Consider the following sequence of reaction: \(\mathrm{Ph}-\mathrm{COOH} \stackrel{\mathrm{PCl}_{5}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{NH}_{3}}{\longrightarrow} \mathrm{B} \stackrel{\mathrm{POCl}_{3} / \Delta}{\longrightarrow}\) \(\mathrm{C} \stackrel{\mathrm{H}_{2} / \mathrm{Ni}}{\longrightarrow} \mathrm{D}\) The final product \(D\) is : (1) Benzonitrile (2) Benzylamine (3) Aniline (4) Benzamide

The synthesis of acid chloride is a vital step often seen in organic chemistry. To form an acid chloride, one typically starts with a carboxylic acid. In our sequence, the carboxylic acid used is benzoic acid (\text{Ph-COOH}).
When benzoic acid reacts with phosphorus pentachloride (PCl\(_5\)), it converts into benzoyl chloride (\text{Ph-COCl}).
The reaction can be summarized as follows: You can remember that PCl\(_5\) is a potent chlorinating agent often used to replace the \text{OH} group in a carboxylic acid with a \text{Cl} group, forming an acid chloride.
This reaction is useful because acid chlorides are more reactive and can be further transformed into various other functional groups.

The next important step involves converting the acid chloride into an amide. In our sequence, benzoyl chloride (\text{Ph-COCl}) reacts with ammonia (\text{NH\(_3\)}).
When ammonia reacts with benzoyl chloride, it replaces the chlorine atom, forming an amide called benzamide (\text{Ph-CONH\(_2\)}).
The reaction can be represented as: Recognizing this transformation is crucial because amides are a recurring functional group in both biological molecules and synthetic compounds.
The use of ammonia results in the formation of primary amides, which are simpler and often the target structure in basic organic syntheses.

Following the formation of benzamide, the next step in our sequence is its conversion into a nitrile.
This can be achieved through dehydration, usually by reacting the amide with phosphorus oxychloride (POCl\(_3\)) under heat (Δ).
For benzamide (\text{Ph-CONH\(_2\)}), this reaction produces benzonitrile (\text{Ph-CN}).
The reaction can be generalized as follows: This process is important because nitriles are key intermediates in many organic reactions.
They can be converted into amines, carboxylic acids, and other valuable compounds. The presence of the triple bond in nitriles can also offer unique reactivity patterns.

The final step in our sequence involves the reduction of the nitrile group to an amine.
Benzonitrile (\text{Ph-CN}) undergoes hydrogenation in the presence of a nickel (Ni) catalyst and hydrogen gas (H\(_2\)).
This reaction converts the nitrile into a primary amine, specifically benzylamine (\text{Ph-CH\(_2\)NH\(_2\)}).
The general reaction is as follows: Hydrogenation is a valuable method in organic synthesis allowing for the selective reduction of double and triple bonds. Catalysts like nickel play a crucial role in facilitating these reactions efficiently.
Hydrogenation can be applied to various functional groups, transforming them into more useful or targeted structures for further chemical manipulation.