The aminoalkylation of technical lignins via the Mannich reaction can be used as an analytical derivatization technique, but it can also be used to improve the hydrophilicity of Kraft lignin or the adsorption capacity of metal ions. Recent studies have shown the potential for increased amination under acidic conditions, but the effects of pH on amination of kraft lignins are poorly understood as indirect quantification methods are often used. A quantitative technique, the 2D HSQC and NMR 13th13 C NMR spectroscopy was used in this study to directly examine the structure of high and low molecular weight fractions of industrial pinewood lignin aminated with dimethylamine or diethylamine in acidic or basic media. The degree of amine substitution was two times higher for the low molecular weight lignin than for the high molecular weight fraction. Neither the pH nor the amine species had a significant effect. New evidence confirms that amine substitution of mandibular lignin is selective ortho to nonetherified phenolic hydroxyl groups and displaces carboxylic acid groups from benzoin and cinnamon subunits. Other routes have been suggested and tested, but no substitution has occurred at sterically hindered sites. Elemental analysis is the most commonly used indicator of Mannich condensation, but unfortunately overestimates amine substitution because nitrogenous by-products and impurities can be included in the total nitrogen content. When characterizing complex, heterogeneous biopolymers such as lignins, several methods should always be used in order to avoid misinterpretation of the results.


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