Use of competition kinetics with fast reactions of grignard reagents

Competition kinetics are useful for estimation of the reactivities of Grign ard reagents if the reaction rates do not differ widely and if exact rates are not needed. If the rate of mixing is slower than the rate of reaction, the ratios between the rates of fast and slow reagents are found to be too small. This is concluded from experiments in which results obtained by comp etition kinetics are compared with results obtained directly by flow stream procedures. A clearer picture of the reactivity ratios is obtained when th e highly reactive reagent is highly diluted with its competitor. A fast rea gent may account for almost all the product even when present as only 1 par t in 100 parts of the competing agent. In this way allylmagnesium bromide i s estimated to react with acetone, benzophenone,. benzaldehyde, and diethyl acetaldehyde ca. 1.5 x 10(5) times faster than does butylmagnesium bromide. The rates found for the four substrates do not differ significantly, and i t seems possible that there is a ceiling over the rate of reaction of this reagent, for example, caused by diffusion control. This may explain that co mpetition kinetics using allylmagnesium bromide have failed to show kinetic isotope effects or effects of polar substituents with isotopically or othe rwise substituted benzophenones. A recently reported alpha-deuterium second ary kinetic isotope effect for the reaction of benzaldehyde with allylmagne sium bromide was observed at -78 degrees C, but was absent at room temperat ure. It is suggested that the reaction of benzophenone and benzaldehyde wit h allylmagnesium bromide has a radical-concerted mechanism since no radical -type products are produced and since no color from an intermediate ketyl i s observed even at -78 degrees C.