Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO
3. It has been described as "unquestionably the most important economically" sulfur oxide.
It is prepared on an industrial scale as a
precursor
Precursor or Precursors may refer to:
* Precursor (religion), a forerunner, predecessor
** The Precursor, John the Baptist
Science and technology
* Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
to
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
.
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 SO
3 is the primary precursor to
acid rain.
Molecular structure and bonding
Monomer
The molecule SO
3 is
trigonal planar. As predicted by
VSEPR theory, its structure belongs to the D
3h point group. The sulfur atom has an
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
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.
The electrical
dipole moment of gaseous sulfur trioxide is zero.
Trimer
Both liquid and gaseous SO
3 exists in an equilibrium between the monomer and the cyclic trimer. The nature of solid SO
3 is complex and at least 3
polymorphs are known, with conversion between them being dependent on traces of water.
Absolutely pure SO
3 freezes at 16.8 °C to give the ''γ''-SO
3 form, which adopts the cyclic trimer configuration
2(''μ''-O)">(=O)2(''μ''-O)sub>3.
Polymer
If SO
3 is condensed above 27 °C, then ''α''-SO
3 forms, which has a melting point of 62.3 °C. ''α''-SO
3 is fibrous in appearance. Structurally, it is the
polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
2(''μ''-O)">(=O)2(''μ''-O)sub>''n''. Each end of the polymer is terminated with OH groups.
''β''-SO
3, 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.
[''Merck Index of Chemicals and Drugs'', 9th ed. monograph 8775]
Relative vapor pressures of solid SO
3 are alpha < beta < gamma at identical temperatures, indicative of their relative
molecular weight
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
s. Liquid sulfur trioxide has a vapor pressure consistent with the gamma form. Thus heating a crystal of ''α''-SO
3 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".
Chemical reactions
Sulfur trioxide undergoes many reactions.
Hydration and hydrofluorination
SO3 is the anhydride of H2SO4. Thus, it is susceptible to hydration:
:SO3 + H2O → H2SO4(Δ''H''f = −200 kJ/ mol)
Gaseous sulfur trioxide fumes profusely even in a relatively dry atmosphere owing to formation of a sulfuric acid mist.
SO3 is aggressively hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...
. The heat of hydration is sufficient that mixtures of SO3 and wood or cotton can ignite. In such cases, SO3 dehydrates these carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s.
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 → 2">O2sub>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. With pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
, it give the sulfur trioxide pyridine complex
Sulfur trioxide pyridine complex is the compound with the formula C5H5NSO3. It is a colourless solid that dissolves in polar organic solvents. It is the adduct formed from the Lewis base pyridine and the Lewis acid sulfur trioxide. The compou ...
. 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 Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution. Aryl sulfonic acids are used as detergents, dye, and drugs.
Stoichiometr ...
. 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:
: SO2 + O2 → SO3(Δ''H'' = −198.4 kJ/mol)
Industrial
Industrially SO3 is made by the contact process. Sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...
is produced by the burning of sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
or iron pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue gi ...
(a sulfide ore of iron). After being purified by electrostatic precipitation, the SO2 is then oxidised by atmospheric oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
at between 400 and 600 °C over a catalyst. A typical catalyst consists of vanadium pentoxide (V2O5) activated with potassium oxide K2O on kieselguhr
Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 μm to les ...
or silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
support. Platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
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
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
.
Laboratory
Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements '' ...
of sodium bisulfate
Sodium bisulfate, also known as sodium hydrogen sulfate, is the sodium salt of the bisulfate anion, with the molecular formula NaHSO4. Sodium bisulfate is an acid salt formed by partial neutralization of sulfuric acid by an equivalent of sodium ...
. Sodium pyrosulfate is an intermediate product:
#Dehydration at 315 °C:
#: 2 NaHSO4 → Na2S2O7 + H2O
#Cracking at 460 °C:
#: Na2S2O7 → Na2SO4 + SO3
In contrast, KHSO4 does not undergo the same reaction.
SO3 may also be prepared by dehydrating sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
with phosphorus pentoxide.
Applications
Sulfur trioxide is a reagent in sulfonation reactions. These processes afford detergents, dye
A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...
s, 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.
Safety
Along with being an oxidizing agent, sulfur trioxide is highly corrosive. It reacts violently with water to produce highly corrosive sulfuric acid.
See also
* Hypervalent molecule
*Sulfur trioxide pyridine complex
Sulfur trioxide pyridine complex is the compound with the formula C5H5NSO3. It is a colourless solid that dissolves in polar organic solvents. It is the adduct formed from the Lewis base pyridine and the Lewis acid sulfur trioxide. The compou ...
References
Sources
NIST Standard Reference Database
{{DEFAULTSORT:Sulfur Trioxide
Sulfur oxides
Acid anhydrides
Acidic oxides
Hazardous air pollutants
Interchalcogens
Hypervalent molecules
Sulfur(VI) compounds