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This question came in my organic chem assignment. The options highlighted are the answers.

The end carbon(adjacent to the carbonyl group) should have more acidic alpha-hydrogen and the alkene formed is more substituted (as compared to the one in F). Still it is given as the minor product. Could someone explain why?

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  • $\begingroup$ The regioselectivity depends on the conditions that you use. See e.g. chemistry.stackexchange.com/q/132890/16683 or chemistry.stackexchange.com/q/17103/16683. The major product here is the so-called "thermodynamic enolate". $\endgroup$
    – orthocresol
    Jan 15, 2021 at 12:37
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    $\begingroup$ The original question here is somewhat misleading in that it assumes that these are the only two products. There is E-/Z- isomerism in what's labelled here the "major" product that must also be accounted for. $\endgroup$
    – Zhe
    Jan 15, 2021 at 13:44
  • $\begingroup$ @orthocresol, thanks for the links. I understand that the removal of the gamma hydrogen results in resonance (leading to a stable product). However, shouldn't the fact that E has more substituted alkene also come into play? $\endgroup$
    – benzene11
    Jan 15, 2021 at 13:55
  • $\begingroup$ @Zhe, thanks for pointing that out. $\endgroup$
    – benzene11
    Jan 15, 2021 at 14:00
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    $\begingroup$ But, in E one double bond is trisubstituted and the other disubstituted, and in F the same is true. Based on that, I don't see why the alkene substitution should affect the overall stability all that much. You would need a significant effect to overcome the preexisting bias, which favours extended conjugation (F) over cross-conjugation (E). Agreed about E/Z isomers; that's a separate issue, though. $\endgroup$
    – orthocresol
    Jan 15, 2021 at 14:07

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A major factor contributing to the stability of the enolate ion formed as the major product (F) is the stability associated with extended conjugation or extended delocalisation via resonance. Due to this, terming (F) as the major product seems justified.

If the product were to be a kinetically-controlled one (KCP), then the presence of three acidic alpha hydrogen atoms could've caused faster formation of product (E) ; it still would not have been the more stable product, but since the major product in KCP-type reactions is controlled more by the kinetics , rather than the thermodynamics, we would possibly expect to see (E) in a larger yield in that case.

Note: Strictly speaking, the problem should mention conditions that can help us to unambiguously determine which pathway (KCP vs. TCP) should be chosen. But a practical advice, garnered from the experience of working problems which seldom perfectly mention the reaction conditions, is that unless mentioned otherwise, it is always wise to first consider the thermodynamically stable product.

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