Prof. McBride, J. Michael, CHEM 125b, Freshman Organic Chemistry II, Spring 2011, YALE Open Courses: Yale University, Chemistry (Accessed 14/09/2014), http://oyc.yale.edu/chemistry/chem-125b/lecture-3 License: Creative Commons BY-NC-SA The reactivity-selectivity principle explains why bromine atoms are more selective that chlorine atoms in abstracting hydrogen atoms from carbon. A free-radical mechanism for adding HBr to alkenes explains its anti-Markovnikov regiospecificity. Careful analysis is required to understand kinetic order for reactions involving catalysts. Termination of radical-chain reactions can make their rate half-order in the initiator. Selectivity due to protonation of radicals and their reaction partners illustrates the importance of ionic charge in determining reaction rates.