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Question: Consider the above reaction, the major product 'P' is:...

Consider the above reaction, the major product 'P' is:

A

CH2=CH-C(OH)(CH3)(CH2CH3)

B

CH3-CH(C2H5)-CO-CH3

C

CH2=CH-CH(OH)-CH(CH3)2

D

CH2=CH-CH2-CO-CH2CH3

Answer

CH2=CH-C(OH)(CH3)(CH2CH3)

Explanation

Solution

The reaction involves the addition of ethyl magnesium bromide (C2H5MgBrC_2H_5MgBr), a Grignard reagent, to but-3-en-2-one, followed by hydrolysis. But-3-en-2-one is an α,β\alpha,\beta-unsaturated ketone. Grignard reagents can undergo two types of addition to α,β\alpha,\beta-unsaturated carbonyl compounds: 1,2-addition (direct addition to the carbonyl carbon) and 1,4-addition (conjugate addition to the β\beta-carbon).

The structure of but-3-en-2-one is CH2=CH−CO−CH3CH_2=CH-CO-CH_3.

1. 1,2-Addition: The ethyl group from C2H5MgBrC_2H_5MgBr attacks the carbonyl carbon. CH2=CH−CO−CH3+C2H5MgBr→CH2=CH−C(OMgBr)(C2H5)−CH3CH_2=CH-CO-CH_3 + C_2H_5MgBr \rightarrow CH_2=CH-C(OMgBr)(C_2H_5)-CH_3 After hydrolysis with H2O,HClH_2O, HCl: CH2=CH−C(OMgBr)(C2H5)−CH3→H2O,HClCH2=CH−C(OH)(C2H5)−CH3CH_2=CH-C(OMgBr)(C_2H_5)-CH_3 \xrightarrow{H_2O, HCl} CH_2=CH-C(OH)(C_2H_5)-CH_3 This product is 3-ethylpent-4-en-3-ol, which matches option (1).

2. 1,4-Addition: The ethyl group from C2H5MgBrC_2H_5MgBr attacks the β\beta-carbon. CH2=CH−CO−CH3+C2H5MgBr→[CH2(C2H5)−CH=C(O−)−CH3]CH_2=CH-CO-CH_3 + C_2H_5MgBr \rightarrow [CH_2(C_2H_5)-CH=C(O^-)-CH_3] After hydrolysis: CH2(C2H5)−CH2−CO−CH3CH_2(C_2H_5)-CH_2-CO-CH_3, which is 3-ethylpentan-3-one. This does not match any of the options.

For simple alkyl Grignard reagents reacting with α,β\alpha,\beta-unsaturated ketones, 1,2-addition is generally favored over 1,4-addition, especially at lower temperatures.

Therefore, the major product 'P' is 3-ethylpent-4-en-3-ol, which corresponds to option (1).