Sulfuryl fluoride

Sulfuryl fluoride is the chemical compound with the formula SO₂F₂. This inorganic gas has properties more similar to sulfur hexafluoride than sulfuryl chloride, being resistant to hydrolysis even up to 150 °C. So inert is this material that suspended molten "sodium metal retains its shiny metallic appearance."^[1]

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Structure and preparation

The molecule is tetrahedral with C_2v symmetry. The S-O distance is 140.5 pm, S-F is 153.0 pm. As predicted by VSEPR, the O-S-O angle is more open than the F-S-F angle, 124° and 97°, respectively.^[1]

It is prepared by direct reaction of fluorine with sulfur dioxide:

SO₂ + F₂ → SO₂F₂

A laboratory-scale synthesis begins with the preparation of potassium fluorosulfite:^[2]

SO₂ + KF → KSO₂F

This salt is then chlorinated to give sulfuryl chloride fluoride:

KSO₂F + Cl₂ → SO₂ClF + KCl

Further heating (180 °C) of potassium fluorosulfite with the sulfuryl chloride fluoride gives the desired product:^[3]

SO₂ClF + KSO₂F → SO₂F₂ + KCl + SO₂

Heating metal fluorosulfonate salts also gives this molecule:^[1]

Ba(OSO₂F)₂ → BaSO₄ + SO₂F₂

Use as a fumigant

SO₂F₂ is of interest as a fumigant with the phase-out of methyl bromide and in view of the risks of phosphine.^[4]

Originally developed by the Dow Chemical Company, SO₂F₂ (sulfuryl fluoride) is in widespread use as a structural fumigant insecticide to control drywood termites, particularly in warm-weather portions of the southwestern and southeastern United States and in Hawaii. Less commonly, it can also be used to control rodents, powder-post beetles, bark beetles, and bedbugs.

Sulfuryl fluoride is currently marketed by two distinct manufacturers, under three different brand names. Vikane (Dow) has been commercially available since the early 1960s, with Zythor (marketed by competitor EnSystex II of North Carolina) being more recently introduced gradually as its use is approved by individual states (in Florida circa 2004, but not in California until October 2006, for example). Dow recently has begun marketing sulfuryl fluoride as a post-harvest fumigant for dry fruits, nuts, and grains under the trade name ProFume. [2]

During application, the building is enclosed in a tight tent and filled with the gas for a period of time, usually at least 16-18 hours, sometimes as long as 72 hours. The building must then be ventilated, generally for at least 6 hours, before occupants can return. Sulfuryl fluoride is colorless, odorless, and leaves no residue. During the fumigation process, a warning agent similar to tear gas is first released into the building to ensure that no occupants remain.

Some pest control experts claim sulfuryl fluoride is the only effective treatment for drywood termites. (Heat is the only other approved method for whole structure treatment for termites in California.^[5]) Because it leaves no residue, sulfuryl fluoride provides no protection from future infestations.

Safety considerations

Sulfuryl fluoride must be transported in a vehicle marked with "Inhalation Hazard 2" placards. Most U.S. states also require a license or certification for the individual applying the fumigant.

References

1. ^ ^{a b c} Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Seel, F. "Potassium Fluorosulfite" Inorganic Syntheses 1967, IX, pages 113-115.
3. ^ Seel, F. "Sulfuryl Chloride Fluoride and Sufluryl Fluoride" Inorganic Syntheses 1967, IX, pages 11-113.
4. ^ Bell, C. H. "Fumigation in the 21st century" "Crop Protection, 2000, 19, 563-569. ISSN 0261-2194. AN 2000:895590
5. ^ [1]