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Structure and properties

Chlorosulfuric acid is a tetrahedral molecule. Its structure was debated for many decades until in 1941 Shrinivasa Dharmatti proved by magnetic susceptibility that chlorine is directly bonded to sulfur.²³

The formula is more descriptively written SO2(OH)Cl, but HSO3Cl is traditional. It is an intermediate, chemically and conceptually, between sulfuryl chloride (SO2Cl2) and sulfuric acid (H2SO4).⁴ The compound is rarely obtained pure. Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:⁵

Synthesis

The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:⁶

It can also be prepared by the method originally used by acid's discoverer Alexander William Williamson in 1854,⁷ namely chlorination of sulfuric acid, written here for pedagogical purposes as HSO3(OH) vs. the usual format H2SO4:

The latter method is more suited for laboratory-scale operations.

Williamson's discovery disproved then-popular hypothesis that sulfuric acid is a compound of water (which was incorrectly assumed to have formula of HO) and sulfur trioxide.⁸

Applications

ClSO2OH is used to prepare alkyl sulfates, which are useful as detergents and as chemical intermediates:⁹

One historical synthesis of saccharin begins with the reaction of toluene with ClSO2OH to give the ortho- and para-toluenesulfonyl chloride derivatives:

Oxidation of the ortho isomer gives the benzoic acid derivative that then is cyclized with ammonia and neutralized with base to afford saccharin.

Chlorosulfonic acid has been used as an anti-contrail agent in Ryan Model 147 reconnaissance drones,¹⁰ and to produce smoke screens.¹¹¹²

Safety

ClSO3H reacts violently with water to yield sulfuric acid and hydrogen chloride, which are corrosive:

Related halosulfuric acids

• Fluorosulfonic acid, FSO2OH, is a related strong acid with a diminished tendency to evolve hydrogen fluoride.
• Bromosulfonic acid, BrSO2OH, is unstable, decomposing at its melting point of 8 °C to give bromine, sulfur dioxide, and sulfuric acid.
• Iodosulfonic acid is not known to occur.

References

1. Cremlyn, R. J. (2002). Chlorosulfonic Acid. Royal Society of Chemistry. ISBN 978-0-85404-498-6. 978-0-85404-498-6 ↩

2. Kirk, Raymond Eller; Othmer, Donald Frederick (1964). Encyclopedia of Chemical Technology. Interscience Publishers. https://books.google.com/books?id=f8tTAAAAMAAJ&q=Dharmatti ↩

3. Dharmatti, S. S. (1941-05-01). "Magnetism and Molecular Structure of Sulphur Compounds". Proceedings of the Indian Academy of Sciences - Section A. 13 (5): 359–370. doi:10.1007/BF03049293. ISSN 0370-0089. https://www.ias.ac.in/describe/article/seca/013/05/0359-0370 ↩

4. Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. pp. 549–550. ↩

5. Bergen, Elvira; Maas, J.; Baunack, F. (2024). "Chlorosulfuric Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–7. doi:10.1002/14356007.a07_017.pub2. ISBN 978-3527306732. 978-3527306732 ↩

6. Bergen, Elvira; Maas, J.; Baunack, F. (2024). "Chlorosulfuric Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–7. doi:10.1002/14356007.a07_017.pub2. ISBN 978-3527306732. 978-3527306732 ↩

7. Kirk, Raymond Eller; Othmer, Donald Frederick (1964). Encyclopedia of Chemical Technology. Interscience Publishers. https://books.google.com/books?id=f8tTAAAAMAAJ&q=Dharmatti ↩

8. "The Quiet Revolution". https://publishing.cdlib.org/ucpressebooks/view?docId=ft5g500723&chunk.id=d0e3659&toc.depth=100&toc.id=d0e3589&brand=ucpress ↩

9. Bergen, Elvira; Maas, J.; Baunack, F. (2024). "Chlorosulfuric Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–7. doi:10.1002/14356007.a07_017.pub2. ISBN 978-3527306732. 978-3527306732 ↩

10. Method and apparatus for suppressing contrails (PDF). United States Patent and Trademark Office. 1970. https://patentimages.storage.googleapis.com/6c/7c/d2/62398f0fc22989/US3517505.pdf ↩

11. The Royal Navy at War (DVD). London: Imperial War Museum. 2005. /wiki/Imperial_War_Museum ↩

12. Amos, Jonathan (2018-04-11). "Nazi legacy found in Norwegian trees". BBC News Online. Retrieved 2018-04-17. https://www.bbc.com/news/science-environment-43727547 ↩