Concept understanding — Gabriel Phthalimide Synthesis
Gabriel Phthalimide Synthesis
You want to make a primary amine — a molecule where an alkyl group is attached to an NH2 group. The obvious route is to react an alkyl halide with ammonia. That gives you a mixture: some primary amine, some secondary amine (two alkyl groups on the nitrogen), some tertiary amine, and even some quaternary ammonium salt. Ammonia is a nucleophile, but once it reacts, the product (the primary amine) is an even better nucleophile than ammonia was. So it keeps attacking more alkyl halide molecules, and you lose control.
The Gabriel synthesis is a clever way to stop that chain reaction. It works by hiding the nitrogen inside a molecule that cannot act as a nucleophile until you want it to.
The intuition
Imagine you have a nitrogen atom that you want to attach exactly one alkyl group to, and then release it as a primary amine. If you put that nitrogen inside a structure that is already "full" — where it has no hydrogen atoms left to be replaced — then it cannot react with more than one alkyl halide molecule. That is the core idea.
Potassium phthalimide is that structure. Phthalimide itself has the formula C6H4(CO)2NH. The nitrogen is flanked by two carbonyl groups, which pull electron density away from it. When you treat phthalimide with a base (usually alcoholic KOH), the N−H bond is deprotonated, giving the potassium salt:
C6H4(CO)2NH+KOH→C6H4(CO)2N−K++H2O
This anion is a good nucleophile. It attacks an alkyl halide (R−X) in an SN2 reaction, giving N-alkylphthalimide:
C6H4(CO)2N−K++R−X→C6H4(CO)2N−R+KX
Now look at that product. The nitrogen already has three bonds: two to the carbonyl carbons and one to the alkyl group. It has no hydrogen left. It cannot react with a second alkyl halide molecule. The secondary and tertiary amine contamination is impossible at this stage.
Watch out
This is an SN2 reaction. It works well with primary alkyl halides. Secondary alkyl halides give poor yields due to steric hindrance and elimination side reactions. Tertiary alkyl halides are useless here — they eliminate instead of substituting.
Releasing the amine
You now have the alkyl group attached to the phthalimide nitrogen. To get the free primary amine, you need to break the two N−C(=O) bonds. This is done by hydrolysis — either acidic or basic.
Alkaline hydrolysis (reflux with aqueous or alcoholic KOH):
C6H4(CO)2N−R+2KOH→C6H4(COOK)2+RNH2
The products are potassium phthalate and the primary amine. The amine is liberated as a free base and can be distilled out or extracted.
Here you get phthalic acid and the amine hydrochloride salt. You then treat the salt with a base to free the amine.
Tip
A modern alternative to hydrolysis is the Ing-Manske procedure: treat the N-alkylphthalimide with hydrazine (NH2NH2). This gives phthalhydrazide and the free amine in one step, under milder conditions. The reaction is:
C6H4(CO)2N−R+NH2NH2→C6H4(CO)2(NH)2+RNH2
The precise statement
Gabriel phthalimide synthesis is a method for preparing pure primary aliphatic amines from alkyl halides, using potassium phthalimide as the nitrogen source. The reaction proceeds in two steps: (1) SN2 alkylation of the phthalimide anion, and (2) hydrolysis (or hydrazinolysis) of the resulting N-alkylphthalimide to release the primary amine. Because the nitrogen in the intermediate has no replaceable hydrogen, it cannot undergo further alkylation, so secondary and tertiary amines are not formed.
PhthalimideKOHPotassium phthalimideR−XN-alkylphthalimideH2O/H+ or OH−RNH2+Phthalic acid
Limitations you must know
Only works well with primary alkyl halides (and to some extent with benzyl and allyl halides).
Aryl halides (like chlorobenzene) do not react — the aryl carbon is too electron-rich for SN2, and the phthalimide anion is not a strong enough nucleophile for addition-elimination.
The method fails for tertiary alkyl halides (elimination dominates) and gives poor yields for secondary ones.
It is not suitable for preparing aromatic primary amines (anilines) directly from aryl halides.
Important
Gabriel phthalimide synthesis is the standard method in organic chemistry to get a pure primary aliphatic amine, uncontaminated by higher substituted amines. If an exam problem asks you to prepare a specific primary amine from an alkyl halide without side products, this is the reaction to write.
The Gabriel phthalimide synthesis is a well-known NCERT Class 12 Amines-chapter method for preparing pure primary amines, searched as "Gabriel phthalimide synthesis mechanism class 12 chemistry" and a regular JEE Main and NEET topic.
Gabriel phthalimide synthesis needs an SN2 step with the halide, which an aryl halide like chlorobenzene cannot undergo -- so this route to aniline fails.
✓Final answer
(b) potassium salt of phthalimide treated with chlorobenzene followed by hydrolysis with aqueous NaOH solution
Step 1. Option (a), Hofmann degradation of benzamide with Br2/NaOH, DOES give aniline directly -- benzamide (C6H5-CONH2) loses its carbonyl carbon as CO2 and gives C6H5-NH2, exactly the general Hofmann degradation rule applied to the aromatic amide.
Step 2. Options (c) and (d), reduction of nitrobenzene with LiAlH4 or with Sn/HCl, both reach aniline by the standard nitro-to-amine reduction route (the book explicitly states Sn/HCl gives aniline, and LiAlH4 is one of the reducing agents this unit lists generally for nitrogen-functional-group reduction).
Step 3. Option (b), the Gabriel phthalimide route, needs potassium phthalimide's nitrogen anion to displace a halide from the alkyl/aryl halide by an SN2 mechanism. Chlorobenzene's C-Cl bond, however, has partial double-bond character from conjugation with the ring and is far too unreactive toward this kind of nucleophilic substitution.
Step 4. Because that SN2 step simply does not happen with an aryl halide, the phthalimide anion is never alkylated, no N-arylphthalimide forms, and the subsequent hydrolysis step has nothing to release -- aniline is never produced by this route, which is precisely why the unit states Gabriel synthesis cannot make aromatic primary amines.
✓Final answer
(b) potassium salt of phthalimide treated with chlorobenzene followed by hydrolysis with aqueous NaOH solution
Recall the ONE structural limitation of Gabriel synthesis stated in the text -- it needs an SN2-reactive alkyl halide, and an aryl halide like chlorobenzene never undergoes that substitution -- while every other listed method (Hofmann degradation, nitro reduction by two different reagents) genuinely reaches aniline.
Assuming any method 'involving KOH and hydrolysis' must work just because it sounds like a general amine-forming sequence -- the specific failure here is the SN2 step with an ARYL halide, not the hydrolysis step, which works perfectly well on its own.