View source for Sulfur trioxide

{{Distinguish|Sulfite}}
{{Chembox
| Verifiedfields = changed
| Watchedfields  = changed
| verifiedrevid  = 476993317
| Name           = 
| ImageFile      = SO3 meso.png
| ImageClass = skin-invert-image
| ImageCaption   = [[Resonance (chemistry)|Resonance structures]]
| ImageFile1     = Sulfur-trioxide-3D-vdW.png
| ImageClass1 = bg-transparent
| ImageSize1     = 100px
| ImageCaption1  = [[Space-filling model]]<ref name="G&E">{{Greenwood&Earnshaw2nd|pages=703-704}}</ref>
| PIN            = Sulfur trioxide
| SystematicName = Sulfonylideneoxidane
| OtherNames     = Sulfuric anhydride, Sulfur(VI) oxide
| IUPACName      = 
| Section1       = {{Chembox Identifiers
| CASNo = 7446-11-9
| CASNo_Ref = {{cascite|correct|CAS}}
| Gmelin = 1448
| PubChem = 24682
| PubChem1 = 22235242
| PubChem1_Comment = <br/>(hemihydrate)
| PubChem4 = 23035042
| index4_label = (monohydrate)
| ChemSpiderID = 23080
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| EINECS = 231-197-3
| UNII_Ref = {{fdacite|changed|FDA}}
| UNII = HH2O7V4LYD
| UNNumber = UN 1829
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 29384
| RTECS = WT4830000
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| SMILES = O=S(=O)=O
| SMILES_Comment = monomer
| SMILES1 = O=S0(=O)OS(=O)(=O)OS(=O)(=O)O0
| SMILES1_Comment = ''γ''-trimer
| SMILES2 = OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)OS(=O)(=O)O
| SMILES2_Comment = ''α''/''β'' polymer
| SMILES4 = O.O=S(=O)=O
| StdInChI = 1S/O3S/c1-4(2)3
| StdInChIKey = AKEJUJNQAAGONA-UHFFFAOYSA-N
| InChI = 1/O3S/c1-4(2)3
| InChIKey = AKEJUJNQAAGONA-UHFFFAOYAX
| InChI4=1S/O3S.H2O/c1-4(2)3;/h;1H2
| InChIKey4 = DEUOBQUHDSDIFY-UHFFFAOYSA-N
 }}
| Section2       = {{Chembox Properties
| Formula = SO<sub>3</sub>
| MolarMass = 80.066{{nbsp}}g/mol<!--2009Weight-->
| Density = 1.92{{nbsp}}g/cm<sup>3</sup>, liquid
| MeltingPtC = 16.9
| MeltingPt_notes =
| BoilingPtC = 45
| Solubility = Reacts to give [[sulfuric acid]]
| Appearance = Colorless to white crystalline solid which will fume in air.<ref>{{
    cite web
    | title=SULFUR TRIOXIDE CAMEO Chemicals NOAA
    | url=https://cameochemicals.noaa.gov/chemical/1560
    | website=Cameochemicals.noaa.gov
  }}</ref> Colorless liquid and gas.<ref name="Lerner">{{
  Cite book
  | title=Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling
  | author=Lerner, L.
  | isbn=9781439813133
  | lccn=2010038460
  | url=https://books.google.com/books?id=VqosZeMjNjEC&pg=PA10
  | page = 10
  | year=2011
  | publisher=CRC Press
}}</ref>
| Odor = Varies. Vapor is pungent; like sulfur dioxide.<ref>{{
    cite web
    | title=Substance:Sulfur trioxide - Learn Chemistry Wiki
    | url=http://www.rsc.org/learn-chemistry/wiki/Substance:Sulfur_trioxide
    | website=Rsc.org
  }}</ref> Mist is odorless.<ref name="Lerner" />
}}
| Section3       = 
| Section4       = {{Chembox Thermochemistry
| DeltaHf = −395.7{{nbsp}}kJ/mol
| Entropy = 256.77{{nbsp}}JK<sup>−1</sup>mol<sup>−1</sup>}}
| Section5       = 
| Section6       = 
| Section7       = {{Chembox Hazards
| ExternalSDS = [https://web.archive.org/web/20111019000350/http://inchem.org/documents/icsc/icsc/eics1202.htm ICSC 1202]
| MainHazards = Highly corrosive, extremely strong dehydrating agent 
| GHSPictograms = {{GHS05}} {{GHS07}}
| GHSSignalWord = '''Danger'''
| HPhrases = {{H-phrases|314|335}}
| PPhrases = {{P-phrases|261|280|305+351+338|310}}<ref>{{cite web |title=Sulfur trioxide 227692 |url=https://www.sigmaaldrich.com/catalog/product/aldrich/227692?lang=en&region=GB |website=SO3 |access-date=1 September 2020 |archive-url=https://archive.org/download/so3-msds/SO3%20MSDS.pdf |archive-date=2020-09-01}}</ref>
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 3
| NFPA-S = W+OX
| FlashPt = Non-flammable
| LD50 =
| LC50 = rat, 4{{nbsp}}hr 375{{nbsp}}mg/m{{sup|3}}{{citation needed|date=April 2015}}
| PEL =
  }}
| Section8       = {{Chembox Related
| OtherAnions =
| OtherCations = [[Selenium trioxide]]<br/>[[Tellurium trioxide]]<br/>[[Polonium trioxide]]
| OtherFunction = [[Sulfur monoxide]]<br/>[[Sulfur dioxide]]
| OtherFunction_label = [[sulfur]] [[oxide]]s
| OtherCompounds = [[Sulfuric acid]]
  }}
}}
'''Sulfur trioxide''' (alternative spelling '''sulphur trioxide''') is the chemical compound with the formula SO<sub>3</sub>. It has been described as "unquestionably the most [economically] important [[sulfur]] [[oxygen|oxide]]".<ref name="G&E" /> It is prepared on an industrial scale as a [[Precursor (chemistry)|precursor]] to [[sulfuric acid]].

