Ozonolysis of Lignin Models in Aqueous Solution: Anisole, 1,2-Dimethoxybenzene, 1,4-Dimethoxybenzene, and 1,3,5-Trimethoxybenzene

The lignin models anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene were reacted with ozone in aqueous solution, and major products were identified and quantified with respect to ozone consumption when reference material was available. Hydroxylation products in yields equivalent to those of singlet oxygen and muconic products (in analogy to the Criegee mechanism) dominate. The formation of quinones points to the release of methanol. Hydroxyl radicals ((OH)-O-center dot, quantified, main precursor: O-3(center dot-)), singlet oxygen (quantified), O-2(center dot-) radicals (quantified), and as counterparts of the (OH)-O-center dot radicals radical cations of these methoxybenzenes must each play an important role as intermediates. In the case of 1,4-dimethoxybenzene, for example, the following products were identified (yields in parentheses when quantified): methyl(2Z,4E)-4-methoxy-6-oxo-hexa-2,4-dienoate 5 (52%), hydroquinone 6 (2%),1,4-benzoquinone 7 (8%), 2,5-dimethoxyhydroquinone 8, 2,5-dimethoxy-1,4-benzoquinone 9, singlet oxygen (6%), hydrogen peroxide (56%), (OH)-O-center dot (similar to 17%), O-2(center dot-) (<= 9%). Gibbs energies for the various potential reaction pathways were calculated with the help of the Jaguar 7.5 program.