The product of the above reaction is : $CH_{3}-CH=CH-CH(CH_3)-CH_{2}-CN \xrightarrow{1. DIBAL-H, 2. H_2O}$

Question

Accepted Answer

To understand why Option 2 is the correct answer, let's analyze the reaction step by step.

### Reaction Analysis:

1. **Reagents Used:**
   - **DIBAL-H (Diisobutylaluminum hydride):** This is a selective reducing agent commonly used to reduce nitriles to aldehydes at low temperatures.
   - **H₂O (Water):** Used to quench the reaction and hydrolyze any aluminum complexes formed during the reduction.

2. **Substrate:**
   - The starting compound is $CH_{3}-CH=CH-CH(CH_3)-CH_{2}-CN$, which contains a nitrile group (-CN) at the end of the carbon chain.

3. **Reaction Mechanism:**
   - **Step 1:** DIBAL-H selectively reduces the nitrile group (-CN) to an aldehyde (-CHO) without affecting the double bond present in the molecule. This is because DIBAL-H is a mild reducing agent and, under controlled conditions, it can stop the reduction at the aldehyde stage.
   - **Step 2:** The addition of water (H₂O) helps to hydrolyze any intermediate aluminum complexes, resulting in the formation of the aldehyde.

### Justification for Option 2:

- **Product Formation:** The nitrile group (-CN) in the starting material is reduced to an aldehyde (-CHO), resulting in the product $CH_{3}-CH=CH-CH(CH_3)-CH_{2}-CHO$. The double bond and the rest of the carbon skeleton remain unchanged.
- **Option 2:** $CH_{3}-CH=CH-CH(CH_3)-CH_{2}-CHO$ correctly represents the product formed after the reduction of the nitrile group to an aldehyde.

### Explanation for Incorrect Options:

1. **Option 1: $CH_{3}-CH_{2}-CH_{2}-CH(CH_3)-CH_{2}-CN$**
   - This option suggests a complete reduction of the double bond and no change to the nitrile group, which is incorrect. DIBAL-H does not reduce alkenes under these conditions.

2. **Option 3: $CH_{3}-CH_{2}-CH_{2}-CH(CH_3)-CH_{2}-CH_{2}-CN$**
   - This option implies a complete reduction of the double bond and an extension of the carbon chain, which is not possible with DIBAL-H. The nitrile group remains unchanged, which is incorrect.

3. **Option 4: $CH_{3}-CH=CH-CH(CH_3)-CH_{2}-COOH$**
   - This option suggests the formation of a carboxylic acid, which would require a stronger oxidizing agent or hydrolysis under acidic conditions, not DIBAL-H. DIBAL-H specifically reduces nitriles to aldehydes, not carboxylic acids.

In conclusion, Option 2 is the correct answer because it accurately reflects the transformation of the nitrile group to an aldehyde while preserving the rest of the molecular structure, consistent with the action of DIBAL-H.