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 should be filled with: "dimers" (or "hydrogen-bonded dimers")** ## Step-by-step reasoning: 1. **Hydrogen bonding capacity**: Carboxylic acids ($R-COOH$) have a highly polar $-OH$ group that forms strong intermolecular hydrogen bonds. 2. **Dimer formation mechanism**: Two carboxylic acid molecules associate through **hydrogen bonding between the carboxyl groups**: $$R-COOH \cdots H-O-C(=O)-R$$ This creates a cyclic structure with two hydrogen bonds per dimer. 3. **Why in vapour phase?**: Even in the gas phase, carboxylic acids don't fully dissociate into monomers because hydrogen bonding is sufficiently strong to keep molecules paired. 4. **Why in aprotic solvents?** - Aprotic solvents (like $CCl_4$, benzene, or diethyl ether) **cannot donate hydrogen bonds** - They cannot solvate the $-OH$ group through hydrogen bonding - Therefore, carboxylic acids remain associated as dimers rather than dissolving as isolated monomers 5. **Contrast with protic solvents**: In protic solvents like water or alcohols, the solvent competes for hydrogen bonding and breaks apart the dimers, forming monomers. **Answer: Dimers**