Question

Conversion of cyclohexene to cyclohexanol can be conveniently achieved by (A) \(\mathrm{NaOH}+\mathrm{H}_{2} \mathrm{O}\) (B) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (C) Hydroboration, oxidation (D) Hydroboration, Hydrolysis

Short answer

The correct reagent/condition for converting cyclohexene to cyclohexanol is \(\textbf{(C) Hydroboration, oxidation}\), as this two-step reaction facilitates the addition of an \(\ce{-OH}\) group to the alkene, forming the desired alcohol product.

Step-by-step solution

Option A: NaOH + H2O

Sodium hydroxide (NaOH) is a strong base. Coupled with water, NaOH can hydrolyze esters and amides, but not alkenes. Therefore, this option is not suitable for the conversion of cyclohexene to cyclohexanol.

Option B: Br₂ / H₂O

Bromine (Br₂) and water (H₂O) can convert alkenes to bromohydrins via a halohydrin formation reaction. However, this is not the desired product in our case, as we want to convert cyclohexene to cyclohexanol and not a bromohydrin. Therefore, this option is also not suitable.

Option C: Hydroboration, Oxidation

Hydroboration-oxidation is a two-step reaction that converts an alkene to an alcohol. The first step is hydroboration, where a borane (BH₃) adds to the alkene. Then, oxidation using hydrogen peroxide (H₂O₂) and a hydroxide base forms the alcohol as the final product. This two-step reaction is suitable for our purpose, as it produces cyclohexanol from cyclohexene.

Option D: Hydroboration, Hydrolysis

The hydroboration part of this reaction is similar to that in the hydroboration-oxidation reaction. However, the second step is hydrolysis, and this would not produce an alcohol. Instead, it would lead to the formation of a borate ester, which is not our desired product. Based on the analysis of the reagents and reactions, the correct answer is: \( \boxed{\textbf{(C) Hydroboration, oxidation}} \)

Key concepts

Hydroboration-Oxidation Reaction

The hydroboration-oxidation reaction is a well-known two-step process used in organic chemistry to transform alkenes into alcohols. This transformation is particularly appealing because it introduces the alcohol group in an anti-Markovnikov manner, which simply means it attaches to the less substituted carbon atom in the double bond.

In the first step, hydroboration, a borane compound such as diborane (BH₃) adds across the double bond of the alkene. The boron atom bonds to the less substituted carbon while the hydrogen adds to the adjacent carbon atom. This step proceeds through a concerted mechanism and occurs without the formation of a carbocation intermediate, which makes it a stereospecific syn-addition, meaning that the boron and hydrogen add to the same side of the alkene.

The second step, oxidation, typically uses hydrogen peroxide (H₂O₂) in the presence of a base such as sodium hydroxide (NaOH). The peroxide reacts with the boron compound to form a peroxide intermediate, which then rearranges to give an alcohol while regenerating the boron compound in the form of borate. Throughout these steps, the hydroboration-oxidation reaction preserves the stereochemistry of the molecule, which is crucial for producing specific enantiomers in enantioselective syntheses.

Alkene to Alcohol Conversion

Alkene to alcohol conversion is a common transformation in synthetic organic chemistry. Alcohols are versatile functional groups that are foundational to numerous biological molecules and industrial chemicals. There are several methods to convert an alkene to an alcohol, and one such preferred method is hydroboration-oxidation.

Alternative methods may include acid-catalyzed hydration, oxymercuration-demercuration, and epoxidation followed by hydrolysis. Each of these methods has specific advantages and selectivity patterns. For instance, acid-catalyzed hydration typically follows Markovnikov's rule and gives the alcohol at the more substituted carbon atom.

Furthermore, each of these reactions might require different conditions, reagents, and procedures, which may affect the overall yield and stereochemistry of the final alcohol. Understanding the intricacies of these reactions and their mechanisms allows chemists to tailor the synthesis process to yield a specific stereochemical outcome, making it crucial for creating substances with the desired biological or physical properties.

Reaction Mechanisms

Reaction mechanisms in organic chemistry are step-by-step descriptions of how a chemical reaction proceeds at the molecular level. They provide insight into the sequence of bond-making and bond-breaking events that lead to the formation of products from reactants.

A good grasp of reaction mechanisms can help students predict the outcome of chemical reactions, including stereochemistry and regiochemistry, which describe the spatial arrangement of atoms and the regions where the chemical reactions occur, respectively. It can also aid in understanding reaction kinetics, energy changes throughout the reaction, and why certain catalysts work.

For example, in the hydroboration-oxidation mechanism, understanding that the hydroboration step proceeds via syn-addition helps predict that the product will have specific stereochemistry. Similarly, knowledge about the oxidation step, involving the formation and subsequent rearrangement of a peroxide intermediate, allows one to determine why the alcohol product is obtained rather than any other functional group. Reaction mechanisms are therefore a crucial component of a chemist's toolkit, opening a window to the intricacies of chemical behavior and the logic behind the design of synthetic pathways.