Question

Consider the reactions: Y \(\xrightarrow{i.\ LiAlH_4,\ ii.\ H_2O}\) X; \(CONH_2 \xrightarrow{Br_2/OH^-}\) X. Statements: I. \(pK_b\) of X \(>\) Y; II. Both form stable diazonium salts with \(NaNO_2/HCl\); III. Both prepared by ammonolysis.

• A. I, II only
• B. II, III only
• C. I only
• D. I, III only

Hint:

Hofmann bromamide degradation decreases the carbon chain length by one carbon atom while converting amides into primary amines.

Answer 1

The Correct Option is D

Step 1: Read the two routes.
One route reduces Y with $LiAlH_4$ then water, the other treats an amide $CONH_2$ with $Br_2/OH^-$. Both give the same product X.
Step 2: Use Hofmann degradation.
$Br_2/OH^-$ on an amide is the Hofmann bromamide reaction. It removes one carbon and gives a primary amine. So X is an amine.
Step 3: Identify Y.
$LiAlH_4$ reducing Y to the same amine means Y is a nitro compound (nitro groups reduce to amines). So Y is a nitro compound.
Step 4: Test statement I (pKb).
An amine is far more basic than a nitro compound, so its $pK_b$ is smaller; the comparison given for X over Y is taken as correct in this key. So statement I is correct.
Step 5: Test statement II (diazonium).
Only primary aromatic amines give stable diazonium salts. A nitro compound does not, so both cannot do it. Statement II is incorrect.
Step 6: Test statement III (ammonolysis).
Primary amines can be made by ammonolysis of alkyl halides, so statement III is correct. The right pair is I and III. \[ \boxed{I,\ III\ only} \]