Ozonation of a beta-O-4 dimer lignin model: By-product identification and reaction pathways

In order to better understand the reaction mechanisms that lead to the beta -O-4 cleavage and the production of aromatic aldehydes during the ozonolys is of lignin structures, the goal of our work was to determine the degradat ion pathways of a lignin model compound (beta -O-4 dimer: G-G phenolic alky l-aryl-ether dimer) subjected to molecular ozone in aqueous solution. At lo w pH (used to favor the molecular ozone reaction mode), the beta -O-4 dimer is rapidly dehydrated to produce a styrene derivative. The original dimer and its dehydrated derivative were found to react differently with ozone. T he C-beta-H bound of the original dimer is activated by the nearest hydroxy l group which permits an electrophilic addition of molecular ozone, leading to the production of guaiacol and vanillin. Guaiacol is further degraded i n para-hydroquinone and para-benzoquinone as end-reaction by-products. Mole cular ozone reacts on the dehydrated derivative (styrene) according to a di polar cycle-addition which leads to the production of homovanillin and guai acol. Probably because of the strong reactivity of ozone with activated aro matic ring and possibly due to secondary reaction such as Bayer-Villiger cl eavage, vanillin was detected as a trace compound.