Page 235 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

6.6 HIGHER-VALENT STATES: OXIDES AND OXOACIDS 215

Se
Se Cl
Se Se +
Se Se Se Ph Se Se −
Se Se Se Ph
Se Se
P Se Se Cl
Ph P
Cl Ph
Ph Cl
Ph (6.46)
Cl
Se Ph
Se Se
Se Se Se + P Ph
Se Se
Cl Ph
In our experience, the mechanisms of cyclooctachalcogen ring formation are perceived as
a bit more difficult than nucleophilic breakdown of the same rings. They are certainly quite
challenging exercises in arrow pushing. We hope you’ll get more comfortable with practice!

6.6 HIGHER-VALENT STATES: OXIDES AND OXOACIDS

The two common oxides of sulfur, SO and SO , are typical acidic oxides; they react with
2 3
water to yield sulfurous acid (H SO ) and sulfuric acid (H SO ), respectively:
2 3 2 4
SO + H O → H SO (6.47)
2 2 2 3
SO + H O → H SO 4 (6.48)
2
2
3
A by-product of fossil fuel combustion, SO is a major pollutant, and the above two reac-
2
tions, which take place in cloud droplets, play a key role in the processes leading to acid rain.
Sulfur trioxide is a strong Lewis acid and reacts with Lewis bases (indicated below as L)
such as pyridine and triphenylphosphine to give stable adducts:
− −
O O
2+ +
L S O L S 2+ (6.49)
− −
O O
− O

REVIEW PROBLEM 6.4

Sulfur trioxide reacts with anhydrous HF and HCl to yield the corresponding halo-
sulfonic acids, XSO H:
3
HX + SO → XSO H
3 3
Suggest a mechanism.