Deuterium (D) is the hydrogen isotope of mass number 2, with a proton and a neutron in i...

Deuterium (D) is the hydrogen isotope of mass number 2, with a proton and a neutron in its nucleus. The chemistry of deuterium is nearly identical to the chemistry of hydrogen, except that the C-D bond is slightly stronger than the C-H bond by 5.0 kJ mol (1.2 kcal mol). Reaction rates tend to be slower if a C-D bond (as opposed to a C-H bond) is broken in a rate-limiting step. This effect, called a kinetic isotope effect, is clearly seen in the chlorination of methane. Methane undergoes freeradical chlorination 12 times as fast as tetradeuteriomethane (CD₄).

(a) Draw the transition state for the rate-limiting step of each of these reactions, showing how a bond to hydrogen or deuterium is being broken in this step.

(b) Monochlorination of deuterioethane (C₂H₅D) leads to a mixture containing 93% C₂H₄DCl and 7% C₂H₅Cl

Calculate the relative rates of abstraction per hydrogen and deuterium in the chlorination of deuterioethane.

(c) Consider the thermodynamics of the chlorination of methane and the chlorination of ethane, and use the Hammond postulate to explain why one of these reactions has a much larger isotope effect than the other.