Halide Effects in the Hydrolysis Reactions of (±)-7β,8α-Dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo- [a]pyrene

Rates of reaction of (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (DE-2) have been determined in 1:9 dioxane−water solutions containing 1.0 M KCl, 0.5 M KBr, and 0.1 M NaI over the pH range 4−13. These pH−rate profiles are more complicated than those for reaction of DE-2 in 0.2 M NaClO4 solutions and are interpreted in part by mechanisms in which halide ion attacks the diol epoxide as a nucleophile at intermediate pH, resulting in the formation of a trans-halohydrin. Reaction of DE-2 in these halide solutions at pH < ca. 5 occurs by rate-limiting carbocation formation, followed by capture of the intermediate carbocation by halide ion. The relative magnitudes of the rate constants for reaction of the intermediate carbocation with halide ions are estimated from product studies. The halohydrins are unstable intermediates and react quickly in subsequent reactions to yield tetrols in a ratio different than that formed from reaction of the carbocation with solvent. Nucleophilic attacks of 1.0 M Cl-, 0.5 M Br-, and 0.1 M I- on DE-2 are the principal reactions in the pH range ca. 6−9, leading to intermediate trans-halohydrins that hydrolyze to tetrols. At pH ca. 9−11, halohydrin formed from attack of halide ion on DE-2 reverts back to epoxide, leading to a negative break in the pH−rate profile. The main product-forming reaction of DE-2 at pH 11.3 is the spontaneous reaction. At pH > 12, the rate of reaction of DE-2 increases due to a second-order reaction of HO- with DE-2.