Nitrobenzene and Cupric Oxide Oxidations

Abstract

Nitrobenzene oxidation of lignins in alkaline solution was introduced in 1939 by Freudenberg and co-workers (Freudenberg 1939, Freudenberg and Lautsch 1939, Freudenberg et al. 1940) to obtain further evidence for the aromatic nature of lignins, which had been established earlier by potassium permanganate oxidation (Freudenberg et al. 1936, 1937, Freudenberg and Müller 1938) (Sect. 6.3). Vanillin (2) (Fig. 6.2.1) was produced in a yield of about 25% (based on Klason lignin) on oxidation of extractive-free spruce (Picea abies) wood meal with nitrobenzene in 2 M sodium hydroxide solution at 160°C for 3 h. When the wood residue from the oxidation was treated again under the same conditions, about 2% additional vanillin was obtained, in addition to insignificant amounts of guaiacol (6), vanillic acid (10), 5-carboxyvanillin (13) (Fig. 6.2.1), and some unidentified phenols. The method applied was a modified Bischler procedure for conversion of isoeugenol into vanillin. Earlier, Bischler produced vanillin (2) in a yield of about 90mol% by oxidation of isoeugenol with nitrobenzene in alkaline solution at high temperature (Freudenberg et al. 1940).