HALIDE EFFECTS IN THE HYDROLYSIS REACTIONS OF (+ ,10-ALPHA-EPOXY-7,8,9,10-TETRAHYDROBENZO[A]PYRENE/

Rates of reaction of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-e poxy-7,8, (DE-2) have been determined in 1:9 dioxane-water solutions c ontaining 1.0 M KCI, 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 mechani sms in which halide ion attacks the diol epoxide as a nucleophile at i ntermediate pH, resulting in the formation of a trans-halohydrin. Reac tion of DE-2 in these halide solutions at pH < ca. 5 occurs by rate-li miting carbocation formation, followed by capture of the intermediate carbocation by halide ion. The relative magnitudes of the rate constan ts for reaction of the intermediate carbocation with halide ions are e stimated from product studies. The halohydrins are unstable intermedia tes and react quickly in subsequent reactions to yield tetrols in a ra tio different than that formed from reaction of the carbocation with s olvent. Nucleophilic attacks of 1.0 M Cl-, 0.5 M Br-, and 0.1M I- on D E-2 are the principal reactions in the pH range cat 6-9, leading to in termediate 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 reactio n. At pH > 12, the rate of reaction of DE-S increases due to a second- order reaction of HO- with DE-2.