Reaction Analysis: Electrophilic Addition to Alkenes

The given reaction involves an alkene (cyclopentene) reacting with diborane (B₂H₆) followed by oxidation with hydrogen peroxide (H₂O₂) in a basic medium (NaOH). This is a classic two-step process known as Hydroboration-Oxidation.

Step-by-Step Mechanism and Solution

Step 1: Hydroboration

Diborane (B₂H₆) dissociates to form borane (BH₃), which acts as the electrophile. The alkene undergoes syn addition across the double bond. The key feature of hydroboration is that it occurs with anti-Markovnikov regiochemistry and involves a syn addition (both new bonds form on the same face of the double bond).

For cyclopentene, the addition is not regioselective because the molecule is symmetric; both carbons of the double bond are equivalent. The product of this step is an alkylborane, trialkylborane.

Reaction: $R-\mathrm{CH}=\mathrm{CH}-R+B_2{H}_6\to (R-{\mathrm{CH}}_2-\mathrm{CH}-R{)}_{3}B$

Step 2: Oxidation

The alkylborane is oxidized by hydrogen peroxide in a basic solution (NaOH). The base deprotonates H₂O₂ to form the hydroperoxide ion (HOO^-), which attacks the boron atom. This step replaces the boron group with a hydroxyl group ($-\mathrm{OH}$), with retention of configuration.

The overall result is the addition of water (H-OH) across the double bond, but in an anti-Markovnikov fashion and with syn stereochemistry.

Reaction: $(R-{\mathrm{CH}}_2-\mathrm{CH}-R{)}_{3}B\to H_2{O}_2,\mathrm{NaOH}R-{\mathrm{CH}}_2-\mathrm{CH}-R+B(\mathrm{OH}{)}_3$

OH

Final Answer:

For a symmetric alkene like cyclopentene, the hydroboration-oxidation reaction yields the corresponding alcohol where the OH group is attached to one carbon and the H is attached to the other. Since the alkene is symmetric, the product is cyclopentanol. The correct structure is a cyclopentane ring with an -OH group attached.

This corresponds to the first option:

Related Topics

• Electrophilic Addition Reactions
• Markovnikov's Rule and Anti-Markovnikov Addition
• Alkene Reaction Mechanisms
• Stereochemistry of Addition Reactions (Syn vs Anti)
• Organoborane Chemistry

Key Formulae and Theory

Hydroboration-Oxidation Overall Reaction:

$R-\mathrm{CH}=\mathrm{CH}-R^{\prime }\to H_2{O}_2,\mathrm{NaOH}R-{\mathrm{CH}}_2-\mathrm{CH}-R^{\prime }$

B₂H₆ OH

Key Theory: The reaction proceeds via a four-membered transition state, which dictates the anti-Markovnikov addition (the boron atom bonds to the less substituted carbon) and syn stereochemistry. The oxidation step occurs with retention of configuration at the carbon.