The reaction in which a nucleophile replaces an existing group (the leaving group) in a molecule is called a nucleophilic substitution reaction. Haloalkanes are the substrates in these reactions. In this type of reaction, a nucleophile reacts with a haloalkane (the substrate) having a partial positive charge on the carbon atom bonded to the halogen. A substitution reaction takes place and the halogen atom, called the leaving group, departs as a halide ion. Since the substitution reaction is initiated by a nucleophile, it is called a nucleophilic substitution reaction.
It is one of the most useful classes of organic reactions of alkyl halides in which the halogen is bonded to an sp3 hybridised carbon.
Ethyl bromide is heated with potassium phthalimide and the product is hydrolysed with alkali. Which amine is formed?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ethylamine
This is the Gabriel phthalimide synthesis of primary amines. In the terms of the passage, the phthalimide anion is the nucleophile, ethyl bromide is the substrate and bromide is the leaving group.
Phthalimide with ethanolic KOH gives potassium phthalimide:
\( \displaystyle \ce{C6H4(CO)2NH + KOH} \xrightarrow{\text{ethanol}} \ce{C6H4(CO)2NK + H2O} \)
On heating with ethyl bromide, the phthalimide anion replaces bromine and gives N-ethylphthalimide:
\( \displaystyle \ce{C6H4(CO)2NK + C2H5Br} \xrightarrow{\text{heat}} \ce{C6H4(CO)2N-C2H5 + KBr} \)
Alkaline hydrolysis then frees the primary amine, ethylamine, and leaves the phthalate salt:
\( \displaystyle \ce{C6H4(CO)2N-C2H5 + 2NaOH -> C6H4(COONa)2 + C2H5NH2} \)
Only one ethyl group can be attached to the nitrogen, because phthalimide has a single N–H, so only the primary amine forms.
Why the other options are incorrect
Option (1) Diethylamine — Incorrect. A secondary amine needs two alkyl groups on nitrogen; the phthalimide nitrogen takes up only one, so the Gabriel synthesis gives only primary amines.
Option (3) Triethylamine — Incorrect. A tertiary amine needs three alkyl groups on nitrogen; for the same reason the method cannot give it.
Option (4) Aniline — Incorrect.
The alkyl group of the amine comes from the halide, here an ethyl group.
Aniline, an aromatic primary amine, cannot be prepared by this method at all, because aryl halides do not undergo nucleophilic substitution with the phthalimide anion.
NCERT (2026-27 edition):
5. Gabriel phthalimide synthesis – “Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine.”