CUET UG Chemistry Booster -3 Carboxylic Acids (Structure, Properties, Reactions & Uses)

Carboxylic acids possess a carbonyl group, but unlike aldehydes and ketones, they do not readily undergo nucleophilic addition because resonance involving the hydroxyl oxygen reduces the positive character on the carbonyl carbon. The –COOH group also withdraws electron density through both inductive and resonance effects. Therefore, statements A, B and D are correct.

Structure: [ HOOC-CH_2-CH_2-COOH ] This can be written as: [ HOOC-(CH_2)_2-COOH ] Hence, succinic acid is butanedioic acid.

[ HOOC-(CH_2)_4-COOH ] The IUPAC name is hexanedioic acid. Its common name is adipic acid, an important industrial chemical used in polymer production.

The carboxyl group withdraws electron density through both −I and −R effects. As a result: • The aromatic ring becomes less reactive. • Electrophiles preferentially enter the meta position. • Friedel-Crafts alkylation/acylation generally does not occur. • AlCl₃ coordinates strongly with the carboxyl group, further deactivating the ring. Thus A, B, C and D are correct.

IUPAC Name — Common Name Ethanedioic acid — Oxalic acid Propanedioic acid — Malonic acid Butanedioic acid — Succinic acid Pentanedioic acid — Glutaric acid Therefore: 1-d, 2-a, 3-b, 4-c

The structure: HOOC-CH2-CH(COOH)CH2-COOH can be viewed as a propane chain where each carbon bears a carboxyl group: CH2(COOH)-CH(COOH)-CH2(COOH) So the correct IUPAC name is: Propane-1,2,3-tricarboxylic acid

In carboxylic acids, the lone pair on the hydroxyl oxygen participates in resonance with the carbonyl group. This delocalization reduces the pure double-bond character of the C=O bond, giving it some partial single-bond character compared with ketones.

The carboxylate ion is highly stabilized because its negative charge is equally delocalized over two oxygen atoms through two equivalent resonance structures. In phenoxide ion, the charge is delocalized partly onto less electronegative carbon atoms of the benzene ring. Therefore, carboxylate ion is more stable than phenoxide ion, making carboxylic acids stronger acids than phenols. Thus, all four statements are correct.

Alkylbenzenes undergo side-chain oxidation with alkaline KMnO₄. The alkyl side chain is oxidized completely to the carboxyl group, while the benzene ring remains unchanged. Acidic hydrolysis gives the corresponding aromatic carboxylic acid. Example: C6​H5​CH3​KMnO4​/OH−​C6​H5​COO−H+​C6​H5​COOH3 . Why Other Options Are Incorrect • Option B: No electrophilic substitution occurs. • Option C: No radical halogenation occurs. • Option D: No nucleophilic substitution occurs.

The usual oxidation sequence is: Alkane Primary Alcohol Aldehyde Carboxylic Acid So the correct order is: (D), (B), (C), (A)

The conversion occurs as: CH3CH2Cl {KCN} CH3CH2CN (Propanenitrile) [ CH_3CH_2CN \xrightarrow{H^+/OH^-} CH_3CH_2COOH ] (Propanoic acid) Since the nitrile carbon becomes part of the carboxylic acid group, the carbon chain increases by one carbon atom. Therefore, chloroethane is the correct starting material.

The carbon attached to magnesium in a Grignard reagent behaves like a carbanion. RMgX + CO2 RCOO-MgX+ The nucleophilic carbon attacks the electrophilic carbon of carbon dioxide, producing a carboxylate salt. Acidification subsequently forms a carboxylic acid. Hence, the reaction is classified as nucleophilic addition.

The passage states that higher carboxylic acids are wax-like solids and are practically odourless because they have very low volatility. Odour is perceived only when molecules enter the vapour phase and reach olfactory receptors. Since these acids do not vaporize readily, they show little or no smell.

Carboxylic acid molecules associate through strong intermolecular hydrogen bonding. Two molecules often form cyclic dimers: [ (RCOOH)_2 ] These hydrogen bonds are strong enough that association is not completely destroyed even in the vapour phase. This explains their high boiling points.

Acidity increases when electron-withdrawing groups stabilize the conjugate base. Order of electron-withdrawing effect: (NO2)2 > NO2 > H > OCH3 Therefore: 3,4-Dinitrobenzoic acid 4-Nitrobenzoic acid Benzoic acid 4-Methoxybenzoic acid Since stronger acids have lower pKa values, the increasing order of pKa is: (D) < (A) < (B) < (C)

The acidity of haloacetic acids depends on the electron-withdrawing inductive effect of the halogen attached to the α-carbon. Order of −I effect: [ F > Cl > Br > I ] Greater electron withdrawal stabilizes the carboxylate ion and increases acidity. Therefore: [ ICH_2COOH < BrCH_2COOH < ClCH_2COOH < FCH_2COOH ] Thus, the increasing order of acidity is: [ (D) < (C) < (B) < (A) ]

The Fischer esterification mechanism involves: 1. Protonation of the carbonyl oxygen. 2. Nucleophilic attack by alcohol. 3. Formation of a tetrahedral intermediate. 4. Proton transfer converting –OH into –OH₂⁺. 5. Elimination of water. 6. Deprotonation to form the ester. Hence all four statements are correct.

In Kolbe electrolysis: [ RCOO^- \rightarrow R^\bullet + CO_2 ] The radicals formed at the anode combine: [ R^\bullet + R^\bullet \rightarrow R-R ] Thus hydrocarbons are produced through electrolysis of sodium or potassium carboxylates.

Compound — Industrial Role Sodium benzoate — Food preservative Hexanedioic acid (Adipic acid) — Nylon-6,6 manufacture Esters of benzoic acid — Perfumery blending Methanoic acid — Electroplating and leather industry Therefore: 1-b, 2-d, 3-a, 4-c

Higher fatty acids such as stearic acid and palmitic acid form sodium or potassium salts when treated with alkali. Examples: [ C_{17}H_{35}COONa ] (Sodium stearate) These salts possess a hydrophobic tail and hydrophilic head, making them effective surfactants used in soaps and detergents.