How are SN2 transition states stabilised by adjacent double bonds and carbonyl groups?

Question

It is my understanding that transition states in an S_N2 reaction have some filled and some empty orbital character, as we have the backside of the **C-X σ*** beginning to fill with electrons from the **HOMO** of the nucleophile, forming a **C-Nu σ** bond in the process. If we have an adjacent carbonyl group to the **α** carbon, I have learnt that the TS is then only stabilised by the **C-O π***, i.e. by electron withdrawal and not by the **C-O π**. However, with adjacent double bonds, I have learnt that the TS is stabilised by both the **C-C π** and the **C-C π***. I was wondering why this would be the case, as looking at the shape of the orbitals, there seems no reason that the **C-O π** could not align itself so that the partly-empty p orbital in the TS is parallel to the lobes of this MO, therefore stabilising the TS as new lower energy MOs form. Also, with regard to the best representation of these structures, I always see partial negative charges on the Leaving group and the Nucleophile in the transition state, but never the positive charge located on the **α** carbon. Surely if with adjacent carbonyl groups we only have stabilisation by electron withdrawal, it is valid to put the partial positive charge there?

Answer 1

The SN2 transition state involves the back-side attack by the nucleophile on substrate producing a trigonal bipyramidal- 5-coordinate carbon – if only for a femtosecond or two. The carbon has a partial formal positive charge- any bonding that stabilizes the partial charge and the planar configuration for the substituents of the carbon that are "not" involved as attacking and leaving..Adjacent double bonds stabilize an sp2 geometry between the substituents and central carbon....those not the leaving or attacking entities. The carbonyl group would usually be a resonance stabilization of the partial positive charge that makes this carbon interesting to the attacking nucleophile- I am not so certain that this is a transition state but rather a precursor to the nucleophile attack. The adjacent carbons being in sp2 molecular orbitals does seem to involve a transition state where the electron in this double bond finds itself in a p orbital rather than a mixed s with p molecular orbital as the transition. The pi electron is nudged by the nuleophile into an unfilled p orbital enabling the 5-coordinate carbon- this electron is no longer bound to the adjacent carbon the s character stabilizes the partial positive charge centrally around the carbon rather than in sp type lobe regions for the femotsecond period until the leaving group is pushed out by the nucleophile and the electrons relax back to the ground state molecular orbitals in the product.