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

Question

The major product formed during hydroborationoxidation of 1 -methylcyclopentene is

Step-by-Step Solution

Answer

The major product is 1-methylcyclopentanol.

Step 1: Understand the Reaction

Hydroboration-oxidation is a two-step reaction process used to convert alkenes into alcohols. In this process, the alkene undergoes hydroboration, followed by oxidation.

Step 2: Identify the Reactants

The given starting material is 1-methylcyclopentene, which is a cyclopentene ring with a methyl group attached to the first carbon.

Step 3: Hydroboration Mechanism

In hydroboration, borane (BH3) or a borane derivative adds across the double bond in a syn-addition manner, where the boron attaches to the less hindered carbon and hydrogen attaches to the more hindered carbon of the double bond.

Step 4: Analyze Regioselectivity

The regioselectivity of hydroboration results in anti-Markovnikov addition because the boron attaches to the less substituted carbon. For 1-methylcyclopentene, the boron will add to the carbon of the double bond that is not attached to the methyl group.

Step 5: Perform Oxidation

After hydroboration, the organoborane is oxidized using hydrogen peroxide in the presence of a base (e.g., NaOH). The boron is replaced by an -OH group, maintaining the stereochemistry.

Step 6: Determine the Major Product

The major product is cyclopentanol with an additional hydroxyl group (OH) on the carbon that was less substituted in the original alkene, resulting in 1-methylcyclopentanol after oxidation.

Key Concepts

Reaction Mechanism

The hydroboration-oxidation reaction is a handy tool that transforms alkenes into alcohols through a two-step process. This sequence begins with hydroboration, where borane (BH₃) adds to the alkene. The way borane interacts with the double bond is crucial. In a synchronic fashion, both the boron and hydrogen add across the double bond. The next step is oxidation. Here, the boron atom that was added is replaced with a hydroxyl group (-OH) using hydrogen peroxide in the presence of a base, typically sodium hydroxide (NaOH). This clever sequence lends itself to creating primary alcohols due to its predictable pattern of reaction.

Regioselectivity

Regioselectivity is an important feature in hydroboration-oxidation, guiding which carbon atom in the double bond the boron and hydrogen will attach to. Generally, the boron adheres to the less hindered carbon — the less substituted end of the double bond. With 1-methylcyclopentene, regioselectivity steers the boron to the carbon not bonded to the methyl group. This choice results in the major product forming on a specific carbon, simplifying the reaction's prediction.

Anti-Markovnikov Addition

The term 'anti-Markovnikov' describes the unique regioselectivity principle seen in hydroboration-oxidation. Unlike Markovnikov's rule, where a hydrogen atom would join the less substituted carbon, producing the more stable carbocation, the boron in hydroboration attaches to the less substituted carbon. This distinction arises because boron, unlike a carbocation, is surrounded by electrons and doesn't require stabilization, thus leading to a different product distribution. Specifically, for 1-methylcyclopentene, boron joins the carbon with fewer surrounding groups.

Alkenes to Alcohols

Transforming alkenes into alcohols is the essence of the hydroboration-oxidation reaction. Through the initial step of adding borane to the alkene, a transition from an unsaturated to a fully saturated intermediate occurs. Subsequently, oxidation substitutes the boron with a hydroxyl group, converting the intermediate into an alcohol. With 1-methylcyclopentene, these steps ultimately yield 1-methylcyclopentanol, highlighting the efficiency of this transformation from alkene to alcohol.

Stereochemistry

Stereochemistry is a key aspect in the hydroboration-oxidation transformation. During the hydroboration step, boron and hydrogen add to the same side of the double bond. This is known as syn-addition, which greatly influences the stereochemistry of the resultant alcohol. When oxidation takes place, the spatial configuration around the newly formed C-OH bond remains. Therefore, for our example starting from 1-methylcyclopentene, the stereochemical outcome is predictable, maintaining the syn addition and dictating the stereochemistry of the resulting alcohol.