The molecules cis-dichloroethylene and transdichloroethylene shown on p. 422 can be interconverted by heating or irradiation.
(a) Starting with cis-dichloroethylene, show that rotating the CPC bond by 1808 will break only the pi bond but will leave the sigma bond intact. Explain the formation of trans-dichloroethylene from this process. (Treat the rotation as two stepwise 908 rotations.)
(b) Account for the difference in the bond enthalpies for the pi bond (about 270 kJ/mol) and the sigma bond (about 350 kJ/mol).
(c) Calculate the longest wavelength of light needed to bring about this conversion?