ALKALINE-HYDROLYSIS OF NONPHENOLIC BETA-O-4 LIGNIN MODEL DIMERS - FURTHER-STUDIES OF THE SUBSTITUENT EFFECT ON THE LEAVING PHENOXIDE

Reaction rates and activation parameters for the alkaline hydrolysis o f nonphenolic beta-O-4 lignin models stiutedphenoxy)-1-(3,4-dimethoxyp henyl)-1-ethanol] were compared to 2-methoxylated Bring analogues run earlier. The Hammett plot with the best fit used sigma(-) values to gi ve a rho of 1.9 +/- 0.2, while the 2-methoxylsubstituted compounds had a rho of 2.3 +/- 0.3. Compounds with a methoxyl group at the ortho po sition of the leaving phenoxide ion hydrolyzed slightly faster than th e nonmethoxylated analogues, indicating that the 2-methoxy group is a weak electron-with-drawer. The strong electron-withdrawing 4-CHO and 4 -CF3 substituents gave significantly different activation parameters t han the unsubstituted compound. Experiments with O-18-labeled water su ggested that our earlier proposed SNAr mechanism for the alkaline hydr olysis of strong electron-withdrawing compounds is nut correct. It app ears that all nonphenolic beta-O-4 dimers are hydrolyzed by a neighbor ing group mechanism.