The oxidation of d‐galactose into mucic acid (galactaric acid): experimental and computational insights towards a bio‐based platform chemical

Aldaric acids are becoming valuable platform chemicals in bioeconomy and circular economy approaches. Among them, mucic acid (meso-galactaric acid) is gaining industrial attention as a bio-based precursor of adipic acid found in polyamides, and of 2,5-furan dicarboxylic acid, a valuable substitute for terephthalic acid in polyesters. A combination of detailed experimental protocols and conditions with DFT computational calculations for the synthesis of mucic acid from galactose are herein described. The product is obtained by reaction with diluted aqueous nitric acid as the oxidizing agent and the reaction medium. Optimized reaction conditions require 5 M nitric acid, 95 °C, galactose/HNO3 molar ratio 1 : 9, 100 g/L galactose concentration in the reaction medium. Computational calculations suggest that the oxidation mechanism may involve a contextual oxygen protonation and nitrate nucleophilic attack, leading to the oxidized product. The proposed mechanism requires a stoichiometry of 6 moles of nitric acid per galactose mole, albeit both experimental and computational data evidence that proton excess may be necessary to start the reaction. The reported study gives useful insights for the production of galactaric acid.