Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
Both make hydrogen bonds, but the acid makes MORE of them per pair of molecules. A carboxylic acid has two spots: the O-H can donate a hydrogen bond, and the C=O oxygen can accept one. So two acid molecules lock together with TWO hydrogen bonds, forming a closed ring (a dimer). An alcohol has only one O-H and no strong second acceptor, so it links with fewer, less organised hydrogen bonds. More and stronger bonds means more heat is needed to separate the molecules, so the acid boils higher.
It is INTERMOLECULAR (between two different molecules), not intramolecular. The dimer is made of two separate acid molecules held together. This matters for NEET: the 2018 question's correct answer is 'intermolecular H-bonding', and the wrong tempting choice is 'intramolecular H-bonding'. Intramolecular H-bonding (like in o-nitrophenol) would actually LOWER boiling point, so do not pick it here.
A dimer means two molecules joined into one bigger unit. In a carboxylic acid, two -COOH groups face each other and form a flat eight-membered ring using two O-H...O=C hydrogen bonds. Because the acid now behaves like a heavier double-sized particle, it is harder to push into the gas phase, so the boiling point rises. NCERT says most carboxylic acids exist as dimers in the vapour phase or in aprotic solvents.
Hydrocarbon (alkane) < aldehyde/ketone < alcohol < carboxylic acid. Alkanes only have weak van der Waals forces. Aldehydes and ketones add dipole-dipole attraction from the polar C=O. Alcohols add one intermolecular hydrogen bond. Carboxylic acids add TWO hydrogen bonds in a dimer, so they sit at the top.
Not fully. NCERT clearly states the hydrogen bonds are not broken completely even in the vapour phase, so the acid still travels as a dimer in the gas. This is why the molecular mass measured in the vapour or in benzene comes out about double the expected value (for example acetic acid appears to have mass ~120 instead of 60).
Carboxylic acids have higher boiling points than aldehydes, ketones and even alcohols of comparable molecular mass. It is due to their:
Statement I: The boiling points of aldehydes and ketones are higher than hydrocarbons of comparable molecular masses because of weak molecular association due to dipole-dipole interactions. Statement II: The boiling points of aldehydes and ketones are lower than the alcohols of similar molecular masses due to the absence of H-bonding. Choose the most appropriate answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because two molecules bond through two intermolecular hydrogen bonds to form a cyclic dimer. This double bonding is stronger than the single hydrogen bond in alcohols and much stronger than the dipole-dipole forces in aldehydes and ketones, so more heat is needed to boil the acid.
Yes, roughly. In benzene (a low-dielectric solvent) acetic acid dimerises through hydrogen bonding, so the number of free particles is halved and the measured molar mass comes out about double (~120 for acetic acid). NCERT uses this exact example.
Intermolecular. The dimer links two separate molecules. Never answer intramolecular for the boiling-point question, because intramolecular H-bonding lowers boiling point.
Alkane (weak van der Waals) < aldehyde and ketone (dipole-dipole) < alcohol (one H-bond) < carboxylic acid (dimer, two H-bonds). Remembering this single ladder answers most NEET boiling-point questions in this unit.
