Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 192 18 4 Alkyl groups are electron-releasing groups. This decreases the magnitude of the partial positive charge on the carbonyl carbon atom. 5 In aldehydes, there is only one alkyl group bonded to the carbonyl carbon, while there are two in ketones. H R CRO : R R CRO : : δ+ δ+ 6 Due to the presence of two large alkyl groups in ketones, there is more steric hindrance to the approach of the nucleophile. CRO : : R R Nu– 7 Generally, the reactivity of carbonyl compounds towards nucleophilic addition decreases in this order. Reaction with Hydrogen Cyanide 1 An example is the addition of aqueous hydrogen cyanide to an aldehyde or ketone to form an addition compound called a cyanohydrin. 2 For example: H & CH3!CRO + HCN !!: CH3!C!CN & & H OH Ethanal (Acetaldehyde) 2-hydroxypropanenitrile (Acetaldehyde cyanohydrin) CN & CH3!C! + HCN !!: CH3!C! ∫ & O OH Phenylethanone 2-phenyl-2-hydroxypropanenitrile 3 Hydrolysis of the cyanohydrins with dilute sulphuric acid produces an important class of compounds called α-hydroxy acids (AHA). Alkyl groups are electron releasing groups. Steric hindrance C O > R H H C O > R R H C O > R H C O > R R R C O > R R C O R Info Chem A little NaCN is added as a catalyst. 2017/P3/Q19
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 193 18 For example: CH3 CH3 & & !C!CN + 2H2O + H+ !: !C!COOH + NH4 + & & OH OH 2-phenyl-2-hydroxypropanenitril 2-phenyl-2-hydroxypropanoic acid (2-phenyl-α-hydroxypropanoic acid) 4 Note that the cyanohydrin produced has one more carbon atom compared to the aldehyde or ketone. Hence, this is another convenient way to increase the number of carbon atoms in a molecule by one. 5 The rate of reaction of carbonyl compounds with aqueous hydrogen cyanide is accelerated in basic medium, but retarded in acidic medium. 6 To account for this observation, the following mechanism is proposed: • Dissociation of aqueous hydrogen cyanide. HCN(aq) H+(aq) + CN– (aq) • Attack of the nucleophile, CN– on the electron-deficient carbonyl carbon atom. C O RR !!: NC!!C :CN– CH O– 3 CH3 H H slow • Abstraction of a H+ ion forms HCN to complete the reaction. NC! + HCN !: CH3!C!OH + CN– C O– CH3 H H CN fast 7 The rate equation for the reaction is: Rate = k[CH3CHO][CN– ] 8 In the presence of alkali, the dissociation of HCN is more complete: HCN + NaOH !!: Na+ + CN– + H2O This increases the concentration of CN– , which in turn causes the rate of reaction to increase. Exam Tips Exam Tips This is a convenient way to increase the number of carbon atoms by one. The intermediate is a nucleophile. Mechanism of nucleophilic substitution NaOH increases the dissociation of HCN. INFO Nucleophilic Addition with HCN
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 194 18 9 However, in acidic solution, the presence of H+ decreases the degree of dissociation of HCN. HCN L H+ + CN– This causes the concentration of CN– to decrease which leads to a decrease in the rate. Acid supresses the dissociation of HCN. Quick Check 18.5 1 Draw the structures of the cyanohydrins produced when the following carbonyl compounds react with hydrogen cyanide. (a) Methanal (d) Diphenylketone (b) Benzaldehyde (e) Cyclohexanon (c) 4-chloropentanal 2 Propanone reacts with aqueous hydrogen cyanide in the presence of a little sodium cyanide to form a cyanohydrin. (a) Write a balanced equation for the reaction. (b) Suggest the mechanism for the reaction. (c) Explain the role of sodium cyanide in the reaction. Oxidation 1 Aldehydes are easily oxidised to carboxylic acids by oxidising agents such as acidified potassium dichromate(VI) or acidified potassium manganate(VII) with heat. C O R O + [O] !: R!!C R !!OH R H 2 For example, CH3CHO + [O] !!: CH3COOH Ethanal Ethanoic acid !!CHO + [O] !!: !!COOH Benzaldehyde Benzoic acid 3 On the other hand, ketones are more difficult to oxidise. They require prolong heating with concentrated solution of acidified potassium dichromate(VI) or potassium manganate(VII). Sometimes, concentrated nitric acid is used instead. 4 Oxidation of ketones always involves cleavage of strong carboncarbon bonds. Decolourisation of KMnO4 2017/P3/Q10 Info Chem • Benzaldehyde, a colourless liquid is oxidised by air to benzoic acid, a white solid. • In alkaline medium, the carboxylate ion, RCOO– is formed.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 195 18 For example, CH3!!C!!CH3 + 4[O] !!: CH3COOH + CO2 + H2O ∫ O 5 An industrial application of this reaction is the oxidation of cyclohexanone by concentrated nitric acid to produce 1,6-hexadioic acid (adipic acid), one of the monomers used for the manufacture of nylon-6,6. O ' + 3[O] !!: HOOC!CH2CH2CH2CH2!COOH Differentiation between Aldehydes and Ketones Oxidation by Tollens' Reagent 1 Tollens' reagent is a colourless solution containing the diamminesilver complex ion, [Ag(NH3)2]+. It is formed by the action of excess ammonia on aqueous silver nitrate. 2 When an aldehyde is warmed gently with Tollens' reagent in a test tube, the silver ion is reduced to metallic silver which sticks to the wall of the test tube as a reflective layer. This is referred to as the silver mirror test. R!C!H + 2[Ag(NH3)2]+ + 2OH– !!: ∫ O R!C!O– + 2Ag(s) + NH4 + + 3NH3 + H2O ∫ O 3 For example, CH3CHO + 2[Ag(NH3)2]+ + 2OH– !!∆: CH3COO– + 2Ag + NH4 + + 3NH3 + H2O !C!H + 2[Ag(NH3)2]+ + 2OH– !!∆: ∫ O !C!O– + 2Ag(s) + 3NH3 + NH4 + + H2O ∫ O The acid produced contains less carbon atoms than the starting material. Info Chem A simplified equation is R9C9H + Ag2O 9: ' O RCOOH + 2Ag The silver mirror test for aldehydes 2009/P1/Q31 2017/P3/Q19 VIDEO Silver Mirror Test
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 196 18 4 Ketones are not oxidised by Tollens' reagent. Hence, the silver mirror test can be used to differentiate between aldehydes and ketones. Oxidation by Fehling’s Solution 1 Fehling’s solution is a dark blue solution containing the copper(II) ion complexed with an alkaline solution of 2,3-dihydroxybutanedioate ions (or the tartrate ions). – OOC!CH!CH!COO– & & OH OH Tartrate ion 2 When an aldehyde is heated with Fehling’s solution, a reddishbrown precipitate of copper(I) oxide is formed. RCHO + 2Cu2+ + 5OH– !!: RCOO– + Cu2O + 3H2O 3 For example, CH3CH2CHO + 2Cu2+ + 5OH– !: CH3CH2COO– + Cu2O + 3H2O Propanal Propanoate ion 4 Ketones and benzaldehyde do not react with Fehling’s solution. Triiodomethane Test 1 Methyl carbonyl compounds with the general formula of CH3!C!R (where R = H, alkyl or aryl) ∫ O react with an alkaline solution of iodine when heated to produce a yellow precipitate of triiodomethane (or iodoform). 2 The general equation of reaction is: CH3!C!R + 3I2 + 4NaOH !Δ : RCOO– + CHI3 + 3NaI + 3H2O ∫ O 3 For example, CH3!C!CH3 + 3I2 + 4NaOH !Δ : CH3COO– + CHI3 + 3NaI + 3H2O ∫ O 4 Ethanal (or acetaldehyde) is the only aldehyde that gives a positive iodoform test. CH3!C!H ∫ O Exam Tips Ketones give negative observation with Tollens' reagent. Formation of reddish-brown precipitate Exam Tips Exam Tips Ketones and benzaldehyde do not react with Fehling's solution. Triiodomethane is also known as iodoform. Test for methyl carbonyl compounds Info Chem O R RCOO– CH3 – C!R CHI3 2017/P3/Q10 2008/P1/Q34 2013/P3/Q12 VIDEO Fehling’s Solution Test
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 197 18 Quick Check 18.6 State which of the following compounds will give a positive iodoform test. (a) Methanal (f) 2-butanone (b) Ethanal (g) Diphenylketone (c) Propanal (h) Cyclohexanal (d) Benzaldehyde (i) 4-hydroxy-4-methyl-2-pentanone (e) Phenylethanone (j) 1-phenylpropanone 18.7 Carbohydrates 1 Carbohydrates are a group of naturally occurring compounds containing carbonyl group. They are found in abundance in the plant world. 2 Examples of carbohydrates are glucose, starch, cellulose, sucrose and fructose. 3 The name ‘carbohydrate’ means ‘hydrates of carbon’ and is derived from the general formula of Cn(H2O)m. 4 Two examples of carbohydrates that conform to that general formula are: Glucose: C6H12O6 or C6(H2O)6 Sucrose: C12H22O11 or C12(H2O)11 5 However, not all carbohydrates have this general formula. Some have too few or too many oxygen atoms in their molecule, while some carbohydrates also contain other elements (such as nitrogen) in their structure. 6 Generally, carbohydrates can be classified as monosaccharides, disaccharides and polysaccharides. Monosaccharides 1 Monosaccharides are compounds that contain one ‘sugar’ unit in their structures. 2 They have the general formula of (CH2O)n, with one of the carbon atoms being the carbonyl group of an aldehyde or ketone. 3 Monosaccharides where n = 6 are known as hexose. 4 Monosaccharides containing an aldehyde group are again classified as aldoses; while those with a ketone group are called ketoses. 5 Two important hexose are glucose and fructose. Both have the molecular formula of C6H12O6. Glucose is an aldose, while fructose is a ketose. Glucose and fructose are isomers. Info Chem Carbohydrates are defined as polyhydroxy carbonyl compounds. Info Chem Sucrose is a dimer of glucose and fructose. 2009/P2/Q4(d) 2015/P3/Q20(b) 2011/P1/Q49 2015/P3/Q10
