It is observed that rate of reaction of two isotopologues of benzene, $\ce{C6D6}$ and $\ce{C6H6}$, is the same for electrophilic aromatic reactions, as the deprotonation is not the rate limiting step. Instead the carbocation is usually limiting.

Do iodination and sulphonation somehow do show different rates with these isotopologues? Is $\ce{C6H6}$ the faster of the two, due to stronger C-D bond (smaller size of D)? If this is correct what is the reason?

Does the protonation becomes the rate limiting step? if yes, then why in only with iodination and sulphonation?

Update : Found potential energy graphs for suphonation and nitration and it does seem like protonation either becomes the rate determining step or just smaller than carbocation hill, but why in sulphonation, the deprotonation curve was high is still a mystery.

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    $\begingroup$ The slow step is electrophilic attack. Deprotonation after electrophilic attack is likely not the slow stop in any of those cases. $\endgroup$ – SendersReagent Feb 5 '16 at 20:09
  • $\begingroup$ -----Update---- $\endgroup$ – Mrigank Feb 6 '16 at 15:45

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