is hydrogen. (where R = H, alkyl or aryl) R C O H RR For example: H!C!H and CH3!C!H ∫ ∫ O O 5 In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups. (where R = alkyl or aryl) R C O R RR For example: CH3!C!CH3 and !C!CH3 ∫ ∫ O O 6 The carbonyl carbon undergoes sp2 hybridisation, and the carbon-oxygen bond consists of a σ bond and a π bond. 7 Note that aldehydes and ketones are isomeric, with the general formula of CnH2nO. Exam Tips Exam Tips In aldehydes, the carbonyl group is at the end of the carbon chain.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 183 18 18.2 Nomenclature Aldehydes 1 In the IUPAC system, the longest carbon chain that contains the aldehyde group is called the parent chain by substituting the suffix ‘e’ in the corresponding alkane by ‘al’. 2 The carbonyl carbon is always assigned as carbon-1. 3 Examples are [the names in brackets are the common names]: H!CRO & !CH2!CRO H & Methanal (Formaldehyde) H Phenylethanal (Phenylacetaldehyde) CH3!CRO & H Ethanal (Acetaldehyde) CH3CH2!CRO 1 2 3 4 5 6 Wrong 6 5 4 3 2 1 Correct CH3!CH!CH2CH2CH2!CRO & & CH3 H 5-methylhexanal & H Propanal 4 For aromatic aldehydes where the aldehyde group is bonded directly to the benzene ring, they are called benzaldehydes. For example: H R O CH3 C 4-methylbenzaldehyde C O H R Benzaldehyde (Benzenecarbaldehyde) 5 For unsaturated aldehydes, the presence of the carbon-carbon double bond is indicated by the infix -‘en’-. However, the numbering of the carbon atom always starts from the carbonyl carbon. For example: 3 2 1 3 2 1 CH2RCH!C!H !CRC!CHO ∫ & & O H H 2-propenal (Acrolein) 3-phenyl-2-propenal (Cinnamaldehyde)
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 184 18 Ketones 1 In the IUPAC system, ketones are named by choosing the longest chain that contains the carbonyl group as the parent chain. 2 Substitute the suffix ‘-e’ of the corresponding alkane to ‘-one’. 3 The chain is numbered from the direction that gives the carbonyl carbon the smaller number. 4 The first member of ketones is named propanone (common name: acetone). CH3!!C!!CH3 ∫ O 5 Other examples are: CH3!CH2!C!CH3 !C!CH3 ∫ ∫ O O Butanone Phenylethanone (Acetophenone) CH3CCH2CH2CH3 ∫ !C! O ∫ 2-pentanone O Diphenylmethanone (Benzophenone) CH3CH2CCH2CH3 ∫ O 3-pentanone !CH2!C!CH3 ∫ O O ∫ 1-phenyl-2-propanone C!CH2CH2CH3 1-cyclohexyl-1-butanone 1 Name the following compounds according to the IUPAC convention. (a) C CH3 H CH3 COCH3 RRC (c) C CH3 CHO Br CH3 RRC (b) O CH2CH2CH3 RR (d) O CH3 RR Quick Check 18.1
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 185 18 (e) CH2CH2CCH2CHCH3 OH O R 2 Draw the structural formulae for the following compounds. (a) 1-iodo-2-propanone (d) 2,2-dimethylpentanal (b) 2,2-dimethylcyclohexanal (e) 3-methyl-3-buten-2-one (c) 3-hydroxybutanal 18.3 Structural and Optical Isomerism in Carbonyl Compounds 1 Aldehydes with four or more carbon atoms exhibit structural isomerism. 2 Structural isomerism in aldehyde is due to the arrangement of the carbon chain. This is because the aldehyde group must always be at the beginning of the chain. An example is C4H8O. CH3CH2CH29C9H Butanal ' O CH39CH9C9H 2-methylpropanal & ' CH3 O 3 Ketones with five or more carbon atoms exhibit structural isomerism. An example is C5H10O. CH3CH2CH2—C—CH3 2-pentanone ' O CH3CH2—C—CH2CH3 3-pentanone ' O CH3—CH—C—CH3 3-methyl-2-butanone & ' CH3 O
