Question

What are the reagents X and Y respectively used in the following reaction sequence to convert a nitrile to a methyl group? 

 

• A. ZnCl\(_2\); Conc. HCl : (i) SnCl\(_2\), HCl (ii) H\(_3\)O\(^+\)
• B. (i) SnCl\(_2\), HCl (ii) H\(_3\)O\(^+\) : Zn - Hg|Conc. HCl
• C. LiAlH\(_4\) : NaBH\(_4\)
• D. (i) DIBAL-H (ii) H\(_2\)O : LiAlH\(_4\)

Hint:

When selecting reagents for multiple step reactions, consider the overall pathway and the compatibility of each step with the next to ensure successful conversion without over-reduction or unwanted side reactions.

Answer 1

The Correct Option is B

Step 1: Examine the reaction conditions and requirements.

The conversion of a nitrile to a methyl group involves significant reduction, typically requiring strong reducing agents and conditions that facilitate both reduction and hydrolysis.

Step 2: Analyze the selected option.

SnCl\(_2\) in the presence of HCl (reagent X) is used initially to reduce the nitrile group partially, likely to an imine.

H\(_3\)O\(^+\) (part of reagent Y) then hydrolyzes the imine to an amine.

The final step involves the use of Zn-Hg amalgam in concentrated HCl (additional part of reagent Y), which is a classic Clemmensen reduction, reducing the amine further to a methyl group. This sequence of reactions is particularly effective for fully reducing nitrile to a primary alkyl group without over-reduction or unwanted side reactions.

Answer 2

To solve this problem, we need to identify the reagents X and Y that are used in the given reaction sequence:

1. Understanding the Reaction:
The reaction involves a compound with a nitrile group (R-C≡N) reacting with X and Y to form an alkyl group (R-CH₃). This type of reaction typically involves a reduction of the nitrile group (C≡N) to an amine or an aldehyde, and subsequent reduction or substitution to yield a methyl group.

2. Evaluating the Options:
Let's look at the reaction steps and identify the reagents:

• Option 1: ZnCl₂ and conc. HCl are typically used for reduction of nitrile to an imine or for other related transformations, but this does not match the required reaction type.
• Option 2: SnCl₂ and HCl (along with the presence of H₃O⁺) are used in reduction reactions, specifically for the reduction of nitrile groups to aldehydes or primary amines. Zn-Hg/conc. HCl is a known reagent for the reduction of nitriles to aldehydes, which corresponds well with the reaction type described in the question.
• Option 3: LiAlH₄ and NaBH4 are used for reducing carbonyl compounds (like aldehydes and ketones), but this does not match the nitrile reduction described in the question.
• Option 4: DIBAL-H and H₂O, along with LiAlH₄, are used for selective reductions of esters and nitriles to aldehydes or amines, but the reagents in this option are not appropriate for the reaction in question.

3. Conclusion:
Based on the typical reaction sequence for the reduction of nitriles to aldehydes, Option B (SnCl₂, HCl, and Zn-Hg) is the most appropriate set of reagents for this reaction.

Final Answer:
The correct answer is Option B: (i) SnCl₂, HCl (ii) H₃O⁺; (ii) Zn-Hg | Conc. HCl.