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 198 18 The structure of glucose (melting point = 146 °C) and fructose (melting point = 102 °C) are as follows: Or CH2OH!(CHOH)4!C!H ∫ O H!C*!OH HO!C*!H H!C*!OH H!C*!OH CH2OH C H !!!!! RO Glucose Or CH2OH!(CHOH)3!C!CH2OH ∫ O CRO HO!C*!H H!C*!OH H!C*!OH CH2OH CH2OH !!!!! Fructose 6 Glucose and fructose can also exist in the more stable cyclic form. C H H! ! C O H H H!C!OH ! ! ! C OH H! ! C OH H! ! C OH H! ! 3 5 6 4 1 2 H OH 4 ! HO H 3! 1 2 H 5 ! 6 O H CH2 OH C HOCH2 Glucose Fructose 7 An aqueous solution of glucose and fructose usually contains about 1% open-chain structure while 99% are in the cyclic form. Physical Properties of Monosaccharides 1 Monosaccharides such as glucose and fructose are colourless crystalline solids. 2 Due to the presence of polar O!H bonds, all monosaccharides are soluble in water. 3 They are only slightly soluble in non-polar solvents. 4 They are all sweet to the taste. Glucose as a Reducing Sugar 1 Due to the presence of aldehyde group in the molecule, glucose has many chemical reactions similar to that of the aldehydes. Glucose is a reducing sugar. The chiral carbons are indicated by an asterisk ‘*’. Cyclic structures of glucose and fructose 2015/P3/Q13
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 199 18 SUMMARY SUMMARY 2 Glucose reduces the silver ion in Tollens' reagent to metallic silver in the ‘silver mirror reaction’. CH2OH(CHOH)4CHO + 2[Ag(NH3)2]+ + 2OH– !Δ : CH2OH(CHOH)4COO– + 2Ag + NH4 + + 3NH3 + H2O 3 Glucose reacts with Fehling’s solution to produce a reddishbrown precipitate of copper(I) oxide: CH2OH(CHOH)4CHO + 2Cu2+ + 5OH– !Δ : CH2OH(CHOH)4COO– + Cu2O + 3H2O 4 Glucose reacts with 2,4-dinitrophenylhydrazine to give an orange precipitate. O2N CH2OH!(CHOH)4!CRO + H2 N!NH! ! NO2 & H O2N CH2OH!(CHOH)4!CRN!NH! !NO2 + H2O & H Quick Check 18.7 Draw the structures of the organic products formed when glucose reacts with the following reagents under suitable conditions. 1 HCN in the presence of NaCN 3 Phosphorus(V) chloride 2 Lithium aluminium hydride in ether 4 Acidified potassium manganate(VII) The silver mirror test 1 The functional group of aldehydes and ketones is carbonyl group, CRO. 2 The main reactions of aldehydes and ketones are: • nucleophilic addition • condensation • oxidation • reduction 3 Aldehydes are more reactive towards nucleophilic addition than ketones. 4 The confirmatory test for carbonyl group is by using 2,4-dinitrophenylhydrazine. A coloured precipitate will be formed. 5 Aldehydes react with Tollens' reagent and Fehling’s solution, but ketones do not. 6 Carbohydrates are naturally occurring carbonyl compounds. 7 Glucose (an aldose) and fructose (a ketose) are reducing sugars. 8 Sucrose, starch and cellulose are not reducing sugars. Exam Tips Exam Tips Hydrolysis of sucrose produces glucose and fructose. C11H22O11 + H2O : C6H12O6 + C6H12O6
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 200 18 Reactions of Aldehydes and Ketones Reagent Product Remarks HCN(aq), room temperature Presence of NaCN as catalyst H(R) & R!C!CN & OH Nucleophilic additions. Reaction is slow in acidic medium. Reaction is fast in alkaline medium. Both aldehydes and ketones give the same reaction. 2,4-dinitrophenylhydrazine, room temperature R R ! CRN!NH! !NO2 NO2 Condensation. Formation of coloured precipitate. Test for aldehydes and ketones. Acidified KMnO4 or alkaline KMnO4, heat RCOOH Aldehydes are easily oxidised to RCOOH. Ketones need prolong refluxing. Cleavage of molecule occurs. Fehling’s solution, heat RCOO– + Cu2O A brick-red precipitate with aldehydes. Ketones have no reaction with Fehling’s solution. Tollens' reagent, heat RCOO– + Ag Silver mirror with aldehydes. Ketones have no reaction with Tollens' reagent. H2/Ni or LiAlH4, heat RCH2OH R!CH!R & OH Aldehydes are reduced to primary alcohols. Ketones are reduced to secondary alcohols. Triiodomethane Test Carbonyl compounds with the following structure will give a yellow precipitate when warmed with an alkaline solution of iodine. CH3!C!R (R = H, alkyl or aryl group) ∫ O STPM PRACTICE 18 Objective Questions 1 A compound has the following properties. • It decolourises acidified potassium manganate(VII). • It gives positive iodoform test. • It does not give white fumes with phosphorus(V). The compound is most likely to be A ethanal B 1-propanol C propanone D propanal 2 Compound X has the structure below: C H CH2 COCH3 ONa O O Which reagent will not react with X under suitable conditions? A Cl2 in the presence of ultra-violet light B Tollen’s reagent C I2 in NaOH D Dilute H2SO4
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 201 18 3 Which of the following compounds reacts by nucleophilic addition? A 3-pentanone B 1-propanol C Methylbenzene D Ethyl ethanoate 4 The structural formula of fructose is shown below: CH2 OH CH2 OH O OH OH HO Which statement is not true about fructose? A Its molecular formula is C6H12O6. B It decolourises acidified potassium manganate(VII). C It reacts with 2,4-dinitrophenylhydrazine to produce an orange precipitate. D It is isomeric with glucose. 5 All the following reagents can be used to distinguish ethanal from propanone except A alkaline iodine B Tollens’ reagent C Fehling’s solution D hot, concentrated potassium manganate(VII) 6 Which statement about glucose is not correct? A Its molecular formula is (CH2O)6. B There are four chiral carbon atoms in a molecule of glucose. C It is isomeric with fructose. D It reacts with 2,4-dinitrophenylhydrazine but not with Fehling’s solution. 7 Which of the following would produce a carbonyl compound? A (CH3)2C(CH3)2 and ozone followed by hydrolysis B C6H5MgBr and C2H5OH C C6H5OH and LiAlH4 in ether D 2-Methyl-2-propanol and acidified potassium manganate(VII) 8 Which of the following would attack the carbonyl carbon atom? A Free radical C Nucleophile B Electrophile D Lewis acid 9 In the reaction between ethanal and aqueous hydrogen cyanide in the presence of sodium cyanide, the intermediate A is a free radical B is a nucleophile C is a pentavalent carbon atom D carries a positive charge 10 Glucose, C6H12O6 is a polyhydroxy carbonyl compound. Which is true about glucose? A It does not react with Tollen’s reagent. B It can undergo polymerisation. C It cannot be reduced. D It is a ketohexose. 11 Compound X gives a silver mirror when warmed with Tollens' reagent and a yellow precipitate with alkaline iodine. X could be I ethanol III propanone II ethanal A I C III B II D I and II 12 Which reagent can be used to differentiate between the two following compounds? CHO O CH2 CH2 A 2,4-dinitrophenylhydrazine B Bromine in tetrachloromethane C Ammoniacal silver nitrate D Acidified potassium manganate(VII) 13 Which of the following statements are true of the reaction between propanone and aqueous hydrogen cyanide? I The product is optically active. II The reaction is accelerated in the presence of a little base. III The intermediate is a nucleophile. A I and II C II and III B I and III D I, II and III 14 Organic compounds X and Y belong to the same homologous series. The chemical properties of X and Y are as follows: (i) Both X and Y decolourises hot acidified KMnO4 (ii) X reacts with alkaline iodine to give a yellow precipitate but Y does not.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 202 18 Which are the possible structural formulae of X and Y? X Y A CH3CHO CH3CH2CHO B C6H5CHO C6H5CH2COCH3 C HCOOH H2C2O4 D CH3CH2OH (CH3)3COH 15 A carbonyl compound is warmed with an alkaline solution of ammonical silver nitrate. Which statement is correct about the reaction? A The reaction involved is electrophilic substitution. B An orange brown precipitate is formed. C The carbonyl compound undergoes reduction. D The reaction can be classified as redox. 16 Which of the following statements is not correct regarding benzaldehyde? 9CHO A It decolourises acidified potassium manganate(VII). B It reacts with Fehling's solution to give an orange-brown solid. C It gives a reddish-brown solid with 2,4-dinitrophenylhydrazine. D It gives a 'silver mirror' with Tollens' reagent. 17 An organic molecule, R, has the following properties: • It forms an orange precipitate with Fehling's solution. • It does not react with thionyl chloride. • It decolourises acidified potassium manganate(VII). R could be A HOOC—CH2—C—CH—C2H5 ' & O OH B CH3—CH—CH=CH—C—H & ' Br O C HOOC—CH2—C—CH3 ' O D HO—CH2—CH=CH—C—H ' O Structured and Essay Questions 1 Identify the following compounds. (a) A, C8H8O, reacts with 2,4-dinitrophenylhydrazine to give an orange precipitate. A, on boiling with acidified potassium manganate(VII) produces a dicarboxylic acid. (b) B, C8H10O, gives C, C8H8O on oxidation. C reacts with 2,4-dinitrophenylhydrazine. Both B and C give a yellow precipitate when warmed with alkaline iodine. (c) D, C3H6O gives a yellow precipitate with 2,4-dinitrophenylhydrazine. When D is warmed with alkaline iodine, a yellow precipitate is formed. (d) E, C4H8O reacts with lithium aluminium hydride in ether to produce F, C4H10O. Both E and F give positive iodoform test. 2 Suggest how you would differentiate between the following pairs of compounds. (a) Propanal and propanone (c) 3-pentanone and 2-pentanone (b) Methanal and ethanal (d) Propanone and cyclohexanone 3 A compound X has the following composition by mass: C, 80.0%; H, 6.7%; O, 13.3% When 0.305 g of X is vaporised at 207 °C, the vapour occupies a volume of 0.10 dm3 under a pressure of 101 kPa.
