Raffles Institution H2 Chemistry Carboxylic Acids and Derivatives (Student)

1 RAFFLES INSTITUTION YEAR 6 H2 CHEMISTRY 2025 Lecture Notes 19 – Carboxylic Acids & Derivatives _______________________________________________________________________ A Content I Carboxylic acids (exemplified by ethanoic acid and benzoic acid) (i) Formation from primary alcohols and nitriles (ii) Salt, ester and acyl chloride formation II Acyl chlorides (exemplified by ethanoyl chloride) (i) Ease of hydrolysis compared with alkyl and aryl chlorides (ii) Reaction with alcohols, phenols and primary amines III Esters (exemplified by ethyl ethanoate and phenyl benzoate) (i) Formation from carboxylic acids and from acyl chlorides (ii) Hydrolysis (under acidic and under basic conditions) B Learning outcomes Candidates should be able to: (a) describe the formation of carboxylic acids from alcohols, aldehydes and nitriles (b) describe the reactions of carboxylic acids in the formation of (i) salts (ii) esters on condensation with alcohols, using ethyl ethanoate as an example (iii) acyl chlorides, using ethanoyl chloride as an example (iv) primary alcohols, via reduction with lithium aluminium hydride, using ethanol as an example (c) explain the acidity of carboxylic acids and of chlorine -substituted ethanoic acids in terms of their structures (d) describe the hydrolysis of acyl chlorides (e) describe the condensation reactions of acyl chlorides with alcohols, phenols and primary amines (f) explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and aryl chlorides (g) describe the formation of esters from the condensation reaction of acyl chlorides, using phenyl benzoate as an example (h) describe the acid and base hydrolyses of esters C References and Websites 1 A-Level Chemistry (by Ramsden) 2 Organic Chemistry (by David Klein) 3 www.chemguide.co.uk

2 1 CARBOXYLIC ACIDS 1.1 Structure Carboxylic acids may be represented as RCOOH, , where R is H or alkyl or aryl group. The functional group of carboxylic acids: carboxyl group ( −COOH). The name is a combination of the carbonyl and the hydroxyl groups. 1.2 Nomenclature Carboxylic acids are named by taking the name of the appropriate hydrocarbon, dropping the –e and adding the suffix –oic acid. ➢ The longest chain bearing the carboxyl group is considered the parent structure. ➢ Numbering of the carbon chain starts from the carboxyl carbon (principal functional group). ➢ When two carboxyl groups are present, the suffix –dioic acid is used. e.g. HOOC–COOH ethanedioic acid HOOCCH2COOH propanedioic acid Examples: methanoic acid (formic acid) ethanoic acid (acetic acid) propenoic acid (acrylic acid) CH3 CH C OH O OH 2-hydroxypropanoic acid (lactic acid) benzenecarboxylic acid (benzoic acid) ethanedioic acid (oxalic acid) HO2C CO2H Carboxyl carbon atom is sp2 hybridised • 3 x sp2 hybrid orbitals form  bonds with the 2 oxygen atoms and the C atom of the R group or H atom. • 1 x unhybridised p -orbital forms a  bond with the p-orbital of O atom of the C=O The molecule is trigonal planar with respect to the carboxyl carbon. C O OH R R C O O H

