Carboxylic Acids & Their Derivatives

Carboxylic Acids – Introduction

Saturated monocarboxylic acids have the general formula C_nH_2nO₂.

Preparation of Carboxylic Acids

1. Oxidation of Alcohols & Alkenes

• R–CH₂OH → R–COOH (same number of carbon atoms)
• Oxidative cleavage of alkenes also gives carboxylic acids

2. Using Grignard Reagents (Carbonation)

• R–MgX + CO₂ → R–COO–MgX → R–COOH (after hydrolysis)
• Aryl Grignard reagents give aromatic carboxylic acids

3. Hydrolysis of Cyanides

• R–X → R–CN → R–COOH
• Product has one carbon more than parent alkyl halide

4. Heating Geminal Dicarboxylic Acids

• R–CH(COOH)₂ → R–CH₂–COOH + CO₂

5. Preparation of Aromatic Carboxylic Acids

• Oxidation of alkyl benzene using KMnO₄
• Carbon attached to benzene must have at least one hydrogen

Properties of Carboxylic Acids

• Higher boiling point than corresponding alcohols
• Exist as dimers due to intermolecular hydrogen bonding
• Dimerisation occurs even in vapour phase

Acidic Strength of Carboxylic Acids

• Ionisation: R–COOH + H₂O ⇌ R–COO⁻ + H₃O⁺
• Strength depends on ease of proton loss
• EWG increases acidity; EDG decreases acidity

Order of Acidic Strength

• HCOOH > CH₃COOH > C₂H₅COOH
• FCH₂COOH > ClCH₂COOH > BrCH₂COOH
• CCl₃COOH > Cl₂CHCOOH > ClCH₂COOH
• RCOOH > H₂O > ROH > HC≡CH > NH₃

Ortho Effect (Benzoic Acids)

• All ortho-substituted benzoic acids are stronger than benzoic acid
• Due to steric effect, inductive effect and H-bonding
• Nitrobenzoic acid: o > p > m
• Hydroxy / chloro / methyl benzoic acids: o > m > p

Chemical Reactions of Carboxylic Acids

Hell–Volhard–Zelinsky (HVZ) Reaction

• α-hydrogen substituted by halogen
• Reagents: X₂ / red P (X = Cl, Br)

Esterification

• R–COOH + R′–OH ⇌ R–COOR′ + H₂O (H⁺)
• SN2 reaction
• Rate decreases with increasing size of R group
• Reactivity: HCOOH > CH₃COOH > higher acids

General Reactions

• With Na → RCOONa + H₂
• With NaOH → RCOONa + H₂O
• With NaHCO₃ → brisk effervescence of CO₂
• With PCl₅, SOCl₂ → acid chlorides
• With LiAlH₄ → primary alcohols

Reactions of Benzoic Acid

• –COOH group is deactivating and meta-directing
• Undergoes nitration, bromination and sulphonation at meta position

Tests for Carboxylic Acids

• Turn blue litmus red
• Effervescence with NaHCO₃ / Na₂CO₃
• Formic acid gives positive Tollens’ and Fehling’s tests

Acid Derivatives

Types

• Acid chlorides: R–COCl
• Acid anhydrides: (RCO)₂O
• Esters: R–COOR′
• Amides: R–CONH₂

Acid Chlorides (Acyl Halides)

Preparation

• RCOOH + PCl₅ / PCl₃ / SOCl₂ → RCOCl
• (RCO)₂O + PCl₅ → RCOCl

Properties & Reactions

• Lower boiling point than acids
• Highly reactive due to weak C–Cl bond
• With H₂O → acid
• With alcohol → ester
• With NH₃ → amide

Acid Anhydrides

Preparation

• Dehydration of acids using P₂O₅
• Acetic anhydride prepared by heating acetic acid to 1073 K

Properties

• Higher boiling point than acid chlorides
• Less reactive than acid chlorides

Esters

Preparation

• Esterification of acids
• From acid chlorides

Reactions

• Hydrolysis (acidic and alkaline)
• Trans-esterification
• Claisen condensation (esters with α-hydrogen)

Amides

Properties

• Highest boiling point among acid derivatives
• Least reactive due to strong C–N bond
• Amphoteric in nature

Important Reactions

• Hydrolysis → acids or salts
• Dehydration → nitriles (P₄O₁₀)
• Reduction → 1° amines (LiAlH₄)
• Hofmann bromamide reaction → amine with one carbon less