Reactions of Sulfur Trioxide

Ethyl pentafluoroisopropenyl ether reacts in its resonance structure with sulfur trioxide in such a way that the sulfur atom of sulfur trioxide becomes attached to the terminal difluoromethylene group. The oxygen atom of sulfur trioxide combines with the methylene carbon of the ethyl group, and thus ethyl 2-oxopentafluoropropane-1 -sulfonate is formed. Its transesterification with trifluoroacetic acid affords 2-oxopentafluoropro-panesulfonic acid and ethyl trifluoroacetate [44], 

Sulfur trioxide can add to the triple bond of perfluoro-terr-butylacetylene in two directions. Since it has only six electrons in the valence shell, it acts as a Lewis acid and combines with the more electronegative end of 

As in Explanation 27, sulfur of sulfur trioxide becomes attached to the more electronegative (electrophilic) carbon of the double bond of perflu-oroisobutylene. The electron density of the central carbon is increased by back-donation of electrons of the difluoromethylene group, and by electron withdrawal of the two trifluoromethyl groups. The intermediate four-membered cyclic sulfate rearranges to 2,2-bis(trifluoromethyl)-l-fluoroethenyl fluorosulfate 46.  

When an excess of sulfur trioxide is used, two molecules of sulfur trioxide add to the double bond of perfluoroisobutylene in such a way that the sulfur atom becomes attached to the more electrophilic central carbon of the double bond to form a six-membered cyclic disulfate that rearranges to 2,2-bis(trifluoromethyl)-l-fluoroethenyl fluoropyrosulfate [46], 

Sulfur trioxide opens the epoxide ring by attaching its oxygen atom to the sterically more accessible difluoromethylene carbon. The propylene oxide oxygen combines with the sulfur atom and forms a five-membered cyclic sulfate [47], 

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Benzene. The reaction of sulfur trioxide and ben2ene in an inert solvent is very fast at low temperatures. Yields of 90% ben2enesulfonic acid can be expected. Increased yields of about 95% can be reali2ed when the solvent is sulfur dioxide. In contrast, the use of concentrated sulfuric acid causes the sulfonation reaction to reach reflux equiUbrium after almost 30 hours at only an 80% yield. The by-product is water, which dilutes the sulfuric acid estabhshing an equiUbrium. 

A basic research study on combustion of sulfur led to the postulation that sulfur trioxide may actually be the primary combustion product and that sulfur dioxide may then be produced by the further reaction of sulfur trioxide with sulfur vapor ki the oxygen-deficient region of the flame (261). 

Other chlorinated solvents such as tetrachloroethylene or chloroform may be used in place of carbon tetrachloride. Caution The reaction of sulfur trioxide with chlorinated solvents has been reported to give phosgene and other toxic products. Adequate venting of all by-product gases is essential. 

Most of the material presented in this section are reactions of sulfur trioxide. This compound is ambivalent and frequently forms a carbon-sulfur bond (true sulfonation), but it can form a carbon-oxygen bond as well. Examples of both types of bonding are included... 

Polyfluorovinyl ethers form unstable (i-sultones with sulfur trioxide The p sultones isomerize at 25 °C to give p carbonylsulfonate esters and acids [16, 17 The reaction of sulfur trioxide with ethyl pentafluoroisopropenyl ether to furnish 2-ketopentafluoropropanesulfonic acid and ethyl 2-ketopentafluoro-propanesulfonate is an example (equation 7)... 

In the most common process for making sulfuric acid, sulfur is burned at a very high temperature in dry air to make sulfur dioxide, which is then reacted with more oxygen over a catalyst to make sulfur trioxide. Reaction of sulfur trioxide with water produces sulfuric acid. 

There are two important general types of reaction by which six-membered heterocycles containing two or more heteroatoms can be transformed into other six-membered heterocyclic systems, namely reactions which involve an ANRORC mechanism, and reactions which proceed by a Diels-Alder/retro-Diels-Alder type of mechanism. Transformation of 1,3-oxazinones and -thiazinones into pyrimidones (equations 193 and 194) has been extensively used, especially in the conversion of isatoic anhydride into quinazolinones (e.g. equation 195). 2,4-Diaryl-l,2,3,5-oxathiadiazines, which are readily accessible by reaction of sulfur trioxide with aryl isocyanates, are useful precursors to pyrimidines and 1,3,5-triazines (equation 196). 

