Derivatives of Sulfur and Phosphorus Acids

A compound in which one of the hydroxy groups of sulfuric acid has been replaced by a carbon group is called a sulfonic acid. 

Sulfonic acids, like sulfuric acid, are much stronger acids than carboxylic acids. However, their chemical behavior resembles that of carboxylic acids in many other respects. Sulfonic acids form the same type of derivatives, sulfonyl chlorides, esters, amides, and so on, as do carboxylic acids. These derivatives are intercon-verted by nucleophilic substitution reactions that resemble those of carboxylic acid derivatives. 

The preparation of tosylate and other sulfonate esters for use as leaving groups in nucleophilic substitution reactions (see Section 8.9) employs the reaction of a sulfonyl chloride (an acid chloride of a sulfonic acid) with an alcohol. Another example is shown in the following equation. Note the similarity of this reaction to the reaction of an acyl chloride with an alcohol to form an ester. 

As expected, the reaction of a sulfonyl chloride with ammonia produces a sulfonamide. This reaction is used in the preparation of sulfanilamide, a sulfa drug that was one of the first antibacterial agents  

In the second step of this process, the carboxylic acid amide is hydrolyzed without cleaving the sulfonamide group, illustrating the lower reactivity of the latter. However, sulfonamides can be hydrolyzed by using more vigorous conditions, as illustrated in the following example  

---