α-Substituent Effects Modulate the Reactivity of C2 Monocarboxylic Acids for Efficient Hemicellulose Removal from Lignocellulose

Efficient and selective fractionation of lignocellulose is essential for downstream valorization, yet pretreatment with conventional monocarboxylic acids is generally inefficient. The inductive effects of α-substituents in C2 monocarboxylic acids (C2-MCAs) may govern lignocellulose deconstruction, but underlying molecular mechanisms remain unclear. Herein, glyoxylic acid (GoA) pretreatment of Eucalyptus was evaluated and benchmarked against glycolic acid (GcA) and acetic acid (AA). GoA removed 86.21% of hemicellulose (150 °C, 4% GoA, and 30 min) and exhibited the lowest apparent activation energy (12.74 kJ/mol), compared with GcA (59.85 kJ/mol) and AA (124.98 kJ/mol). Electrostatic potential and energy-decomposition analyses indicated enhanced proton-donating ability and the strongest interaction with a lignin-carbohydrate complex (−62.69 kJ/mol) for GoA. This suggested that stronger preassociation may facilitate hemicellulose removal. GoA pretreatment reduced lignin β-O-4 abundance and molecular weight while increasing phenolic hydroxyl content. These findings establish a structure–activity relationship between C2-MCAs and hemicellulose separation, providing a molecular basis for efficient organic-acid pretreatment.