STUDIES ON THE MECHANISM OF THE PHOTOYELLOWING OF BLEACHED MECHANICAL AND CHEMIMECHANICAL PULPS .4. THE ROLE OF RING-CONJUGATED ETHYLENIC GROUPS

The 4-O-methyl ethers of isoeuigenol (I) and coniferyl alcohol (II) were examined for their ability to produce and/or react with singlet oxygen or a hydroxyl radical on irradiation with visible light to form veratraldehyde and veratric acid. Compound III proved to be a poor photosensitizer for the light-induced formation of singlet oxygen. Exposure of I and II to singlet oxygen for 24 h led to low (< approximately 1%) net conversions to veratraldehyde. Irradiation of I and II on a filter paper surface with a 1000 W mercury-tungsten light source had the effect of reducing brightness by less than 2.5 units. As a result of this irradiation, the ethylenic double bonds in these two compounds were ruptured to a minor extent as indicated by the recovery of veratraldehyde in yields of approximately 7% (I) and 1.4% (II), respectively. When exposed to the same light source, veratraldeheyde underwent extensive photoyellowing and was converted to veratric acid (1.7%) and other unidentified products. Results of experiments designed to identify the reactants (e.g. O-1(2) and .OH) responsible for the small extent of photooxidation observed were contradictory.