Why O and N promote SN1 but C=O inhibit SN1 although both are electron-withdrawing?

[![enter image description here](https://i.stack.imgur.com/TOPHd.png)](https://i.stack.imgur.com/TOPHd.png)


**Source:
Jonathan Clayden, Nick Greeves, Stuart Warren, Organic Chemistry 2th Edition, 339.**


The general idea I got so far, is that, carbocation will be more stable if the + charge at central C is dispersed.


How can a very electronegative atom bonded to C bearing leaving group promote SN1? That electronegative atom should intensify the + charge, making the central C more electon-hungry, making the carbocation less stable, no?


C=O inhibits SN1 because it is electron-withdrawing, but why not O and N inhibit SN1 too? They too are electron-withdrawing.




Those carbocations are termed as 'stable' as the O/N atom establishes a '2p-2p back bond' with the empty p orbital on the carbon atom carrying the positive charge. This helps the carbocation stabilize as the O/N atom shares its electron density to the starving cation. This effect is FAR more dominating than the pulling inductive effect of the O/N atom.



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> C=O inhibits SN1 because it is electron-withdrawing, but why not O and N inhibit SN1 too? >They too are electron-withdrawing.
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I am not quite sure what you are referring to, but I think you are talking about a carbonyl group on the alpha (just adjacent) carbon to the carbocation. That destabilizes the cation as the Oxygen atom cannot back-bond now with the cation. Now it pulls electrons owing to its negative inductive effect, or its electronegativity, you might say.


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