Question

Alkene \(\mathrm{R}-\mathrm{CH}=\mathrm{CH}_{2}\) reacts with \(\mathrm{B}_{2} \mathrm{H}_{6}\) in the presence of \(\mathrm{H}_{2} \mathrm{O}_{2}\) to give (a) [R]C(C)=O (b) [R]OC(C)O (c) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{CHO}\) (d) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\)

Short answer

The correct product is (d) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\.\)

Step-by-step solution

Understand the Reaction Type

The reaction of an alkene with diborane (B_2H_6 d) followed by hydrogen peroxide (H_2O_2 d) is a hydroboration-oxidation reaction. This reaction typically converts an alkene into an alcohol where the OH group is added to the less substituted carbon.

Analyze the Alkene Structure

The given alkene is R-CH=CH_2 d, meaning R is some arbitrary alkyl group while one carbon has a hydrogen (the terminal carbon of the double bond). This is a terminal alkene.

Determine the Reaction Product

Upon hydroboration-oxidation, the OH group adds to the less substituted carbon in Markovnikov fashion, which in the case of a terminal alkene results in the product R-CH_2-CH_2-OH d, an alcohol.

Match the Product to Given Options

Comparing the expected product R-CH_2-CH_2-OH d with the given options, it corresponds to option (d).

Key concepts

Alkene Reactions

In the world of organic chemistry, **alkene reactions** are fundamental, serving as a basis for understanding more complex transformations. Alkenes, characterized by their carbon-carbon double bond, are reactive due to the electrophilic nature of the double bond. This makes alkenes highly receptive to reactions, especially those that involve addition across the double bond. Here, we talk about one specific type of reaction: hydroboration-oxidation.
Alkene reactions like hydroboration-oxidation involve two main steps:

• The alkene initially reacts with diborane (\(\textrm{B}_2\textrm{H}_6\)), leading to the formation of a trialkyl borane intermediate.
• This intermediate is then oxidized using hydrogen peroxide (\(\textrm{H}_2\textrm{O}_2\)) and sodium hydroxide (\(\textrm{NaOH}\)), to form an alcohol.

This two-step method is essential for converting simple alkenes into more functional alcohol structures. In our particular example, the alkene \(\textrm{R}-\textrm{CH}=\textrm{CH}_2\) reacts with diborane followed by an oxidizing agent to finally yield \(\textrm{R}-\textrm{CH}_2-\textrm{CH}_2-\textrm{OH}\) as the alcohol product.

Organic Chemistry

**Organic chemistry** is the study of carbon-based compounds and is considered one of the cornerstones of chemical science. It encompasses a vast array of substances including alkanes, alkenes, aromatics, and functionalized derivatives of such molecules. One of the key aspects of organic chemistry is understanding how these compounds interact and react with one another.
Within this domain, alkenes play a critical role due to their unique reactive double bonds. They serve as building blocks in syntheses across the chemical industry and academia. Hydroboration-oxidation is a classic reaction studied in organic chemistry courses to exemplify how these unsaturated molecules can turn into more complex structures. The hydroboration step involves the addition of borane to the alkene, while the oxidation step transforms the borane compound into an alcohol.
Hydroboration-oxidation reactions showcase several concepts fundamental to organic chemistry such as:

• Stereochemistry: The syn addition of B-H to the alkene bonds in a concerted mechanism.
• Regiochemistry: Understanding how the boron atom attaches to the less substituted carbon atom of the alkene.
• Oxidation: Transition from the borane to the alcohol through oxidative cleavage.

These aspects combined give students a comprehensive view of how functional transformations occur in organic molecules.

Markovnikov Addition

The term **Markovnikov addition** refers to the rule discovered by Vladimir Markovnikov in 1869, which predicts the regioselectivity of hydrogen halide additions to alkenes. According to this rule, during the addition of a molecule which adds hydrogen and another group (like \(\textrm{X}\)) across the double bond, the hydrogen atom bonds to the less substituted carbon atom, while the \(\textrm{X}\) group bonds to the more substituted carbon atom.
In the context of hydroboration-oxidation, however, the addition of \(\textrm{BH}_3\) follows an "anti-Markovnikov" trend, meaning that the boron atom bonds to the less substituted carbon. This unique behavior is due to the concerted mechanism of hydroboration, where both boron and hydrogen add simultaneously, prioritizing steric factors over Markovnikov's original rule.
Several features make this an "anti-Markovnikov" reaction:

• Boron's preference is dictated by sterics, reducing crowding at the more substituted carbon.
• The reaction follows a syn-addition mechanism, where hydrogen and boron add to the same side of the alkene, enhancing selectivity.

Understanding the nuances of such reaction trends helps students grasp when rules like Markovnikov Addition can have exceptions, allowing them to predict reaction outcomes accurately in organic synthesis.