Sulfur trioxide resonance forms

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],... 

Write the Lewis structures of the three resonance forms of sulfur trioxide, S03, then combine them in a fourth composite structure showing the delocalization of the pair of electrons over all atoms in the molecule. 

Carbon dioxide is a molecule with two atoms attached (SN = 2) to the central atom via double bonds. The electrons in each double bond must be between C and O, and the repulsion between these electron groups forces a linear structure on the molecule. Sulfur trioxide has three atoms bound to the sulfur (SN = 3), with equivalent partial doublebond character between sulfur and each oxygen, a conclusion rendered by analysis of its resonance forms. The best positions for the oxygens to minimize electron-electron repulsions in this molecule are at the corners of an equilateral triangle, with O—S—O bond angles of 120°. The multiple bonding does not affect the geometry, because aU three bonds are equivalent in terms of bond order. 

Sulfonation (Section 22.1 B) The electrophile is either sulfur trioxide, SOj, or HSO3+ depending on experimental conditions. The mechanism involves reaction of SO3 as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the aiylsulfonic add product. 

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