Reactions of excited cyclic hydrocarbons

Work has also been done on the unimolecular reactions of hot cyclic hydrocarbon molecules, which are molecules produced with excessive amounts of energy. This subject has been reviewed by Frey and only a brief account is given here. Such hot molecules are commonly produced by the addition of methylene radicals to certain molecules. Methylene radicals are conveniently generated by the action of light on ketene or on diazomethane, viz. 

By using light of various wavelengths it is possible to vary the amount of kinetic energy in the methylene radical produced. 

Methylene radicals may be caused to react with various molecules, and since the reations that occur are usually highly exothermic, molecules of very high energy are produced. Methylene radicals add, for example, to the double bond in ethylene to give excited cyclopropane, viz. 

If Vt is the rate of production of excited cyclopropane, assuming a stationary concentration of excited molecules leads to the conclusion that the rates of formation of normal cyclopropane and propene are 

An assumption as to k (e.g. that the efficiency of deactivation is unity) thus allows one to determine At2 from the observed yields. At low pressures the yield of cyclopropane is expected to fall to zero, and this is found to be the case experimentally. At high pressures the equation leads to the conclusion that only cyclopropane will be formed this is not quite correct since propene is also found to some extent from the direct insertion of methylene into ethylene, viz. 

---