Why can't this also give a substitution product?

[PocketRocket]

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Im going through the roadmaps of the Destroyer and I stumbled across a roadblock, pun intended...🙂

In the roadmap this is the only product that is formed but the strong nucleophile(weakened by the protic solvent) and the protic solvent can also account for an SN1 reaction. No? I just noticed the product is conjugated, is that the reason?


Thanks

 

[BYU4you]

Secondary leaving groups with a strong nucleophile do E2s that's pretty much it

 

[FeralisExtremum]

When the leaving group is secondary and the nucleophile/base is strong, the product can be a bit tricky to figure out because E2 and SN2 are both viable - but as a rough rule the protic solvent will favor the elimination (E2) and an aprotic solvent will favor a substitution (SN2).

SN1 wouldn't really be viable regardless, due to the charged/strong nucleophile. The protic solvent would stabilize it a bit but SN2 would still happen over SN1.

 

[100%permwinner]

The polar protic solvent does decrease strength of the nucleophile by hydrogen bonding, making it more difficult to attack the secondary carbon. It is reasonable to prefer E2 over SN2. The conjugation of pi bonds may also explain why E2 is preferred over SN2.
I agree with Feralis that we should rule out SN1 or E1 because the condition is basic. In general, anions form in basic conditions and cations form in acidic conditions.

 

[PocketRocket]

ok awesome sauce guys, thank you!

 

[DAT Destroyer]

Im going through the roadmaps of the Destroyer and I stumbled across a roadblock, pun intended...🙂 View attachment 190851In the roadmap this is the only product that is formed but the strong nucleophile(weakened by the protic solvent) and the protic solvent can also account for an SN1 reaction. No? I just noticed the product is conjugated, is that the reason?


Thanks

This reaction is almost exclusively by the E2 mechanism. We have a very strong base on a secondary halide and the final product is a conjugated compound. The perfect storm indeed!

Hope this helps

Dr. Romano