Question

The major product formed during hydroborationoxidation of 1 -methylcyclopentene is

Short answer

The major product is 1-methylcyclopentanol.

Step-by-step solution

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.

Identify the Reactants

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

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.

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.

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.

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.