Identify the reagent (A) used in the following reaction.
Choose the correct answer from the options given below
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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: Read the reaction goal. An alkyl halide R-X is being turned into a primary amine $\text{R-NH}_2$; we must name reagent A. Step 2: Recall how primary amines form from halides. Treating an alkyl halide with excess ammonia ($\text{NH}_3$) gives a primary amine by nucleophilic substitution - this is ammonolysis (Hofmann's method). Step 3: Trace the mechanism. The lone pair on $\text{NH}_3$ attacks the carbon bearing the halogen, displacing $\text{X}^-$: $\text{R-X} + \text{NH}_3 \rightarrow \text{R-NH}_3^+\text{X}^- \xrightarrow{-\text{HX}} \text{R-NH}_2$. Step 4: Rule out RNH$_2$. A primary amine as reagent would substitute again to give a secondary amine, not a fresh primary amine. Step 5: Rule out water and methanol. $\text{H}_2\text{O}$ would give an alcohol R-OH, and $\text{CH}_3\text{OH}$ would give an ether $\text{R-O-CH}_3$ - neither gives an amine. Step 6: Pick the reagent. Only ammonia produces the primary amine, so A is $\text{NH}_3$ - option (1). \[ \boxed{\text{Reagent A} = \text{NH}_3} \]