The most common synthetic methods involving formation of three or more bonds and leading to polyheteroatom systems are of the [2 + 2+2] type. Condensations employing a wide range of different two-atom fragments have been reported, and examples involving the use of sulfur trioxide, nitriles, ketones, isocyanates, ketenes, thioketenes and cyanates as the two-atom components are known. Of these, the reactions of sulfur trioxide and isocyanates are probably the more important, and a few illustrative examples are given here. 

Some reactions of sulfur trioxide with dialkylcyanamides (77ZC222), trichloroacetonitrile (77ZOR222) and a combination of nitriles and alkenes (66USP3235549) are given in Scheme 29. In the reaction with dialkylcyanamides a second product (252 X = NR2) is also formed. 

An analogous product (252 X = Cl) is formed in the reaction of sulfur trioxide with cyanogen chloride (68AG(E)172). Aryl cyanates also condense with sulfur trioxide to give 1,2,3,5-oxathiadiazines analogous to those formed from dialkylcyanamides (67CB3736). 

Reaction of sulfur trioxide with water - care is needed. Test the solution to show that it is acidic (using both universal indicator and magnesium powder), and that it contains sulfate ions (by adding acidified barium chloride solution). 

Several detailed theoretical studies have been conducted on the reaction of sulfur trioxide with alkenes to afford /3-sultones. The first publication <1998JA6468> details the reaction of sulfur trioxide with ethylene to afford /3-sultone 9 via transition state 8 (Scheme 1). Important bond lengths are shown (pm). Density functional and correlated ab initio calculations indicate that the reaction proceeds by a concerted [2+2] pathway. 

The second publication detailed molecular modeling studies on the reaction of sulfur trioxide with a series of fluoroalkenes and found that the reaction proceeds via formation of stable 7t-complexes prior to transition state formation <1999J(P2)1819>. 

The addition of the S=0 bond across the C=S bond took part in the reaction of sulfur trioxide with methylidene sulfur tetrafluoride <1995HAC611>. The resulting 1,2,4-oxadithietane 16 was isolated as a stable colorless solid (Scheme 35). 

Reaction of Sulfur Trioxide with Perfluoro-tert-butylacetylene... 

What is the structure of the product of the reaction of sulfur trioxide with perfluoro-tert-butylacetylene ... 

A study of the reaction of sulfur trioxide with water clusters has also been made (4). The ion clusters observed in these experiments were a series of protonated water clusters H+(H20)n with n up to 14 and another less abundant series of the form S03(H20)nH+ with n up to 9. The H+(H20)n series could result from the ionization of unreacted water clusters and also from the ionization of S03(H20)m clusters. The relative distribution of the H+(H20)n series, however, was not appreciably affected by the introduction of SO3. The distribution was very similar... 

Sultones such as 2 are formed in high yield from the cycloaddition reaction of sulfur trioxide and polyfluorinated alkenes. e.g. 

The /J-sultone formed in the reaction of sulfur trioxide with tetrafluoroethenc itself is a key intermediate in the synthesis of Nafion polymers, which have commercial use as ion-exchange membranes. Furthermore, perfluorovinylsulfonyl fluoride (3) is transformed into mainly the /Tsultone 4 when heated with sulfur trioxide at elevated temperature. ... 

Oxathietane 2,2-dioxides (e.g., 448) are isolated in good yields from the reaction of sulfur trioxide with fluorinated alkenes, fluorinated dienes, ° and from the reaction of the sulfur trioxide donor 499 with fluorinated alkenes. Zwitterionic intermediates ( C-C-S020 ) are suggested. An explosion has been reported in the addition of sulfur trioxide to tetrafluoro-ethylene. Tetrafluoroethane /3-sultone (498) also is obtained by treatment of fluorosulfonyldifluoroacetic anhydride with antimony trichloride. ... 

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