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 The Cativa process is a method for the production of acetic acid by the carbonylation of methanol. The technology, which is similar to the Monsanto process, was developed by BP Chemicals and is under license by BP Plc. The process is based on an ...

, 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 BASF SE () is a German multinational chemical company and the largest chemical producer in the world. Its headquarters is located in Ludwigshafen, Germany. The BASF Group comprises subsidiaries and joint ventures in more than 80 countries ...

, 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 Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and s ...

step is the

oxidative addition Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxid ...

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 Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...

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 In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

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 In organic chemistry, acetyl is a functional group with the chemical formula and the structure . It is sometimes represented by the symbol Ac (not to be confused with the element actinium). In IUPAC nomenclature, acetyl is called ethanoyl, ...

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 In chemistry, a catalytic cycle is a multistep reaction mechanism that involves a catalyst. The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, bioinorganic chemistry, materials s ...

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

of methyl iodide is proposed as the

rate-determining step In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of the ...

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

methyl acetate Methyl acetate, also known as MeOAc, acetic acid methyl ester or methyl ethanoate, is a carboxylate ester with the formula CH3COOCH3. It is a flammable liquid with a characteristically pleasant smell reminiscent of some glues and nail polish remo ...

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