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<sub>3</sub> is the primary precursor to [[acid rain]].<ref>{{cite journal
| title = Toward elimination of descrepancies between theory and experiment: The rate constant of the atmospheric conversion of SO<sub>3</sub> to H<sub>2</sub>SO<sub>4</sub>
|author1=Thomas Loerting|author1-link=Thomas Loerting |author2=Klaus R. Liedl | journal = Proceedings of the National Academy of Sciences of the United States of America
| year = 2000
| volume = 97
| issue = 16
| pages = 8874–8878
| doi = 10.1073/pnas.97.16.8874
| pmc = 16788
 | pmid=10922048
|bibcode=2000PNAS...97.8874L |doi-access=free }}</ref>

==Molecular structure and bonding==
===Monomer===
The molecule SO<sub>3</sub> is [[trigonal planar]]. As predicted by [[VSEPR theory]], its structure belongs to the D<sub>3h</sub> [[molecular symmetry|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).<ref>{{cite book |last1=Housecroft |first1=Catherine E. |last2=Sharpe |first2=Alan G. |title=Inorganic Chemistry |date=2012 |publisher=Pearson |location=Essex, England |page=575 |edition=4}}</ref> When the formal charge is non-zero, the S-O bonding is assumed to be delocalized. In any case the three S-O [[bond length]]s are equal to one another, at 1.42 Å.<ref name="G&E" /> The electrical [[dipole#molecular dipoles|dipole moment]] of gaseous sulfur trioxide is zero.

[[File:Sulfur-trioxide-trimer-from-xtal-1967-3D-balls-B.png|thumb|left|200px|[[Ball-and-stick model]] of the cyclic trimer of SO<sub>3</sub> <!--c***internally inconsistentShows S3O9 which is not at 120 degrees, so this a) different molecule (mismatch on gr/mole standard above) and b) does match upper verified standard bonding-also abovert***rrueuo-->]]

===Trimer===
Both liquid and gaseous<ref>{{cite journal |last1=Lovejoy |first1=R. W. |last2=Colwell |first2=J. H. |last3=Eggers |first3=D. F. |last4=Halsey |first4=G. D. |title=Infrared Spectrum and Thermodynamic Properties of Gaseous Sulfur Trioxide |journal=The Journal of Chemical Physics |date=February 1962 |volume=36 |issue=3 |pages=612–617 |doi=10.1063/1.1732581|bibcode=1962JChPh..36..612L }}</ref> SO<sub>3</sub> exists in an equilibrium between the monomer and the cyclic trimer. The nature of solid SO<sub>3</sub> is complex and at least 3 [[Polymorphism (materials science)|polymorphs]] are known, with conversion between them being dependent on traces of water.<ref>{{Holleman&Wiberg}}</ref>

