Carboxylic acids exist as … in the vapour phase or in the aprotic solvents. Fill in the blank with b — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

# Carboxylic Acids in Vapour Phase and Aprotic Solvents **The blank is filled with: Dimers (or hydrogen-bonded dimers)** ## Step-by-Step Reasoning: 1. **Structure of carboxylic acids:** Carboxylic acids ($R-COOH$) contain a highly polar $-COOH$ group capable of strong hydrogen bonding. 2. **In vapour phase:** - Molecules are far apart with weak intermolecular forces from dipole-dipole interactions - However, carboxylic acids form stable **hydrogen-bonded dimers** through: $$2 \, R-COOH \rightarrow (R-COOH)_2$$ - Each $-OH$ of one molecule hydrogen bonds with the $C=O$ of another, creating a cyclic structure 3. **In aprotic solvents** (e.g., benzene, $CCl_4$): - These solvents cannot donate or accept hydrogen bonds - Carboxylic acids cannot interact with the solvent through H-bonding - They self-associate into **dimers** to stabilize through mutual hydrogen bonding 4. **Why dimers are stable:** - Each hydrogen bond contributes ~5-8 kcal/mol - Two hydrogen bonds per dimer (~10-16 kcal/mol) make the structure thermodynamically favourable - Breaks down only at high temperatures or in protic/polar solvents **Answer: Dimers** (hydrogen-bonded dimers or associated dimers)