Question

Consider the following set of reactions.
$\text{C}_6\text{H}_5\text{CN} \xrightarrow{\text{B}} \text{Y} \xrightarrow{\text{A}} \text{X} (\text{reacts with } 2,4\text{-DNP})$
$\text{C}_6\text{H}_5\text{CN} \xrightarrow{\text{A}} \text{X} (\text{dissolves in dil. } \text{HCl})$
What are A and B respectively?

• A. $\text{LiAlH}_4, \text{H}_2\text{O}; \text{H}_2/\text{Ni}$
• B. $\text{Na}/\text{Hg}, \text{C}_2\text{H}_5\text{OH}; \text{DIBAL-H}, \text{H}_2\text{O}$
• C. $\text{DIBAL-H}, \text{H}_2\text{O}; \text{LiAlH}_4, \text{H}_2\text{O}$
• D. $\text{Na}/\text{Hg}, \text{C}_2\text{H}_5\text{OH}; \text{H}_2/\text{Ni}$

Hint:

The choice of reducing agent is crucial in organic synthesis: - Partial Reduction of Nitriles ($\text{R-CN} \to \text{R-CHO}$): Use $\text{DIBAL-H}$ at low temperature, followed by hydrolysis. - Complete Reduction of Nitriles ($\text{R-CN} \to \text{R-CH}_2\text{NH}_2$): Use $\text{LiAlH}_4$ or $\text{Na}/\text{Hg}/\text{C}_2\text{H}_5\text{OH}$ or catalytic hydrogenation.

Answer 1

The Correct Option is B

To solve this problem, we need to determine the reagents A and B for the given reactions involving \(\text{C}_6\text{H}_5\text{CN}\):

1. \(\text{C}_6\text{H}_5\text{CN} \xrightarrow{\text{B}} \text{Y} \xrightarrow{\text{A}} \text{X} \, (\text{reacts with } 2,4\text{-DNP})\)
2. \(\text{C}_6\text{H}_5\text{CN} \xrightarrow{\text{A}} \text{X} \, (\text{dissolves in dil. } \text{HCl})\)

Let's evaluate the reactions step-by-step:

Step 1: Understanding the reaction conditions and product behavior

The compound \(\text{X}\) reacts with 2,4-DNP and dissolves in dilute HCl, indicating that it is likely a carbonyl compound (could be an aldehyde or ketone).

Step 2: Identify A and B

• Reagent B should convert the cyanide group \((\text{CN})\) into a primary imine, which subsequently can be reduced to an aldehyde.
• Reagent A should convert the imine to a carbonyl compound, which also dissolves in dilute HCl.

Analysis of Reagents:

1. Option 1: \(\text{LiAlH}_4, \text{H}_2\text{O}; \text{H}_2/\text{Ni}\)
   • \(\text{LiAlH}_4\) reduces nitriles directly to amines, which do not react with 2,4-DNP.
   • \(\text{H}_2/\text{Ni}\) is a catalytic hydrogenation reagent not suitable for selective reductions required here.
2. Option 2: \(\text{Na}/\text{Hg}, \text{C}_2\text{H}_5\text{OH}; \text{DIBAL-H}, \text{H}_2\text{O}\)
   • \(\text{Na}/\text{Hg}, \text{C}_2\text{H}_5\text{OH}\) selectively reduces nitriles to imines, which DIBAL-H can then reduce to aldehydes.
   • \(\text{DIBAL-H}\) reduces the nitrile to an aldehyde, which reacts with 2,4-DNP and dissolves in dilute HCl.
3. Option 3: \(\text{DIBAL-H}, \text{H}_2\text{O}; \text{LiAlH}_4, \text{H}_2\text{O}\)
   • \(\text{DIBAL-H}\) first does work, but \(\text{LiAlH}_4\) will further reduce any product to an alcohol, not suitable.
4. Option 4: \(\text{Na}/\text{Hg}, \text{C}_2\text{H}_5\text{OH}; \text{H}_2/\text{Ni}\)
   • Similar selective reduction issues as Option 1.

Conclusion:

The correct option is Option 2, where B is \(\text{Na}/\text{Hg}, \text{C}_2\text{H}_5\text{OH}\) and A is \(\text{DIBAL-H}, \text{H}_2\text{O}\).