When D2 reacts with ethylene (C2H4) in the presence of a finely divided catalyst, ethane with two deuteriums, CH2D - CH2D, is formed. (Deuterium, D, is an isotope of hydrogen of mass 2). Very little ethane forms in which two deuteriums are bound to one carbon (for example, CH3 - CHD2). Use the sequence of steps involved in the reaction (Figure 14.24) to explain why this is so?