Monsanto process
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The Monsanto process in organometallic chemistry is a major route to acetic acid and rhodium-catalysed. The Monsanto process operates at a pressure of 30-60 atm and a temperature of 150-200˚C and gives a selectivity greater than 99%. The Monsanto process has largely been supplanted by an iridium-based catalytic carbonylation of methanol, which is greener and more economical (see acetic acid article for ref.).
[edit] Catalytic Cycle
The catalytically active species is the anion cis-[Rh(CO)2I2]- (1). The catalytic cycle has been shown to involve six steps, two of which are purely organic. The first step involving the metal is the oxidative addition of methyl iodide (by reaction of methanol with catalytic Hydrogen iodide) to cis-[Rh(CO)2I2]- to form the hexacoordinate species [(CH3)Rh(CO)2I3]- (2). This rapidly transforms, via the migration of a methyl group to the carbonyl ligand, into the pentacoordinate acetyl complex [(CH3CO)Rh(CO)I3]- (3). It next reacts with carbon monoxide to form the six coordinate dicarbonyl complex (4) which decomposes by reductive elimination to form the acetic acid iodine compound (CH3COI) and regenerate the active form of the catalyst. Acetic acid iodine is then hydrolyzed to acetic acid.
The reaction has been shown to be first order with respect to methyl iodide and the catalyst (1). Hence the rate-determining step of the catalytic cycle has been proposed to be the oxidative addition of methyl iodide to the catalyst (1). It is believed that this occurs via the nucleophilic attack by the rhodium centre on the carbon of methyl iodide.
