Why Aromatic Primary Amines Cannot Be Made by Gabriel Synthesis
Chemistry · Amines · NEET
Gabriel synthesis makes only aliphatic primary amines. It needs an SN2 reaction between the phthalimide anion (a nucleophile) and an alkyl halide. Aryl halides like chlorobenzene do NOT undergo SN2, so aniline (an aromatic primary amine) cannot be made this way. Memory hook: "Gabriel loves alkyl, hates aryl."
Gabriel synthesis works with alkyl halides (sp3 carbon, SN2 possible) but fails with aryl halides (sp2 carbon, no SN2), so aniline cannot be made.
Your doubts, answered
Why exactly can't aniline be made by Gabriel synthesis?
The key step is SN2 attack of the phthalimide anion on the halide. Aryl halides (like chlorobenzene) do not undergo SN2 substitution: the C-X bond is short and strong (partial double-bond character from lone-pair resonance into the ring), the sp2 carbon is hard to approach from the back, and the electron-rich benzene ring repels the incoming nucleophile. No substitution means no N-aryl phthalimide, so no aniline.
So which amines DOES Gabriel synthesis give?
Only aliphatic (and araliphatic like benzyl) PRIMARY amines. The alkyl halide's carbon is sp3 and open to back-side SN2 attack. It cannot make secondary or tertiary amines either, because only one alkyl group can attach to the phthalimide nitrogen.
Is benzylamine aromatic? Can Gabriel make it?
Benzylamine (C6H5-CH2-NH2) is NOT an aromatic amine; the -NH2 sits on an sp3 CH2 carbon, not on the ring. Its starting material benzyl chloride is a normal SN2 substrate, so Gabriel synthesis works for benzylamine. 'Aromatic amine' means NH2 directly on the ring, like aniline.
What is the difference between aryl halide and alkyl halide here?
Alkyl halide = halogen on sp3 carbon (CH3-Cl) -> good SN2 substrate -> Gabriel works. Aryl halide = halogen on sp2 ring carbon (C6H5-Cl) -> C-X has double-bond character, ring hinders back attack -> no SN2 -> Gabriel fails. This single difference is the whole answer.
How are aromatic primary amines like aniline actually prepared?
By reduction of nitrobenzene: nitrobenzene + H2/Ni (or Fe/HCl or Sn/HCl) gives aniline. This is the NCERT route for aromatic primary amines, since Gabriel and Hoffmann-bromamide routes have limits.
⚠️ The NEET trap ✗ Aniline can be prepared by Gabriel synthesis because it is a primary amine. ✓ Being a primary amine is not enough. Gabriel needs an SN2 step, and aryl halides do not do SN2, so aniline (aromatic primary amine) cannot be made by Gabriel. 🧠 NTA tests 'primary amine' vs 'aliphatic primary amine'. Gabriel = aliphatic primary amines only.
Real NEET questions
NEET 2024
Given below are two statements:
Statement I: Aniline does not undergo Friedel-Crafts alkylation reaction.
Statement II: Aniline cannot be prepared through Gabriel synthesis.
In the light of the above statements, choose the correct answer:
A · Both Statement I and Statement II are false
B · Statement I is correct but Statement II is false
C · Statement I is incorrect but Statement II is true
D · Both Statement I and Statement II are true ✓
Solution: Statement II is true: Gabriel synthesis needs an SN2 attack of phthalimide anion on the halide, but aryl halides (chlorobenzene) do not undergo nucleophilic substitution, so aniline cannot form. Statement I is also true: aniline's basic N lone pair reacts with the Lewis acid AlCl3 to form a salt, making the ring positively charged and deactivated, so Friedel-Crafts alkylation fails. Both true, answer (D).
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
State one reason aromatic primary amines cannot be prepared by Gabriel synthesis.
Aryl halides do not undergo nucleophilic (SN2) substitution with the phthalimide anion, so the N-aryl bond never forms. This is the exact NCERT reason.
Can Gabriel synthesis prepare secondary or tertiary amines?
No. Only one alkyl group can attach to the phthalimide nitrogen, so Gabriel gives only primary amines.
What reagent hydrolyses the N-alkyl phthalimide to release the amine?
Alkaline hydrolysis (aqueous KOH/NaOH) or reaction with hydrazine releases the primary amine and regenerates the phthalimide/phthalhydrazide.
Why do aryl halides not undergo SN2?
The C-X bond has partial double-bond character (lone-pair resonance into the ring), the sp2 carbon blocks back-side attack, and the electron-rich ring repels the nucleophile.
