Stabilization and Electron withdrawing groups

[Aletheia]

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I read in a question that electron withdrawing groups destabilize carbocations if a carbocation is near the electron withdrawing group. I also read that withdrawing groups increase acidity (in phenol, the electrons are stabilizing by resonance and withdrawing groups weaken the OH bond and stabilize the ion). I was confused as to how this works exactly. I am wondering if someone can please explain to me how exactly electron withdrawing groups destabilize carbocations.

 

[TrainHopper413]

.if i understand the question correctly, i would say that electron withdrawing groups (EWGs) destabilize carbocations because in general EWG take electrons away from close sources. This weakens bonds. The closer an atom is to an EWG, the more strongly it feels the pull. Since, carbocations are by definition positive carbons, and carbons prefer to be neutral, when electrons are taken away (i.e. some of the negative charge that is helping to balance out the carbocation is taken away) from an already positive carbon, the carbon becomes even more positive still. This ..destabilizes its bonds as well as ..increases the likelihood that this area could react (attacked by a nucleophile).

Acidity is affected in the same manner; the electrons on a -CH3 bond for instance pulled towards an EWG, means that the C-H bond holding the H's in place are less strong. I seem to recall when taking orgo that there are charts that show the relative strength of various groups and show how they affect acidity. .

 

[PiBond]

I read in a question that electron withdrawing groups destabilize carbocations if a carbocation is near the electron withdrawing group. I also read that withdrawing groups increase acidity (in phenol, the electrons are stabilizing by resonance and withdrawing groups weaken the OH bond and stabilize the ion). I was confused as to how this works exactly. I am wondering if someone can please explain to me how exactly electron withdrawing groups destabilize carbocations.

Carbocations are electron deficient; note the positive charge on the carbon. Therefore, any group that pulls electrons away is further destabilizing it. In organic chemistry we do not like localized charges--we like to spread them out if possible. Therefore to stabilize carbocations we like to use electron donating groups like alkyl groups.

Electron withdrawing groups do increase acidity by delocalizing the negative charge once the electron is plucked off. By pulling electrons away they prevent a localized negative charge. Acidity is related to the stability of the anion.

 

[Aletheia]

Great! Thanks. 🙂