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Alkaline Hydrolysis of Nonphenolic ß-O-4 Lignin Model Dimers: Further Studies of the Substituent Effect on the Leaving Phenoxide

Reaction rates and activation parameters for the alkaline hydrolysis of nonphenolic β-O-4 lignin models [2-(4-substituted phenoxy)-1-(3,4-dimethoxyphenyl)-1-ethanol] were compared to 2-methoxylated B-ring analogues run earlier. The Hammett plot with the best fit used σ - values to give a ρ of 1.9 ± 0.2. while the 2-methoxyl-substituted compounds had a ρ of 2.3 ± 0.3. Compounds with a methoxyl group at the ortho position of the leaving phenoxide ion hydrolyzed slightly faster than the non-methoxylated analogues, indicating that the 2-methoxy group is a weak electron-withdrawer. The strong electron-withdrawing 4-CHO and 4-CF 3 substituents gave significantly different activation parameters than the unsubstituted compound. Experiments with 18 O-labeled water suggested that our earlier proposed S N Ar mechanism for the alkaline hydrolysis of strong electron-withdrawing compounds is not correct. It appears that all nonphenolic β-O-4 dimers are hydrolyzed by a neighboring group mechanism.