Aldehydes, Ketones and Carboxylic Acids — Part 2: Carboxylic acids: visual lesson, CBSE Class 12 Chemistry

Lesson

Carboxylic acids

Part 2 — the –COOH group: preparation, acidity, reactions and uses.

In this lesson you will

Write methods of preparing carboxylic acids from alcohols, aldehydes, nitriles, Grignard reagents and alkylbenzenes
Explain the physical properties and acidity of carboxylic acids and the effect of substituents
Write reactions of carboxylic acids, including formation of acid derivatives, reduction, decarboxylation and the HVZ reaction
State uses of methanoic, ethanoic and benzoic acids and their esters

Table of common carboxylic acids with their common names and sources or uses
Acid	Common name	Source or use
Methanoic acid	Formic acid	Ant stings; leather tanning
Ethanoic acid	Acetic acid	Vinegar (about 5%)
Butanoic acid	Butyric acid	Rancid butter smell
Benzoic acid	—	Sodium benzoate food preservative
Hexanedioic acid	Adipic acid	Making nylon-6,6

Concept

Naming and preparing carboxylic acids

IUPAC: replace -e by -oic acid (the COOH carbon is C1): ethanoic acid, 3-methylbutanoic acid, benzoic acid. Preparation: oxidation of 1° alcohols and aldehydes (KMnO₄, K₂Cr₂O₇); oxidation of alkylbenzenes (any side chain → –COOH); hydrolysis of nitriles and amides; Grignard reagent + CO₂, then acid; hydrolysis of esters, anhydrides and acid chlorides.

\ce{CH3CN + 2H2O + HCl -> CH3COOH + NH4Cl}

\ce{CH3MgBr + CO2 -> CH3COOMgBr}

Table of preparation methods for carboxylic acids with reagents and example products
From	Reagent	Example product
Primary alcohol	Alkaline KMnO₄, then H⁺	Propanoic acid from propan-1-ol
Alkylbenzene	KMnO₄, KOH, heat; then H⁺	Benzoic acid from ethylbenzene
Nitrile	H₃O⁺, heat	Ethanoic acid from ethanenitrile
Grignard reagent	Dry ice (CO₂), then H₃O⁺	Ethanoic acid from CH₃MgBr

Worked example

Worked example: preparations with chain length

Problem. Prepare (a) propanoic acid from bromoethane in two ways; (b) benzoic acid from propylbenzene.

(a) Route 1: KCN gives propanenitrile ( $\ce{CH3CH2CN}$ ), then acid hydrolysis → propanoic acid. Route 2: Mg in dry ether gives $\ce{CH3CH2MgBr}$ , then $\ce{CO2}$ and $\ce{H3O+}$ → propanoic acid.

(b) Hot alkaline KMnO₄, then acid: the whole propyl chain is oxidised to –COOH.

Cookies