[TBeezee]

Hello. I have been studying free radical halogenation recently and I have a few questions...

First, as a propagation step - is it possible for a halogen free radical to abstract a halogen atom from product forming the orginal halogen molecule again? I assume if this happens at all that it is less favorable than normal hydrogen abstraction since the reaction must be driven mostly towards the formation of product... but if so, why should it be less favorable? If it does not happen at all, why not?

My second question concerns the general trend that going down group 7, halogen reactivity decreases so that only chlorine and bromine can be used, with fluorine being too reactive for practical purposes and iodine not being reactive enough. The explanation I have been able to find so far only gives the values of DHs for the reactions (fluorine is too exothermic and iodine being too endothermic overall). Could somebody please give more information as to why these differences exist? I guess that the answer relates to the size and electronegativity of the halogen involved... but I am not sure!

Thank you very much!

[0000000]

often the radical is formed by the catalyst dividing the molecule in to 2 radicals, they have only one reaction centre, the orbital with one electron, that so when one of them is added to a organic molecule they lose the active "centre" forming a bond, that so the reaction of forming a halogen molecule is not favored.

the hipper activity of F and the opposite with I is connected directly with the electronegativity therefore with the radii of the atom and with the electrons in the last layer.

[TBeezee]

Tomce, thanks very much for your answers. I wonder, regarding again the first question: surely the case of the active center is the same for hydrogen? i.e., hydrogen does not have that radical ability or reaction center when originally bound to the starting material, yet can be abstracted all the same by the strong reactivity of the halogen radical (during the first step of propagation). Why then, can hydrogen be abstracted by a radical while a halogen atom can not?

[Mitch]

Addressing the first part of your question. Your confusion lies for not doing #1 in my signature. Halogen radicals will readily react with other halogens, but the net effect is regeneration of a reactive radical.

[0000000]

On the other had, the substitution of the hydrogen atom lies to right at equilibrium process because of several electo-prperties of the H and X radicals