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Question

Find y in this question : enter image description here

The $\ce{ -CH2-Cl}$ below according to me will not take part the reaction (or please correct me) thus my answer is y=2 but actually it's y=3

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  • $\begingroup$ This is surely from an older version of MS Chauhan. I remember this being an error in the book which was corrected in the recent edition. $\endgroup$ Jun 22, 2021 at 3:08

1 Answer 1

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The third Grignard reagent molecule could extract a proton from the methylene group which is in an electron withdrawing environment. This type of enhanced acididity on carbon is referred to by the term "carbon acid" .

While this reference deals with carbonyl groups, conjugation with a phenyl ring can do the same thing to the methylene group.

We do not see any change of the methylene group in the final product because it's just deprotonated, and the acid workup brings the proton back. We would, however, see the third Grignard reagent being protonated to its parent hydrocarbon, in this case methane which bubbles off.

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    $\begingroup$ What about the third equivalent of MeMgBr coupling with the benzyl chloride to produce an ethyl group. Alternatively, ethoxide may cause SN2 displacement of chloride or, less likely, the intermediate magnesium alkoxide of the product may displace chloride and effect polymerization. $\endgroup$
    – user55119
    Mar 19, 2018 at 14:21
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    $\begingroup$ Possible, but apparently not considered by the text. Grignard reagents are known to preferably act as Bronsted-Lowry bases instead of nucleophiles at the drop of a hat, and conjugation of the methylene group with an aromatic ring drops the hat. The resulting deprotonation would kill the coupling. Here It is noted that benzyl chloride +Mg gives 98% Grignard reagent and only 2% coupling. $\endgroup$ Mar 19, 2018 at 14:49
  • $\begingroup$ If allyl MgX is prepared under standard conditions, the yield is modest owing to coupling. Must be run dilute. $\endgroup$
    – user55119
    Mar 19, 2018 at 15:24

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