Sulfuryl chloride fluoride

Sulfuryl chloride fluoride (SO₂ClF) is a strong Lewis acid and a useful solvent and reagent. As shown in the hazard information table, it must be handled with care. Its boiling point is in the range that allows its use as a gas or a liquid.

In the 1930s, Harold Simmons Booth* and Carl V. Herrmann at Western Reserve University (Cleveland; later Case Western Reserve University) performed pioneering work on SO₂ClF. They synthesized it via the reaction of sulfuryl chloride (SO₂Cl₂) and antimony trifluoride (SbF₃) in the presence of antimony pentachloride (SbCl₅). They also measured several properties of SO₂ClF, including boiling point, melting point, liquid density, vapor pressure from –89 to +9 ºC, heat of vaporization, and surface tension.

But the real champion of SO₂ClF was the 1994 chemistry Nobelist George A. Olah. In 1967, also at Case Western, Olah and Joachim Lukas used the solvent system SO₂ClF–fluorosulfonic acid (FSO₃H)–antimony pentafluoride (SbF₅) to generate stable alkylcarbonium ions (now called carbocations)—from simple ones such as Me₃C⁺ and Me₂EtC⁺ through cations containing as many as nine carbon atoms. Acid mixtures like the ones they used came to be known as superacids.

In 1976, Olah, Mark R. Bruce, and John Welch developed an improved synthesis of SO₂ClF by adding SO₂Cl₂ to pyridinium poly(hydrogen fluoride) under mild conditions. By the early 1980s, now at the University of Southern California (Los Angeles), Olah and co-workers developed SO₂ClF as a versatile reagent. Some SO₂ClF applications include

• a dehydrating agent for converting aldoximes to nitriles;
• in combination with triethylamine, a reagent for making amides from carboxylic acids and primary amines and esters from carboxylic acids and alcohols;
• in combination with sulfur trioxide, an oxidant for converting tertiary phosphines and arsines to their corresponding oxides; and
• in combination with SO₂Cl₂, a reagent for converting enol silyl ethers to α-chloro ketones.

Finally, in 1992, V. Prakash Reddy, Donald R. Bellew, and G. K. Surya Prakash* at USC reported another useful synthesis of SO₂ClF, from SO₂Cl₂ and ammonium or potassium fluoride in the presence of trifluoroacetic acid. Surya Prakash was a student of Olah and is his successor as the director of USC’s Loker Hydrocarbon Research Institute, the location of much of Olah’s carbocation research.