AIIMS 2003 Chemistry Grignard Reagents Assertion Reason Question

To analyze the question regarding hydroxyketones and Grignard reagents, let's break down both the assertion and the reason provided.

Assertion: Hydroxyketones are not directly used in Grignard reaction.

Explanation:

Grignard reagents, which are organomagnesium compounds (e.g., RMgX), are highly reactive and can react with a variety of functional groups. Specifically, they are nucleophiles that can attack electrophilic centers. However, hydroxyketones contain a hydroxyl group (-OH) which is a functional group that can behave as a weak acid.

When a Grignard reagent encounters a hydroxyketone, it will preferentially react with the hydroxyl group rather than the carbonyl group. The reaction between a Grignard reagent and an alcohol (such as the hydroxyl group in hydroxyketones) leads to the formation of an alkoxide and a by-product of the corresponding hydrocarbon (alkane). This side reaction effectively renders the carbonyl carbon of the hydroxyketone inaccessible for further reactions with the Grignard reagent. Therefore, hydroxyketones cannot be directly used in Grignard reactions because they would not yield the desired products.

Reason: Grignard reagents react with hydroxyl group.

Explanation:

This reason supports the assertion. Grignard reagents indeed react with hydroxyl groups. The reaction can be represented as follows:

$$R-MgX + R'OH \rightarrow R'OMgX + RH$$

Here, $R$ is the alkyl or aryl group from the Grignard reagent, and $R'$ is the hydrocarbon chain from the hydroxyl compound (hydroxyketone in this case). This reaction results in the formation of an alkoxide (R'OMgX) and an alkane (RH), consuming the Grignard reagent in the process. Thus, the presence of a hydroxyl group in a hydroxyketone prevents the Grignard reagent from performing its intended nucleophilic addition to the carbonyl carbon, confirming that hydroxyketones cannot be used directly in Grignard reactions.

Conclusion on the Options:

• Correct Answer: A (Both the assertion and reason are true, and the reason correctly explains the assertion)
• Options B, C, D are incorrect as they would suggest that either the assertion or reason is false or that the reason does not adequately support the assertion. However, since both statements are true and the reason correctly explains why the assertion is valid, Option A is the only correct choice.

Summary:

In summary, hydroxyketones cannot be used directly in Grignard reactions due to the reactivity of Grignard reagents with hydroxyl groups. This results in the consumption of the Grignard reagent before it can react with the carbonyl carbon of the hydroxyketone. Thus, the correct answer is A, as both the assertion and the reason are true and logically connected.