Question

Explain the kinetic and thermodynamics control in the sulphonation of naphthalene

Answer 1

Answer:

In contrast to many other electrophilic aromatic substitution reactions, aromatic sulfonation is reversible, in other words it is an equilibrium. If you use a large excess of SO3SOX3 you push the reaction to the sulfonation side; if you heat the sulfonated product in the absence of SO3SOX3 it will de-sulfonate. This makes sulfonation a nice method to 1) control where a second substituent will be introduced or 2) serve as a protecting group (block second substituent introduction) - and then when you are done, you simply de-sulfonate.

Now to your question

If we look at the intermediates leading to the 2 isomers we see that we can draw 2 resonance structures that preserve aromaticity in one ring for the transition state leading to naphthalene-1-sulfonic acid, but only one resonance structure where the other aromatic ring remains intact for the 2-isomer (draw the resonance structures to convince yourself).