JEE Chemistry Aldehydes Ketones and Carboxylic Acids Notes Set 06

Carboxylic Acids: Preparation and Properties

1. Preparation: oxidation of primary alcohols and aldehydes, side-chain oxidation of alkylbenzenes with KMnO₄ (any alkyl chain gives –COOH), hydrolysis of nitriles, amides and esters, and the Grignard reagent with CO₂.
2. Carboxylic acids have higher boiling points than alcohols of similar mass because they exist as hydrogen-bonded dimers. Lower acids are soluble in water.
3. Acidity: the carboxylate ion is stabilised by delocalisation, so acids are much stronger than alcohols and phenols (pKa about 4–5). Electron-withdrawing groups increase acidity (ClCH₂COOH > CH₃COOH) and electron-donating groups decrease it. Formic acid (pKa 3.75) is stronger than acetic acid (4.76) and benzoic acid (4.2).
4. Carboxylic acids liberate CO₂ from NaHCO₃. With SOCl₂ or PCl₅ they give acid chlorides, with alcohols (H⁺) esters, with NH₃ on heating amides, and LiAlH₄ or B₂H₆ reduces them to primary alcohols.
5. Hell–Volhard–Zelinsky reaction: RCH₂COOH with Cl₂ or Br₂ and red P gives the α-halo acid. Halogenation takes place at the α-carbon only.
6. Decarboxylation: sodium salts heated with soda lime give alkanes with one carbon less (RCOONa + NaOH → RH + Na₂CO₃). β-Keto acids lose CO₂ very easily on gentle heating.
7. Hunsdiecker reaction: the silver salt with Br₂ gives RBr with one carbon fewer.

Acid Derivatives

1. The common derivatives are acid chlorides, anhydrides, esters and amides. They undergo nucleophilic acyl substitution (addition of a nucleophile followed by loss of the leaving group). Reactivity order: acid chloride > anhydride > ester > amide.
2. Electron-withdrawing substituents on the ring increase the rate of hydrolysis; electron-donating groups decrease it (4-nitrobenzoyl chloride hydrolyses faster than benzoyl chloride).
3. Acid chlorides: made from acids with SOCl₂ or PCl₅. They react with water to give acids, with alcohols to give esters, with NH₃ to give amides, with sodium carboxylates to give anhydrides, and with phenol to give phenyl esters. H₂/Pd–BaSO₄ reduces them to aldehydes (Rosenmund), and R₂Cd converts them into ketones.
4. Anhydrides acylate alcohols, phenols and amines. Esters are hydrolysed by acids (reversible) or by alkali (saponification, irreversible), reduced by LiAlH₄ to alcohols or by DIBAL-H to aldehydes, and give tertiary alcohols with excess Grignard reagents. Transesterification exchanges the alkoxy group, and hydrazine converts esters into hydrazides.
5. Amides are reduced by LiAlH₄ to amines (same number of carbons) and dehydrated by P₂O₅ to nitriles. Hofmann bromamide reaction: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O, giving an amine with one carbon fewer through an isocyanate intermediate.
6. Primary amines react with acetyl chloride to give N-substituted acetamides (acylation). The Kolbe–Schmidt and Fries rearrangement reactions are also used to make aromatic hydroxy acids and ketones.
7. Useful products: aspirin (acetylsalicylic acid), oil of wintergreen (methyl salicylate), and phenolphthalein (from phthalic anhydride and phenol).