KINETICS OF ADDITION OF METHYLMAGNESIUM BROMIDE TO 4-METHYLMERCAPTOACETOPHENONE IN DIETHYL ETHER

The ultraviolet spectrum of reacting solutions of 4-methylmercaptoacetophenone in ether solutions of methylmagnesium bromide at 25.0βdg has been found to exhibit an absorbance, βLDmax 336 mβmU (* 1.5 X 104), corresponding to complex formation with an apparent equilibrium constant of 15 1./mol. The pseudo-first-order rate constant for the addition of methylmagnesium bromide to the carbonyl group increases from 0.3 to 1.3 sec−1 with an increase in the Grignard concentration from 0.05 to 0.6 M. Reaction schemes involving conversion of the complex to product which provide a descriptive summary of these data are outlined. A twofold decrease in pseudo-first-order rate constant with an increase in ketone concentration by a factor of ca. 10, esr signals, and an absorption spectrum, βLDmax 334 and 560 mmU, observed during the reaction of methylmagnesium bromide with benzophenone, as well as the formation of benzopinacol in 14% yield from 9.6 X 10−3 M ketone and 2 M Grignard, suggest the presence of a ketyl-like side reaction in this system. © 1968, American Chemical Society. All rights reserved.