Thiophene-3-sulfonic acid

Thiophene-2-sulfonic acid is a strong acid, similar to benzenesulfonic acid. It forms a sulfonyl chloride with phosphoryl chloride which on reduction with zinc yields thiophene-2-sulfinic acid. 

Thiophene-2-sulfonic acid, 5-acetamido-synthesis, 4, 932 Thiophene-3-sulfonic acid synthesis, 4, 932 Thiophene-2-sulfonyl chloride, synthesis, 4, 932... 

In a commercially useful reaction, sulfuric acid can be used to sulfonate thiophene at room temperature Benzene is not attacked under these conditions, and needs a reaction with oleum at 60 °C before it is sulfonated. Thiophene-2-sulfonic acid, formed in this way, dissolves in dilute alkali, allowing thiophene in coal tar distillate to be easily removed from benzene. Thiophene can be generated from its sulfonic acid by heating the latter in steam. 

The text states that electrophilic aromatic substitution in furan, thiophene, and pyrrole occurs at C-2. The sulfonation of thiophene gives thiophene-2-sulfonic acid. 

As discussed in the introduction, the production of thiophene-2-sulfonic acid by sulfuric acid sulfonation of the heterocycle has been long known use of the pyridine-sulfur-tfioxide complex is probably the best method. 2-Chlorosulfonation and 2-thiocyanation are similarly efficient. 

Aroylthiophenes have been sulfonated with cone. H2S04/acetic anhydride to give 4-aroyl-thiophene-2-sulfonic acids in good yield <89JHC1793>. Reaction of the above substrates with chloro-sulfonic acid led to intractable materials. 

Sulfonation of thiophene by sulfuric acid is very well known [144]. Thiophene-2-sulfonic acid was one of the first isolated compounds in the thiophene family... 

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