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 bond by 5.0 kJ / mol (1.2 kcal mol) Reaction rates tend to be slower if a bond (as opposed to a 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 free-radical chlorination 12 times as fast as tetra-deuteriomethane
(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) Mono-chlorination of deuterio-ethane leads to a mixture containing 93% and 7%
Calculate the relative rates of abstraction per hydrogen and deuterium in the chlorination of deuterio-ethane.
(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.