Monsanto rate-limiting Step

The Monsanto process has been thoroughly studied, and the relatively simple catalytic mechanism in Scheme 17.1, consisting of only anionic rhodium species, has been proposed. Virtually any source of rhodium or of iodidemay be introduced as a "precatalyst" to generate [Rh(CO)2lJ and Mel imder catalytic conditions. High-pressure IR spectroscopy has shown that the major rhodium species in the catalytic solutions is [Rh(CO)2l2]"(v Q = 2055 and 1985 cm" ). The overall rate law for the formation of acetic acid contains a linear dependence on rhodium and methyl iodide and no dependence on the concentrations of reactants (CO and methanol) and product (acetic acid). Consistent with the rate law, the turnover-limiting step has been proposed to be the oxidative addition of Mel to [Rh(CO)jIj]". 

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