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 ...
of
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''-
2I2">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''-
2I2">h(CO)2I2sup>− to form the
hexacoordinate species
3)Rh(CO)2I3">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
3CO)Rh(CO)I3">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 (CH
3C(O)I). The
catalytic cycle involves two non-organometallic steps: conversion of methanol to
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 ...
and the hydrolysis of the
acetyl iodide to
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 ...
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
2I2">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
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 ...
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.
: CH
3CO
2CH
3 + CO → (CH
3CO)
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