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 203 18 X reacts with iodine in sodium hydroxide to give a yellow precipitate. X reacts with 2,4-dinitrophenylhydrazine to give an orange precipitate. X on reduction produces Y which gives a white fume with phosphorus(V) chloride. When Y is heated with excess concentrated sulphuric acid, Z is produced. Z decolourises bromine in tetrachloromethane. Z can be polymerised. (a) Calculate the empirical formula of X. (b) Determine the molecular formula of X. (c) What is the yellow precipitate? (d) Identify X, Y and Z. (e) Draw a repeating unit for the polymer obtained from Z. 4 Suggest how you would carry out the following conversions. (a) 2-butanone to 2-chlorobutane (b) Propanone to 2-methyl-2-hydroxypropanoic acid 5 A hydrocarbon A (relative molecular mass = 56) contains 85.7% by mass of carbon. A reacts with hydrobromic acid to produce B which is optically active. Hydrolysis of B with aqueous sodium hydroxide produces C, which on oxidation produces D. (a) Identify A, B, C and D. (b) Both C and D gives a yellow precipitate when warmed with alkaline iodine. Write equations for the reactions. (c) What is observed when 2,4-dinitrophenylhydrazine is added to compound D? Draw the structure of the product. 6 Aqueous hydrogen cyanide, in the presence of a little sodium cyanide, reacts with butanone an addition compound called cyanohydrine. (a) Write a balanced equation for the reaction. (b) Describe the mechanism of the reaction and state what is the role of sodium cyanide in the reaction. (c) The cyanohydrine produced above can exist in two isomeric form. Draw the structure of the two isomers and name the type of isomerism involved. (d) Give the structure of the product formed when the cyanohydrins is boiled with dilute sulphuric acid. 7 Compound A is found in cinnamon stick use in cooking. !CHRCH!C H RO A (a) Give the IUPAC name for A. (b) A can exist in two isomeric forms. Draw the structure of the two isomers and state the type of isomerism involved. (c) Using A as example, show how you would carry out the following types of reactions. (i) Nucleophilic addition (ii) Oxidation of the carbonyl group only (iii) Electrophilic addition (iv) Electrophilic substitution (v) Reduction of the carbonyl group only (d) State how you would differentiate between A and the product in (c)(v). 8 Give the structure of the products and state what type of reaction occurs between the following compounds. (a) Propanone and hydrogen cyanide (b) Ethanal and 2,4-dinitrophenylhydrazine (c) Butanal and Tollens' reagent (d) Propenal and lithium aluminium hydride
Chemistry Term 3 STPM Chapter 18 Carbonyl Compounds 204 18 9 Compound W (molecular formula of C5H10O) reacts with 2,4-dinitrophenylhydrazine but has no effect on Tollens' reagent. W reacts with alkaline iodine to give a yellow precipitate. (a) What functional group is present in W? (b) Draw all possible structures of W. (c) Which of the isomers in (b) will produce butanoic acid on prolong boiling with a concentrated solution of acidified potassium manganate? 10 Glucose, C6H12O6, is a naturally occurring carbonyl compound. (a) Draw the structure of glucose. (b) Name one major use of glucose. (c) In view of the structure of glucose, explain how the presence of glucose in human blood can be detected. Name the reagents and conditions used. (d) Write equations for the reactions between glucose and (i) lithium aluminium hydride (ii) 2,4-dinitrophenylhydrazine. 11 The molecular formula of compound J is given below: R CH2CH O O R Give the formula of the products formed (if any) when J reacts with each of the following reagents. (a) LiAlH4 (c) Alkaline iodine (b) Tollens' reagent (d) Acidified potassium manganate(VII) 12 Molecule X has the structure below: CH3 !CH"CH!C!CH2 COOH ' O (a) Give the IUPAC name of X. (b) X reacts with aqueous hydrogen cyanide, in the presence of a little sodium cyanide, to form compund Y. (i) Draw the structural formula of Y. (ii) Is Y optically active? Explain your answer. (iii) Name the type of reaction taking place. (c) Draw the structural formulae of the organic product(s) formed when X reacts with the following reagents. (i) C2H5MgI followed by H3O+ (ii) LiAlH4 in ether followed by H3O+ (iii) H2 in the presence of nickel and heat. 13 Write balanced equations for all the reactions involved in the preparation of the following carbonyl compounds. (a) Propanone from propene (c) Phenylethanone from benzene (b) Butanal from 1-iodobutane (d) Ethanal from 2-butene
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 19 CHAPTER CARBOXYLIC ACIDS AND 19 THEIR DERIVATIVES Carboxylic Acids Uses of Carboxylic Acids Nomenclature • Aliphatic acids • Unsaturated acids • Cyclic acids • Dicarboxylic acids • Aromatic acids • Fatty acids Physical Properties • Boiling and melting points • Solubility Preparation of Carboxylic Acids • Oxidation of primary alcohols and aldehydes • Alkyl benzene • Hydrolysis of nitriles Chemical Properties of Carboxylic Acids • As weak acids • Reactions of carboxylic acids as weak acids • Conversion to acyl chlorides • Reduction Derivatives of Carboxylic Acids Acyl Chlorides • Nomenclature • Reactions of acyl chlorides Esters • Nomenclature • Physical properties • Preparation of esters • Reactions of esters • Uses of esters Amides • Nomenclature • Preparation of amides • Reactions of amides • Dehydration Comparison between Derivatives of Carboxylic Acid Special Reactions of Methanoic Acid and Ethanedioic Acids • Oxidation • Dehydration Concept Map
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 19 Learning earning Outcomes Students should be able to: Carboxylic acids • write the general formula of aliphatic and aromatic carboxylic acids; • name carboxylic acids according to the IUPAC nomenclature and their common names from C1 to C6; • describe structural and optical isomerism in carboxylic acids; • state the physical properties of carboxylic acids; • write the equations for the formation of carboxylic acids from alcohols, aldehydes and nitriles; • describe the acidic properties of carboxylic acids as exemplified by their reactions with metals and bases to form salts; • explain substitution of the −OH in carboxylic acids by nucleophiles −OR and −Cl to form esters and acyl chlorides respectively; • describe the reduction of carboxylic acids to primary alcohols; • describe the oxidation and dehydration of methanoic and ethanedioic acids (oxalic acid); • state the uses of carboxylic acids in food, perfume and polymer industries. Acyl chlorides • write the general formula for acyl chlorides; • name acyl chlorides according to the IUPAC nomenclature; • describe structural and optical isomerism in acyl chlorides; • state the physical properties of acyl chlorides; • explain the ease of hydrolysis of acyl chlorides compared to chloroalkanes; • describe the reactions of acyl chlorides with alcohols, phenols and primary amines. Esters • write the general formula for esters; • name esters according to the IUPAC nomenclature; • describe structural and optical isomerism in esters; • state the physical properties of esters; • describe the preparation of esters by the reactions of acyl chlorides with alcohols and phenols; • describe the acid and base hydrolysis of esters; • describe the reduction of esters to primary alcohols; • state the uses of esters as flavourings, preservatives and solvents. Amides • write the general formula for amides; • name amides according to the IUPAC nomenclature; • describe structural and optical isomerism in amides; • state the physical properties of amides; • describe the preparation of amides by the reaction of acyl chlorides with primary amines; • describe the acid and base hydrolysis of amides. 206
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 207 19 19.1 Carboxylic Acids 1 Carboxylic acids with the general formula of CnH2nO2 are organic compounds containing carboxyl (or acid) group, O ∫ ————— Carboxyl group !C!OH which is a combination of carbonyl and hydroxyl groups. 2 The carboxyl group can also be written as !COOH or !CO2H. 3 The structural formula of monoprotic carboxylic acids is RCOOH (where R = H, alkyl or aryl group). R C O OH RR 4 Note that the ‘carbonyl group’ in carboxylic acids does not have the characteristics of aldehydes and ketones. 5 Examples of carboxylic acids are found in vinegar (ethanoic acid), food preservative (benzoic acid), coagulation of latex (formic acid), palm oil (palmitic acid), animal fat (stearic acid) and many others. Nomenclature of Carboxylic Acids Aliphatic Acids 1 Names of carboxylic acids are derived from the longest carbon chain containing the carboxyl group and by replacing the suffix ‘e’ from the corresponding alkanes with the suffix ‘oic’ followed by the word ‘acid’. 2 The carbon carrying the carboxyl group is always labelled as carbon-1. For example: HCOOH Methanoic acid (Formic acid) CH3COOH Ethanoic acid (Acetic acid) CH3CH2CH2CH2COOH Pentanoic acid CH3CH2CH2CHCOOH 2-chloropentanoic acid & (not 4-chloropentanoic acid) Cl !CH2COOH Phenylethanoic acid HO!CH2CH2CH2COOH 4-hydroxybutanoic acid 2016/P3/Q17 2014/P3/Q3
