Nucleophilic Substitution at a Tetrahedral Center

Nonbonding electron pairs are good electron sources, especially lone pairs of anions. The following list gives the relative nucleophilicity of many lone pair nucleophiles in an Sn2 reaction on methyl iodide. An anion is always a better nucleophile than its neutral counterpart methoxide is almost 2 million times more nucleophilic than methanol. 

We might suspect that the more basic species would also be more nucleophilic, but nucleophihcity does not parallel basicity exactly because softness must be considered. The harder ions are more tightly solvated in polar solvents and cannot serve as a nucleophile as well as a poorly solvated softer ion. To verify this, we need only to look at the following list and see that iodide at pATabH -10 is over a thousand times more nucleophilic than the much more basic acetate, pATabH pairs of the same 

Relative nucleophilicity toward CH3I (in CH3OH as solvent)  

Steric hindrance decreases nucleophilicity. As the nucleophile becomes larger, its bulk tends to get in the way of its acting as a nucleophile, especially when the electrophile is also large. For this reason the larger tert-butoxide, (CH3)3CO, is a much poorer nucleophile than the smaller methoxide, CH30 . 

Electron availability decreases as the orbital the electrons occupy is made more stable. As the hybridization of the nitrogen lone pair goes from sp to sp to sp, the lone pair gets less basic and less nucleophilic as indicated by the following trend. 

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