Sulfur trioxide

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 may be assigned a formal charge value as low as 0 (if all three sulfur-oxygen bonds are assumed to be double bonds) or as high as +2 (if the Octet Rule is assumed).[7] When the formal charge is non-zero, the S-O bonding is assumed to be delocalized. In any case the three S-O bond lengths are equal to one another, at 1.42 Å.[1] The electrical dipole moment of gaseous sulfur trioxide is zero.

Ball-and-stick model of the cyclic trimer of SO₃

Both liquid and gaseous[8] SO₃ exists in an equilibrium between the monomer and the cyclic trimer. The nature of solid SO₃ is complex and at least 3 polymorphs are known, with conversion between them being dependent on traces of water.[9]

Absolutely pure SO₃ freezes at 16.8 °C to give the γ-SO₃ form, which adopts the cyclic trimer configuration [S(=O)₂(μ-O)]₃.[10][1]

An ampoule of sulfur trioxide

If SO₃ is condensed above 27 °C, then α-SO₃ forms, which has a melting point of 62.3 °C. α-SO₃ is fibrous in appearance. Structurally, it is the polymer [S(=O)₂(μ-O)]_n. Each end of the polymer is terminated with OH groups.[1] β-SO₃, like the alpha form, is fibrous but of different molecular weight, consisting of an hydroxyl-capped polymer, but melts at 32.5 °C. Both the gamma and the beta forms are metastable, eventually converting to the stable alpha form if left standing for sufficient time. This conversion is caused by traces of water.[11]

Relative vapor pressures of solid SO₃ are alpha < beta < gamma at identical temperatures, indicative of their relative molecular weights. Liquid sulfur trioxide has a vapor pressure consistent with the gamma form. Thus heating a crystal of α-SO₃ to its melting point results in a sudden increase in vapor pressure, which can be forceful enough to shatter a glass vessel in which it is heated. This effect is known as the "alpha explosion".[11]

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

SO₃ + H₂O → H₂SO₄ (ΔH_f = −200 kJ/mol)[12]

Gaseous sulfur trioxide fumes profusely even in a relatively dry atmosphere owing to formation of a sulfuric acid mist. SO₃ is aggressively hygroscopic. The heat of hydration is sufficient that mixtures of SO₃ and wood or cotton can ignite. In such cases, SO₃ dehydrates these carbohydrates.[11]

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

SO₃ + HF → FSO₃H

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

2 SO₃ + N₂O₅ → [NO₂]₂S₂O₇

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

SO₃ + SCl₂ → SOCl₂ + SO₂

SO₃ is a strong Lewis acid readily forming adducts with Lewis bases.[13] With pyridine, it give the sulfur trioxide pyridine complex. Related adducts form from dioxane and trimethylamine.

Sulfur trioxide is a potent sulfonating agent, i.e. it adds SO₃ groups to substrates. Often the substrates are organic, as in aromatic sulfonation.[14] For activated substrates, Lewis base adducts of sulfur trioxide are effective sulfonating agents.[15]

Industrially SO₃ is made by the contact process. Sulfur dioxide is produced by the burning of sulfur or iron pyrite (a sulfide ore of iron). After being purified by electrostatic precipitation, the SO₂ is then oxidised by atmospheric oxygen at between 400 and 600 °C over a catalyst. A typical catalyst consists of vanadium pentoxide (V₂O₅) activated with potassium oxide K₂O on kieselguhr or silica support. Platinum also works very well but is too expensive and is poisoned (rendered ineffective) much more easily by impurities.[16] The majority of sulfur trioxide made in this way is converted into sulfuric acid.

Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate. Sodium pyrosulfate is an intermediate product:[17]

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.[17]

SO₃ may also be prepared by dehydrating sulfuric acid with phosphorus pentoxide.[18]