Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

The major product formed during hydroborationoxidation of 1 -methylcyclopentene is (a) CC1CCCC1O (b) CC1(C=O)CCCC1 (c) CC1(O)CCCC1 (d) CC1CCCC1O

Step-by-Step Solution

Answer

The major product is (c) CC1(O)CCCC1.

Step 1: Understand the Mechanism of Hydroboration-Oxidation

Hydroboration-oxidation of alkenes is a two-step reaction. In the first step, the alkene undergoes hydroboration where boron (BH3) adds to the less substituted carbon of the double bond. In the second step, this is followed by oxidation with hydrogen peroxide (H2O2) and aqueous NaOH, which replaces the boron with a hydroxyl group (-OH). Importantly, the -OH group adds to the less substituted carbon, leading to anti-Markovnikov orientation.

Step 2: Identify the Substrate: 1-Methylcyclopentene

1-Methylcyclopentene is a cyclic alkene with a methyl group attached to a cyclopentene ring. The double bond is between one of the carbons of the ring and a carbon bearing the methyl group.

Step 3: Predict the Addition in Hydroboration

In the hydroboration step, the boron atom from BH3 adds to the less substituted carbon of the double bond, which is the one within the ring, while hydrogen adds to the methyl-bearing carbon due to steric and electronic effects.

Step 4: Carry out the Oxidation

In the oxidation step, the boron is replaced by a hydroxyl group (-OH) in an anti-Markovnikov fashion. Since boron initially attached to the less substituted carbon, the hydroxyl group will now be on that carbon atom after oxidation.

Step 5: Determine the Major Product

The major product formed is an alcohol where the hydroxyl group replaces the boron atom. Since the -OH group ends up on the less substituted carbon of the original alkene, the structure of the product is cyclopentanol with a methyl group attached, correctly represented as option (c), which is the structural isomer where the hydroxyl group and the methyl group are on adjacent carbons.

Key Concepts

Anti-Markovnikov orientation

In organic chemistry, the concept of Markovnikov's rule is often applied to understand how hydrogen halides add to alkenes. However, in the case of hydroboration-oxidation, the product forms via anti-Markovnikov orientation. This unique orientation means that during the first stage of hydroboration-oxidation, boron attaches to the less substituted carbon atom.

• This is contrary to typical additions across a double bond, where more substituted carbon atoms usually gain new attachments.
• The added boron is later replaced by an -OH group during the oxidation step.

The result is a final product where the hydroxyl group is also attached to the less substituted carbon.
This reaction is pivotal in the formation of alcohols, allowing for precise control over the position of the hydroxyl group in the molecule.

1-methylcyclopentene

1-Methylcyclopentene is a specific type of cyclic alkene. The ring in this compound is made up of five carbon atoms, where one of these carbons holds a single methyl group.

• The double bond resides between the carbon with the methyl group and another carbon within the cyclopentane ring.
• This specific positioning is crucial as it affects which carbon atoms in the structure are considered more or less substituted.

The methyl group attached to the ring contributes to this compound's stereochemistry, impacting how external chemicals, like BH3 and subsequent -OH groups, add during reactions like hydroboration-oxidation. Understanding the makeup of 1-methylcyclopentene is vital for predicting the structure of potential reaction products.

Alcohol formation

Alcohols are characterized by containing one or more hydroxyl (-OH) groups attached to carbon. During the hydroboration-oxidation of alkenes, an alcohol is synthesized in a multi-step reaction process.

• Initially, boron from BH3 binds to the less substituted carbon of the alkene.
• Upon oxidation, the boron atom is exchanged with a hydroxyl group.

This anti-Markovnikov addition ensures that the -OH group attaches to the less substituted carbon.
This precise method converts alkenes such as 1-methylcyclopentene into alcohols, fulfilling roles in various synthetic pathways. Alcohols formed in this way are significant in both industrial and laboratory settings, contributing to many further transformations and applications.