Sulfur trioxide

Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO₃. It has been described as "unquestionably the most [economically] important sulfur oxide".

Molecular structure and bonding

Monomer

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). 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. 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.

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.

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".

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. With pyridine, it gives 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, as in aromatic sulfonation. For activated substrates, Lewis base adducts of sulfur trioxide are effective sulfonating agents.

Preparation

The direct oxidation of sulfur dioxide to sulfur trioxide in air proceeds very slowly:

: 2 SO₂ + O₂ → 2 SO₃(ΔH = −198.4 kJ/mol)

Industrial

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.

The majority of sulfur trioxide made in this way is converted into sulfuric acid.

Laboratory

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

1. Dehydration at 315 °C:
2. : 2 NaHSO₄ → Na₂S₂O₇ + H₂O
3. Cracking at 460 °C:
4. : Na₂S₂O₇ → Na₂SO₄ + SO₃

The latter occurs at much lower temperatures (45–60 °C) in the presence of catalytic H₂SO₄. In contrast, KHSO₄ undergoes the same reactions at a higher temperature.

Applications

Sulfur trioxide is a reagent in sulfonation reactions. Dimethyl sulfate is produced commercially by the reaction of dimethyl ether with sulfur trioxide:

:

Sulfate esters are used as detergents, dyes, and pharmaceuticals. Sulfur trioxide is generated in situ from sulfuric acid or is used as a solution in the acid.

B₂O₃ stabilized sulfur trioxide was traded by Baker & Adamson under the tradename "Sulfan" in the 20th century<!-- at least 1960s to 1980s -->.

Sources

• NIST Standard Reference Database
• ChemSub Online