Nucleophilic substitution base strengths, correlation with

It is not possible to construct an invariant nucleophilicity order because different substrates and different conditions lead to different orders of nucleophilicity, but an overall approximate order is NH2 > PhaC > PhNH (aryne mechanism) > ArS > RO > R2NH > ArO > OH > ArNHi > NH3 > 1 > Br > Cl > H2O > ROH. As with aliphatic nucleophilic substitution, nucleophilicity is generally dependent on base strength and nucleophilicity increases as the attacking atom moves down a column of the periodic table, but there are some surprising exceptions, for example, OH, a stronger base than ArO , is a poorer nucleophile. In a series of similar nucleophiles, such as substituted anilines, nucleophilicity is correlated with base strength. Oddly, the cyanide ion is not a nucleophile for aromatic systems, except for sulfonic acid salts and in the von Richter (13-30) and Rosenmund-von Braun (13-8) reactions, which are special cases. 

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