So what is the effect of a deactivaing substituent and an activating substituent already on the benzene ring, respecively, on the rate of the EAS reaction? I'm not talking about meta and o/p directors, but rather the effect on the rate of the reaction by the presence of one of these groups already on the benzene ring. I hope this makes sense. Haha.
For electrophillic aromatic substitution, the rate determining step is the electrophillic addition....
The step is (obviously) endergonic. Ok, if we draw a potential energy diagram for this rate determining step, we see that the transition state is closer in energy to the intermediate (product of this slow step)...
According to hammond's postulate, the transition state resembles the intermediate to a greater extent; more so than the starting aromatic compound.
In a nutshell, this means that factors contributing to intermediate stability also contribute to transition state stability; since according to hammond's postulate, the transition state resembles the intermediate to a greater extent.
Activators, lead to more stable intermediates, and therefore, are preceeded by more stable transition states (for the rate determining step). A more stable transition state means a lower activation energy for the slow step, and thus, the reaction is faster.
*keep in mind that the above application of hammond's postulate is for an endergonic step. For an exergonic step, the TS resembles the starting compound to a greater extent (by virtue of their PE). Thus, intermediate stability has little bearing on TS stability beause they are "not alike."*