3 1.3 Physical Properties 1.3.1 Volatility Comparison with compounds of other homologous series ➢ Carboxylic acids have higher boiling points than the corresponding alkanes and alcohols of similar size of electron cloud. Compound Mr Boiling point / °C Reason CH3CH2CH2CH3 58 −0.5 Boiling point: CH3COOH > CH3CH2CH2CH3 The hydrogen bonds between ethanoic acid molecules are stronger and require more energy to break compared to the instantaneous dipole - induced dipole interactions between butane molecules. CH3COOH 60 118 Compound Mr Boiling point / °C Reason CH3CH2CH2OH 60 97 Boiling point: CH3COOH > CH3CH2CH2OH Both ethanol and ethanoic acid are capable of forming intermolecular hydrogen bonds. The hydrogen bonds between ethanoic acid molecules are stronger and require more energy to break because the –OH group in ethanoic acid is more polarised due to the presence of the electron-withdrawing C=O group. CH3COOH 60 118 ➢ Carboxylic acid molecules dimerise in the vapour state and in non -polar solvents, forming two hydrogen bonds between each pair of molecules. This accounts for the observation that the relative molecular mass of ethanoic acid when dissolved in benzene is approximately 120, but is 60 when dissolved in water. Comparison within the homologous series Compound Boiling point / °C Reason HCOOH 101 The boiling point increases with increasing length of alkyl chain of the carboxylic acids due to an increase in electron cloud size and the polarisability of the electron cloud of each carboxylic acid, leading to stronger instantaneous dipole-induced dipole interactions between molecules. CH3COOH 118 CH3CH2COOH 141 CH3CH2CH2COOH 164 The two intermolecular hydrogen bonds are formed between the carboxyl oxygen atom of one molecule and the hydroxyl hydrogen atom of another molecule. Intermolecular hydrogen bonding for a generic acid.

4 1.3.2 Solubility ➢ The first four members of the aliphatic acids are completely miscible with water because of: 1. the ability of the –COOH group to form hydrogen bonds with water molecules. 2. the partial dissociation of carboxylic acids in water to form ions, i.e. H3O+ and RCOO –, which are capable of forming ion-dipole interactions with water molecules. RCOOH + H2O RCOO− + H3O+ ➢ As the length of the non-polar hydrocarbon chain increases, solubility in water decreases. ➢ Benzoic acid dissolves readily in hot water but forms a white crystalline solid when cooled as it is only slightly soluble in cold water. 1.4 Preparation of carboxylic acids Candidates should be able to describe the formation of carboxylic acids from alcohols, aldehydes and nitriles. Carboxylic acids may be prepared in a number of ways, namely (a) hydrolysis (b) oxidation 1.4.1 Hydrolysis of nitriles Reagents & conditions Balanced equations Acidic hydrolysis HCl(aq), heat or H2SO4(aq), heat Alkaline hydrolysis NaOH(aq), heat, followed by acidification with HCl(aq), room temperature δ+ δ– a

5 1.4.2 Hydrolysis of acyl chlorides (refer to Section 2.3.1.1) 1.4.3 Hydrolysis of esters (refer to Section 2.3.2.1) 1.4.4 Hydrolysis of amides (refer to Organic Nitrogen Compounds notes) 1.4.5 Oxidation of primary alcohols and aldehydes Primary alcohols are oxidised via aldehydes to carboxylic acids. Reagents / conditions : KMnO4 (aq), H2SO4 (aq), heat/heat under reflux or K2Cr2O7 (aq), H2SO4 (aq), heat/heat under reflux 1.4.6 Oxidative cleavage of alkenes Reagents / conditions : KMnO4 (aq), H2SO4(aq), heat/heat under reflux (will not work with K2Cr2O7 (aq), H2SO4 (aq), heat) Example: CH3CH=CHCH3 + 4[O] 2CH3COOH 1.4.7 Side-chain oxidation of an alkylbenzene to form benzoic acid Reagents / conditions : KMnO4 (aq), H2SO4(aq), heat/heat under reflux (will not work with K2Cr2O7 (aq), H2SO4 (aq), heat) 4 2 4KMnO (aq), H SO (aq), heat ⎯⎯⎯⎯⎯⎯⎯⎯⎯ →

6 1.4.8 Comparison of oxidation by hot acidified KMnO4 and K2Cr2O7 KMnO4 and K2Cr2O7 do not have the same oxidising abilities even under the same conditions. It is also important to take note of how the different benzene side chains react when subjected to strong oxidation. (Note: NR = No reaction) K2Cr2O7(aq), H2SO4(aq), heat KMnO4(aq), H2SO4(aq), heat Alkene NR CH3COOH + CO2 + H2O Alkylbenzene NR NR NR 1° alcohols 2° alcohols 3° alcohols NR NR Aldehyde