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

# Solution: Carboxylic Acids in Vapor Phase/Aprotic Solvents **Step 1: Understand carboxylic acid structure** Carboxylic acids contain the $-COOH$ group with both a hydrogen bond donor (O-H) and acceptor (C=O). **Step 2: Analyze behavior in vapor phase** In the vapor phase, molecules are far apart with minimal intermolecular interactions. Carboxylic acids cannot form extensive hydrogen bonding networks like they do in aqueous or polar protic solvents. **Step 3: Analyze behavior in aprotic solvents** Aprotic solvents (like $CCl_4$, benzene, $CS_2$) cannot donate hydrogen bonds to the $-OH$ group. The carboxylic acid molecules must interact with each other instead. **Step 4: Identify the association mechanism** In both cases, carboxylic acids form **hydrogen-bonded dimers** through intermolecular O-H···O=C interactions: $$2 \, R-COOH \rightleftharpoons (R-COOH)_2$$ Each molecule acts as both H-bond donor and acceptor, creating cyclic dimeric structures stabilized by two hydrogen bonds. **Answer: DIMERS** (or dimer/dimeric form) This explains why carboxylic acids have anomalously high boiling points and densities in vapor/aprotic conditions compared to structurally similar compounds.