Sulfur trioxide

Structure and bonding

Gaseous SO₃ is a trigonal planar molecule of D_3h symmetry, as predicted by VSEPR theory. SO₃ belongs to the D_3h point group.

In terms of electron-counting formalism, the sulfur atom has an oxidation state of +6 and a formal charge of +2. The Lewis structure consists of an S=O double bond and two S–O dative bonds without utilizing d-orbitals.

The electrical dipole moment of gaseous sulfur trioxide is zero. This is a consequence of the 120° angle between the S-O bonds.

Chemical reactions

SO₃ is the anhydride of H₂SO₄. Thus, the following reaction occurs:

The reaction occurs both rapidly and exothermically, too violently to be used in large-scale manufacturing. At or above 340 °C, sulfuric acid, sulfur trioxide, and water coexist in significant equilibrium concentrations.

Sulfur trioxide also reacts with sulfur dichloride to yield the useful reagent, thionyl chloride.

SO₃ is a strong Lewis acid readily forming crystalline complexes with pyridine, dioxane, and trimethylamine. These adducts be used as sulfonating agents.

Preparation

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. Cracking at 460 °C:

In contrast, KHSO₄ does not undergo the same reaction.

Industrially SO₃ is made by the contact process. Sulfur dioxide, which in turn 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 sulphur trioxide made in this way is converted into sulfuric acid not by the direct addition of water, with which it forms a fine mist, but by absorption in concentrated sulfuric acid and dilution with water of the produced oleum.

Structure of solid SO3

The nature of solid SO₃ is a surprisingly complex area because of structural changes caused by traces of water. Upon condensation of the gas, absolutely pure SO₃ condenses into a trimer, which is often called γ-SO₃. This molecular form is a colorless solid with a melting point of 16.8 °C. It adopts a cyclic structure described as [S(=O)₂(μ-O)]₃.

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. 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.

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".

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.

Applications

Sulfur trioxide is an essential reagent in sulfonation reactions. These processes afford detergents, dyes, and pharmaceuticals. The sulfur trioxide is generate in situ from sulfuric acid or is used as a solution in the acid.

Safety

Sulfur trioxide will cause serious burns on both inhalation and ingestion because it is highly corrosive and hygroscopic in nature. SO₃ should be handled with extreme care as it reacts with water violently and produces highly corrosive sulfuric acid.