Question

What is the major product in the reaction sequence given below?

• A. Butylbenzene
• B. 4-phenylbutan-1-ol
• C. 4-phenylbutan-1-amine
• D. 4-phenylbutanal

Hint:

Remember:
\[ \text{R-CN} \xrightarrow{\text{(1) DIBAL-H, (2) } \text{H}_2\text{O}} \text{R-CHO} \]
\[ \text{R-CHO} \xrightarrow{\text{Zn-Hg/HCl}} \text{R-CH}_3 \]
Combined, this sequence converts a nitrile group ($-\text{CN}$) into a methyl group ($-\text{CH}_3$).

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
The question asks us to determine the final major product of a multi-step reaction starting with 4-phenylbutanenitrile ($\text{Ph-CH}_2\text{-CH}_2\text{-CH}_2\text{-CN}$).
Step 2 : Key Formulas and Approach:
We will analyze the reaction sequence step-by-step:

Diisobutylaluminum hydride ($\text{DIBAL-H}$, 1 equivalent) is a selective reducing agent that reduces nitriles to imines.

Acidic hydrolysis ($\text{H}_2\text{O}$) converts the imine intermediate into an aldehyde.

Clemmensen reduction ($\text{Zn-Hg/conc. HCl}$) reduces the carbonyl group ($-CHO$) of the aldehyde to a methylene group ($-\text{CH}_3$).
Step 3 : Detailed Explanation:
Let us trace each step of the reaction:

Steps (i) and (ii): DIBAL-H reduction followed by Hydrolysis
The starting material is 4-phenylbutanenitrile:
\[ \text{Ph}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CN} \]
Treatment with one equivalent of $\text{DIBAL-H}$ reduces the nitrile group selectively to an imine intermediate:
\[ \text{Ph}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}=\text{NH} \]
Subsequent addition of water hydrolyzes this imine to yield the corresponding aldehyde, 4-phenylbutanal:
\[ \text{Ph}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CHO} \]

Step (iii): Clemmensen Reduction
The aldehyde is treated with zinc amalgam ($\text{Zn-Hg}$) and concentrated hydrochloric acid ($\text{HCl}$).
This is the classic Clemmensen reduction, which converts the carbonyl group ($C=O$) of aldehydes and ketones into a hydrocarbon group ($\text{CH}_2$):
\[ \text{Ph}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CHO} \xrightarrow{\text{Zn-Hg/conc. HCl}} \text{Ph}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \]
The resulting product is butylbenzene.

Step 4 : Final Answer:
The final major product of the reaction sequence is butylbenzene (an alkane), as shown in Option (A).