An infrared absorption study of autoxidized methyl linoleate

Summary

A study has been made of the infrared absorption spectra of autoxidized methyl linoleate in samples ranging from PV 1 to PV 940 m.e./kg. Principal changes occur in the frequency range 3400–3550 cm⁻¹ where bonded−OH groups absorb and at 1650–1775 cm⁻¹ where >c=0 groups absorb.

Two maxima were observed in oxidized samples in the −OH absorption range: one sharp and distinct at 3467–70 cm⁻¹ which increased in intensity with increase in PV and a broad band which increased with increasing PV until it resolved into a true maximum at 3430 cm⁻¹. Reduction of typical oxidized samples with KI reagent resulted in disappearance of the 3430 cm⁻¹ band and appearance of a new band above 3500 cm⁻¹. The band at 3430 cm⁻¹ was attributed to −OOH groups associated by hydrogen bridging. The band at 3467 cm⁻¹ and the band appearing above 3500 cm⁻¹ were attributed to −OH groups, the band at the higher frequency resulting directly from reduction of a hydroperoxide.

Absorption due to ketone and aldehyde carbonyl groups appeared only as an indefinite shoulder on the band due to the ester carbonyl. These were resolved by using the intensity of the sample with PV 1 as I_o and that for the oxidized samples as I. A plot of Log I_o/I then revealed three maxima. These indicate the presence of two and possibly three carbonyl containing substances other than the ester carbonyl in autoxidized methyl linoleate.

Absorption in the two critical frequency ranges of fractions of autoxidized methyl linoleate eluted from an adsorption column correlate with interpretations made from ultraviolet absorption studies of the same substances.