a.
What is the relative abstraction of H and D ?
b. Why free-radical chlorination of `CH_(4)` is nearly 11 times faster than `CD_(4)` ?

Explanation

a. i. Number of H atoms abstracted by D =5 and percentage of compound formed `=93%` . Therefore, reactivity factor for replacement of H atoms `=93//5 =18.6`
ii. Number of D abstracted by C1 = 1 and percentage of compound formed `=7%`. Therefore, relative factor for D replacement `=7//1 =7`
iii. Relative reactivity of H : D abstraction `=18.6//7 =2.7 : 1`, i.e., each H is abstracted `2.7` times faster than D.
b. Chemically, H and D are identicsl but (C ----D) bonds are slightly stronger than (C----H) bonds, therefore, the energy of activation of D is greater than that of H. Since abstraction is a slow step, removal of H will be faster.
Relative abstraction of `H : D = 2.7 : 1`
Relative abstraction of `4H : D ~~ 10.8 : 1`
`~~ 11 : 1`