Alkylpyridines tautomerism

Analogous to the tautomerism of the hydroxy- and amino-pyridines (Sections 3.2.3.7.1 and 3.2.3.5.5), there are alternative tautomeric alkylidene forms of the 2- and 4-alkylpyridines e.g. 609 for 2-picoline). Although the proportion of alkylidene form at equilibrium is very small (as is discussed in Section 2.2.5.1.2), it can be important as a reactive intermediate (see above). 

Under proton or Lewis acid catalysis, 2- and 4-alkylpyridines of the above type are in equilibrium with the tautomeric methylene bases 57 and 58, which can function as enamines ... 

The acidity is, of course, increased when R and R in (17) are electron-attracting, and when the nitrogen atom is protonated or quaternized. Some comparative data are available for phenyl-substituted compounds in the last conditionii2 xhe system of ionic and tautomeric equilibria shown occurs with, for example, a 4-alkylpyridine. Ka has, of course, been measured in many cases, and Kt is commonly very large in favour of the NM (alkyl-pyridine) form rather than the Z (methide) form (pp. 137, 154). The remaining constants Kiy Kz and Kz are unknown. K can be approximated by the equivalent constant for the quaternary system ... 

As noticed before (p. 137), the tautomeric equilibrium vastly favours the alkylpyridine NM) forms, and even with electron-attracting acyl groups present the alkyl hydrogen atom is extremely weakly acidic. The evidence, so far as it goes, gives the order of acidity GH(4) > GH(2) > GH(3>. 

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