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Electrophilic Substitution and the Sulfonation of Benzene

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic substitution reaction between benzene and sulfuric acid (or sulfur trioxide). If you want this mechanism explained to you in detail, there is a link at the bottom of the page.

The Electrophilic Substitution Reaction Between Benzene and Sulfuric acid

The Facts

There are two equivalent ways of sulfonating benzene:

\text{C}_6\text{H}_6 + \text{H}_2\text{SO}_4 \longrightarrow \text{C}_6\text{H}_5\text{SO}_3\text{H} + \text{H}_2\text{O}


The product is benzenesulfonic acid.

The electrophile is actually sulfur trioxide, SO3, and you may find the equation for the sulfonation reaction written:

Note: Which version of this equation you use will depend on what question you are being asked. If the question refers to the reaction with sulfuric acid, then you must use that one. If the question refers to SO3 as the electrophile, then you could use this one.

The Formation of the Electrophile

The sulfur trioxide electrophile arises in one of two ways depending on which sort of acid you are using.

Concentrated sulfuric acid contains traces of SO3 due to slight dissociation of the acid.

\text{H}_2\text{SO}_4 \xrightleftharpoons{} \text{H}_2\text{O} + \text{SO}_3

Fuming sulfuric acid, H2S2O7, can be thought of as a solution of SO3 in sulfuric acid – and so is a much richer source of the SO3.

Sulfur trioxide is an electrophile because it is a highly polar molecule with a fair amount of positive charge on the sulfur atom. It is this which is attracted to the ring electrons.

The Electrophilic Substitution Mechanism

Stage one

Stage two

The second stage of the reaction involves a transfer of the hydrogen from the ring to the negative oxygen.