Radical bromination and relative reactivity of hydrogens

[mushaboom]

I just did a lab where we tested the relative reactivities of hydrogens by radical brominating ethyl benzene, toluene, methyl cyclohexane, cyclohexane, and t-butyl benzene. One set of reactions was done under bright light and the other was done under our weak fume hood light. In my report, I'm supposed to talk about how only some of the reactions needed light while others didn't. This is tough because all of the reactions took place in light, and all of them reacted at least to some extent. I assume that toluene and ethyl benzene are the ones that didn't need light, but I can't explain why. I looked this up online and read that aromatic compounds can only be substituted in the presence of an aluminum or iron catalyst, which we did not use. Are there other ways as to why the reactions occurred the way they did?

[aelek]

From what I know, you need light in order to gain radicals, so all of the reactions should proceed in light. Also, it is difficult to add halogens to aromatics because the resulting compounds are unstable, maybe that's why you need more radicals (more light) to get benzene, toluene and t-butyl benzene going... Just a guess, though

[rktpro]

Whether one product dominates can be predicted by relating the stability of free radical intermediate. As far as aluminium and iron are concerned, the make electrophiles that are not radicals. Electrophiles out of chlorine, bromine etc.

Edited May 19, 2014 by rktpro