11.2 Preparation of Benzoic Acid and Further Oxidation of Carboxylic Acids

Preparation of Benzoic Acid

1. Oxidation of Alkylbenzenes with Hot Alkaline KMnO₄

The most important method for preparing benzoic acid in the FBISE syllabus is the oxidation of alkylbenzenes (compounds with an alkyl side chain attached to a benzene ring) using hot alkaline potassium permanganate (KMnO₄/OH⁻), followed by acidification (H₃O⁺).

Key rule: Regardless of the length of the alkyl side chain, the entire chain is oxidised down to a single $-\mathrm{COOH}$ group, as long as the carbon directly bonded to the ring (the benzylic carbon) carries at least one hydrogen atom.

Example 1 — Methylbenzene (Toluene)

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}{\mathrm{H}}_3\stackrel{{\text{hot KMnO}}_4/{\text{OH}}^{-},\text{then }{\mathrm{H}}_3{\mathrm{O}}^{+}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}$

Methylbenzene → Benzoic acid

Example 2 — Ethylbenzene

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3\stackrel{{\text{hot KMnO}}_4/{\text{OH}}^{-},\text{then }{\mathrm{H}}_3{\mathrm{O}}^{+}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}$

The two-carbon ethyl group is oxidised entirely to $-\mathrm{COOH}$; the extra carbon is lost as $\mathrm{C}{\mathrm{O}}_2$.

Example 3 — Propylbenzene

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3\stackrel{{\text{hot KMnO}}_4/{\text{OH}}^{-},\text{then }{\mathrm{H}}_3{\mathrm{O}}^{+}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}$

The entire propyl chain is degraded to $-\mathrm{COOH}$.

Why tert-Butylbenzene Does NOT React

tert-Butylbenzene, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}(\mathrm{C}{\mathrm{H}}_3{)}_3$, cannot be oxidised to benzoic acid because the benzylic carbon carries no hydrogen atoms. A benzylic C–H bond is required to initiate the oxidation.

2. From Grignard Reagent + CO₂

Phenylmagnesium bromide (a Grignard reagent) reacts with dry ice ($\mathrm{C}{\mathrm{O}}_2$) to give a carboxylate salt, which on acid hydrolysis yields benzoic acid:

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{MgBr}+\mathrm{C}{\mathrm{O}}_2\stackrel{\text{dry ether}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOMgBr}\stackrel{{\mathrm{H}}_3{\mathrm{O}}^{+}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}$

3. Hydrolysis of Benzonitrile

Benzonitrile (${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CN}$) undergoes acid hydrolysis (reflux with dilute $\mathrm{HCl}$ or ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$) or alkaline hydrolysis to give benzoic acid:

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CN}+2{\mathrm{H}}_2\mathrm{O}\stackrel{\text{dil. }{\mathrm{H}}^{+}/\Delta }{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}+\mathrm{N}{\mathrm{H}}_4^{+}$

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Further Oxidation of Carboxylic Acids (SLO C-12-E-35)

Most carboxylic acids resist further oxidation. However, two important exceptions exist:

a. Methanoic Acid (Formic Acid), HCOOH

Methanoic acid is unique among carboxylic acids because it contains an aldehyde-like $\mathrm{C}-\mathrm{H}$ bond in its structure ($\mathrm{H}-\mathrm{COOH}$). It can therefore be further oxidised to $\mathrm{C}{\mathrm{O}}_2$ and ${\mathrm{H}}_2\mathrm{O}$.

Oxidising agents that oxidise HCOOH:

• Fehling's reagent (brick-red precipitate of $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$)
• Tollens' reagent (silver mirror)
• Acidified $\mathrm{KMn}{\mathrm{O}}_4$ (decolourised)
• Acidified ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ (orange → green)

$\mathrm{HCOOH}\stackrel{[\mathrm{O}]}{\to }\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$

This behaviour is used to distinguish methanoic acid from other carboxylic acids.

b. Ethanedioic Acid (Oxalic Acid), HOOCCOOH

Ethanedioic acid is oxidised by warm acidified $\mathrm{KMn}{\mathrm{O}}_4$ to carbon dioxide and water:

$\mathrm{HOOCCOOH}\stackrel{{\text{warm acidified KMnO}}_4}{\to }2C{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$

This reaction is used in volumetric analysis (titration of oxalic acid against $\mathrm{KMn}{\mathrm{O}}_4$).