Give reasons for the following: (3 × 1)

(a) Carboxylic acids have higher boiling point than alcohols of comparable molecular masses.

(b) Alpha (α) hydrogens of aldehydes and ketones are acidic in nature.

(c) Nucleophilic addition of ammonia and its derivatives does not occur with carbonyl group in strongly acidic medium.

Q: Give reasons for the following: (3 × 1) (a) Carboxylic acids have higher boiling point than alcohols of comparable molecular masses. (b) Alpha (α) hydrogens of aldehydes and ketones are acidic in nature. (c) Nucleophilic addition of ammonia and its derivatives does not occur with carbonyl group in strongly acidic medium.

**(a) Carboxylic acids boil higher than comparable alcohols:** Carboxylic acid molecules form **stronger and more extensive intermolecular hydrogen bonds** than alcohols. In fact two acid molecules pair up as a stable **cyclic dimer** held by two H-bonds. - This effectively doubles the molecular mass in the liquid/vapour and needs more energy to break, so the boiling point is higher than an alcohol of similar mass. **(b) α-Hydrogens are acidic:** The carbonyl group (>C=O) is strongly **electron-withdrawing (–I effect)**, and after the α-H leaves, the resulting carbanion is **stabilised by resonance** with the C=O (enolate ion, negative charge delocalised onto oxygen). - This stabilisation of the conjugate base makes the α-hydrogen acidic. **(c) No addition of NH₃/derivatives in strongly acidic medium:** Ammonia and its derivatives (the nucleophile) have a lone pair on nitrogen. In a **strongly acidic** medium this nitrogen gets **protonated** to –N⁺H₃, losing its lone pair. - With no lone pair, it can no longer act as a nucleophile and attack the carbonyl carbon, so the addition is stopped. (These reactions work best at a mildly acidic, controlled pH ≈ 3.5–4.5.)

Aldehydes, Ketones and Carboxylic Acids

Properties of Carbonyl and Carboxylic Compounds

Official Answer

(a) Carboxylic acids boil higher than comparable alcohols:

Carboxylic acid molecules form stronger and more extensive intermolecular hydrogen bonds than alcohols. In fact two acid molecules pair up as a stable cyclic dimer held by two H-bonds.

This effectively doubles the molecular mass in the liquid/vapour and needs more energy to break, so the boiling point is higher than an alcohol of similar mass.

(b) α-Hydrogens are acidic:

The carbonyl group (>C=O) is strongly electron-withdrawing (–I effect), and after the α-H leaves, the resulting carbanion is stabilised by resonance with the C=O (enolate ion, negative charge delocalised onto oxygen).

This stabilisation of the conjugate base makes the α-hydrogen acidic.

(c) No addition of NH₃/derivatives in strongly acidic medium:

Ammonia and its derivatives (the nucleophile) have a lone pair on nitrogen. In a strongly acidic medium this nitrogen gets protonated to –N⁺H₃, losing its lone pair.

With no lone pair, it can no longer act as a nucleophile and attack the carbonyl carbon, so the addition is stopped. (These reactions work best at a mildly acidic, controlled pH ≈ 3.5–4.5.)

hydrogen bondingcyclic dimerboiling pointα-hydrogen acidityenolate resonanceelectron-withdrawing carbonylprotonation of nucleophilenucleophilic addition pH

Marking Scheme

11 mark (a): stronger/extensive intermolecular hydrogen bonding, existence as a cyclic dimer (two H-bonds) → higher boiling point.
21 mark (b): –I / electron-withdrawing carbonyl and resonance stabilisation of the resulting carbanion (enolate) makes α-H acidic.
31 mark (c): in strongly acidic medium the ammonia derivative gets protonated, loses its lone pair, so it can no longer act as a nucleophile (optimum pH ~3.5–4.5).

Hint

Think 'conjugate/association stability': (a) cyclic dimer via two H-bonds, (b) resonance-stabilised enolate, (c) strong acid protonates the N lone pair and kills the nucleophile.

Quick Oral Answer

Carboxylic acids form cyclic dimers through two strong hydrogen bonds so they boil higher than comparable alcohols; α-hydrogens are acidic because the electron-withdrawing C=O and resonance stabilise the resulting enolate carbanion; and in strongly acidic medium ammonia derivatives get protonated, lose their nitrogen lone pair, and can no longer attack the carbonyl carbon.

Analysis & Explanation

All three parts hinge on one idea: the polar, electron-pulling C=O / –COOH group and how it interacts with its neighbours.

(a) Hydrogen bonding — quality, not just presence:

Both acids and alcohols H-bond, but the –COOH group has an acidic O–H and a carbonyl O, letting two molecules lock into a six-membered cyclic dimer through two hydrogen bonds. This is a stronger, more organised association than the chain-like H-bonding of alcohols, so acids need more energy (higher b.p.) to separate.

(b) Acidity of α-H — stabilise the conjugate base:

Acidity always comes down to how stable the anion left behind is. The carbanion at the α-carbon is next to C=O, so the lone pair delocalises onto the electronegative oxygen (enolate resonance). A stabilised base ⇒ the α-H is comparatively acidic (pKa ≈ 19–20), which is the basis of aldol and related reactions.

(c) pH control of nucleophilic addition — exam favourite:

Ammonia derivatives (hydroxylamine, hydrazine, etc.) need their N lone pair to attack the carbonyl. Too much acid protonates the nitrogen (kills the nucleophile); too little acid means the carbonyl is not activated for attack. Hence an optimum weakly acidic pH is used — a classic 'why not strongly acidic?' trap.

Real-world link: The dimerisation of acids (part a) is why even small carboxylic acids like acetic acid have surprisingly high boiling points and persist as dimers in the vapour phase.

Common Mistakes

1Saying acids H-bond but alcohols do not — both do; the point is the acid's stronger, dimer-forming hydrogen bonding.
2Attributing α-H acidity only to the –I effect and forgetting the resonance (enolate) stabilisation of the carbanion, which is the main reason.
3For part (c), saying strong acid destroys the carbonyl — actually the strong acid protonates the nitrogen nucleophile, removing its lone pair.

Interesting Facts

Acetic acid exists as hydrogen-bonded cyclic dimers even in the vapour phase, so its measured vapour density corresponds to roughly twice the formula mass.

The weakly acidic α-hydrogen (pKa ≈ 20) is what makes the aldol condensation possible — a reaction used industrially to build larger carbon skeletons such as in the synthesis of pentaerythritol.

The bell-shaped rate-vs-pH curve for carbonyl + amine addition (fastest near pH 3.5–4.5) is a textbook example of an 'optimum pH', mirroring how enzyme activity peaks at a specific pH.

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Frequently Asked Questions

Why is a carboxylic acid's boiling point higher than an alcohol of similar molecular mass?

Carboxylic acids form stronger and more extensive intermolecular hydrogen bonds than alcohols. Two acid molecules associate into a stable cyclic dimer held together by two hydrogen bonds, so they behave as much heavier particles and need more energy to vaporise, giving a higher boiling point than comparable alcohols.

Why must nucleophilic addition of ammonia derivatives to carbonyls be done at controlled (not strongly acidic) pH?

The nucleophile needs the lone pair on nitrogen to attack the carbonyl carbon. In strongly acidic medium the nitrogen gets protonated and loses that lone pair, so no addition occurs. A little acid, however, protonates the carbonyl oxygen and activates it. Hence a mildly acidic optimum pH (about 3.5–4.5) gives the fastest reaction.