SO3
Sulfur trioxide (SO₃) is an inorganic chemical compound with the molecular formula SO₃, consisting of one sulfur atom bonded to three oxygen atoms, and it functions as the anhydride of sulfuric acid.[1] In its gaseous monomeric form, SO₃ exhibits a trigonal planar molecular geometry with D_{3h} symmetry, featuring three equivalent S=O double bonds and no lone pairs on the central sulfur atom.[2] In the solid state, SO₃ forms polymeric structures such as cyclic trimers (gamma form, most stable at room temperature) or chain polymers. The compound appears as a colorless to white crystalline solid that readily fumes in air due to its hygroscopic nature and violent reaction with water to produce sulfuric acid (H₂SO₄) and significant heat.[3] SO₃ exists in three main polymorphic modifications—alpha (α), beta (β), and gamma (γ)—with varying physical properties; for instance, the α-form has a melting point of approximately 62°C (144°F), while the γ-form melts at around 17°C (62°F), and the boiling point is 45°C (113°F) across forms.[4] Its molecular weight is 80.06 g/mol, and it has a vapor density of 2.76 relative to air, making it denser and prone to accumulation in low areas.[3] SO₃ is primarily produced industrially through the contact process, where sulfur dioxide (SO₂) is oxidized by oxygen in the presence of a vanadium pentoxide (V₂O₅) catalyst at elevated temperatures (around 400–450°C) and near-atmospheric pressure (1–2 atm).[1] This gaseous SO₃ is then absorbed into concentrated sulfuric acid to form oleum (H₂SO₄·SO₃), which is subsequently diluted to yield sulfuric acid, the most widely used industrial chemical.[4] Beyond sulfuric acid production, SO₃ serves as a sulfating agent in the manufacture of detergents, explosives, and dyes; it is also employed as a disinfectant, preservative, and in textile processing and battery production.[4] In organic chemistry, it acts as a reagent for sulfonation reactions, introducing sulfonic acid groups into aromatic compounds.[1] Despite its industrial significance, SO₃ is highly reactive and hazardous, classified as a strong oxidizer and corrosive substance that can cause severe burns to skin, eyes, and respiratory tissues upon contact or inhalation.[3] It reacts explosively with water and violently with organic materials, bases, and reducing agents, posing fire and explosion risks; exposure limits are set at 0.2 mg/m³ for an 8-hour time-weighted average by occupational standards.[4] In the environment, airborne SO₃ contributes to acid rain formation by converting to sulfuric acid aerosols.[1]Structure and bonding
Gaseous monomer
The isolated sulfur trioxide (SO₃) molecule in the gas phase adopts a trigonal planar geometry with D₃ₕ point group symmetry. This arrangement features three equivalent S–O bonds with a length of approximately 1.42 Å and O–S–O bond angles of 120°. These structural parameters were determined through gas-phase electron diffraction studies, confirming the planarity and equivalence of the bonds within experimental error.[5] The geometry aligns with predictions from valence shell electron pair repulsion (VSEPR) theory, classifying SO₃ as an AX₃ species. Here, the central sulfur atom is surrounded by three bonding domains and no lone pairs, resulting in minimal electron pair repulsion that favors a flat, equilateral triangular arrangement to maximize separation between the bonding pairs. SO₃ possesses 24 valence electrons (6 from sulfur and 18 from the three oxygens). In the conventional Lewis structure, sulfur exhibits an expanded octet, formally sharing 12 electrons through three double bonds, which accounts for the observed bond lengths shorter than a typical single S–O bond. The bonding involves three σ bonds formed by overlap of sp²-hybridized sulfur orbitals with oxygen p or sp² orbitals, complemented by π bonding primarily from lateral overlap of sulfur p orbitals with oxygen p orbitals (pπ–pπ interactions). Sulfur d orbitals contribute mainly as polarization functions to refine the electron density rather than forming direct covalent bonds, consistent with high-level quantum chemical analyses. A basic molecular orbital diagram for SO₃ reveals a filled σ bonding framework from the sp² hybrids, delocalized π bonding MOs from p orbitals, and non-bonding oxygen lone pairs as the HOMO, supporting the stability of the monomeric form. Due to the high symmetry of the trigonal planar structure, the individual S–O bond dipoles cancel out, yielding a net dipole moment of zero Debye.Cyclic trimer
