[lrw1793]

When the sulfonation of benzene was carried out in 80% H2SO4, a primary
kinetic isotope effect was not observed, but when the same reaction was
performed in 100% H2SO4, there was found to be a significant primary
kinetic isotope effect. On the basis of these data, identify the rate determining
step in each case.

So the benzene ring is isotopically labelled with deuterium, and when the concentration is 80% the RDS is the attack of the benzene ring by SO3, but when the concentration is 100% it is the loss of the proton to reform the aromatic ring. But why is this the case? Many thanks!

[orgopete]

I think this is saying in 80% sulfuric acid there isn't much SO3. The rate of SO3 formation is slower than rearomatization. In 100% sulfuric acid, there is more SO3 present, so that doesn't limit the rate. The addition is presumably fast and reversible, therefore the rate limiting step is loss of a proton and rearomatization.