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A reaction in which a hydrogen atom in an aromatic hydrocarbon compound such as a benzene molecule is replaced by a sulfonic acid group in a sulfuric acid molecule. Sulfonation process A reaction process that introduces a sulfonic or sulfonyl chloride group into an organic molecule. In the process of sulfonation, the hydrogen substituted on the carbon atom by the sulfonic group is called direct sulfonation; the substitution of the halogen or nitro group on the carbon atom by the sulfonic group is called indirect sulfonation. The sulfonating agent usually uses concentrated sulfuric acid or oleum as the sulfonating agent, and sometimes sulfur trioxide, chlorosulfonic acid, sulfur dioxide plus chlorine gas, sulfur dioxide plus oxygen, and sodium sulfite are used as the sulfonating agent.