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 186 18 4 Aldehydes with three carbon atoms or more are isomeric with ketones. For example, C3H6O: CH3CH2—C—H Propanal ' O CH3—C—CH3 Propanone ' O 5 Carbonyl compounds with a chiral carbon atom exhibit optical isomerism. For example, the aldehyde with the molecular formula of C5H10O: C* CHO C2H5 CH3 CH3 C2H5 H C* CHO H 18.4 Physical Properties 1 Oxygen is more electronegative than carbon (electronegativity of 3.5 and 2.5 respectively). As a result, the carbon-oxygen double bond of the carbonyl group is polarised as shown below. C O RR δ+ δ– C!!O 2 As such, the intermolecular forces are stronger, and their boiling points are higher than the corresponding alkanes and other non-polar compounds of comparable molecular mass. 3 However, carbonyl compounds cannot form intermolecular hydrogen bonds with their own molecules. Therefore, their intermolecular forces are weaker, and their boiling points are lower than the corresponding alcohols. 4 The table below lists the boiling points of five compounds of comparable molecular mass. Compound Structural formula Relative molecular mass Boiling point/°C Diethyl ether C2H5OC2H5 74 34 Pentane CH3CH2CH2CH2CH3 72 36 Butanal CH3CH2CH2CH ∫ O 72 76 Butanone CH3CH2CCH3 ∫ O 72 80 1-butanol CH3CH2CH2CH2OH 74 117 Info Chem Carbonyl compounds cannot form hydrogen bonds with their own molecules.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 187 18 5 Methanal (formaldehyde) with a boiling point of –21 °C is a gas at room conditions. All other common aldehydes and ketones are liquids at room conditions. 6 The carbonyl groups of aldehydes and ketones can form hydrogen bond with water molecule: C O O H : : H R δ+ δ– 7 The lower molecular mass aldehydes and ketones are miscible with water. However, the solubility decreases as the size of the non-polar hydrophobic hydrocarbon group increases. 8 Methanal (formaldehyde), ethanal (acetaldehyde) and propanone (acetone) are completely miscible with water in any proportions. 9 A 40% aqueous solution of methanal called ‘formalin’ is used to preserve biological specimens. 10 Carbonyl compounds have a pleasant smell and are used in the perfume industries and also as food flavourings. 18.4 Preparation of Carbonyl Compounds Oxidation of Alcohols 1 Primary and secondary alcohols are oxidised to aldehydes and ketones respectively by acidified potassium dichromate(VI) or acidified potassium manganate(VII) with warming. O!!H & & R!!C!!H + [O] !!: R!!C!!H + H2O & ∫ & O H O!!H & & R!!C!!R + [O] !!: R!!C!!R + H2O & ∫ & O H Exam Tips Only methanal is a gas at room conditions. Info Chem Carbonyl compounds can form hydrogen bonds with water. Formalin Info Chem Ketones are more resistant to further oxidation. 2011/P1/Q35 2016/P3/Q9
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 188 18 2 However, the aldehydes formed from the oxidation of primary alcohols are easily oxidised to carboxylic acids. R!!C!!H + [O] !!: R!!C!!OH ∫ ∫ O O To overcome this, the alcohol is warmed with a dilute acidified solution of potassium dichromate(VI), and the temperature of the mixture is maintained between the boiling points of the alcohol and the aldehyde. The aldehyde is distilled over as soon as it is formed. 3 Examples are: CH3CH2OH + [O] !!: CH3CHO + H2O !CH2OH + [O] !!: !CRO + H2O & H + [O] !!: + H2O OH RO Cyclohexanone Friedel-Craft Acylation 1 This is a good method for the preparation of aromatic ketones. !!: + HCl H Cl RO + R!!C R!!C R !! O 2 For example, benzene reacts with ethanoyl chloride in the presence of anhydrous aluminium chloride to produce phenylethanone (acetophenone). + HCl H + CH3COCl !!: CH3!!C R !! O Info Chem Care must be taken to prevent further oxidation of the aldehydes formed to carboxylic acids. Preparation of aromatic ketones 2013/P3/Q11 Exam Tips Exam Tips When the vapour of an alcohol is passed over heated copper, dehydrogenation occurs and a carbonyl compound is formed. E.g. CH3CH2OH !: CH3—C—H + H2 ‖ O Cu Δ