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 208 19 Unsaturated Acids Unsaturated carboxylic acids are named as alkenoic acids. Keep in mind that the carboxyl carbon is still labelled as C-1. CH2RCHCOOH Propenoic acid (Acrylic acid) CH3CHRCHCOOH 2-butenoic acid (Crotonic acid) CH3CHRCHCH2COOH 3-pentenoic acid !CHRCHCOOH 3-phenylpropenoic acid (Cinnamic acid) Cyclic Acids Aliphatic cyclic carboxylic acids are named cycloalkanecarboxylic acids. The carboxyl carbon atom is also attached to carbon-1 of the cyclic structure and is not itself numbered. For example: !COOH !COOH Cyclopentanecarboxylic acid Cyclohexanecarboxylic acid 1 23 COOH 3-cyclohexenecarboxylic acid Dicarboxylic Acids 1 Dicarboxylic acids are named by adding the suffix ‘dioic acid’ to the name of the carbon chain that contains the two carboxyl groups. For example: HOOC!COOH Ethanedioic acid (Oxalic acid) HOOCCH2COOH Propanedioic acid (Malonic acid) HOOC(CH2)2COOH Butanedioic acid (Succinic acid) HOOC! !COOH Cyclohexane-1,4-dioic acid Aromatic Acids 1 The simplest aromatic acid is benzoic acid (benzenecarboxylic acid) with the following structure. COOH O O ∫ ∫ HO!C!C!OH Oxalic acid Benzoic acid is a white solid that is insoluble in cold water but soluble in hot water.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 209 19 2 Derivatives of benzoic acid are named by using numbers to locate the position of the substituents. For example: COOH COOH COOH OH COOH 2-hydroxybenzoic acid 1,2-benzenedicarboxylic acid NO2 (Salicylic acid) (Phthalic acid) 4-nitrobenzoic acid Fatty Acids 1 Long chain carboxylic acids are called fatty acids. They occur mostly in plants and animals in the form of the triesters of propanetriol or glycerol. O ∫ CH2!O!C!R & & O CH2OCOR & ∫ & CH!O!C!R Or CHOCOR & & & CH2OCOR & CH2!O!C!R ∫ O 2 An example is CH2OCOC15H31 & CHOCOC15H31 & CH2OCOC15H31 Propane-1,2,3-trihexadecanoate 3 The fatty acids are obtained by the hydrolysis of fats and oils. For example: CH2OCOC15H31 CH2OH & & CHOCOC15H31 + 3NaOH !Δ : CHOH + 3C15H31COONa & & CH2OCOC15H31 CH2OH 4 Other examples of fatty acids are CH3(CH2)14COOH Hexadecanoic acid (palmitic acid) CH3(CH2)16COOH Octadecanoic acid (stearic acid) CH3(CH2)7CHRCH(CH2)7COOH Oleic acid Salicylic acid is used to prepare aspirin. O COOH O!C R !CH3 ! Long chain carboxylic acid that usually contains even number of carbon atoms. CH2OH & CHOH & CH2OH Glycerol
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 210 19 Structural and Optical Isomerism in Carboxylic Acids Structural Isomerism 1 The occurrence of structural isomerism is due to the arrangement of the carbon chain as the acid group —COOH must be at carbon number one. 2 Carboxylic acids with four or more carbon atoms exhibit structural isomerism. An example is C4H8O2: CH3CH2CH2COOH Butanoic acid CH3—CH—COOH 2-methylpropanoic acid & CH3 3 Note that carboxylic acids are isomeric with esters. Optical Isomerism 1 Carboxylic acids with five or more carbon atoms can exhibit optical isomerism. An example is 2-methylbutanoic acid, C5H10O2: CH3 C2H5 C* COOH H C* HOOC C2H5 CH3 H Physical Properties Boiling Point and Melting Point 1 The O!H bond in the carboxyl group is polar: R C O O!!H RR δ– δ+ 2 Aliphatic acids with up to eight carbon atoms are liquids at room conditions. Higher members are waxy solids. 3 In liquid and solid states or when dissolved in non-polar solvents, carboxylic acids undergo association by hydrogen bonding into dimers. R C O O!!H O H!!O C!!R RR RR 4 As a result, carboxylic acids have significant higher boiling/ melting points than other classes of organic compounds with comparable molecular mass. Acids can form intermolecular hydrogen bonds. 2017/P3/Q16(a)
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 211 19 This is shown in the table below. Compound Molecular mass/g mol–1 Boiling point/°C CH3COOH 60 118 CH3CH2CH2OH 60 96 CH3CH2CHO 58 48 CH3CH2CH2COOH 88 165 CH3CH2CH2CH2CH2OH 88 137 CH3CH2CH2CH2CHO 86 102 Example 19.1 Explain the difference in the boiling point of 1-propanol (96 °C) and ethanoic acid (118 °C). Solution Both 1-propanol and ethanoic acid can undergo intermolecular hydrogen bonding due to the presence of the polar O!H group. However, the hydrogen bonds in ethanoic acid are stronger than those in 1-propanol. This is because the O!H bond in ethanoic acid is more polarised than the O!H bond in 1-propanol due to the presence of a very electronegative carbonyl oxygen atom. CH3 C O O!!H ;! !!Q Solubility 1 Carboxylic acids can form hydrogen bonding with water through both the carbonyl group and the hydroxyl group. R C O H H H O!!H O O RR H 2 The first four carboxylic acids (methanoic, ethanoic, propanoic and butanoic acids) are completely miscible with water. However, the solubility decreases with increasing size of the hydrophobic R group. R C O O!!H RR Water insoluble Water soluble Carboxylic acids form stronger hydrogen bonds compared to alcohols.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 212 19 3 Benzoic acid is insoluble in cold water but dissolves in hot water. 4 Solubility in water increases with the number of carboxyl groups in the molecule as they can form more hydrogen bonding with water molecules. 5 All carboxylic acids are soluble in non-polar solvents and exist as dimers through formation of intermolecular hydrogen bonding. Preparation of Carboxylic Acids Oxidation of Primary Alcohols and Aldehydes 1 Primary alcohols and aldehydes are oxidised to carboxylic acids when heated with acidified solutions of potassium dichromate(VI) or potassium manganate(VII). RCH2OH + 2[O] !!: RCOOH + H2O RCHO + [O] !!: RCOOH 2 For example: CH3CH2CH2CH2OH + 2[O] !!: CH3CH2CH2COOH + H2O CH3CH2CH2CHO + [O] !!: CH3CH2CH2COOH C + [O] !: O H RR COOH Benzoic acid, a white crystalline solid is soluble in hot water. Quick Check 19.1 Draw the structural formula of the compound with the given molecular formula, which on oxidation gives the carboxylic acid shown. 1 C6H12O !!: CH3CH2CH2CH2CH2COOH 2 C6H14O !!: CH3CH2CH2CH2CH2COOH 3 C6H14O2 !!: HOOC!CH2CH2CH2CH2!COOH Oxidation of Alkyl Benzene 1 This is a convenient way to prepare benzoic acid, as any alkyl group that is bonded to the benzene ring will be oxidised by heating with acidified solutions of potassium manganate(VII) or potassium dichromate(VI) to !COOH. R + [O] !: COOH Exam Tips Exam Tips Alkyl benzene that does not contain any benzylic hydrogen is not oxidisable.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 213 19 2 The excess carbon and hydrogen atoms will be oxidised to carbon dioxide and water respectively. For example: CH3 + 3[O] !: COOH + H2O Hydrolysis of Nitriles 1 Nitriles are compounds with the !C#N group. 2 They are usually prepared by heating a mixture of a haloalkane and potassium cyanide dissolved in ethanol. R!X + KCN !!: RCN + KX For example: CH3CH2CH2I + KCN !!: CH3CH2CH2CN + KI 1-iodopropane Butanenitrile 3 Boiling a nitrile with dilute sulphuric acid produces a carboxylic acid. RCN + 2H2O + H+ !!: RCOOH + NH4 + For example: CN + 2H2O + H+ !: COOH + NH4 + 4 Boiling the nitrile with aqueous sodium hydroxide produces the salt of the carboxylic acid. Acidification produces the free acid. RCN + H2O + NaOH !!: RCOO– Na+ + NH3 For example: CH3CH2CH2CN + NaOH + H2O !: CH3CH2CH2COO– Na+ + NH3 Butanenitrile Sodium butanoate CH3CH2CH2COO– Na+ + H+ !!: CH3CH2CH2COOH + Na+ Quick Check 19.2 Show how you would carry out the following conversions. 1 Cyclohexanol to cyclohexanecarboxylic acid 2 2-methyl-2-propanol to 2,2-dimethylpropanoic acid Oxidation of ethylbenzene: 9C2H5 + 6[O] 9: 9COOH + CO2 + 2H2O Acidic hydrolysis Alkaline hydrolysis 2014/P3/Q10
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 214 19 Chemical Properties of Carboxylic Acids As Weak Acids 1 Carboxylic acids are organic acids. Due to the strong O!H bond, they undergo partial dissociation in water, making them weak acids. RCOOH + H2O L RCOO– + H3O+ For example: CH3COOH + H2O L CH3COO– + H3O+ COOH + H2O L COO– + H3O+ 2 Carboxylic acids are weaker acids than the mineral acids (HCl, HNO3, H2SO4), but are stronger than alcohols and phenols. 3 The acid dissociation constant, Ka and pKa of some carboxylic acids and hydroxy compounds are given below. Compound Structural formula Ka/mol dm–3 pKa Methanoic acid HCOOH 1.78 × 10–4 3.75 Ethanoic acid CH3COOH 1.74 × 10–5 4.76 Propanoic acid CH3CH2COOH 1.35 × 10–5 4.87 Butanoic acid CH3CH2CH2COOH 1.58 × 10–5 4.80 Benzoic acid !!COOH 6.31 × 10–5 4.20 Ethanol CH3CH2OH 1.00 × 10–16 16.0 Phenol !!OH 1.00 × 10–10 10.0 (Note: The larger the value of Ka or the smaller the value of pKa, the stronger the acid.) 4 The higher acidity of carboxylic acids is due to two factors: (a) The presence of an electron-withdrawing carbonyl group helps to weaken the O!H bond. This increases their degree of dissociation compared to alcohols which do not have electron-withdrawing groups. R C O O!!H ;! !!Q (b) The negative charge on the carboxylate ion, RCOO– , can delocalise between the two oxygen atoms as shown by the following resonance structures. They are stronger acids than alcohols and phenols. Delocalisation of the negative charge 2011/P2/Q4(a)(i) 2012/P1/Q49 2014/P3/Q19
