VJC 2025 CAD Lecture Notes

1 VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT JC2 H2 CHEMISTRY CARBOXYLIC ACIDS & DERIVATIVES _______________________________________________________________________________ Lecture Outline Part I: Carboxylic Acids 1 Introduction 2 Physical Properties 2.1 Boiling Point 2.2 Solubility 2.3 Acidity 3 Laboratory Preparation 3.1 Oxidation of Primary Alcohols and Aldehydes 3.2 Hydrolysis of Nitriles 3.3 Benzoic Acid Formation from Alkylbenzene 4 Reactions 4.1 Salt Formation 4.2 Condensation to form Ester 4.3 Conversion to Acyl Chlorides 4.4 Reduction to Primary Alcohols 4.5 Decarboxylation to Alkanes 5 Special Reactions (Dehydration and Oxidation) 5.1 Methanoic Acid 5.2 Ethanedioic Acid 6 Summary of Reactions of Carboxylic Acids Part II: Acid Derivatives (Esters and Acyl Chlorides) 1 Introduction 2 Physical Properties 2.1 Boiling Point 2.2 Solubility 3 Reactions 3.1 Esters 3.1.1 Hydrolysis 3.2 Acyl Chlorides 3.2.1 Hydrolysis 3.2.2 Condensation with Alcohols and Phenols to form Esters 3.2.3 Condensation with Ammonia and Amines to form Amides 4 Summary of Reactions of Acid Derivatives Lesson 1 2 Pages 1 – 14 15 – 20 Complete by 30 Mar 30 Mar scan this QR code to ask questions related to the lecture

2 Assessment Objectives Students 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 hydrolysis of esters References (a) Introduction to Organic Chemistry by GI Brown (b) Principles of Organic Chemistry by Peter RS Murray (c) Organic Chemistry by Morrison and Boyd (d) Organic Chemistry by TW Graham Solomons

3 Part I: Carboxylic Acids 1 Introduction Structure • General formula of monocarboxylic acids is CnH2nO2 • Functional group is a carboxyl functional group, –COOH or –CO2H, which contains both a carbonyl group and a hydroxyl group; hence the name. • Carboxylic acids are represented as RCO2H, where R = H, alkyl or aryl group. Nomenclature 1. find the longest alkyl chain, which is the parent chain, that contains the carboxyl group 2. the ‘e’ of the –ane of the alkane with the same number of carbon atoms as the parent chain is replaced by ‘oic acid’. Note that it is not required to number the position of the carboxyl group as it is assigned position 1 within the chain. 3. identify substituent(s), if any, on the parent chain and include its position as prefixes to the name of the parent chain Examples IUPAC name (common name) structural formula skeletal formula methanoic acid (formic acid) HCO2H ethanoic acid (acetic acid) CH3CO2H benzoic acid (benzenecarboxylic acid) C6H5CO2H 2-hydroxypropanoic acid (lactic acid) CH3CH(OH)CO2H prop–2–enoic acid (acrylic acid) CH2=CHCO2H octadecanoic acid (stearic acid) CH3(CH2)16CO2H

4 2 Physical Properties 2.1 Boiling Point • The lower members are liquids with pungent odours. • Example: ethanoic acid (vinegar), butanoic acid (rancid butter, human sweat) Fun Fact: Body odour begins with secretions from the apocrine glands, located mostly in the armpits. Skin bacteria use these secretions to produce energy and waste products, among which is 3–methyl–2–hexenoic acid, CH 3CH2CH2C(CH3)=CHCO2H, which is largely respons ible for body odour. Each individual ’s sweat glands also produce a characteristic blend of carboxylic acids which can be detected by the sensitive n ose of a dog. • Boiling point increases with increasing molecular size and number of electrons per molecule (Recall: Molecular size increases with Mr). compound Mr boiling point / °C CH3CO2H 60 118 CH3(CH2)2CO2H 88 164 • Carboxylic acids are less volatile (higher boiling point) than alcohols of similar Mr. compound Mr boiling point / °C CH3CO2H 60 118 CH3CH(OH)CH3 60 97 o Carboxylic acids form stronger hydrogen bonding than alcohols because the O–H bond is more polarised due to the presence of electron –withdrawing inductive C=O group. o Furthermore, carboxylic acid molecules dimerise in the liquid state and in non –polar solvents, forming two hydrogen bonds between each pair of molecules. dimer o However, in polar solvents such as water, th e molecules exist as monomers and dissociate to some extent into H+ and RCO2–. RCO2H RCO2– + H+

5 2.2 Solubility • Lower acids are completely miscible with water because of the ability of the –CO2H group to form hydrogen bonds with water. • As the length of the non-polar hydrocarbon chain increases, solubility in water decreases. Example: benzoic acid is only slightly soluble in cold water but dissolves readily in hot water. • In contrast, the solubility of the carboxylic acids in non-polar solvents increases as the non- polar hydrocarbon chain length increase. 2.3 Acidity Students should be able to c) explain the acidity of carboxylic acids and of chlorine–substituted ethanoic acids in terms of their structures • Carboxylic acids are generally considered weak acids. • Being a weak acid, carboxylic acids partially dissociate by the following equation: RCO2H H+ + RCO2– • The carboxylate anion can be resonance stabilised due to the de localisation of electrons around the –COO– group. RCO2H H+ + RCO2– resonance stabilisation is minimal highly stabilised by resonance Note: o C–O bonds are equal in length. The negative charge is equally distributed between the two oxygen atoms, leading to a stabilisation of the carboxylate anion.

6 Strength of carboxylic acids Consider the dissociation of a weak acid HA: HA H+ + A– When A– is more stable, there is a greater tendency for HA to dissociate. HA would then be a stronger acid. Thus, the strength of the carboxylic acid RCO 2H depends o n the stability of RCO2– ion. An electron–withdrawing inductive group in RCO2– reduces the electron density of the negative charge on the carboxylate anion by charge dispersal. This stabilises the RCO2– and increases the strength of the acid. An electron–donating inductive group does the reverse. • pKa of some carboxylic acids formula pKa formula pKa HCO2H 3.75 ClCH2CO2H 2.86 CH3CO2H 4.76 Cl2CHCO2H 1.29 CH3CH2CO2H 4.82 Cl3CCO2H 0.65 CH3CH2CH2CO2H 4.87 FCH2CO2H 2.60 C6H5CO2H 4.20 NCCH2CO2H 2.50 Recall: • smaller pKa  larger Ka  greater extent of acid dissociation  stronger acid Worked Example 1 Why are carboxylic acids stronger acids than alcohols in general? Answer: RCO2H H+ + RCO2– --- (1) ROH H+ + RO– ---- (2) In RCO2– ion, the .................... in the –CO2– group .................... with one another, allowing delocalisation and resulting in RCO2– ion having a ...................... structure. This helps to disperse the negative charge on the O atom, hence .................... the RCO2– ion and .................... the strength of the acid RCO2H. The RO– ion is ......................... by ................................................ inductive effect of the R group, which causes the negative charge to be .......................... The first equilibr