The cyclic trimer of sulfur trioxide, known as γ-SO3, consists of discrete [S(=O)2(μ-O)]3 units, where each sulfur atom is bonded to two terminal oxygen atoms via double bonds and shares bridging oxygen atoms with adjacent sulfurs, forming a nearly planar six-membered S3O3 ring with exocyclic oxygens. This oligomeric structure arises as a polymerization product of the gaseous monomer upon condensation. The gamma phase has a melting point of 16.8 °C.[2] The crystal structure of γ-SO3 is orthorhombic, belonging to the Pnma space group, with unit cell dimensions a = 10.88 Å, b = 7.37 Å, c = 8.47 Å, and four formula units per cell (Z = 4).[6] In this arrangement, the bridging S-O bonds are longer than the terminal S=O bonds due to the lower bond order in the shared oxygens. The phase forms a colorless solid that appears ice-like. Under standard conditions, γ-SO3 is metastable and slowly converts to the stable polymeric alpha form, though the conversion is kinetically slow. Infrared spectroscopy in matrix isolation confirms the cyclic trimer through characteristic vibrational modes associated with asymmetric and symmetric S-O stretches, distinguishing it from monomeric and dimeric species.[7] The gamma form has the highest vapor pressure among the solid phases.Chain polymer
The less stable α and β phases of solid sulfur trioxide (SO3) adopt chain polymer structures consisting of infinite chains of SO4 tetrahedra linked by bridging oxygen atoms. In the α-phase, these form helical chains with each sulfur atom coordinated to two terminal oxygen atoms and two bridging oxygens, crystallizing in the orthorhombic system. The alpha phase is the thermodynamically stable polymorph with a melting point of 62.3 °C and a density of 2.60 g/cm³. However, upon heating to this temperature, it converts to the gamma form in the liquid state and rapidly vaporizes, as the boiling point of SO3 is 45 °C. The β-phase features a similar infinite chain structure but with altered packing of the chains and forms as feathery crystals upon cooling the liquid below ~32.5 °C in the presence of trace moisture; it has a melting point of approximately 32.5 °C and is metastable, converting to the alpha phase upon standing.[8][2][9] Bond lengths in these polymeric chains show bridging S–O distances of approximately 1.55 Å and terminal S=O distances of about 1.40 Å, consistent with the tetrahedral coordination around sulfur.[10] The alpha phase has the lowest vapor pressure among the solid forms. X-ray diffraction analyses confirm the infinite chain conformation and helical arrangement in the α-phase, as well as the related structural features in the β-phase.Physical properties
States of matter
Sulfur trioxide (SO₃) exhibits complex phase behavior due to its polymorphism in the solid state and the tendency to form monomeric vapor, liquid, and various crystalline forms. In the gaseous state, SO₃ exists as the monomeric molecule, with a boiling point of 44.8 °C at standard atmospheric pressure. The vapor is denser than air, with a relative density of 2.8.[9][12] The liquid state of SO₃ is colorless and fuming, with a density of 1.922 g/cm³ at 20 °C. Its viscosity is 1.3 cP at 38 °C, indicating relatively low flow resistance compared to similar polar liquids. Liquid SO₃ is miscible with concentrated sulfuric acid but reacts violently with water, rendering it effectively insoluble.[13][12][9] In the solid state, SO₃ forms three primary polymorphs, each with distinct melting points and structural characteristics derived from its trimeric and polymeric arrangements. The gamma (γ) form, consisting of cyclic trimers, melts at 16.8 °C. The beta (β) form, a fibrous polymer, melts at 32.5 °C. The alpha (α) form, a cross-linked polymer, melts at 62.3 °C and is the most stable solid phase under equilibrium conditions.[12][9]| Polymorph | Structure | Melting Point (°C) |
|---|---|---|
| γ (gamma) | Cyclic trimer | 16.8 |
| β (beta) | Fibrous polymer | 32.5 |
| α (alpha) | Cross-linked polymer | 62.3 |