SN2 question

[Ryltar]

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According to destroyer, in SN2 the sp3 carbon goes to sp2, then back to sp3. This doesn't seem to make sense to me, since there is a pentacoordinate transition state.

 

[ECUpirate19]

unless it formed a double bond in the intermediate and then it was broken in the final product. Otherwise id have to agree with you that it doesnt make much since for it to change like that

 

[NeoAkira]

According to destroyer, in SN2 the sp3 carbon goes to sp2, then back to sp3. This doesn't seem to make sense to me, since there is a pentacoordinate transition state.

The trick here I think is how one interprets the semi-bonds formed in the transition state. If you think about it, during the transition state the central carbon is neither full attached nor unattached to the leaving group and nucleophile. In effect, the bonds between the carbon and those two groups isn't completely formed, so I think destroyer simply says they don't count because they're not full bonds. If that were the case, then the central carbon would only be fully attached to 3 groups and thus technically be sp2.

Now I'm not saying I agree with them or anything, but if this is their logic then it works.

 

[Ryltar]

Thanks. I think destroyer has it drawn with the nucleophile and leaving group attached by semi bonds (abbreviated with dotted lines) to the central carbon in the transition state. That makes a bit more sense.