Carboxylic Acids and Esters

20min Part 4 / Ch7 / Lesson 4

Prerequisites: 4-7-2 , 4-7-3

Objectives

Carboxylic acids R-COOH: organic acids containing the carboxyl group (-COOH).

Weak acids (e.g., acetic acid CH₃COOH)
Dissociate in water: $\mathrm{R\text{-}COOH \rightleftharpoons R\text{-}COO^- + H^+}$
Neutralize bases; react with metals to evolve hydrogen

Name	Formula	Carbons	Key feature
Formic acid	HCOOH	1	Has reducing power (contains -CHO equivalent)
Acetic acid	CH₃COOH	2	Main component of vinegar
Oxalic acid	(COOH)₂	2	Diprotic acid; reducing agent
Lactic acid	CH₃CH(OH)COOH	3	Hydroxy acid (-OH + -COOH)
Benzoic acid	C₆H₅COOH	7	Aromatic carboxylic acid

Representative carboxylic acids

Formic acid is special: HCOOH contains a structure equivalent to -CHO, so it gives positive silver mirror and Fehling’s tests.

$\mathrm{R\text{-}COOH + R'\text{-}OH \underset{H_2SO_4}{\rightleftharpoons} R\text{-}COOR' + H_2O}$

Esters and their aromas

Fats and oils = esters of glycerol + 3 molecules of fatty acid

Fats (solid): rich in saturated fatty acids (stearic acid C₁₇H₃₅COOH, etc.)
Oils (liquid): rich in unsaturated fatty acids (oleic acid C₁₇H₃₃COOH, etc.)

Unsaturated oil + H₂ (Ni catalyst) → hardened fat (principle behind margarine)

Soap making (saponification)

Saponification: fat/oil + NaOH → glycerol + sodium fatty acid salt (soap)

Soap has a hydrophilic end (-COO⁻) and a hydrophobic end (long alkyl chain), functioning as a surfactant.

Q1 Esterification is a reaction between...

Q2 Why does formic acid give a positive silver mirror test?

Q3 What is saponification?

Q1. Write the esterification equation for acetic acid and ethanol, and name two ways to shift the equilibrium toward ester formation.

Solution

$\mathrm{CH_3COOH + C_2H_5OH \underset{H_2SO_4}{\rightleftharpoons} CH_3COOC_2H_5 + H_2O}$

Ways to shift equilibrium toward products:

Add conc. H₂SO₄: removes water (dehydrating agent) → shifts equilibrium right
Use excess of one reactant: add more alcohol or acid → Le Chatelier’s principle