Identify the reagent (A) used in the following reaction.
Choose the correct answer from the options given below
Show Hint
To introduce a simple amino group ($\text{-NH}_2$) onto an aliphatic alkyl group, your primary nucleophile must be ammonia ($\text{NH}_3$). Remember that using excess ammonia prevents further substitution stages that lead to secondary or tertiary amines.
Step 1: Understanding the Question:
The question asks us to identify the chemical reagent (A) required to carry out a nucleophilic substitution reaction on an alkyl halide to produce a primary amine ($\text{R-NH}_2$).
Step 2: Key Formula or Approach:
The reaction shows a substrate with a leaving group being converted into a primary amine structure. Alkyl halides react with excess ammonia ($\text{NH}_3$) via an $\text{S}_\text{N}2$ nucleophilic substitution mechanism to yield primary amines. This process is known as Hoffmann's ammonolysis of alkyl halides.
Step 3: Detailed Explanation:
Let's analyze the substitution process using the options provided:
1. When an alkyl halide (R-X) is treated with ammonia ($\text{NH}_3$), the lone pair on the nitrogen atom acts as a nucleophile and attacks the electrophilic carbon atom bonded to the halogen, displacing the halide ion:
$$\text{R-X} + \text{NH}_3 \rightarrow \text{R-NH}_3^+\text{X}^- \xrightarrow{-\text{HX}} \text{R-NH}_2$$
This directly matches the formation of the primary amine shown in the structural diagram.
2. If $\text{RNH}_2$ (a primary amine) were used as the reagent, the reaction would instead produce a secondary amine ($\text{R}_2\text{NH}$).
3. If $\text{H}_2\text{O}$ were used, nucleophilic substitution would yield an alcohol (R-OH).
4. If $\text{CH}_3\text{OH}$ were used, it would act as a nucleophile to generate an ether ($\text{R-O-CH}_3$).
Therefore, the starting reagent must be ammonia ($\text{NH}_3$).
Step 4: Final Answer:
The reagent (A) used in the reaction is $\text{NH}_3$, which corresponds to option (A).
