Question

Which amine cannot be obtained by Gabriel pthalimide amine synthesis?

Short answer

Aniline (\(C_6H_5NH_2\)) cannot be obtained via Gabriel pthalimide synthesis because it is an aromatic amine, and the reaction mechanism is limited to producing primary amines. Additionally, the high reactivity of aromatic compounds makes synthesizing aniline through a nucleophilic substitution reaction with an alkyl halide highly difficult and inefficient, as it would lead to side reactions and undesirable products.

Step-by-step solution

Understanding Gabriel pthalimide synthesis mechanism

Gabriel pthalimide amine synthesis is a process that allows the production of primary amines. The reaction begins with the formation of pthalimide salt, followed by the nucleophilic substitution reaction on an alkyl halide, and finally hydrolysis of the pthalimide derivative. The reaction mechanism has the following general form: Pthalimide + Base → Pthalimide salt Pthalimide salt + Alkyl halide → N-alkyl pthalimide N-alkyl pthalimide + (Aqueous acid or base treatment)→ Primary amine Based on the mechanism, the method is limited to producing primary amines because the initial substitution involves the nucleophilic attack of the pthalimide anion on an alkyl halide.

Identifying an amine that cannot be synthesized through Gabriel pthalimide synthesis

As this reaction produces primary amines, any amine that is not a primary amine would not be possible to synthesize through this method. Examples of amines that cannot be synthesized by Gabriel pthalimide amine synthesis include secondary amines, tertiary amines, and aromatic amines. Secondary amine: \[R-NR'_2\] Tertiary amine: \[R-NR'_3\] Aromatic amine: \[Ar-NH_2\] When selecting an amine that cannot be obtained via Gabriel pthalimide synthesis, we can pick any example from the secondary, tertiary, or aromatic amines. For this exercise, let's choose an aromatic amine as an example to demonstrate the limitation in Gabriel pthalimide synthesis.

Example amine

The example amine is aniline (Ar-NH₂): Aniline: \[C_6H_5NH_2\] Aniline cannot be obtained via Gabriel pthalimide synthesis because it is an aromatic amine, and the reaction mechanism limits us to only primary amines. Furthermore, due to the high reactivity of aromatic compounds, any attempts to synthesize aniline through a nucleophilic substitution reaction with an alkyl halide would be extremely difficult and inefficient, as it would lead to side reactions and undesirable products.

Key concepts

Primary Amines

Primary amines are a class of organic compounds where a nitrogen atom is bonded to one alkyl or aryl group and two hydrogen atoms. The general formula for primary amines is \( RNH_2 \).

• The nitrogen atom in primary amines features a lone pair of electrons, enabling it to participate in various reactions.
• They are known for their ability to act as nucleophiles due to this lone pair.
• Primary amines are typically more reactive than secondary and tertiary amines.

Gabriel pthalimide synthesis focuses on producing primary amines. The method is robust and avoids multistep processes, directly delivering primary amine derivatives, unlike other synthesis methods that may yield secondary or tertiary amines as side products. This quality makes the Gabriel synthesis particularly valuable for creating simple, linear amino compounds.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental mechanism in organic chemistry where a nucleophile replaces a leaving group in a molecule. In the context of Gabriel pthalimide synthesis, this involves the pthalimide anion acting as the nucleophile.
Upon treating pthalimide with a strong base, it forms a pthalimide salt, preparing it to attack electrophilic centers like those in alkyl halides.

• The pthalimide salt features a negatively charged nitrogen that seeks electrophiles to form a new bond.
• In Gabriel synthesis, this nucleophilic attack is directed towards an alkyl halide, replacing its leaving group and forming N-alkyl pthalimide.

This key step establishes the backbone for forming primary amines, as the substitution reaction between the alkyl halide and the pthalimide salt is crucial. Nucleophilic substitution in pthalimide synthesis is highly selective, minimizing side reactions and enhancing the yield of pure primary amines.

Aromatic Amines

Aromatic amines, unlike aliphatic amines, feature nitrogen atoms bonded to aromatic rings, such as benzene. Aniline is a prime example, with the formula \( C_6H_5NH_2 \).

• Aromatic amines are known for their stability and lower reactivity compared to aliphatic amines.
• The aromatic ring acts as a stabilizing force, delocalizing electron density and hampering typical nucleophilic reactions.

In Gabriel pthalimide synthesis, producing aromatic amines like aniline poses a significant challenge. Aromatic compounds resist nucleophilic substitution because the aromatic ring prefers to retain its stable electron configuration. Additionally, typical nucleophilic substitutions in these contexts require overly harsh conditions, leading to other undesirable side reactions, thus preventing effective synthesis using this method.

Reaction Mechanism

Understanding the reaction mechanism of Gabriel pthalimide synthesis is vital to grasp why it is specific to primary amines. This detailed mechanism consists of three main stages:

• Formation of Pthalimide Salt: This step involves deprotonating pthalimide with a strong base to form the pthalimide salt, a nucleophile.
• Nucleophilic Attack: The pthalimide salt attacks the electrophilic carbon of an alkyl halide, establishing the N-alkyl pthalimide intermediate.
• Hydrolysis: Finally, this intermediate undergoes hydrolysis, breaking the N-alkyl bond to liberate the primary amine.

The entire process is notable for its specificity towards forming primary amines due to the selective nature of the nucleophilic substitution step, combined with the simplistic hydrolysis condition that doesn't favor more complex structural transformations, as would be necessary for producing secondary or tertiary amines.