CD 12.4 Reactions of Arenes: Sulfonation of Benzene

This chemical ideas chapter is a little bigger than most, hence why I've broken it down into the separate reactions. The video above (Thanks to T Holbrook, go give him a thumbs up if you like his video) shows the reaction in a little more detail than needed, but if you want a thorough understanding go and watch it.

For those that want the simplest explanation that will get you the marks in the exam, read on.

The general equation for the reaction is:

The sulfuric acid provides the electrophile for the reaction, which is, SO₃. The video above explains how that happens but for the exam we just need to know that the electrophile is SO₃ and that it comes from the sulfuric acid.

It's an electrophile because the oxygen atoms surrounding the sulfur atom are electronegative, leaving the sulfur atom with a slightly positive charge.

Hence it's attracted towards the electron dense benzene.
It's the sulfur that attacks the benzene ring and binds to the carbon. With the addition of a hydrogen ion (formed from the sulfuric acid), benzenesulfonic acid is formed.

Benzenesulfonic acid can form salts in alkaline solutions. This can be particularly helpful as the ionic groups allow the molecule to be soluble.

That's it for sulfonation anyway, it's a relatively short part of the chapter. There's not been any past exam questions on sulfonation, but that doesn't mean it's not going to come up! Just remember that it's an electrophilic substitution reaction!