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HETEROCYCLES - AROMATICITY
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6-Membered heteroaromatics possessing -OH or -SH groups in 2,4-positions preferentially exist in solution in the keto or thioketo forms. In solution, compounds with amino groups in 2,4-positions preferentially exist in the amino form, rather than as an imino tautomer.

heterocycle tautomerism 01

[1] 2-Hydroxypyridine

[2] 2-Pyridone

[3] 4-Hydroxypyridine

[4] 4-Pyridone

[5] 2-aminopyridine

In the case of five-membered rings, the balance between thiol and thione is usually shifted to the left.

tautomerism heterocycles 02

When tautomers are in solution, the equilibrium displacement depends on:

- The concentration of the solution

- The nature of the solvent, which can generate hydrogen bonds with the tautomers.

It should also be taken into account that the tautomeric equilibrium depends little on the aromaticity of each tautomer, being often displaced towards the less aromatic tautomer.

The determination of the most favorable tautomer is carried out by means of spectroscopy (NMR, IR and UV). In this way it is known that 2-hydroxypyridine is not found as such, but as 2-pyridone.