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 215 19 R C O O– R C O O This delocalisation stabilises the carboxylate ion. 5 The presence of electron-withdrawing group further helps weaken the O!H bond and stabilises the carboxylate ion, as can be seen by comparing the Ka values of substituted ethanoic acids. Compound Name Ka/mol dm–3 pKa CH3COOH Acetic acid 1.80 × 10–5 4.74 ClCH2COOH Chloroacetic acid 1.41 × 10–3 2.85 Cl2CHCOOH Dichloroacetic acid 3.31 × 10–2 1.48 Cl3CCOOH Trichloroacetic acid 2.00 × 10–1 0.70 H H Cl & & q Cl;C!C!O– Cl;C!C!O– Cl;C!C!O– & ∫ p ∫ p ∫ H O Cl O Cl O Chloroethanoate Dichloroethanoate Trichloroethanoate 6 Another example of inductive effect on the strength of carboxylic acids is seen in benzoic acid. Compound Name Ka/mol dm–3 pKa CH3COOH Ethanoic acid 1.80 × 10–5 4.74 C6H5COOH Benzoic acid 6.46 × 10–5 4.19 The benzene ring is an electron-withdrawing group. This further weakens the O!H bond. ;C!OH ∫ O Furthermore, the negative charge on the benzoate ion can delocalise into the benzene ring and stabilises the anion. C C OOO O As a result, benzoic acid is a stronger acid than the aliphatic acids. Exam Tips Delocalisation helps to reduce the electron-density on any one of the oxygen atoms. Exam Tips Exam Tips Electron-withdrawing groups increase the strength of the acids. Delocalisation of the negative charge into the benzene ring Resonance structure of the carboxylate ion: O– R – C O O R – C O– O R – C O 4 p –
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 216 19 7 The presence of electron-withdrawing substituent increases the acidity of benzoic acid by stabilising the benzoate ion, while electron-donating group decreases the acidity of benzoic acid by destabilising the benzoate ion. W; !COO– W stabilises the benzoate ion D: !COO– D destabilises the benzoate ion 8 The acid dissociation constants of benzoic acid and some substituted benzoic acid are given in the table below. Compound Name Ka/mol dm–3 pKa HOC6H4COOH 4-hydroxybenzoic acid 3.30 × 10–5 4.48 CH3C6H4COOH 4-methylbenzoic acid 4.3 × 10–5 4.34 C6H5COOH Benzoic acid 6.46 × 10–5 4.19 ClC6H4COOH 4-chlorobenzoic acid 1.00 × 10–4 4.00 O2NC6H4COOH 4-nitrobenzoic acid 3.80 × 10–4 3.42 The structures of the four substituted benzoic acid are HO: !COOH Cl; !COOH 4-hydroxybenzoic acid 4-chlorobenzoic acid CH3: !COOH O2N; !COOH 4-methylbenzoic acid 4-nitrobenzoic acid Example 19.2 The pKa of chloroethanoic acid and 4-chlorobutanoic acid are 2.86 and 4.53 respectively. Explain the difference in their acid strength. Solution 4-chlorobutanoic acid is a weaker acid than chloroethanoic acid because it has a larger pKa value. Both the acids contain chlorine which is an electron-withdrawing group. Cl;CH2!COOH Cl;CH2CH2CH2!COOH Info Chem The lower the pKa value, the stronger the acid.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 217 19 However, the electron-withdrawing effect of the chlorine atom in chloroethanoic acid is stronger because it is closer to the !COOH group. The stronger electron-withdrawing effect in chloroethanoic acid also helps stabilise the carboxylate ion more. Cl;CH2!COO– Cl;CH2CH2CH2!COO– Quick Check 19.3 1 Arrange the following compounds in the order of increasing acid strength. Benzoic acid, 2-methylbenzoic acid, cyclohexanoic acid, 4-aminobenzoic acid, 2-nitrobenzoic acid. 2 The pKa for ethanoic acid and chlorine-substituted ethanoic acids are given in the table below. Acid Structure pKa Ethanoic acid CH3COOH 4.76 Chloroethanoic acid ClCH2COOH 2.86 Dichloroethanoic acid Cl2CHCOOH 1.48 Trichloroethanoic acid Cl3CCOOH 0.70 Account for the difference in their pKa values. Reactions of Carboxylic Acids as Weak Acids 1 Aqueous solutions of carboxylic acids turn blue litmus paper red. 2 They react with electropositive metals to liberate hydrogen gas. 2RCOOH + Zn !!: (RCOO)2Zn + H2 For example: 2CH3COOH + Zn !!: (CH3COO)2Zn + H2 Zinc ethanoate 3 They liberate carbon dioxide from carbonates and hydrogen carbonates. 2RCOOH + Na2CO3 !!: 2RCOO– Na+ + CO2 + H2O RCOOH + NaHCO3 !!: RCOO– Na+ + CO2 + H2O For example: Sodium benzoate is a fungal growth inhibitor, and is often added to baked foods to increase their shelf-life. 2008/P1/Q35 General properties of the acids 2 !!COOH + Na2CO3 !: 2 !!COO– Na+ + CO2 + H2O Sodium benzoate !!COOH + NaHCO3 !: !!COO– Na+ + CO2 + H2O Use of sodium benzoate Info Chem When the anhydrous salt of a carboxylic acid is heated with sodalime, an alkane is formed. RCOONa(s) + NaOH : RH + Na2CO3
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 218 19 4 They neutralise base to form salt and water only. RCOOH + NaOH !!: RCOO– Na+ + H2O For example: CH3COOH + NaOH !!: CH3COO– Na+ + H2O 2CH3COOH + CaO !!: (CH3COO)2Ca + H2O Quick Check 19.4 1 Draw the structures of the organic products in the following reactions. (a) 4-chlorobenzoic acid with aqueous sodium hydroxide (b) Sodium benzoate with dilute sulphuric acid (c) 4-hydroxybenzoic acid with sodium (d) 2-chlorobenzoic acid with aqueous sodium carbonate 2 Write balanced equations for the reactions of ethanedioic acid (oxalic acid) with (a) magnesium (c) aqueous ammonia (b) potassium (d) calcium hydroxide Conversion to Acyl Chlorides 1 The functional group of an acyl chloride (or acid chloride) is a carbonyl group bonded to a chlorine atom. R!C!Cl ∫ O 2 Carboxylic acids react with phosphorus(V) chloride or thionyl chloride to form acyl chlorides. R!C!OH + PCl5 !!: R!C!Cl + POCl3 + HCl ∫ ∫ O O R!C!OH + SOCl2 !!: R!C!Cl + SO2 + HCl ∫ ∫ O O In both cases, steamy white fumes of hydrogen chloride are released. 3 For example: CH3COOH + PCl5 !!: CH3COCl + POCl3 + HCl Ethanoyl chloride Benzoyl chloride Exam Tips Exam Tips This is an example of nucleophilic substitution. COOH + SOCl2 !!: !!C!!Cl + SO2 + HCl O R INFO Reactions of Carboxylic Acids
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 219 19 Quick Check 19.5 Draw the structures of the acyl chlorides formed when the following compounds react with phosphorus(V) chloride. (a) Butanoic acid (c) 2-hydroxypropanoic acid (b) Ethanedioic acid (Oxalic acid) (d) 1,4-benzenedicarboxylic acid Esterification 1 When a carboxylic acid is boiled under reflux with an alcohol in the presence of a little concentrated sulphuric acid as a catalyst, an ester is produced. R!C!OH + R9!O!H L R!C!O!R9 + H2O ∫ ∫ O O 2 For example: CH3COOH + CH3CH2OH L CH3COOCH2CH3 + H2O Ethyl ethanoate COOH + CH3CH2OH L !COOC2H5+ H2O Benzoic acid Ethyl benzoate Quick Check 19.6 1 Draw the structures of the esters formed from the following reactions. (a) Benzoic acid + methanol (b) Ethanedioic acid + ethanol (c) Ethanoic acid + cyclohexanol (d) 4-hydroxybenzoic acid + 2-propanol Reduction 1 Carboxylic acids are reduced to primary alcohols. RCOOH + 4[H] !!: RCH2OH + H2O The reducing agent used is usually lithium aluminium hydride in ether. 2 For example: CH3COOH + 4[H] !!: CH3CH2OH + H2O !COOH + 4[H] !: !CH2OH + H2O The reaction is reversible. Info Chem R!C!OH O !: R!CH2!OH R [H] 2008/P1/Q39 2013/P3/Q13 Exam Tips Exam Tips This is a nucleophilic substitution.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 220 19 Quick Check 19.7 Draw the structures of the alcohols formed when the following acids are reduced by lithium aluminium hydride. (a) Methanoic acid (e) 3-cyclopentenecarboxylic acid (b) Phenylethanoic acid (f) 2-hydroxypropanoic acid (c) Propenoic acid (g) Cyclohexanecarboxylic acid (d) 1,6-hexadioic acid Oxidation and Dehydration of Methanoic Acid and Ethanedioic Acid Methanoic acid, HCOOH and ethanedioic acid, HOOC!COOH have some reactions that are not shared by other carboxylic acids. Oxidation 1 Methanoic acid and ethanedioic acid are oxidised by acidified potassium dichromate(VI) or potassium manganate(VII) to carbon dioxide and water. HCOOH + [O] !Δ : H2O + CO2 HOOC!COOH + [O] !Δ : 2CO2 + H2O 2 However, the rate of oxidation of ethanedioic acid is very slow at room temperature. The oxidation is usually carried out at around 70 °C. Dehydration 1 When heated with concentrated sulphuric acid, methanoic acid and ethanedioic acid undergo dehydration to carbon monoxide and carbon dioxide respectively. HCOOH !Δ : H2O + CO HOOC!COOH !Δ : H2O + CO2 + CO 2 As a result, no charring occurs and no solid residue is left. Other Reactions of Methanoic Acid 1 The methanoic acid molecule contains both the carbonyl group of an aldehyde and the carboxyl group of an acid. H!!C !!OH ∫ O Aldehyde group Carboxyl group Ethanedioic acid (a.k.a. oxalic acid) is sometimes written as H2C2O4. Decolourisation of KMnO4 Info Chem Oxidation of oxalic acid is very slow at room temperature. Exam Tips Exam Tips No solid residue is left.