Question

Propose a mechanism for the sulfonation of pyridine, and point out why sulfonation occurs at 3-position.

Short answer

Any electrophilic attack, including sulfonation, is preferred at the 3-position of pyridine because the intermediate is more stable than the intermediate from attack at either 2-position or the 4-position. In three of the resonating structures formed the positive charge is not present on the nitrogen atom.

The nitrogen of pyridine is basic and in the strong acid mixture, it will be protonated as shown in the mechanism. That is the part of reason why pyridine is so sluggish to react as ring already has positive charge so attack of electrophile is slowed.

Step-by-step solution

Electrophilic substitution at pyridine:

Pyridine is a six-membered ring containing nitrogen and will undergo electrophilic substitution in which the electrophile will be more likely to attach to the third carbon to produce the more stabilized carbocation intermediate on which the positive charge never ends up on the electronegative nitrogen atom.

Mechanism of sulfonation at pyridine:

Sulfonation occurs at 3-position of pyridine and resonance structures are formed after the attack of an electrophile. Carbocation is formed as an intermediate and is stable. HSO₄ ^- acts as a base and abstracts the proton for charge neutralization and in the next step acid-base reaction occurs with between sulphuric acid and SO₃ ^-, and the resultant product is obtained. Any electrophilic attack, including sulfonation, is preferred at the 3-position of pyridine because the intermediate is more stable than the intermediate from attack at either 2-position or the 4-position.