Amines are prepared by Hoffimann's method, in which ammonolysis of alkyl halide is done. Which alkyl halide cannot be used in this reaction? (1) \(3^{\circ}\) halide (2) vinyl halide (3) \(1^{\circ}\) halide (4) aryl halide (a) (1) and (2) (b) (1), (2) and (4) (c) (1) and (4) (d) (2) only

Amines are versatile compounds that play crucial roles in organic chemistry and biochemistry. The preparation of amines frequently involves the conversion of other functional groups, a commonly employed method amongst these being Hoffmann's Ammonolysis. In this process, an alkyl halide undergoes a reaction with ammonia (NH₃), leading to the displacement of the halide ion and the formation of an amine. This reaction is ideal for producing primary amines and occurs under relatively mild conditions.

During the preparation of amines via this route, it's important to choose the appropriate starting alkyl halide. Primary alkyl halides are most suitable as they have the least steric hindrance and thus, provide a more accessible reaction site for the ammonia molecule. Tertiary alkyl halides might still participate but often lead to side reactions, such as eliminations. As seen in the textbook exercise, secondary alkyl halides are also less favorable for this reaction.

Alkyl halides are important intermediates in organic synthesis because they can react in various ways to form many different types of compounds, among which amines are a prime example. An alkyl halide consists of an alkyl group attached to a halogen atom. The nature of the alkyl group and the degree of substitution (primary, secondary, tertiary, or other special cases like vinyl or aryl) determine its reactivity in different types of reactions.

Hoffmann's ammonolysis, as highlighted in the exercise, requires an alkyl halide that is prone to nucleophilic attack. Tertiary halides, with their high degree of steric bulk, are less favorable. Vinyl halides and aryl halides are particularly unreactive in such nucleophilic substitution reactions because the halogen is attached to an sp²-hybridized carbon atom, imparting partial double-bond character to the bond and thus stabilizing it against the nucleophilic attack.

Nucleophilic substitution is a fundamental class of reactions in organic chemistry where an electron-rich nucleophile targets a positive or partially positive carbon atom, displacing a leaving group. This mechanism is pivotal for creating new chemical bonds and generating complex molecules from simpler ones.

In the context of Hoffmann's ammonolysis, the ammonia molecule acts as the nucleophile, attacking the electrophilic carbon of the alkyl halide. The reactivity in this type of reaction decreases as the degree of carbon substitution increases, due to sterics -- in other words, the more crowded the carbon atom, the harder it is for the nucleophile to access and react. Hence, primary alkyl halides are generally the most reactive towards nucleophilic substitution, while vinyl and aryl halides, with their stronger C-halide bonds due to resonance and lower electrophilicity, resist such reactions. The selectivity of this process underpins the utility of Hoffmann's method in synthesizing amines selectively and efficiently.