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 221 19 2 As a result, methanoic acid has some reactions that are similar to those of the aldehydes. 3 Methanoic acid reacts with Fehling’s solution upon heating to give a reddish-brown precipitate of copper(I) oxide. HCOOH + 2Cu2+ + 4OH– !Δ : Cu2O + 3H2O + CO2 4 Methanoic acid reacts with Tollen’s reagent to give a ‘silver mirror’. HCOOH + Ag2O !Δ : 2Ag + H2O + CO2 5 Methanoic acid also reacts with aqueous mercury(II) chloride to give a white precipitate of mercury(I) chloride. HCOOH + 2HgCl2 !Δ : Hg2Cl2 + CO2 + 2HCl 6 In the presence of excess methanoic acid, mercury(I) chloride is further reduced to mercury. HCOOH + Hg2Cl2 !Δ : 2Hg + CO2 + 2HCl 7 However, methanoic acid does not react with hydrogen cyanide or 2,4-dinitrophenylhydrazine. Info Chem Methanoic acid does not undergo nucleophilic addition. Quick Check 19.8 How would you differentiate between the following pairs of compounds? (a) Ethanoic acid and ethanedioic acid (b) Ethanedioic acid and methanoic acid Uses of Carboxylic Acids 1 Ethanoic acid is found in vinegar. 2 Benzoic acid is used as a food preservative. 3 Formic acid (methanoic acid) is used to coagulate latex. 4 Fatty acids (long-chain acids) are used in the manufacture of soaps. 5 1,4-benzenedicarboxylic acid is a monomer for Terylene, a polyester. 19.2 Acyl Chloride 1 The three derivatives of carboxylic acids that will be studied are (a) acyl chlorides (or acid chloride) (b) ester (c) amide
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 222 19 2 All of them contain acyl group derived from the !COOH group of the carboxylic acid. R C O Y RR 3 Acyl chlorides are compounds with the general formula of R!!C !!Cl ∫ ∫ O 4 It can be thought of as a derivative of carboxylic acids when the !OH group in the carboxyl group is replaced by a chlorine atom. R!!C!!OH R!!C!!Cl ∫ ∫ O O 5 It is most conveniently prepared by the reaction of phosphorus(V) chloride or thionyl chloride with an anhydrous carboxylic acid. RCOOH + PCl5 !!: RCOCl + POCl3 + HCl RCOOH + SOCl2 !!: RCOCl + SO2 + HCl Nomenclature 1 In the IUPAC convention, the names of acyl chlorides are derived from their corresponding carboxylic acids by replacing the suffix ‘ic acid’ to ‘yl chloride’. 2 For example: Structure of acyl chloride Name of acid Name of acyl chloride CH3COCl Ethanoic acid Ethanoyl chloride CH3CH2COCl Propanoic acid Propanoyl chloride ClCO(CH2)4COCl Hexanedioic acid Hexanedioyl chloride !!COCl Benzoic acid Benzoyl chloride Structural and Optical Isomerism in Acyl Chlorides 1 Structural isomerism occurs in acyl chlorides with four or more carbon atoms. An example is C4H7ClO: CH3CH2CH2COCl Butanoyl chloride CH3—CH—COCl 2-methylpropanoyl chloride & CH3
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 223 19 2 Optical isomerism occurs in acyl chlorides with five or more carbon atoms. For example, C5H9ClO: C* COCl C2H5 CH3 H CH3 C2H5 C* ClOC H Physical Properties of Acyl Chlorides 1 Most acyl chlorides are colourless fuming liquids due to hydrolysis with water vapour in the air. 2 Acyl chlorides react vigorously with water at room conditions to produce a mixture of carboxylic acid and hydrochloric acid. Hence, it is not possible to get an aqueous solution of acyl chlorides. 3 As acyl chlorides are very reactive compounds, precautions should be taken while handling them because they can react with water on the surface of the eye producing hydrochloric and organic acids causing irritation to the eye. Similar problems can result if one inhales acyl chloride vapour. 4 Acyl chlorides are polar molecules. The intermolecular forces between the molecules are stronger than those between nonpolar molecules such as alkanes. 5 However, acyl chlorides cannot form intermolecular hydrogen bonding. As a result, their boiling points are lower than alcohols of similar molar mass. For example: Compound C4H7OH CH3COCl Molar mass/g mol–1 72 78.5 Boiling point/°C 117 51 Nucleophilic Substitution of Acyl Chlorides 1 The carbonyl group in the molecule is highly polarised. As a result, the chlorine-carrying carbon atom acquires a much larger positive charge than their haloalkane counterparts. R δ– Cl Cδ+ Q Oδ– ;! 2 Hence, acyl chlorides are more susceptible to attack by nucleophile than the corresponding haloalkanes in nucleophilic substitution reactions. C!!Cl !: Nu– : O R RR C!!Nu + Cl– O R RR Nucleophilic substitution 2016/P3/Q11 Presence of two electronwithdrawing atoms: Cl and O.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 224 19 3 Nucleophilic substitution of acyl chlorides can be represented by the general equation below. C!!Cl + H!!Y !: O R RR C!!Y + HCl O R RR 4 In the reactions, the hydrogen atom of the other reagent is substituted by an acyl group: R!!C R !! O Acyl chlorides are called acylating agents. Example 19.3 Explain why aromatic acyl chlorides such as benzoyl chloride are less reactive towards nucleophilic substitution than the aliphatic acyl chlorides. Solution Aromatic acyl chlorides (e.g. benzoyl chloride) are less reactive towards nucleophilic substitution due to the steric hindrance of the large benzene ring. The high electron density of the benzene ring makes the approach of the nucleophile more difficult. C Cl O RR δ+ Nu– Furthermore, the high electron density of the benzene ring also reduces the size of the positive charge on the carbonyl carbon atom. Hydrolysis 1 Acyl chlorides react rapidly with water to form corresponding carboxylic acids with the liberation of white fumes of hydrogen chloride. No heating is required. C!!Cl + H!!OH !: O R RR R!!CRR !!OH + HCl O As a result, acyl chlorides fume in moist air. Formation of carboxylic acids 2008/P1/Q32 2010/P1/Q37 2013/P3/Q9
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 225 19 2 Examples are: CH3COCl + H2O !!: CH3COOH + HCl 3 For comparison, boiling with aqueous sodium hydroxide is needed to hydrolyse the haloalkanes. CH3CH2CH2CH2Cl + NaOH !Δ : CH3CH2CH2CH2OH + NaCl 1-chlorobutane 1-butanol Reaction with Alcohols and Phenols 1 Acyl chlorides react with alcohols at room temperature to produce esters. C!!Cl + H!!ORfi !: O R RR C!!ORfi + HCl O R RR Acyl chlorides are so reactive that no catalyst is required. 2 Examples are: 3 Reaction with phenol is usually slow under normal conditions. However, in the presence of a base, the reaction can occur at room conditions. The function of the base is to convert phenol into the more reactive phenoxide ion. Exam Tips On hydrolysis, one mole of acyl chloride produces two moles of acids: hydrochloric acid and carboxylic acid. !: !!C R !!OH + HCl O !!C R !!Cl + H2O O Exam Tips Exam Tips Ease of hydrolysis: RCOCl RCl Formation of esters CH3!!C + CH3CH2OH !: O Cl RR CH3!!C + HCl O O!!CH2CH3 RR Ethanoyl chloride Ethanol Ethyl ethanoate Exam Tips Exam Tips Acylation of phenol occurs in alkaline medium. + NaOH !: + H2O OH O– Na+ + C O O– Na+ !: RR Cl !!C!!O!! + NaCl O R Benzoyl chloride Sodium phenoxide Phenyl benzoate
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 226 19 Reaction with Ammonia and Amines 1 Acyl chlorides react with concentrated ammonia, at room temperature to form amides. R!C!Cl + H!NH2 !!: R!C!NH2 + HCl ∫ ∫ O O Amide HCl + NH3 !!: NH4Cl The overall reaction is RCOCl + 2NH3 !!: RCONH2 + NH4Cl 2 For example: C!!Cl + 2NH3 !: O CH3 RR C!!NH2 + NH4Cl O CH3 RR Ethanoyl chloride Ethanamide C!!Cl + 2NH3 !: O RR C!!NH2 + NH4Cl O RR Benzoyl chloride Benzamide 3 Acyl chlorides react with primary and secondary amines to produce substituted amides. Formation of amides Room temperature R!C!Cl + 2H!N!R9 !!!!!!: R!C!N!R9 + R9NH3 +Cl– ∫ & ∫ & O H O H Primary amine N-substituted amide Room temperature R!C!Cl + 2H!N!R9 !!!!!!: R!C!N!R9 + R9R0NH2 +Cl– ∫ & ∫ & O R0 O R0 Secondary amine N,N-disubstituted amide 2015/P3/Q10, Q11 INFO Reactions of Acyl Chlorides
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 227 19 4 For example: Example 19.4 Explain why acyl chlorides do not react with tertiary amines. Solution This is because there is no hydrogen attached to the nitrogen atom in the amine for substitution to occur. R & R!N!R Example 19.5 Aspirin, a pain-killer is manufactured by the reaction between ethanoyl chloride and 2-hydroxybenzoic acid. Draw the structure of aspirin. Solution CH3!C!Cl + !: COOH O!H O COOH O!C!CH3 R + HCl O R Quick Check 19.9 1 Write an equation to show the reaction between benzoyl chloride and (a) sodium hydroxide (e) phenylamine, (b) dilute sulphuric acid NH2 (c) concentrated ammonia (d) ethylmethylamine, CH3NHC2H5 (f) phenol Room temperature CH3!C!Cl + 2H!N!CH3 !!!!!!: CH3!C!N!CH3 + CH3NH3 +Cl– ∫ & ∫ & O H O H Ethanoyl chloride Methylamine N-methylethanamide Room temperature CH3!C!Cl + 2H!N!CH3 !!!!!!: CH3!C!N!CH3 + (CH3)2NH2 +Cl– ∫ & ∫ & O CH3 O CH3 Dimethylamine N,N-dimethylethanamide Info Chem • 2-Hydroxybenzoic acid is also known as salicylic acid • Aspirin is also known as acethylsalicylate.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 228 19 19.3 Esters 1 Esters have the general formula of CnH2nO2. (where R = H, alkyl or aryl; R9 = alkyl or aryl) C ORfi O R RR 2 Esters are either liquids or solids with a sweet fruity smell. Nomenclature 1 In the IUPAC system, the alkyl or aryl group that is bonded to oxygen (R9) is named first followed by the name of the acid in which the suffix ‘ic’ is replaced by ‘ate’. 2 Examples are: H!C!OCH3 ∫ O Methyl methanoate !C R !OCH3 O (Methyl formate) Methyl benzoate H!C!OCH2CH3 ∫ O Ethyl methanoate CH3!C!OCH3 !C R !O! O ∫ Phenyl benzoate O Methyl ethanoate (Methyl acetate) CH3!C!OCH2CH3 CH3O!C!C!OCH3 ∫ ∫ ∫ O O O Ethyl ethanoate Dimethyl ethanedioate (Dimethyl oxalate) 3 Esters are isomeric with carboxylic acids with the same number of carbon atoms. For example, methyl methanoate is isomeric with ethanoic acid. H!C!OCH3 CH3!C!OH ∫ ∫ O O Info Chem Esters and carboxylic acids have the same general formula. 2015/P3/Q16, Q20 2016/P3/Q12
