Question

Which of the following steps is not present in Gabriel phthalimide synthesis?

• A. Treating phthalimide with alcoholic KOH
• B. Heating potassium phthalimide with alkyl halide
• C. Alkaline hydrolysis of N-alkylphthalimide
• D. Heating phthalic acid with NaOH

Answer 1

The correct option is D Heating phthalic acid with NaOH

The reactant involved in the Gabriel phthalimide synthesis is phthalimide which reacts with ethanolic $KOH$ to give potassium phthalimide.
Heating potassium phthalimide with alkyl halide produces N-alkyl phthalimide.
Alkaline hydrolysis of N-alkyl phthalimide produces the corresponding primary amine.
MECHANISM
Step 1: When potassium hydroxide is introduced to the phthalimide, an acid-base reaction occur. The hydroxide ion deprotonates the imide. The resulting proton is more acidic than any simple amine, generating a strong nucleophile – the potassium phthalimide​ ion.

Step 2:
The nucleophilic potassium imide ion attacks the electrophilic carbon of the alkyl halide. The nitrogen atom subsequently replaces the halogen (Fluorine, Chlorine, Bromine or Iodine) in the alkyl halide and bonds with the carbon itself. This results in the formation of an N-Alkyl Phthalimide.
Step 3:
The product N-Alkyl phthalimide is hydrolysed with aq. $NaOH$ to form primary amine, i.e. Alkylamine.
A short trick to form the product in Gabriel's phthalimide synthesis

So, (d) is correct.