Sulfonic acid
A sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula R−S(=O)2−OH, where R is an organic alkyl or aryl group and the S(=O)2−OH group a sulfonyl hydroxide. 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 hypothetical compound sulfurous acid. Salts or esters of sulfonic acids are called sulfonates.
A sulfonic acid is produced by the process of sulfonation. Usually the sulfonating agent is sulfur trioxide. A particularly large scale application of this method is the production of alkylbenzenesulfonic acids:
In this reaction, sulfur trioxide is an electrophile and the arene undergoes electrophilic aromatic substitution.
Thiols can be oxidized to sulfonic acids:
Certain sulfonic acids, such as perfluorooctanesulfonic acid, are prepared by electrophilic fluorination of preformed sulfonic acids. The net conversion can be represented simplistically:
Sulfonic acids are much stronger acids than the corresponding carboxylic acids. p-Toluenesulfonic acid, with a pKa of −2.8, is about a million times stronger acid than benzoic acid, with a pKa of 4.2. Similarly, methanesulfonic acid, pKa = −1.9, is also about one million times stronger acid than acetic acid. Because of their polarity, sulfonic acids tend to be crystalline solids. They are also usually colourless and nonoxidizing, which is convenient. Because of their high acidity, sulfonic acids are often soluble in water or exhibit detergent-like properties.
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