2-Methylbutane, bromination chlorination

Bromination of an optically active form of the corresponding chloro compound (l-chloro-2-methylbutane) also results in an optically active product, and retention of configuration. It may be that an actual bridged radical is formed, but a somewhat less concrete interaction seems more likely, as halogenation with the more reactive chlorine is found to lead wholly to racemisation. 

This example demonstrates that the rule of thumb is an estimate at best. Most importantly, it is a reminder that the tertiary product will not necessarily be the primary product. Bromine is more selective than chlorine and substituting bromine for chlorine in the same reaction will result in predominately 2-bromo-2-methylbutane. Fluorine, on the other hand, is so reactive that the primary7 product would pre-dominate. 

Because homolysis of the bromine-bromine bond in Br2 is more facile (DH° = 193kJmoT [46.1 kcal moT ]) than the bond between chlorine atoms in CI2 (DH° = 242.4 kJmol [58.9kcalmoT ]), it should come as no surprise that when bromine is allowed to react with 2-methylbutane, only 2-bromo-2-methylbutane... 

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