Absolutely pure SO<sub>3</sub> freezes at 16.8&nbsp;°C to give the ''γ''-SO<sub>3</sub> form, which adopts the cyclic trimer configuration [S(=O)<sub>2</sub>(''μ''-O)]<sub>3</sub>.<ref>{{cite journal |last1=Westrik |first1=R. |last2=Mac Gillavry |first2=C. H. |title=The crystal structure of the ice-like form of sulphur trioxide (γ-modification) |journal=Recueil des Travaux Chimiques des Pays-Bas |date=1941 |volume=60 |issue=11 |pages=794–810 |doi=10.1002/recl.19410601102}}</ref><ref name="G&E" />

===Polymer===
[[File:Schwefeltrioxid Ampulle.jpg|thumb|200px|left|An [[ampoule]] of sulfur trioxide]]

If SO<sub>3</sub> is condensed above 27&nbsp;°C, then ''α''-SO<sub>3</sub> forms, which has a melting point of 62.3&nbsp;°C. ''α''-SO<sub>3</sub> is fibrous in appearance. Structurally, it is the [[polymer]] [S(=O)<sub>2</sub>(''μ''-O)]<sub>''n''</sub>. Each end of the polymer is terminated with OH groups.<ref name="G&E" /> ''β''-SO<sub>3</sub>, like the alpha form, is fibrous but of different molecular weight, consisting of an hydroxyl-capped polymer, but melts at 32.5&nbsp;°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.<ref name="Merck">''Merck Index of Chemicals and Drugs'', 9th ed. monograph 8775</ref>

Relative vapor pressures of solid SO<sub>3</sub> are alpha &lt; beta &lt; gamma at identical temperatures, indicative of their relative [[molecular weight]]s. Liquid sulfur trioxide has a vapor pressure consistent with the gamma form. Thus heating a crystal of  ''α''-SO<sub>3</sub> 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".<ref name="Merck"/>

==Chemical reactions==
Sulfur trioxide undergoes many reactions.<ref name=G&E/>

===Hydration and hydrofluorination===
SO<sub>3</sub> is the [[Inorganic anhydride|anhydride]] of H<sub>2</sub>SO<sub>4</sub>. Thus, it is susceptible to hydration:
:SO<sub>3</sub> + H<sub>2</sub>O  →  H<sub>2</sub>SO<sub>4</sub>{{pad|2em}}(Δ<sub>f</sub>''H'' = −200 [[kilojoule|kJ]]/[[mole (unit)|mol]])<ref>{{cite web |title=The Manufacture of Sulfuric Acid and Superphosphate |publisher=Chemical Processes in New Zealand |url=http://nzic.org.nz/ChemProcesses/production/1B.pdf |access-date=2016-04-22 |archive-date=2018-01-27 |archive-url=https://web.archive.org/web/20180127124034/http://nzic.org.nz/ChemProcesses/production/1B.pdf |url-status=dead }}</ref>

Gaseous sulfur trioxide fumes profusely even in a relatively dry atmosphere owing to formation of a sulfuric acid mist.
SO<sub>3</sub> is aggressively [[hygroscopic]]. The heat of hydration is sufficient that mixtures of SO<sub>3</sub> and wood or cotton can ignite. In such cases, SO<sub>3</sub> dehydrates these [[carbohydrate]]s.<ref name="Merck"/>

Akin to the behavior of H<sub>2</sub>O, [[hydrogen fluoride]] adds to give [[fluorosulfuric acid]]:
:SO<sub>3</sub> + HF  →  FSO<sub>3</sub>H

===Deoxygenation===
SO<sub>3</sub> reacts with dinitrogen pentoxide to give the [[nitronium]] salt of pyrosulfate:
:2 SO<sub>3</sub> + N<sub>2</sub>O<sub>5</sub> → [NO<sub>2</sub>]<sub>2</sub>S<sub>2</sub>O<sub>7</sub>

===Oxidant===
Sulfur trioxide is an [[Oxidizing agent|oxidant]]. It oxidizes [[sulfur dichloride]] to [[thionyl chloride]].
:SO<sub>3</sub> + SCl<sub>2</sub> → SOCl<sub>2</sub> + SO<sub>2</sub>

===Lewis acid===
SO<sub>3</sub> is a strong [[Lewis acid]] readily forming adducts with Lewis bases.<ref>{{Cotton&Wilkinson6th}}</ref>  With [[pyridine]], it gives the [[sulfur trioxide pyridine complex]].  Related adducts form from [[dioxane]] and [[trimethylamine]].

