Substituent effects on the dehydration of arene hydrates in aqueous solution

Abstract

Rate constants have been determined by UV spectrophotometry at 25 °C for the acid‐catalyzed dehydration of different types of monocyclic arene hydrates including those substituted at the 1‐, 2‐ or 3‐positions. General acid catalysis was not observed, and linear plots of pseudo‐first‐order rate constants for dehydration against hydronium concentration were obtained. A Hammett plot of the second‐order rate constants for acid‐catalyzed dehydration, kH (M−1s−1), of unsubstituted‐ (8a), 3‐substituted (8b, 8c, 8d, 8e) and 1‐substituted‐benzene hydrates (14f and 14h) shows an excellent correlation with σ+ values and yields a large negative ρ‐value of –6.5. The results are consistent with rate‐determining formation of a benzenium ion in which direct mesomeric interaction with the substituent occurs, presumably permitted by the coplanar arrangement of the diene and carbocation centre in the intermediate. Data points for 2‐substituted arene hydrates (13f, 13g, 13h, 13i) deviate negatively from the Hammett plot as direct mesomeric interaction with the substituent is not possible in the corresponding benzenium intermediates. Copyright © 2013 John Wiley & Sons, Ltd.