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. 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 amide group can be involved in hydrogen bonding to other nitrogen- and oxygen-containing species. The proton attached to the nitrogen can also move to the oxygen to give the second tautomer form, 2-hydroxypyridine. This lactam lactim tautomerism can also be exhibited in many related compounds. 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 determination of which of the two tautomeric forms is present in solution has been the subject of many publications. The energy difference appears to be very small and is dependent on the polarity of the solvent. Non-polar solvents favour the formation of 2-hydroxypyridine whereas polar solvents such as alcohols and water favour the formation of 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.