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 189 18 Ozonolysis of Alkenes 1 Alkenes (dissolved in an inert solvent) react with ozone gas in the presence of zinc and dilute sulphuric acid to produce carbonyl compounds. 2 In the reaction, the carbon-carbon double bond breaks to form two fragments as shown below. C C RR + O3 + H2O !: C R R CO + O + H2O2 3 For example: C C R + O3 + H2O !: C R R O + C O + H2O2 CH3 H CH3 CH3 CH3 CH3 CH3 H Quick Check 18.2 1 Draw the structures of the carbonyl compounds formed (if any) when the following alcohols are subjected to oxidation by dilute potassium dichromate(VI). (a) Methanol (c) 1-methylcyclohexanol (b) Ethanol (d) 1-phenylethanol 2 Draw the structures of the carbonyl compounds formed when the following alkenes are subjected to ozonolysis. (a) Ethene (d) Cyclohexene (b) 1-butene (e) 1-methylcyclopentene (c) Phenylethene 18.6 Reactions of Carbonyl Compounds 1 Since both aldehydes and ketones have the same functional group, they exhibit many similar chemical properties. 2 One of the most important reactions of carbonyl compounds is addition. 3 Due to the polarisation of the carbon-oxygen double bond, the carbonyl carbon atom is readily attacked by nucleophiles. As a result, carbonyl compounds undergo nucleophilic addition. 4 Apart from nucleophilic addition, other important reactions of carbonyl compounds are (a) reduction (b) condensation (c) oxidation (d) triiodomethane test 2009/P1/Q34 2013/P3/Q19 2012/P1/Q35 2014/P3/Q17
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 190 18 Reduction of Carbonyl Compounds 1 Aldehydes and ketones are reduced to primary alcohols and secondary alcohols respectively. RCHO + 2[H] !!: RCH2OH RCOR + 2[H] !!: R!CH!R & OH 2 The reducing agents used are: (a) Hydrogen gas in the presence of heated nickel or platinum (catalytic reduction) (b) Lithium aluminium hydride, LiAlH4 in ether 3 Catalytic reduction has a disadvantage in that, it also reacts with carbon-carbon double and triple bonds. 4 Lithium aluminium hydride is a powerful reducing agent. It reduces not only the carbonyl group of aldehydes and ketones, but also the carboxylic acids and their derivatives. 5 Examples of reduction: CH3CHO + 2[H] !!: CH3CH2OH CH3!C!CH3 + 2[H] !!: CH3!CH!CH3 ∫ & O OH Info Chem Catalytic reduction has the disadvantage that the hydrogen will react with other functional groups such as C=C. Quick Check 18.3 1 Draw the structural formulae of the products formed when the following compounds undergo reduction under suitable conditions. (a) HCHO (c) H!C!CH2CH2!C!CH3 (b) ∫ ∫ !C!CH3 O O ∫ (d) HOCH2CH2C!H O ∫ O 2 What aldehydes or ketones produce these alcohols on reduction with lithium aluminium hydride. (a) Cyclohexanol (c) 2,5-heptadiol (b) 2-phenylethanol (d) 4-chlorophenylmethanol Reaction with 2,4-dinitrophenylhydrazine 1 The structure of 2,4-dinitrophenylhydrazine is: H!!N!!NH!! !!NO2 H NO2 !! 2008/P1/Q34 This is a condensation reaction. Exam Tips Exam Tips • Lithium aluminium hydride, LiAlH4 does not react with C=C or CC bonds. H ǀ —C— + 4[H] : —C— + H2O ‖ ǀ O H Info Chem Carbonyl compounds are reduced to hydrocarbons by zinc amalgam and concentrated HCI:
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 191 18 2 Carbonyl compounds react with 2,4-dinitrophenylhydrazine in methanol at room temperature to form yellow/orange crystalline solids of 2,4-dinitrophenylhydrazone. C O R + N!NH! C N R !NH! NO2 H H !NO2 !: NO2 !NO2 + H2O 3 This reaction is used to test the presence of carbonyl compounds, which give a coloured precipitate with 2,4-dinitrophenylhydrazine. For example, C O R + N!NH! C N R !NH! NO2 H H !NO2 !: NO2 !NO2 + H2O CH3 CH3 CH3 CH3 propanone 2,4-dinitrophenylhydrazone Quick Check 18.4 Draw the structures of the products formed when phenylethanone reacts with 2,4-dinitrophenylhydrazine. Nucleophilic Addition 1 The nucleophilic addition of carbonyl compounds can be represented by C O RR !!: Nu!!C Nu– : O– δ+ δ– 2 The reaction is completed when the intermediate picks up a proton. + H+ Nu!!C !!: O– Nu!!C OH 3 The reactivity of nucleophilic addition depends on two factors. (a) The magnitude of the partial positive charge on the carbonyl carbon atom (b) Steric hindrance Coloured crystalline solid 2010/P1/Q35 INFO Reaction with 2,4-Dinitrophenylhydrazine