Kinetics and mechanism of iodination of 1-methylpyrazole
A spectrophotometric method was used to determine the rate law in buffered aqueous iodine-iodide solutions. Three iodinating agents other than iodine were considered: direct attack by hypoiodous acid, acid catalyzed attack by hypoiodous acid, and the iodinium ion. The first two were discarded because of the observed dependence of the rate on the buffer concentration and the observed independence of the rate on the hydrogen ion concentration.
The postulated mechanism is in agreement with the mechanisms for aniline and pyrazole. The rate determining step is the removal of the proton from the intermediate by the buffer. However, it was found that iodine was the major iodinating species and that the iodinium ion had a much smaller effect.
A qualitative discussion of the reactivities of pyrazole, J-methylpyrazole., and l-methylpyrazole was made. An attempt was made to show possible reasons for the observed results.