Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?
Gabriel phthalimide synthesis is used for the preparation of primary amines. In this method, phthalimide is treated with ethanolic potassium hydroxide to form the potassium salt of phthalimide, which then reacts with alkyl halides through nucleophilic substitution.
Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide. The nucleophilic substitution reaction requires the halide to be susceptible to attack by the nucleophile (phthalimide anion), but aryl halides are unreactive under these conditions.
Explanation
The textbook explicitly states that 'Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.' This is the key reason - the Gabriel synthesis relies on a nucleophilic substitution mechanism where the phthalimide anion attacks an alkyl halide. Aryl halides (aromatic halides) cannot undergo this reaction due to the nature of the C-X bond in aromatic systems, which has partial double bond character from resonance, making it resistant to nucleophilic attack.
Solution Steps
Step 1: Identify that Gabriel synthesis involves nucleophilic substitution reaction between phthalimide anion and alkyl halide.
Step 2: State that aryl halides do not undergo nucleophilic substitution with the phthalimide anion.
Step 3: Conclude that aromatic primary amines cannot be prepared by this method.