The total number of amines among the following which can be synthesized by Gabriel synthesis is __________.

Solution

Chemistry ma’am · 3 steps

Gabriel needs an S_N2 on a halide: alkyl and benzyl carbons work, a ring carbon does not.

1. How Gabriel works. Potassium phthalimide attacks an alkyl halide by S_N2, and hydrolysis then frees the primary amine. So the amine forms only if its R-X precursor can undergo S_N2 substitution.

   Phth-N⁻ K⁺ + R-X →(S_N2) Phth-N-R →(hydrolysis) R-NH₂

   Amine	Halide needed	SN2 possible?
   a. (CH₃)₂CHCH₂NH₂	isobutyl halide (primary)	yes
   b. CH₃CH₂NH₂	ethyl halide (primary)	yes
   c. C₆H₅CH₂NH₂	benzyl halide	yes, and very fast
   d. C₆H₅NH₂	aryl halide	no

2. Why aniline fails. In an aryl halide the C-X bond has partial double-bond character from resonance, and the ring blocks backside attack. Phthalimide anion cannot displace the halogen, so aniline cannot be made this way.
3. So. Amines a, b and c can be made by Gabriel synthesis, giving 3.

The common slip: 4. Counting aniline as well gives 4, but aryl halides do not undergo S_N2 with phthalimide anion.

The idea to keep. Gabriel synthesis gives only primary amines, and only from halides that do S_N2. Primary alkyl and benzyl amines work; aryl amines like aniline never do.

Answer from the NTA final answer key. Solution by PrepWiser.