===Sulfonating agent===
Sulfur trioxide is a potent [[Sulfonate|sulfonating agent]], i.e. it adds SO<sub>3</sub> groups to substrates. Often the substrates are organic, as in [[aromatic sulfonation]].<ref>{{cite journal |doi=10.15227/orgsyn.036.0083|title=α-Sulfopalmitic Acid|first1=J. K.|last1=Weil|first2=R. G.|last2=Bistline Jr.|first3=A. J. |last3=Stirton|journal=Organic Syntheses|year=1956|volume=36|page=83}}</ref>  For activated substrates, Lewis base adducts of sulfur trioxide are effective sulfonating agents.<ref>{{cite journal |doi=10.15227/orgsyn.034.0085|title=Sodium β-Styrenesulfonate and β-Styrenesulfonyl Chloride|first1=Christian S.|last1=Rondestvedt Jr.|first2=F. G.|last2=Bordwell|journal=Organic Syntheses|year=1954|volume=34|page=85}}</ref>

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

: 2 SO<sub>2</sub> + O<sub>2</sub> → 2 SO<sub>3</sub>{{pad|2em}}(Δ''H'' = −198.4 kJ/mol)

===Industrial===
Industrially SO<sub>3</sub> 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 SO<sub>2</sub> is then oxidised by atmospheric [[oxygen]] at between 400 and 600&nbsp;°C over a catalyst. A typical catalyst consists of [[Vanadium(V) oxide|vanadium pentoxide]] (V<sub>2</sub>O<sub>5</sub>) activated with [[potassium oxide]] K<sub>2</sub>O on [[Diatomaceous earth|kieselguhr]] or [[Silicon dioxide|silica]] support. [[Platinum]] also works very well but is too expensive and is poisoned (rendered ineffective) much more easily by impurities.<ref>Hermann Müller "Sulfuric Acid and Sulfur Trioxide" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. 2000 {{doi|10.1002/14356007.a25_635}}</ref>
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:<ref name="vries">{{cite journal |author=K.J. de Vries |author2=P.J. Gellings |title=The thermal decomposition of potassium and sodium-pyrosulfate |journal=Journal of Inorganic and Nuclear Chemistry |volume=31 |issue=5 |date=May 1969 |pages=1307–1313 |url=https://research.utwente.nl/en/publications/the-thermal-decomposition-of-potassium-and-sodium-pyrosulfate |doi=10.1016/0022-1902(69)80241-1}}</ref>

#Dehydration at 315&nbsp;°C:
#: 2 NaHSO<sub>4</sub>  →  Na<sub>2</sub>S<sub>2</sub>O<sub>7</sub> + H<sub>2</sub>O
#Cracking at 460&nbsp;°C:
#: Na<sub>2</sub>S<sub>2</sub>O<sub>7</sub>  →  Na<sub>2</sub>SO<sub>4</sub> + SO<sub>3</sub>

The latter occurs at much lower temperatures (45&ndash;60&nbsp;°C) in the presence of catalytic [[sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]].<ref>{{cite web|title=Preparation of Sulfur Trioxide and Oleum|author=GarageChemist|url=https://www.sciencemadness.org/member_publications/SO3_and_oleum.pdf|pages=1–2}}</ref>  In contrast, KHSO<sub>4</sub> undergoes the same reactions at a higher temperature.<ref name="vries"/>

Another two step method involving a salt pyrolysis starts with concentrated sulfuric acid and anhydrous tin tetrachloride:

# Reaction between tin tetrachloride and sulfuric acid in a 1:2 molar mixture at near reflux (114&nbsp;°C):
#: SnCl<sub>4</sub> + 2 H<sub>2</sub>SO<sub>4</sub>  →  Sn(SO<sub>4</sub>)<sub>2</sub> + 4 HCl 
# Pyrolysis of anhydrous tin(IV) sulfate at 150&nbsp;°C - 200&nbsp;°C:
#:Sn(SO<sub>4</sub>)<sub>2</sub>  →  SnO<sub>2</sub> + 2 SO<sub>3</sub>

