So if we have an alpha halogen ketone, why is an Sn1 type mechanism so unfavorable for that species?

I don't understand. It seems like the C+ would be resonance stabilized by the carbonyl's double bond.

  • $\begingroup$ Welcome to Chemistry.SE. Would you mind to explain how a carbocation next to an electrophilic centre is stabilized by resonance? $\endgroup$ Commented Apr 8, 2015 at 6:51
  • $\begingroup$ Could you push the pi electrons in the carbonyl to form an alkene from the alpha carbon to the carbonyl carbon and a + charged oxygen? $\endgroup$
    – user15437
    Commented Apr 8, 2015 at 7:08
  • 3
    $\begingroup$ A plus charge on oxygen is highly unfavourable situation even more worse than carbocation $\endgroup$
    – DSinghvi
    Commented Apr 8, 2015 at 9:30

1 Answer 1


The carbon atom of the carbonyl group has a partial positive charge due to electron withdrawal by the electronegative oxygen atom. An $S_{N}1$ mechanism would result in an intermediate with a positive charge next to a partial positive charge, which would be high in energy.


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