Base strength mesomeric effects

Amino groups are strong mesomeric electron donors and hence base strengthening. The order of base strength is 4-amino > 2-amino (increased importance of opposing inductive effect) > 3-amino (minimal mesomeric effect). 

Conversely, if a. para substituent stabilizes the conjugate base of an acid-base pair rather more than it stabilizes the benzoate ion, more positive substituent constants are required to achieve linearity in Hammett plots. Examples of this are acid dissociations of phenols and anilinium ions, where mesomerically electron-withdrawing substituents (Y = —NO2, —C N) are more effective in enhancing acid strength than they are in benzoic acid, because charge delocalization of the type [15] is not possible in the benzoate anion. 

Electron-releasing substituents generally increase the basic strength 2-methyl- (pK 5.97), 3-methyl (5.68) and 4-methylpyridine (6.02) illustrate this. The basicities of pyridines carrying groups that can interact mesomerically as well as inductively vary in more complex ways, for example 2-methoxypyridine (3.3) is a weaker, but 4-methoxypyridine (6.6) a stronger base than pyridine the effect of inductive with-... 

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