Cheletropic Addition of SO2

A cheletropic reaction has been defined as a process in which two sigma bonds which are directed to a single atom of a ring are made or broken concertedly. The number of pi-bonds decrease or increase by one, depending on the direction of the reaction (ring-forming or ring-opening). An example is the reversible addition of sulfur dioxide to 1,3-butadiene shown here.

ΔHº = –16.5 kcal/mole

The direction taken by this reaction depends upon the temperature. Below 100º C the equilibrium favors the addition product, the standard heat of reaction being –16.5 kcal/mole. Above 100º C the cyclic sulfone decomposes to 1,3-butadiene. The equilibrium constant is close to unity at 100º C. If we assume this is true, the entropy change is easily calculated using the equations:

ΔGº = – RTlnK = – 2.303RTlogK ΔGº = ΔHº – TΔSº

Since ΔG = 0 when Keq = 1, ΔSº = – 16,500/373 = – 44.24 cal/ ºK mole. At 25º C the ΔGº = -3.3 kcal/mole, corresponding to a Keq = 270.