Unlike most other electrophilic aromatic substitutions, sulfonation is often reversible (see Section 17-4). When one sample of toluene is sulfonated at 0 °C and another sample is sulfonated at 100 °C, the following ratios of substitution products result:
| Reaction Temperature |
| Isomer of the Product 0 °C 100 °C |
| o-toluenesulfonic acid 43% 13% |
| m-toluenesulfonic acid 4% 8% |
| p-toluenesulfonic acid 53% 79% |
(a) Explain the change in the product ratios when the temperature is increased.
(b) Predict what will happen when the product mixture from the reaction at 0 °C is heated to 100 °C.
(c) Because the SO3H group can be added to a benzene ring and removed later, it is sometimes called a blocking group. Show how 2,6-dibromotoluene can be made from toluene using sulfonation and desulfonation as intermediate steps in the synthesis.
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