2-Pyridone

2-Pyridone is an organic compound with the formula C5H4NH(O). It is a colourless solid. It is well known to form hydrogen bonded dimers and it is also a classic case of a compound that exists as tautomers. The second tautomer is 2-hydroxypyridine. This lactam lactim tautomerism can also be exhibited in many related compounds. The amide group can be involved in hydrogen bonding to other nitrogen- and oxygen-containing species. The predominant solid state form is 2-pyridone. This has been confirmed by X-ray crystallography which shows that the hydrogen in solid state is closer to the nitrogen than to the oxygen (because of the low electron density at the hydrogen the exact positioning is difficult), and IR-spectroscopy, which shows that the C=O longitudinal frequency is present whilst the O-H frequencies are absent. The tautomerization has been exhaustively studied. The energy difference appears to be very small. Non-polar solvents favour 2-hydroxypyridine whereas polar solvents such as alcohols and water favour the 2-pyridone. The energy difference for the two tautomers in the gas phase was measured by IR-spectroscopy to be 2.43 to 3.3 kJ/mol for the solid state and 8.95 kJ/mol and 8.83 kJ/mol for the liquid state. The single molecular tautomerisation has a forbidden 1-3 suprafacial transition state and therefore has a high energy barrier for this tautomerisation, which was calculated with theoretical methods to be 125 or 210 kJ/mol. The direct tautomerisation is energetically not favoured. There are other possible mechanisms for this tautomerisation. 2-Pyridone and 2-hydroxypyridine can form dimers with two hydrogen bonds. In the solid state the dimeric form is not present; the 2-pyridones form a helical structure over hydrogen bonds. Some substituted 2-pyridones form the dimer in solid state, for example the 5-methyl-3-carbonitrile-2-pyridone. The determination of all these structures was done by X-ray crystallography. In the solid state the hydrogen is located closer to the nitrogen so it could be considered to be right to call the colourless crystals in the flask 2-pyridone.