Identify the reactions which give aniline as the major product.
Show Hint
- Hoffmann Bromamide Degradation (\(\text{RCONH}_2 \xrightarrow{\text{X}_2/\text{OH}^-} \text{RNH}_2\)) cuts out the \(\text{C}=\text{O}\) group entirely.
- \(\text{NaBH}_4\) cannot reduce aromatic \(-\text{NO}_2\) groups; you need active metals in acid (\(\text{Fe/HCl}, \text{Sn/HCl}\)) or catalytic hydrogenation (\(\text{H}_2/\text{Pd}\)) to do that.
Concept:
Aniline (\(\text{C}_6\text{H}_5\text{NH}_2\)) is a primary aromatic amine. Synthesizing it requires a direct attachment of the amino group to the phenyl ring. We will analyze each reaction pathway systematically to see if aniline is formed as the principal chemical product.
Step 1: Evaluation of Reaction A
Reaction A involves benzonitrile (\(\text{C}_6\text{H}_5\text{CN}\)) reacted with lithium aluminum hydride (\(\text{LiAlH}_4\)), a potent reducing agent:
\[
\text{C}_6\text{H}_5-\text{C}\equiv\text{N} \xrightarrow{\text{LiAlH}_4} \text{C}_6\text{H}_5-\text{CH}_2-\text{NH}_2
\]
The nitrile group undergoes complete reduction to a primary aliphatic amine, yielding
benzylamine rather than aniline. Thus, Reaction A does not yield aniline.
Step 2: Evaluation of Reaction B
Reaction B treats benzamide (\(\text{C}_6\text{H}_5\text{CONH}_2\)) with bromine in the presence of potassium hydroxide (\(\text{KOH}, \text{Br}_2\)). This combination represents the classic
Hoffmann Bromamide Degradation Reaction:
\[
\text{C}_6\text{H}_5-\text{CONH}_2 + \text{Br}_2 + 4\text{KOH} \rightarrow \text{C}_6\text{H}_5-\text{NH}_2 + \text{K}_2\text{CO}_3 + 2\text{KBr} + 2\text{H}_2\text{O}
\]
The reaction successfully degrades the amide carbonyl, shortening the chain by one carbon atom to produce a primary amine. The major product is explicitly
aniline. Hence, Reaction B is a valid path.
Step 3: Evaluation of Reaction C
Reaction C subjects nitrobenzene (\(\text{C}_6\text{H}_5\text{NO}_2\)) to sodium borohydride (\(\text{NaBH}_4\)). \(\text{NaBH}_4\) is a selective, relatively mild reducing agent commonly targeted at carbonyl functions (aldehydes and ketones). It is chemically incapable of reducing aromatic nitro groups down to amines under ambient conditions:
\[
\text{C}_6\text{H}_5-\text{NO}_2 \xrightarrow{\text{NaBH}_4} \text{No Reaction (Nitro group remains unaffected)}
\]
Therefore, Reaction C does not produce aniline.
Step 4: Evaluation of Reaction D
Reaction D involves the acid-catalyzed hydrolysis of acetanilide (\(\text{C}_6\text{H}_5\text{NHCOCH}_3\)) using aqueous HCl and heat (\(\Delta\)):
\[
\text{C}_6\text{H}_5-\text{NH}-\text{CO}-\text{CH}_3 + \text{H}_2\text{O} \xrightarrow{\text{HCl}, \, \Delta} \text{C}_6\text{H}_5-\text{NH}_2 + \text{CH}_3\text{COOH}
\]
This clean nucleophilic acyl substitution breaks the amide bond, yielding
aniline and acetic acid. Hence, Reaction D is a valid path.
Concluding the analyses, pathways
B and
D successfully generate aniline as their primary organic product.
