Solvolytic elimination and hydrolysis promoted by an aromatic hydroxy group. Part 1. The reaction of 2,6-dibromo-4-dibromomethylphenol and of 2,6-dibromo-4-bromomethylene-cyclohexa-2,5-dienone with water in 95% 1,4-dioxane

Abstract

The reaction of 2,6-dibromo-4-dibromomethylphenol with water to give the quinone methide, 2,6-dibromo-4-bromomethylenecyclohexa-2,5-dienone, together with 2,6-dibromo-4-hydroxybenzaldehyde has been studied in 95% 1,4-dioxane. U.v. spectroscopy has been used to study the kinetic form of the reaction. At very low acidities, rapid conversion into the quinone methide occurs through the aryl oxide ion, and is followed by addition and replacement to give the aldehyde. At acidities greater than 10^–3M, reaction of the un-ionised phenol is dominant and follows a conventional S_N1 pathway through a mesomeric carbocationic intermediate. Subsequent loss of a proton from this intermediate to give the quinone methide is competitive with its capture by the solvent (to give in several steps the related aldehyde) and by bromide ion (to give starting material).The mass-law constant for capture of the carbocation by bromide ion is very large (ca. 10⁴ in this solvent), so that relatively small concentrations of hydrogen bromide convert the quinone methide back into the dibromomethylphenol by 1,6-addition. Solvent kinetic isotope effects on the individual reaction stages are reported. The rate of solvolysis of the phenol is much greater than would be predicted by the use of conventional linear free-energy parameters.