Outline the Gabriel synthesis for preparing phenethylamine from phenethyl bromide . Include (i) the reaction of potassium phthalimide with the alkyl bromide, (ii) the hydrazine‑hydrate mediated hydrolysis, and (iii) write the overall balanced equation for the conversion of the phthalimide intermediate to phenethylamine. Also state the name of the intermediate formed after the first step.
Answer
Overall equation: ; the intermediate is N‑phenethylphthalimide.
Step-by-step solution
- Step (i): Nucleophilic substitution – the anionic nitrogen of potassium phthalimide attacks the carbon attached to the bromide, displacing and forming N‑phenethylphthalimide plus . Equation: .
- Identify the product of step (i) as N‑phenethylphthalimide (the phthalimide‑derived intermediate).
- Step (ii): Hydrazine hydrate attacks the imide carbonyls, opening the ring and liberating the primary amine. The reaction gives phenethylamine, phthalhydrazide, and water. Equation: .
- Step (iii): Combine the two steps, canceling the intermediate, to obtain the overall balanced equation:
- .
- State that the key intermediate after the first step is N‑phenethylphthalimide.
Common mistake: Confusing the nucleophilic substitution step with an SN2 mechanism involving instead of the phthalimide anion
Concept tested: Gabriel synthesis of primary amines
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