What are Leaving Groups

In consideration of the relationship between nucleophiles and leaving groups, recall that the pXa values for methanol are 15-16 and the pKa value for hydrochloric acid is — 2.2. Because methanol is less acidic than hydrochloric acid, methoxide ions are expected to be better nucleophiles than chloride ions. This is, in fact, the case and the 

In summary, SN2 reactions are defined by the principles surrounding organic acids and their conjugate bases as discussed in Chapters 2 and 3. Specifically, as discussed in Chapter 3, relative nucleophilicities can be estimated based on pAia values. Finally, relative values can be used to predict whether a reaction will proceed and what component of the starting material will be the leaving group. 

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