Direct sulfonation of toluene with concentrated sulfuric acid gives a mixture of ortho and para sulfonic acids from which about 40% of toluene para sulfonic acid can be isolated as the sodium salt.

We shall use SO₃ as the electrophile in this case and draw the intermediate with the charge at the ipso carbon to show the stabilization from the methyl group.

No other acid is needed because chlorosulfonic acid is a very strong acid indeed and proto nates itself to give the electrophile. This explains why OH is the leaving group rather than Cl and why chlorosulfonation rather than sulfonation is the result.

In drawing the mechanism, we can again get the positive charge onto the tertiary ipso atom. No treatment with NaCl is needed in this reaction as the major product (the ortho acid chloride) is isolated by distillation.

It is fortunate that the ortho acid chloride is the major product in the chlorosulfonation because it is needed in the synthesis of saccharin, the first of the non-fattening sweeteners. The formation of the sulfonamide is like that of an ordinary amide, but the oxidation of the methyl group with potassium permanganate is probably new to you. It’s a rather vigorous reaction, but one which very usefully turns toluene derivatives into benzoic acid derivatives.

●Alkylbenzenes react with electrophiles faster than benzene and give mixtures of ortho- and para-substituted products.

مواضيع ذات صلة

Halogens show evidence of both electron withdrawal and donation

Some substituents withdraw electrons by conjugation

Electron-withdrawing substituents give meta products

Selectivity between ortho and para positions

Making aromatic amines less reactive

A nitrogen lone pair activates even more strongly

Oxygen substituents activate a benzene ring

Reaction conditions

Conjugated alkenes can be electrophilic

Alkenes conjugated with carbonyl groups

Two reasons to avoid a Friedel–Crafts alkylation

Two or more substituents may cooperate or compete