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Question 46

Which of the following conditions or reaction sequence will NOT give acetophenone as the major product?

Benzaldehyde $$\mathrm{(C_6H_5CHO)}$$ reacts with $$\mathrm{CH_3MgBr}$$ to form the secondary alcohol $$\mathrm{C_6H_5CH(OH)CH_3}$$, which on oxidation gives acetophenone $$\mathrm{(C_6H_5COCH_3)}$$.

Acetaldehyde $$\mathrm{(CH_3CHO)}$$ reacts with $$\mathrm{C_6H_5MgBr}$$ to form the same secondary alcohol $$\mathrm{C_6H_5CH(OH)CH_3}$$, which on oxidation again gives acetophenone $$\mathrm{(C_6H_5COCH_3)}$$.

In the reaction of ethyl benzoate $$\mathrm{(C_6H_5COOC_2H_5)}$$ with excess $$\mathrm{CH_3MgBr}$$, the first addition temporarily forms acetophenone $$\mathrm{(C_6H_5COCH_3)}$$.

However, ketones readily react with Grignard reagents. The second equivalent of $$\mathrm{CH_3MgBr}$$ attacks the ketone immediately to form a tertiary alcohol $$\mathrm{C_6H_5C(OH)(CH_3)_2}$$.

Thus, acetophenone is not obtained as the final major product.

Reaction of benzoyl chloride $$\mathrm{(C_6H_5COCl)}$$ with dialkylcadmium $$\mathrm{((CH_3)_2Cd)}$$ stops at the ketone stage because dialkylcadmium reagents are less reactive than Grignard reagents.

Hence, the sequence that does not yield acetophenone as the major product is Option $$\mathrm{(C)}$$.

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