Materials. Picrylimidazole was synthesized from picryl chloride (1 mmol) and imidazole (2 mmol) in 2 mL of N,N-dimethylformamide. After 30 min at room temperature, in the dark, the solution was poured into ice-cooled water, filtered, and washed with water and ethanol. The light yellow crystals decompose at 187-188 " C (lit.36 205 "C). The NMR spectrum agrees with that of the literat~re.~' Attempts to purify the product by recrystallization or column chromatography resulted in decomposition of picrylimidazole; hence, we use the compound freshly synthesized without further purification. The purity of S was checked by UV-visible spectra under acidic and basic conditions. Under basic conditions (NaOH, 0.01 M) it rapidly hydrolyzes, giving a spectrum coincident with that of picric acid (Amx = 358 nm (c = 15000 M-' cm-I)). In acid solution there was no absorption at h = 358 nm. D a h was twice recrystallized from petroleum ether (bp 6C-80 "C), mp 155-156 OC. Imidazole was recrystallized from benzene, mp 89-90 "C. Acetic acid was distilled, bp 118 OC. Dioxane was purified by the method of Fieser'* and was stored over LiAIH4 from which it was distilled as needed. Water twice distilled in a glass apparatus was used throughout.All the inorganic salts were reagent grade commercial reagents and were used without further purification.The buffer solutions were prepared from solutions of one of the buffer species of known concentration. After the required amount of acid or base and compensating electrolyte was added, the pH of the solutions were determined. If the pH of the set with constant buffer ratio differ by more than 5 X lo-' pH unit, a drop of diluted acid or base was added to adjust the pH to that of the more concentrated solution. Four or more (36) Coburn, M. D.; Neuman, P. N.buffer concentrations were used at each pH.UV spectra were recorded on a Beckman 24 spectrophotometer, and the change in optical density during a kinetic run was measured on the same instrument at the maximum absorption of picric acid (A = 358 nm). pH measurements were carried out on a Seybold digital pH meter at 25 "C. Standard buffers prepared according to literat~re'~ were used to calibrate the electrode.Kinetic Procedures. Reactions were initiated by adding the substrate dissolved in dioxane to a solution containing all other constituents. Rate constants were determined by following the appearance of picric acid at 25 OC and p = 1 M. All kinetic runs were carried out under pseudofirst-order conditions with substrate concentrations of about 5 X 10"-4 X 10-4 M. Rate constants are accurate to &3% and were computed from plots of In (OD, -OD,) vs. time. Most reactions were followed to 80-9096 conversion, but for the slowest runs the reaction was followed up to 10-5096 conversion. The infinity value for these reactions was determined by hydrolysis of a portion of the substrate solution at high pH. The hydrolyzed substrate was diluted to a concentration suitable for reading the absorption (OD = 0.8) with a buffer of the same pH as the reaction solut...
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