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Concept
Sulfonic acid
Summary
In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is known as a sulfo group. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound (with the organic substituent replaced by hydrogen) is the parent sulfonic acid, , a tautomer of sulfurous acid, . Salts or esters of sulfonic acids are called sulfonates.
Aryl sulfonic acids are produced by the process of sulfonation. Usually the sulfonating agent is sulfur trioxide. A large scale application of this method is the production of alkylbenzenesulfonic acids:
RC6H5 + SO3 -> RC6H4SO3H
In this reaction, sulfur trioxide is an electrophile and the arene is the nucleophile. The reaction is an example of electrophilic aromatic substitution.
Alkylsulfonic acids can be prepared by many methods. In in sulfoxidation, alkanes are irradiated with a mixture of sulfur dioxide and oxygen. This reaction is employed industrially to produce alkyl sulfonic acids, which are used as surfactants.
RH + SO2 + 1/2 O2 -> RSO3H
Direct reaction of alkanes with sulfur trioxide is not generally useful, except for the conversion methanesulfonic acid to methanedisulfonic acid.
Many alkane sulfonic acids can be obtained by the addition of bisulfite to terminal alkenes. Bisulfite can also be alkylated by alkyl halides:
HSO3- + RCH=CH2 + H+ -> RCH2CH2SO3H
HSO3- + RBr -> RSO3H + Br-
Sulfonic acids can be prepared by oxidation of thiols:
RSH + 3/2 O2 -> RSO3H
This pathway is the basis of the biosynthesis of taurine.
Many sulfonic acids are prepared by hydrolysis of sulfonyl halides and related precursors. Thus, perfluorooctanesulfonic acid is prepared by hydrolysis of the sulfonyl fluoride, which in turn is generated by the electrofluorination of octanesulfonic acid. Similarly the sulfonyl chloride derived from polyethylene is hydrolyzed to the sulfonic acid.
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