Q. 23 The major product formed in the following reaction is

Question Overview

The alkene shown undergoes the following reaction:

Reagents:

1. B₂H₆ / diglyme

2. H₂O₂ / OH⁻

You are asked to identify the major product formed.

This is a classic hydroboration–oxidation reaction, frequently asked in JEE, NEET, and GATE examinations.

Key Concept: Hydroboration–Oxidation

Hydroboration–oxidation of alkenes follows three important rules:

1. Anti-Markovnikov addition
   – OH group attaches to the less substituted carbon

2. Syn addition
   – H and OH add to the same face of the double bond

3. No carbocation rearrangement
   – Reaction proceeds via a concerted mechanism

Step-by-Step Analysis of the Given Alkene

Structure of the Alkene

The starting compound is 1-methylcyclopentene.

• One carbon of the double bond is more substituted (attached to CH₃)

• The other carbon is less substituted

Step 1: Regioselectivity (Anti-Markovnikov)

• Boron attaches to the less substituted carbon

• Hydrogen attaches to the more substituted carbon

• During oxidation, B is replaced by OH

➡️ OH ends up on the less substituted carbon

Step 2: Stereochemistry (Syn Addition)

• Both H and OH add from the same side

• Leads to a specific stereochemical alcohol

Correct Answer

✅ Option (C) is the correct answer

Why?

• OH is on the less substituted carbon (anti-Markovnikov)

• Addition is syn, matching hydroboration stereochemistry

• No rearrangement or ketone formation

Explanation of All Options

Option (A): Incorrect ❌

• Shows OH on the more substituted carbon

• This corresponds to acid-catalyzed hydration, not hydroboration

Option (B): Incorrect ❌

• Shows a ketone

• Hydroboration–oxidation never forms ketones

• Ketones are formed by oxidation of secondary alcohols, not here

Option (C): Correct ✅

• OH on less substituted carbon

• Correct syn stereochemistry

• Matches hydroboration–oxidation rules perfectly

Option (D): Incorrect ❌

• OH placement and stereochemistry are inconsistent

• Violates syn-addition requirement

Final Answer (Exam-Ready Statement)

👉 The major product formed is option (C), produced via anti-Markovnikov, syn addition during hydroboration–oxidation of 1-methylcyclopentene.