Question

Which of the follwing is not the characteristic of the mechanism of free radical halogenation chain reactions?
A. Recation can be terminated prematurely by adding radical scavengers.
B. Initiators such as organic preoxides can catalyze the reaction in the absence of light or heat.
C. If `O_(2)` is present, there is an inhibition period after which the reaction rate decreases.
D. The quantum yield is typically high.

Answer 1

Correct Answer - C
A small amount of `O_(2)` gas slows down the reaction for a period of time, which depends upon the amount of `O_(2)`, after which the reaction proceeds smoothly. It is believed that `O_(2)` reacts with a methyl ( or any other alkyl) radical to form a new free radical:
`.^(.)CH_(3)+O_(2)rarrCH_(3)-O-O`
The `CH_(3)OO^(.)` radical is much less reactive than the `CH_(3)` radical and is helpless to continue the chain. Thus, by combining with a `CH_(3)^(.)` one `O_(2)` molecule breaks a chain and thus, slows down the reaction tremendously. The reaction is free to proceed at its normal rate after all `O_(2)` molecules present have combined with `.^(.)CH_(3)` radicals.
A substance that slows down or stops a reaction even though present in small amount is called an inhibitor. The period of time during which inhibition lasts and after which the reaction proceeds normally is called the inhibition period. Inhibition by `O_(2)` is one of the strong supports for a free radical reaction.
Radical scavengers are substance that combine withe free radicals, thus terminating a chain. Quantum yield is the number of product molecules produced by one photon of light.