The Monsanto process is an industrial method for the manufacture of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

by catalytic

carbonylation Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbon ...

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

. The Monsanto process has largely been supplanted by the Cativa process, a similar

iridium Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density o ...

-based process developed by BP Chemicals Ltd which is more economical and environmentally friendly. This process operates at a

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

of 30–60 atm and a

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

of 150–200 °C and gives a selectivity greater than 99%. It was developed in 1960 by the German chemical company, BASF, and improved by the

Monsanto Company The Monsanto Company () was an American agrochemical and agricultural biotechnology corporation founded in 1901 and headquartered in Creve Coeur, Missouri. Monsanto's best known product is Roundup, a glyphosate-based herbicide, developed i ...

in 1966, which introduced a new catalyst system.

Catalytic cycle

The catalytically active species is the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

''cis''- h(CO)₂I₂sup>− (top of scheme). The first organometallic step is the oxidative addition of

methyl iodide Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...

to ''cis''- h(CO)₂I₂sup>− to form the hexacoordinate species CH₃)Rh(CO)₂I₃sup>−. This anion rapidly transforms, via the migration of a

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ...

group to an adjacent carbonyl

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...

, affording the pentacoordinate acetyl complex CH₃CO)Rh(CO)I₃sup>−. This five-coordinate complex then reacts with

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

to form the six-coordinate dicarbonyl complex, which undergoes reductive elimination to release acetyl iodide (CH₃C(O)I). The catalytic cycle involves two non-organometallic steps: conversion of methanol to

and the hydrolysis of the acetyl iodide to

and hydrogen iodide. The reaction has been shown to be

first-order In mathematics and other formal sciences, first-order or first order most often means either: * "linear" (a polynomial of degree at most one), as in first-order approximation and other calculus uses, where it is contrasted with "polynomials of hig ...

with respect to methyl iodide and h(CO)₂I₂sup>−. Hence the oxidative addition of methyl iodide is proposed as the rate-determining step.

Tennessee Eastman acetic anhydride process

Acetic anhydride Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a co ...

is produced by

of methyl acetate in a process that is similar to the Monsanto acetic acid synthesis. Methyl acetate is used in place of methanol as a source of methyl iodide. : CH₃CO₂CH₃ + CO → (CH₃CO)₂O In this process lithium iodide converts methyl acetate to lithium acetate and methyl iodide, which in turn affords, through carbonylation, acetyl iodide. Acetyl iodide reacts with acetate salts or acetic acid to give the anhydride. Rhodium iodides and lithium salts are employed as catalysts. Because acetic anhydride hydrolyzes, the conversion is conducted under anhydrous conditions in contrast to the Monsanto acetic acid synthesis.

References

External links

* https://web.archive.org/web/20050412040850/http://www.uyseg.org/catalysis/ethacid/ethacid2.htm {{Organometallics Organometallic chemistry Rhodium Catalysis Chemical processes Monsanto