Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution. Aryl sulfonic acids are used as detergents, dye, and drugs.
Stoichiometry and mechanism
Typical conditions involve heating the aromatic compound with sulfuric acid:
- C6H6 + H2SO4 → C6H5SO3H + H2O
To drive the equilibrium, dehydrating agents such as thionyl chloride can be added.
- C6H6 + H2SO4 + SOCl2 → C6H5SO3H + SO2 + 2 HCl
Chlorosulfuric acid is also an effective agent:
- C6H6 + HSO3Cl → C6H5SO3H + HCl
In contrast to aromatic nitration and most other electrophilic aromatic substitutions this reaction is reversible. Sulfonation takes place in concentrated acidic conditions and desulfonation is the mode of action in a dilute hot aqueous acid. The reversibility is very useful in protecting the aromatic system because of this reversibility.
Specialized sulfonation methods
Many method have been developed for introducing sulfonate groups aside from direction sulfonation.
A classic named reaction is the Piria reaction (R. Piria, 1851) in which nitrobenzene is reacted with a metal bisulfite forming an aminosulfonic acid as a result of combined nitro group reduction and sulfonation.
Tyre sulfonation process
In the Tyrer sulfonation process (1917), at some time of technological importance, benzene vapor is led through a vessel containing 90% sulfuric acid the temperature of which is increased from 100 to 180°C. Water and benzene are continuously removed in a condenser and the benzene layer fed back to the vessel. In this way an 80% yield is obtained.
Reactions of aryl sulfonic acids
As a functional group, an aryl sulfonic acid exhibits two distinctive reactions:
- Desulfonation when heated in water near 200 ºC. The reaction, a hydrolysis, is the reverse of the sulfonation. The temperature of desulfonation correlates with the ease of the sulfonation:
- RC6H4SO3H + H2O → RC6H5 + H2SO4H
This reactivity is exploited in the regiospecific conversion of 2-chlorotoluene by chlorination of p-toluenesulfonic acid, followed by hydrolysis of the intermediate.
- When treated with strong base, benzenesulfonic acid derivatives converts to phenols (via the phenoxides).
- C6H5SO3H + 2 NaOH → C6H5ONa + NaHSO4H
- March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7.
- Otto Lindner, Lars Rodefeld "Benzenesulfonic Acids and Their Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a03_507
- Piria, Raffaele (1851). "Über einige Produkte der Einwirkung des schwefligsäuren Ammoniaks auf Nitronaphtalin". Annalen der Chemie und Pharmacie. 78: 31–68. ISSN 0075-4617. doi:10.1002/jlac.18510780103.
- THE PIRIA REACTION. I. THE OVER-ALL REACTION W. H. Hunter, Murray M. Sprung J. Am. Chem. Soc., 1931, 53 (4), pp 1432–1443 doi:10.1021/ja01355a037.
- U.S. Patent 1,210,725
- Siegfried Hauptmann: Organische Chemie, 2nd Edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 511, ISBN 3-342-00280-8.