REACTIONS OF THE ETHYL RADICAL: VI. ADDITION TO CONJUGATED DIENES

Abstract

Arrhenius parameters have been measured for the addition of the ethyl radical to the conjugated diene system in three representative molecular environments. Significant differences are found among the values of the energy of activation for addition, which are: 4.5 ± 0.2 kcal/mole for 2,3-dimethylbutadiene-1,3, 5.2 ± 0.3 kcal/mole for cyclohexadiene-1,3, and 6.6 ± 0.3 kcal/mole for 2,5-dimethylhexadiene-2,4. The increase in the energy of activation in this series is paralleled by an increase in the degree of shielding of the terminal carbon atoms of the conjugated system by substituent groups. The energy of activation for metathesis is significantly lower for cyclohexadiene-1,3 (5.4 ± 0.5 kcal/mole) than for 2,5-dimethylhexadiene-2,4 (7.6 ± 0.4 kcal/mole); the activated hydrogen atoms of the former are all secondary, whereas those of the latter are all primary. The ratio of the rate constants for addition and metathesis at 60° indicate that the radical homopolymerization of cyclohexadiene-1,3 and 2,5-dimethylhexadiene-2,4 should be subject to extensive degradative chain transfer.