Hi, can anyone please explain what selectivity of free radicals is? For example, there is an MCAT question that I recently came across and that can't seem to make sense of the answer. Which of the following halogens will give the greatest percent yield of tertiary alkyl halide when reacted with isobutane in the presence of heat or light? a. F2 b. Cl2 c. Br2 d. Isobutane wont produce a tertiary alkyl halide The answer is C. Br2, but i dont understand why... I applying common sense. Here is what I thought it would work. F is the least selective and the most reactive when is in the free radical form. My common sense tells me that since is very reactive and less selective, it won't care what carbon it binds to, so the tertiary hydrogen in isobutane (which is surrounded by 3 alkyl groups, or 9 hydrogens), will also react with F. Br and Cl are both less reactive and more selective. Hence they will most likely bind to the other primary carbons cause they are readily there. But this seems to not have been the right approach, can you please help me out? Starting by what selectivity is... Thank you.