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 carbonylation also refers to oxidation of protein side chains.
Several industrially useful organic chemicals are prepared by carbonylations, which can be highly selective reactions. Carbonylations produce organic carbonyls, i.e., compounds that contain the C=O functional group such as aldehydes, carboxylic acids, and esters. Carbonylations are the basis of two main types of reactions, hydroformylation and Reppe Chemistry.
- RCH=CH2 + H2 + CO → RCH2CH2CHO
The reaction requires metal catalysts that bind the CO, the H2, and the alkene, allowing these substrates to combine within its coordination sphere.
- RCHO → RH + CO
Reppe Chemistry, named after Walter Reppe, entails addition of carbon monoxide and an acidic hydrogen donor to the organic substrate. Large-scale applications of this type of carbonylation are the Monsanto and Cativa processes, which convert methanol to acetic acid. Acetic anhydride is prepared by a related carbonylation of methyl acetate. In the related hydrocarboxylation and hydroesterification, alkenes and alkynes are the substrates. This method is used in industry to produce propionic acid from ethylene:
- RCH=CH2 + H2O + CO → RCH2CH2CO2H
These reactions require metal catalysts, which bind and activate the CO. In the industrial synthesis of ibuprofen, a benzylic alcohol is converted to the corresponding carboxylic acid via a Pd-catalyzed carbonylation:
- ArCH(CH3)OH + CO → ArCH(CH3)CO2H
Acrylic acid was once prepared by the hydrocarboxylation of acetylene but is now produced by the oxidation of propene. The hydrocarboxylation of alkenes is a prominent example of Reppe chemistry. In industry, propanoic acid is mainly produced by the hydrocarboxylation of ethylene using nickel carbonyl as the catalyst:
- H2C=CH2 + H2O + CO → CH3CH2CO2H
Hydroesterification is like hydrocarboxylation, but uses alcohols instead of water.
The Koch reaction (also the related Koch-Haaf reactions) entails the addition of CO to unsaturated compounds in the presence of strong acids such as sulfuric acid. This method is less frequently used in industry as are the metal-catalyzed reactions, described above. The industrial synthesis of glycolic acid is achieved in this way:
- CH2O + CO + H2O → HOCH2CO2H
- (CH3)2C=CH2 + H2O + CO → (CH3)3CCO2H
- 2 CH3OH + 1/2 O2 + CO → (CH3O)2CO + H2O
Carbonylation in inorganic chemistry
Metal carbonyls, compounds with the formula M(CO)xLy (M = metal; L = other ligands) are prepared by carbonylation of transition metals. Iron and nickel powder react directly with CO to give Fe(CO)5 and Ni(CO)4, respectively. Most other metals form carbonyls less directly, such as from their oxides or halides. Metal carbonyls are widely employed as catalysts in the hydroformylation and Reppe processes discussed above. Inorganic compounds that contain CO ligands can also undergo decarbonylation, often via a photochemical reaction.
The modification of side chains in a few native amino acids such as histidine, cysteine, and lysine in proteins to carbonyl derivatives (aldehydes and ketones) is known as protein carbonylation. Oxidative stress, often metal catalyzed, leads to protein carbonylation.
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