Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?
Solution and Explanation
Gabriel phthalimide synthesis is used for the preparation of aliphatic primary amines. It involves nucleophilic substitution (SN2) of alkyl halides by the anion formed by the phthalimide.
But aryl halides do not undergo nucleophilic substitution with the anion formed by the phthalimide.
Hence, aromatic primary amines cannot be prepared by this process.
Top Questions on Amines
- The product B formed in the following reaction sequence is:
- Identify correct statements: (A) Primary amines do not give diazonium salts when treated with in acidic condition.
(B) Aliphatic and aromatic primary amines on heating with and ethanolic form carbylamines.
(C) Secondary and tertiary amines also give carbylamine test.
(D) Benzenesulfonyl chloride is known as Hinsberg’s reagent.
(E) Tertiary amines react with benzenesulfonyl chloride very easily.
Choose the correct answer from the options given below: Amines have a lone pair of electrons on the nitrogen atom, due to which they behave as Lewis bases. Greater the value of or smaller the value of , stronger is the base. Amines are more basic than alcohols, ethers, esters, etc. The basic character of aliphatic amines should increase with the increase of alkyl substitution. However, it does not occur in a regular manner, as a secondary aliphatic amine is unexpectedly more basic than a tertiary amine in aqueous solutions. Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron-releasing groups such as , , etc., increase the basicity, while electron-withdrawing substituents such as , , halogens, etc., decrease the basicity of amines. The effect of these substituents is more pronounced at the para-position than at the meta-position.
(a) Arrange the following in increasing order of their basic character. Give reason:
\[ \includegraphics[width=0.5\linewidth]{ch29i.png} \]- Amines have a lone pair of electrons on nitrogen atom due to which they behave as Lewis base. Greater the value of or smaller the value of , stronger is the base. Amines are more basic than alcohols, ethers, esters, etc. The basic character of aliphatic amines should increase with the increase of alkyl substitution. But it does not occur in a regular manner as a secondary aliphatic amine is unexpectedly more basic than a tertiary amine in aqueous solutions. Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron releasing groups such as , , etc., increase the basicity while electron-withdrawing substituents such as , –CN, halogens, etc., decrease the basicity of amines. The effect of these substituents is more at the p₊ than at m₋ position.
- Assertion (A): Aromatic primary amines cannot be prepared by Gabriel Phthalimide synthesis. Reason (R): Aryl halides do not undergo nucleophilic substitution reaction with the anion formed by phthalimide.
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Concepts Used:
Preparation of Amines
- Reduction of nitro compounds: Nitro compounds are reduced to amines by reduction with metals in an acidic medium or by passing them through hydrogen gas in the presence of finely divided nickel, palladium, or platinum.
- Ammonolysis of alkyl halides: Alkyl benzyl halide undergoes a nucleophilic substitution reaction while reacting with an ethanolic solution of ammonia where the halogen atom is replaced by an amino group and this process of splitting of the C-X bond by an ammonia molecule is called ammonolysis.
- Reduction of Nitriles: On reduction of nitriles with lithium or aluminum hydride or catalytic hydrogenation are used.
- Reduction of amides: On reduction, the amides with lithium aluminum hydride yield amines.
- Gabriel phthalimide synthesis: Gabriel synthesis is used to prepare primary amines. Phthalimidewhen treated with ethanolic potassium hydroxide, forms potassium salt of phthalimide which when heated with alkyl halide followed by alkaline hydrolysis leads to the formation of the corresponding primary amine.
- Hoffmann bromamide degradation reaction: In the Hoffmann bromamide degradation reaction, the transfer of an alkyl or aryl group takes place from the carbonyl carbon of the amide to the nitrogen atom. The amine so formed, therefore, contains one carbon less than that present in the initial amine.