3.3.1. Nitrobenzene oxidation
The aim of NO is to evaluate the condensation degree (frequency
of C C linkages between aromatic rings) and the relative
proportion between the aromatic moieties in the non-condensed
fractions: p-hydroxyphenyl:guaiacyl:syringyl (H:G:S) as described
elsewhere (Costa et al., 2014; Pinto et al., 2010). H:G:S provides
the basic information about the nature of the lignin. Results are
based on the yields and types of simple phenolic aldehydes and
acids produced in the oxidation.
Condensation degree is inversely correlated with NO total yield
because aryl–aryl covalent linkages are resistant to the depolymerization
induced by NO and thus restricting the monomeric
phenolics production. The evaluation of this parameter is important
since the frequency of C C linkages is also inversely related
with lignin reactivity.
Besides the reliable evaluation of condensation degree of lignin
with basis in the total yield, the NO results allow the quantification
of the maximum individual precursor available to the production
of each aldehyde. This is important as an approach to evaluate
lignin as source of monomeric phenolics, in particular, vanillin and
syringaldehyde, two products with high commercial value (Pinto
et al., 2012).