Alcohols react with Grignard reagent to form:

• A. Alkanes
• B. Alkenes
• C. Alkynes
• D. All of these

When alcohols react with Grignard reagents, the primary product formed is an alkane. So, the correct answer to your question is A. Alkanes. Let's delve into why this is the case and explore the underlying chemistry involved.

The Role of Grignard Reagents

Grignard reagents are organomagnesium compounds, typically represented as R-MgX, where R is an organic group and X is a halogen. They are highly reactive nucleophiles, meaning they readily donate electrons to electrophiles. This property makes them incredibly useful in organic synthesis.

Mechanism of Reaction with Alcohols

When a Grignard reagent encounters an alcohol, it reacts with the hydroxyl (-OH) group. The reaction can be summarized in a few steps:

• Nucleophilic Attack: The Grignard reagent acts as a nucleophile and attacks the hydrogen atom of the hydroxyl group in the alcohol. This leads to the formation of an alkoxide intermediate.
• Proton Transfer: The alkoxide is then protonated, typically by water or another source of protons, resulting in the formation of an alkane.

Example Reaction

For instance, if we take phenylmagnesium bromide (a Grignard reagent) and react it with ethanol, the following occurs:

• Phenylmagnesium bromide (C6H5MgBr) attacks the hydroxyl group of ethanol (C2H5OH).
• This forms phenoxide (C6H5O-) and releases a bromide ion.
• Upon protonation, the phenoxide converts back to phenyl (C6H5H), which is an alkane.

Why Not Alkenes or Alkynes?

Alkenes and alkynes are formed through different types of reactions, such as elimination reactions or dehydrohalogenation processes. The reaction between alcohols and Grignard reagents does not lead to these products because the mechanism primarily involves the nucleophilic attack and subsequent protonation, which does not favor the formation of double or triple bonds.

Summary

In summary, when alcohols react with Grignard reagents, the primary product is indeed an alkane. This reaction highlights the unique reactivity of Grignard reagents and their utility in organic synthesis. Understanding these reactions is crucial for anyone studying organic chemistry, as they form the basis for more complex transformations.