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 & Aprotic Solvents **Understanding the Structure:** Carboxylic acids ($RCOOH$) have a unique property due to their two carbonyl groups in close proximity (the C=O and the O-H of the acid group). **In Vapour Phase or Aprotic Solvents:** In these environments: - No protic solvent (like water) is available to break hydrogen bonds - Carboxylic acids form **hydrogen bonds between two molecules** - Two $RCOOH$ molecules associate through reciprocal O-H···O=C hydrogen bonding $$\ce{2 RCOOH <-> (RCOOH)_2}$$ This creates a cyclic dimeric structure where: - Each carboxylic acid acts as both H-bond donor and acceptor - Two hydrogen bonds hold the dimer together - This is energetically favorable in non-polar environments **Why Dimers Form:** - Aprotic solvents (benzene, $CCl_4$) cannot compete for hydrogen bonding - In vapor phase, there's no solvent interference - Dimerization reduces overall free energy of the system **Answer: DIMERS** (or **dimeric form**) The correct answer is **A** because carboxylic acids characteristically exist as **hydrogen-bonded dimers** in the vapor phase and aprotic solvents due to intermolecular hydrogen bonding between acid molecules.