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 229 19 Methyl ethanoate is isomeric with propanoic acid. CH3!C!OCH3 CH3CH2!C!OH ∫ ∫ O O Structural and Optical Isomerism in Esters 1 Structural isomerism is exhibited by esters with three or more carbon atoms. An example is C3H6O2. HCOOCH2CH3 Ethylmethanoate CH3COOCH3 Methylethanoate 2 However, the general formula of esters, CnH2nO2 is the same as that of carboxylic acids. As a result, esters are also isomeric with carboxylic acid with the same number of carbon atoms. For example, propanoic acid is isomeric with the two esters above. CH3CH2COOH Propanoic acid 3 Optical isomerism occurs in esters with chiral carbons. One example is C6H12O2: CH3 C2H5 C* COOCH3 H C* COOCH3 C2H5 CH3 H Physical Properties 1 Since there are no hydrogen atoms bonded directly to the oxygen atom, esters cannot form intermolecular hydrogen bond with its own molecules. 2 The intermolecular forces are the van der Waals forces. 3 Most esters are liquids at room conditions. Phenyl benzoate is a white solid. They are all insoluble in water. 4 The boiling points of some esters are given in the table below. Ester Formula Melting point/°C Boiling point/°C Methyl methanoate HCOOCH3 –90 32 Methyl ethanoate CH3COOCH3 –99 56 Ethyl ethanoate CH3COOC2H5 –83.6 77 Methyl benzoate C6H5COOCH3 –12.3 200 Ethyl benzoate C6H5COOC2H5 –34 213 Info Chem All esters are insoluble in water. Exam Tips Exam Tips Molecules of esters cannot form hydrogen bonds with one another.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 230 19 Preparation of Esters 1 When a carboxylic acid is boiled under reflux with an alcohol in the presence of concentrated sulphuric acid as a catalyst, an ester is produced. R!C!OH + R9!O!H L R!C!O!R9 + H2O ∫ ∫ O O 2 Another convenient method is to react an acyl chloride with an alcohol or phenol. C!!Cl + H!!ORfi !: O R RR C!!ORfi + HCl O R RR Reactions of Esters Hydrolysis 1 Esters are hydrolysed to their corresponding carboxylic acids and alcohols. The reaction requires acid or base as a catalyst and needs to be boiled under reflux. 2 Alkaline hydrolysis is also known as saponification which is a soap-making process. Acidic hydrolysis is reversible, but alkaline hydrolysis is not. 3 General equations for hydrolysis: H+ R!C!OR9 + H2O L RCOOH + R9OH ∫ O RCOOR9 + NaOH !Δ : RCOO– Na+ + R9OH 4 Examples are: CH3COOC2H5 + H2O L CH3COOH + C2H5OH Ethyl ethanoate Ethanoic acid Ethanol On acidification with dilute sulphuric acid, a free acid is obtained. No heating is required. No catalyst is needed. !!C R !!OCH3 + NaOH !: O !!C!!O– Na+ R + CH3OH O Methyl benzoate Sodium benzoate Methanol The reaction is reversible. The reaction is irreversible. 2014/P3/Q12 2008/P1/Q36 2010/P2/Q10(c) 2011/P2/Q10(a) 2012/P1/Q4(a) 2014/P3/Q11
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 231 19 Example 19.6 Ethylbenzoate is boiled under reflux with aqueous sodium hydroxide. The product of the reaction is then distilled. (a) State the distillate and the residue. (b) State what happens when the cooled residue is acidified with dilute sulphuric acid. Solution (a) Ethylbenzoate is hydrolysed by sodium hydroxide to sodium benzoate and ethanol. !COOC2H5 + NaOH !: !COO– Na+ + C2H5OH Sodium benzoate is an ionic compound while ethanol is a covalent molecule. Hence, on distillation, ethanol will distill out (distillate) leaving behind sodium benzoate (residue). (b) On acidification, colourless crystals of benzoic acid will be precipitated. !COO– Na+ + H+ !: !COOH + Na+ Example 19.7 Suggest how you would differentiate between ethyl methanoate and methyl ethanoate. Solution Boil both the esters separately under reflux with the excess sodium hydroxide. The resulting solutions are then distilled. The distillates are collected. An alkaline solution of iodine is then added to the cooled distillates. The one that gives a yellow precipitate is the one originally containing ethyl methanoate. Hydrolysis of ethyl methanoate produces methanoic acid and ethanol. HCOOCH2CH3 + NaOH !!: HCOO– Na+ + CH3CH2OH The distillate contains ethanol that gives a positive iodoform test with alkaline iodine. CH3CH2OH + 4I2 + 6NaOH !!: CHI3 + HCOO– Na+ + 5NaI + 5H2O Hydrolysis of methyl ethanoate produces ethanoic acid and methanol. CH3COOCH3 + NaOH !!: CH3COO– Na+ + CH3OH The distillate contains methanol that does not give a positive iodoform test. Exam Tips Exam Tips Benzoic acid is soluble in hot water but not in cold water.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 232 19 Saponification 1 Naturally occurring esters are found in fats and oils. 2 They are the esters of propane-1,2,3-triol (also called glycerol or glycerin) in combination with long-chain carboxylic acids (known as fatty acids). R!C!O!CH2 R O R!C!O!CH2 R O R!C!O!CH R O !!! !!! 3 In saponification (soap-making), the fats or oils are first boiled with concentrated alkali. Aqueous sodium chloride is then added to precipitate the sodium salt of the long-chain carboxylic acid which is soap. R!C!O!CH2 R O R!C!O!CH2 R O OH R!C!O!CH R O !!! !!! + 3NaOH !: 3RCOO– Na+ + CH2!CH!CH2 ! OH ! OH ! 4 An example is: Reduction of Esters 1 Esters are reduced by lithium aluminium hydride to two alcohols. 2 The alcohol from the acyl group is always a primary alcohol. R!C!OR9 + 4[H] !!: RCH2OH + R9OH ∫ O Exam Tips Exam Tips NaCl is added to reduce the solubility of the sodium salt of the long-chain carboxylic acid through common ion effect. RCOO–Na+(s) L RCOO–(aq) + Na+(aq) CH3!(CH2)16!C!O!CH2 R O CH3!(CH2)16!C!O!CH R O CH3!(CH2)16!C!O!CH2 R O OH !!! !!! + 3NaOH !: 3CH3(CH2)16COO– Na+ + HOCH2!CH!CH2OH Sodium octadecanoate ! (A soap) O R RfiOH R!C!ORfi RCH2OH 2017/P3/Q11
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 233 19 3 For example: CH3COOCH3 + 4[H] !!: CH3CH2OH + CH3OH Example 19.8 An ester, X on reduction with LiAlH4 produces phenylmethanol and 2-propanol. What is the structure of X? Solution The two alcohols are CH2OH and CH3!CH!CH3 & OH Phenylmethanol is a primary alcohol while 2-propanol is a secondary alcohol. Hence, phenylmethanol is obtained from the carboxyl part of the ester. Therefore, the structure of the ester is C!!O!!CH!!CH3 ∫ & O CH3 Reaction with Ammonia 1 When an ester is heated with concentrated ammonia in ethanol, an amide and an alcohol are formed. O ∫ R!C!OR9 + NH3 !!: R!C!NH2 + R9OH ∫ O 2 For example: CH3COOC2H5 + NH3 !!: CH3CONH2 + C2H5OH Ethanamide Uses of Esters 1 Some esters are used as solvents, preservatives and in the production of polyesters. 2 Esters with sweet fruity smell are used as food flavouring or taste enhancers. 3 Fats and oils are used to make soap. !C R !O! + 4[H] !: !CH2OH + !OH O Phenyl benzoate Phenylmethanol Phenol
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 234 19 Quick Check 19.10 1 Draw the structures of the esters formed from the following reactions. (a) HCOOH + CH2OH (b) HOOC!COOH + C2H5OH (c) CH2OH CH2OH + COOH (d) COCl + CH3 CH2OH 2 Draw the structures of the products formed when the following esters are boiled with aqueous sodium hydroxide followed by acidification with dilute sulphuric acid. (a) HCOOCH3 (b) CH3CH2CH2COOCH2CH3 (c) COO CH3 3 Write equations for the following reactions. (a) CH3CH2COOCHCH3 + NH3 & CH3 (b) C!!O + LiAlH4 ∫ O 4 An ester, Y undergoes reduction to produce two alcohols, ethanol and 2-methyl-1-propanol. Suggest the possible structures for Y. 19.4 Amides Nomenclature 1 The general formula of amide is CnH2n + 1NO. The functional group of amides is acyl group bonded to a nitrogen atom. R!C!NH2 or RCONH2 ∫ O Amide group 2011/P2/Q10(b) 2012/P1/Q37
