# Why are alpha-carbonyl relatively less stable?

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.

• Welcome to Chemistry.SE. Would you mind to explain how a carbocation next to an electrophilic centre is stabilized by resonance? – Klaus-Dieter Warzecha Apr 8 '15 at 6:51
• 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? – user15437 Apr 8 '15 at 7:08
• A plus charge on oxygen is highly unfavourable situation even more worse than carbocation – DSinghvi Apr 8 '15 at 9:30

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.