08-22-2021-0221 - Sulfur Trioxide

Sulfur trioxide (alternative spelling sulphur trioxide, also known as nisso sulfan) is the chemical compound with the formula SO₃. It has been described as "unquestionably the most important economically" sulfur oxide.^[1] It is prepared on an industrial scale as a precursor to sulfuric acid.

Sulfur trioxide exists in several forms - gaseous monomer, crystalline trimer, and solid polymer. Sulfur trioxide is a solid at just below room temperature with a relatively narrow liquid range. Gaseous SO₃ is the primary precursor to acid rain.^[6]

The molecule SO₃ is trigonal planar. As predicted by VSEPR theory, its structure belongs to the D_3h point group. The sulfur atom has an oxidation state of +6 and a formal charge of 0. The S-O bonding is delocalized with all three S-O bond lengthsequal at 1.42 Å.^[1] The electrical dipole moment of gaseous sulfur trioxide is zero.

Laboratory[edit]

Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate. Sodium pyrosulfate is an intermediate product:^[12]

1. Dehydration at 315 °C:

   2 NaHSO₄ → Na₂S₂O₇ + H₂O

2. Cracking at 460 °C:

   Na₂S₂O₇ → Na₂SO₄ + SO₃

In contrast, KHSO₄ does not undergo the same reaction.^[12]

It may also be prepared by dehydrating sulfuric acid with phosphorus pentoxide.^[13]

Names
Preferred IUPAC name Sulfur trioxide
Systematic IUPAC name Sulfonylideneoxidane
Other names Sulfuric anhydride, Sulfur(VI) oxide

Sulfur trioxide undergoes many reactions.^[1]

Hydration and hydrofluorination

SO₃ is the anhydride of H₂SO₄. Thus, it is susceptible to hydration:

SO₃ + H₂O → H₂SO₄ (ΔH_f = −200 kJ mol⁻¹)^[7] Gaseous sulfur trioxide fumes profusely even in a relatively dry atmosphere owing to formation of a sulfuric acid mist.

Akin to the behavior of H₂O, hydrogen fluoride adds to give fluorosulfuric acid:

SO₃ + HF → FSO₃H

Deoxygenation

SO₃ reacts with dinitrogen pentoxide to give the nitronium salt of pyrosulfate:

2 SO₃ + N₂O₅ → [NO₂]₂S₂O₇

Oxidant

Sulfur trioxide is an oxidant. It oxidizes sulfur dichloride to thionyl chloride.

SO₃ + SCl₂ → SOCl₂ + SO₂

Lewis acid

SO₃ is a strong Lewis acid readily forming adducts with Lewis bases.^[8] With pyridine, it give the sulfur trioxide pyridine complex. Related adducts form from dioxane and trimethylamine.

Sulfonating agent

Sulfur trioxide is a potent sulfonating agent, i.e. it adds SO₃ groups to substrates. Often the substrates are organic.^[9] For activated substrates, Lewis base adducts of sulfur trioxide are effective sulfonating agents.^[10]

https://en.wikipedia.org/wiki/Sulfur_trioxide