Solvolytic elimination and hydrolysis promoted by an aromatic hydroxy-group. Part 2. The hydrolysis of 2-bromo-4-dibromomethylphenol in 95% 1,4-dioxane

Abstract

The reaction of 2-bromo-4-dibromomethylphenol with water in slightly aqueous (95%) 1,4-dioxane has been examined kinetically by using u.v. spectroscopy, which provides evidence for the transient formation of the quinone methide, 2-bromo-4-bromomethylenecyclohexa-2,5-dienone, during the hydrolysis. The final product is 3-bromo-4-hydroxybenzaldehyde. The rate of disappearance of starting material is independent of acidity, but is reduced by added or developing bromide ion. The rate of the loss of bromide ion from the phenol is very much greater than that of the corresponding reaction of 2-bromo-4-dibromomethylanisole under the same conditions. An estimate of the value of σ_p-OH⁺(–1.36), made by using these relative rates, is larger than the value (–0.92) based on relative rates of aromatic electrophilic substitution. Solvent kinetic isotope effects on these reactions are reported; the theoretical implications of variation in the substituent parameter for the hydroxy-group is discussed in terms of solvent effects on H–O hyperconjugation.