Chemistry Reference
In-Depth Information
Figure 3.4 Order of increasing nucleophilicity of halide ions is influenced by polarizing influences such
as solvent effects.
associated with a hard base attracts a tight shell of solvent surrounding it. This solvent shell
blocks the hard base from reacting as a nucleophile. On the other hand, this solvent effect is
minimized for a soft base due to its large size and diffuse electron concentration. The
absence of a solvent shell around a soft base enhances its ability to react as a nucleophile.
This effect is illustrated in Figure 3.5. Thus, while solvent effects can influence the order of
nucleophilicity observed for halide ions, the general rule of thumb is that nucleophilicity
increases as we move from the top to the bottom of any given column in the periodic
table of the elements.
Of the factors influencing nucleophilicity, steric effects have perhaps the greatest
influence. Steric effects occur when groups attached to a nucleophilic atom are too large
Figure 3.5 Solvent shells surround hard bases rendering them less reactive nucleophiles than
soft bases.
