The Thermal Decomposition of Hydroxy- and Methoxy-Substituted Anisoles

Article abstract of DOI:10.1021/ja00186a042

Rates of decomposition of anisole and the three isomeric hydroxyanisoles and methoxyanisoles along with phenetole (ethoxybenzene) and o-hydroxyphenetole were determined in a very low pressure pyrolysis apparatus.In all reactions, the predominant process was O-methyl bond homolysis.Therefore, relative rates provide a quantitative measure of the effects of hydroxy and methoxy substituents on bond strengths in anisoles.A large bond-weakening effect was observed for the o-hydroxy group (7 kcal mol^-1), accounting for the exceptional lability of guaiacol observed in thecondensed-phase studies of both Vuori and Virk.The p-hydroxy group weakened the anisole O-methyl bond by 2.5 kcal mol^-1 and meta substitution had little effect.Both o- and p-methoxy groups weakened the O-methyl bond by 4 kcal mol^-1 and m-methoxy substitution weakened this bond by 1 kcal mol^-1.Rates of all the reactions, when extrapolated to lower temperatures, were similar to rates of condensed-phase thermolysis reactions, indicating that homolysis is a key step in the liquid phase reactions.Other findings and conclusions are as follows: (1) Measured substituent effects do not correlate well with published rates of H abstraction from substituted phenols. (2) Bond breaking in anisoles occurs in a direct manner, without initial migration of the methyl group to the ring. (3) The generally observed higher reactivity of o-quinones than p-quinones may be due, in large part, to the special stabilization of the o-hydroxyphenoxy radical. (4) Theoretical (AMI) calculations yield the correct magnitude of the measured bond-weakening effects, but they do not reproduce important trends.