Q.Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?

The key here is to understand why Gabriel phthalimide synthesis works for some amines and not others — and that comes down to the chemistry of the nucleophilic substitution step.

Gabriel phthalimide synthesis is a two-step method: first, phthalimide is treated with alcoholic KOH to form the potassium salt of phthalimide. This salt is a strong nucleophile. In the second step, it attacks an alkyl halide ($R-X$) in an $S_N2$ reaction, giving an N-alkylphthalimide. Finally, hydrolysis (or hydrazinolysis) liberates the primary amine.

The critical step is the $S_N2$ attack. For this to happen, the carbon bearing the halogen must be able to undergo backside attack — it must be sp³-hybridised and not too sterically hindered. Aryl halides (like chlorobenzene) have the halogen attached directly to an sp² carbon of the benzene ring. Such carbons do not undergo $S_N2$ reactions because the p-orbitals of the double bond block the backside approach, and the C–X bond has partial double-bond character due to resonance.

So the assertion is true: you cannot prepare aromatic primary amines (like aniline) this way. The reason is also true: Gabriel phthalimide synthesis is indeed used for aliphatic primary amines. But the reason does not explain why aromatic amines fail — it merely states what the method is used for. The actual explanation lies in the reactivity of aryl halides, not in the classification of the product.

Let’s walk through the logic step by step.

1. Understand the assertion.

   Assertion (A) says aromatic primary amines cannot be prepared by Gabriel phthalimide synthesis. This is correct. If you try to react potassium phthalimide with chlorobenzene, no reaction occurs under normal conditions. Even if you force the reaction (high temperature, catalyst), the product is not aniline — you get side products.

2. Understand the reason.

   Reason (R) says Gabriel phthalimide synthesis is used for the preparation of primary aliphatic amines. This is also correct. The classic examples are the preparation of ethylamine, benzylamine, etc. from their respective alkyl halides.

3. Check if (R) explains (A). …

Understanding Gabriel Phthalimide Synthesis

The Gabriel synthesis is a classic method for preparing primary amines. The strategy is elegant: phthalimide (with $\mathrm{p}K_\mathrm{a} \approx 9$) is deprotonated by base to form a nucleophilic anion, which then attacks an alkyl halide in an $\mathrm{S_N2}$ displacement. Hydrolysis of the resulting N-alkylphthalimide releases the primary amine.

The entire method hinges on a successful nucleophilic substitution step. That's where the limitation appears.

Why Aryl Halides Don't Cooperate

Aryl halides—compounds where a halogen is directly bonded to a benzene ring—are notoriously unreactive toward nucleophilic substitution under normal conditions. Three factors conspire against the reaction:

1. Partial double-bond character: The lone pairs on the halogen overlap with the aromatic $\pi$-system, giving the C–X bond some double-bond character. This strengthens the bond and makes it harder to break.

2. $\mathrm{S_N2}$ geometry is impossible: An $\mathrm{S_N2}$ mechanism requires backside attack at the carbon bearing the leaving group. In an aryl halide, that carbon is $sp^2$-hybridized and embedded in a planar ring—there's no accessible backside. The nucleophile would have to attack through the ring itself.

3. $\mathrm{S_N1}$ is prohibitively unfavorable: Heterolytic cleavage would produce a phenyl cation, an extraordinarily unstable species. The empty $p$-orbital would be orthogonal to the aromatic $\pi$-system, offering no stabilization. The activation energy is far too high.

Evaluating the Statements

Assertion (A): Claims aromatic primary amines can be prepared by Gabriel synthesis.

This is false. The synthesis requires an alkyl halide (or similar electrophile) that undergoes $\mathrm{S_N2}$ substitution with the phthalimide anion. Since aryl halides don't participate in this substitution, you cannot use Gabriel synthesis to make aniline or other aromatic primary amines. …