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2006 IITJEE Previous Year Question

The following is a solution to a "match the following" question from IITJEE 2006.

I'm confused why does it show E1CB mechanism? Isn't the mechanism exclusively for highly acidic Hydrogen atom, which eventually creates a carbanion intermediate? I don't know why the following setup creates such a carbanion intermediate.

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    $\begingroup$ As well you should be confused. A Chem. Absts. search found no hits, even with the para-nitro analog. The question maybe made up to illustrate E1CB and a deuterium isotope effect. $\endgroup$
    – user55119
    Commented May 31, 2023 at 16:13

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Conditions for E1CB mechanism are as follows: -

  1. The leaving group must be bad (Fluorine, etc.)
  2. The carbanion should be resonance stabilized.

Since the leaving group in the question is Bromine (which is a good leaving group), this will not show E1CB. Apparently, the question is wrong.

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    $\begingroup$ The substrate having a good leaving group definitely doesn't prevent E1cB from happening, it just means E2 and E1 can compete. $\endgroup$
    – EpichinoM2
    Commented May 31, 2023 at 21:17
  • $\begingroup$ Your statement is self contradictory or at best all inclusive, a simultaneous E1cB, E2 and E1 mechanism. Maybe that is what is going on. $\endgroup$
    – jimchmst
    Commented Jul 3, 2023 at 16:41
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I agree the info is confusing. If the major product is deuterated it implies a rapid exchange at the benzyl position forming a carbanion that did NOT eliminate. The kinetics would be of interest. Extensive studies of these compounds generally support an E2 mechanism. In any case were the mechanism E1CB formation of the carbanion should give more immediate elimination since the deuterated compound should react much more slowly. A reference should have been cited. Unfortunately I have no access to chem abstracts to research in depth but one reference that pops up states that deuterium exchange does not take place.

If this is a fabricated question it is more than unfortunate it is professional malfeasance. One far out possibility is a paper was misread and a 1-phenylethyl bromide was reacted. This compound might have an active benzyl H that exchanges faster than the E2 reaction. It might be an interesting reaction to study.

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