To further reduce water contamination, Oleum and a slight excess of Tin(IV) Chloride should be used. The slight excess of SnCl<sub>4</sub> can then be separated by carefully heating the solid Tin(IV) Sulfate under a vacuum to no more than 120&nbsp;°C. The excess SO<sub>3</sub> from the Oleum and the remaining SnCl<sub>4</sub> will react during HCl formation and form Tin(IV) Oxide and Sulfuryl Chloride. If an excess of SO<sub>3</sub> in the Oleum is present relative to SnCl<sub>4</sub> , the Tin(IV) Oxide will absorb it and form more Tin(IV) Sulfate.

The advantage of this method over the sodium bisulfate one is that it can produce the pure trimer of SO<sub>3</sub> (since no water is present) while still using safe temperatures for normal borosilicate laboratory glassware. Other dry sulfate salt pyrolysis reactions require higher temperatures which increases the risk of shattering. A disadvantage is that it generates significant quantities of hydrogen chloride gas which needs to be captured as well.

SO<sub>3</sub> may also be prepared by dehydrating [[sulfuric acid]] with [[phosphorus pentoxide]].<ref>{{cite web |title=How to make sulfur trioxide - YouTube |url=https://www.youtube.com/watch?v=zLr-PnSQvX0 |website=www.youtube.com | date=21 September 2017 |access-date=1 September 2020}}</ref>

==Applications==
Sulfur trioxide is a reagent in [[sulfonation]] reactions. [[Dimethyl sulfate]] is produced commercially by the reaction of [[dimethyl ether]] with sulfur trioxide:<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a08_493 |chapter=Dialkyl Sulfates and Alkylsulfuric Acids |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |last1=Weisenberger |first1=Karl |last2=Mayer |first2=Dieter |last3=Sandler |first3=Stanley R. |isbn=978-3-527-30385-4 }}</ref>
:{{chem2|CH3OCH3 + SO3 → (CH3)2SO4 }}

Sulfate esters  are used as [[detergent]]s, [[dye]]s, and [[Medication|pharmaceuticals]]. Sulfur trioxide is generated [[in situ]] from sulfuric acid or is used as a solution in the acid.

{{anchor|Sulfan}}B<sub>2</sub>O<sub>3</sub> stabilized sulfur trioxide was traded by Baker & Adamson under the tradename "''Sulfan''" in the 20th century<!-- at least 1960s to 1980s -->.<ref name="Habashi-Dugdale_1973"/>

==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]]

==References==
{{Reflist|refs=
<ref name="Habashi-Dugdale_1973">{{cite journal |author-first1=Fathi |author-last1=Habashi |author-first2=Raymond |author-last2=Dugdale |title=The Action of Sulfur Trioxide on Chalcopyrite |journal=[[Metallurgical and Materials Transactions]] |volume=B-4 |pages=1553–1556 |date=June 1973 |issue=6 |orig-date=1972-11-06 |doi=10.1007/BF02668007 |bibcode=1973MT......4.1553H |s2cid=93744787 |quote-page=1553 |quote=Sulfur trioxide used was pure, colorless liquid SO3 marketed under the trade name Sulfan by Baker and Adamson |url=https://reader.zlibcdn.com/articles/6427f27508dc5960e6c75b3feac71828/1.jpg<!-- |access-date=2021-07-22 |url-status=live |archive-url=https://web.archive.org/web/20210722110634/https://reader.zlibcdn.com/articles/6427f27508dc5960e6c75b3feac71828/1.jpg |archive-date=2021-07-22 -->|url-access=subscription }}</ref>
}}

==Sources==
* [http://webbook.nist.gov/chemistry/ NIST Standard Reference Database]
* [http://chemsub.online.fr/name/sulfur_trioxide.html ChemSub Online]

{{Oxides}}
{{sulfur compounds}}
{{Authority control}}

{{DEFAULTSORT:Sulfur Trioxide}}
[[Category:Sulfur oxides]]
[[Category:Acid anhydrides]]
[[Category:Acidic oxides]]
[[Category:Interchalcogens]]
[[Category:Hypervalent molecules]]
[[Category:Sulfur(VI) compounds]]