Equation 1 describes the deprotonation of a methyl halide by a methyl anion. The energy change associated with this exothermic reaction can be calculated using a computer. The energy change increases going from fluorine to chlorine to bromine and iodine. This shows that the CH2I- anion is the most stable and least basic of all the halides. Each of these reactions are isodesmic. The energy change in each example also depends on the difference in bond energy of the C-X bond in the base and conjugate acid. In other cases, the difference may be due to steric strain (the blocking of atoms by other atoms). The steric strain is small in fluorine (which is a small diameter atom) but large in iodine (in favor of the anion). So, the energy change grows between each example halide despite the fact that C-F bonds are stronger than C-I bonds.
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