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 235 19 2 Amides are named by dropping the suffix ‘oic acid’ from the IUPAC name of the parent acid and substituted by ‘amide’. For example: O O ∫ ∫ CH3!C!NH2 C!NH2 Ethanamide (Acetamide) O Benzamide ∫ CH3CH2!C!NH2 Propanamide The above compounds are primary amides. 3 For alkyl-substituted amide, the alkyl group is named first, and its location on the nitrogen atom is indicated by the prefix N-. For example: O O CH3 ∫ ∫ & CH3!C!NH!CH3 C!N!CH3 N-methylethanamide (Secondary amide) O N,N-dimethylbenzamide (Tertiary amide) ∫ CH3!C!NH N-phenylethanamide O CH3 ∫ & CH3!C!N!CH2CH3 N-ethyl-N-methylethanamide Example 19.9 Explain why ethanamide (acetamide) is neutral. Solution The structure of ethanamide is CH3 C O N: H H RR The presence of a lone pair of electrons on the nitrogen atom should give ethanamide basic character. However, the lone pair of electrons on the nitrogen atom can be delocalised by interacting with the p electrons of the carboxyl group: CH3 CH3 C N H H H H O O C N p-orbitals that contain lone pairs of electrons Delocalised orbitals This delocalisation causes the lone pair electrons to be less readily available for reaction with hydrogen ions, H+. Primary amides Substituted amides
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 236 19 Structural and Optical Isomerism in Amides 1 Primary amides with three or more carbon atoms are isomeric with secondary and tertiary amides. For example, propanamide C3H7NO: CH3CH2—C—NH2 Propanamide (Primary) ∫ O CH3—C—NH—CH3 N-methylethanamide (Secondary) ∫ O 2 Another example is C4H9NO: CH3CH2CH2—C—NH2 Butanamide (Primary) ∫ O CH3—CH—C—NH2 2-methyl-propanamide (Primary) & ∫ CH3 O CH3CH2—C—NH—CH3 N-methylpropanamide (Secondary) ∫ O CH3—C—N—CH3 N,N-dimethylethanamide (Tertiary) ∫ & O CH3 4 The first member of amides that exhibits optical isomerism has the molecular formula of C5H11NO: C* CONH2 C2H5 CH3 H CH3 C2H5 C* H2NOC H Physical Properties of Amides 1 Due to the presence of a hydrogen atom that is attached to a nitrogen atom, amides can undergo intermolecular hydrogen bonding. C C R N N O H H O R H H
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 237 19 2 As a result, the melting and boiling points of amides are higher compared to that of the acyl chlorides and esters. 3 All amides are colourless crystalline solid except methanamide, HCOONH2 which is a colourless liquid with a melting point of 2.6 °C. 4 The table below lists the melting and boiling points of some common primary amides. Compound HCONH2 CH3CONH2 CH3CH2CONH2 –CONH2 Name Methanamide Ethanamide Propanamide Benzamide Melting point/°C 2.6 82.4 81.3 132.0 Boiling point/°c 193 222 215 290 5 Lower members of amides are soluble in water because they can form hydrogen bonding with water molecules. However, the solubility decreases with the increase in the size of hydrophobic hydrocarbon group. Preparation of Amides From Acyl Chlorides with Ammonia and Amines 1 Acyl chlorides react vigorously with concentrated ammonia at room temperature to produce amides. R!C!Cl + 2H!NH2 !!: R!C!NH2 + NH4Cl ∫ ∫ O O 2 Acyl chlorides react with primary amines to produce secondary amides. 3 Acyl chlorides react with secondary amines to produce tertiary amides. From Ammonium Salts of Carboxylic Acids 1 When the anhydrous ammonium salt of a carboxylic acid is heated strongly, the corresponding amide is produced. O O ∫ ∫ R!!C!!O– NH4 + !Δ : R!!C!!NH2 + H2O Room temperature R!C!Cl + 2H!N!R9 !!!!!!: R!C!N!R9 + R9NH3 +Cl– ∫ & ∫ & O H O H Primary amine Secondary amide Room temperature R!C!Cl + 2H!N!R9 !!!!!!: R!C!N!R9 + R9R0NH2 +Cl– ∫ & ∫ & O R0 O R0 Secondary amine Tertiary amide Dehydration of the ammonium salt of carboxylic acid 2015/P3/Q10
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 238 19 2 For example: CH3COONH4 !Δ : CH3CONH2 + H2O Ammonium ethanoate Ethanamide !COO– NH4 + !!: !C!NH2 + H2O R O Ammonium benzoate Benzamide Δ Reactions of Amides Hydrolysis 1 Boiling an amide with dilute mineral acid gives a carboxylic acid and an ammonium ion. O O ∫ ∫ R!!C!!NH2 + H2O + H+ !!: R!!C!!OH + NH4 + 2 For example: 3 Boiling an amide with alkali produces the sodium salt of a carboxylic acid and ammonia is liberated. O O ∫ ∫ R!!C!!NH2 + NaOH !!: R!!C!!O– Na+ + NH3 4 For example: Reduction 1 Amides are reduced to amines by lithium aluminium hydride. O ∫ R!!C!!NH2 + 4[H] !!: RCH2NH2 + H2O 2 Secondary amides will be reduced to secondary amines. O ∫ R!!C!!NH!!R9 + 4[H] !!: RCH2!!N!!R9 & H O O ∫ ∫ CH3!!C!!NH2 + H2O + H+ !: CH3!!C!!OH + NH4 + Ethanamide Liberation of ammonia O O ∫ ∫ CH3!!C!!NH2 + NaOH !: CH3!!C!!O– Na+ + NH3 Exam Tips Exam Tips O [H] R9C9NH2 99: R9CH29NH2 2008/P1/Q37 2010/P2/Q10(c) 2013/P3/Q20 2014/P3/Q14
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 239 19 3 For example: O ∫ CH3!!C!!NH2 + 4[H] !!: CH3CH2NH2 + H2O Ethanamide Ethylamine O ∫ CH3!C!NH!CH3 + 4[H] !: CH3CH2!NH!CH3 + H2O N-methylethanamide Methylethylamine Dehydration 1 When an amide is heated with phosphorus(V) oxide, a water molecule is eliminated from the structure and a nitrile is formed. P4O10 RCONH2 !!: RCN + H2O 2 For example: P4O10 CH3CONH2 !!: CH3CN + H2O P4O10 CONH2 !!: CN + H2O Quick Check 19.11 1 Write the equation for the reaction between N-phenylethanamide with (a) sodium hydroxide (b) dilute sulphuric acid (c) lithium aluminium hydride (d) Cl2 in bright sunlight 2 Suggest the ways whereby the following amides can be prepared using suitable reagents. (a) CH3CONH2 (b) CONHCH3 3 Suggest how you would carry out the following conversions. CH3CH2CH2CONH2 !!: CH3CH2CH2COOH
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 240 19 SUMMARY SUMMARY Comparison between Derivatives of Carboxylic Acid Derivatives of carboxylic acids Reaction Acid chloride Ester Amide Water Vigorous. White fumes are liberated. Carboxylic acid is formed. No reaction No reaction Acid hydrolysis Vigorous. Forms carboxylic acid. Reversible. Produces carboxylic acid and alcohol. Forms carboxylic acid and ammonium salt. Base hydrolysis Vigorous. Forms salt of carboxylic acid. Non-reversible. Forms sodium salt of carboxylic acid and alcohol. Salt of carboxylic acid. Ammonia gas liberated. Ammonia Forms amide Forms amide No reaction Reduction No reaction Forms two alcohols Forms amine 1 The functional group of carboxylic acids is carboxyl group. 2 Aromatic acids (such as benzoic acid) are stronger acids than the aliphatic acids. 3 Electron-withdrawing groups increase the strength of the acids, while electron-releasing groups decrease the strength of the acids. 4 The test for acids is by using aqueous sodium carbonate – Effervescence occurs. 5 Two acids that can be oxidised are methanoic acid (formic acid) and ethanedioic acid (oxalic acid). 6 Methanoic acid gives positive test with Tollen’s reagent, and reacts with aqueous mercury(II) chloride to give a precipitate. Reactions of Carboxylic Acids Reagent Product Remarks NaOH(aq), room temperature RCOO–Na+ Neutralisation Na2CO3(aq), room temperature RCOO–Na+ + CO2 Neutralisation Effervescence. CO2 liberated. PCl5, room temperature RCOCl Acylation (Nucleophilic substitution) White fumes of HCl are liberated. R’OH, concentrated H2SO4, reflux RCOOR’ Esterification (Nucleophilic substitution) LiAlH4, heat RCH2OH Reduction to primary alcohol Reactions of Methanoic Acid Reagent Product Remarks Acidified KMnO4, warm H2O + CO2 Decolourisation of KMnO4 Tollen’s reagent, heat H2O + CO2 + Ag Silver mirror Fehling’s solution, heat H2O + CO2 + Cu2O Brick-red precipitate is formed. Aqueous mercury(II) chloride Hg2Cl2 Hg White precipitate is formed. In excess of methanoic acid, mercury is formed (silvery liquid). Concentrated H2SO4, heat H2O + CO Dehydration No solid residue is left behind.
Chemistry Term 3 STPM Chapter 19 Carboxylic Acids and their Derivatives 241 19 Reactions of Ethanedioic Acid Reagent Product Remarks Acidified KMnO4, heat CO2 + H2O Decolourisation of KMnO4 Concentrated H2SO4, heat CO2 + CO + H2O Dehydration No solid residue is left behind. 7 The three important derivatives of carboxylic acids are acyl chlorides, esters and amides. 8 Acyl chlorides fume in moist air. 9 Acyl chlorides are used as acylating agent. 10 Esters have sweet fruity smells and are used mainly as food enhancers. 11 Amides are neutral compounds even though they contain the amine group, –NH2. Reactions of Acyl Chlorides Reagent Product Remarks Water, room temperature RCOOH + HCl Nucleophilic substitution (Hydrolysis) White fumes of HCl are given off. R9OH, room conditions RCOOR9 Nucleophilic substitution (Esterification) For phenol, it has to be converted to the more reactive sodium phenoxide, C6H5O–Na+. Concentrated NH3, room conditions RCONH2 Nucleophilic substitution Formation of amide R9NH2, room conditions R!C!NH!R9 ∫ O Nucleophilic substitution Formation of substituted amide Reactions of Esters Reagent Product Remarks NaOH(aq), reflux RCOO–Na+ + R’OH Hydrolysis Dilute H2SO4, reflux RCOOH + R’OH Hydrolysis Concentrated NH3 RCONH2 Formation of amide LiAlH4, heat RCH2OH + R’OH Reduction Reactions of Amides Reagent Product Remarks Dilute H2SO4, boil RCOOH Hydrolysis Dilute NaOH, boil RCOOH Hydrolysis Ammonia gas is liberated. Br2(aq) + KOH(aq), heat RNH2 Hoffman degradation LiAlH4 in ether, heat RCH2NH2 Reduction to primary amine P4O10(s), heat RCN Dehydration to nitrile