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 many types of reactions, including hydroformylation and Reppe Chemistry. These reactions require metal catalysts, which bind and activate the CO. Much of this theme was developed by Walter Reppe.
- RCH=CH2 + H2 + CO → RCH2CH2CHO
The reaction requires metal catalysts that bind CO, forming intermediate metal carbonyls. Many of the commodity carboxylic acids, i.e. propionic, butyric, valeric, etc, as well as many of the commodity alcohols, i.e. propanol, butanol, amyl alcohol, are derived from aldehydes produced by hydroformylation. In this way, hydroformylation is a gateway from alkenes to oxygenates.
- RCHO → RH + CO
Few catalysts are highly active or exhibit broad scope.
Acetic acid and acetic anhydride
Large-scale applications of carbonylation are the Monsanto and Cativa processes, which convert methanol to acetic acid. In another major industrial process, Acetic anhydride is prepared by a related carbonylation of methyl acetate.
- 2 CH3OH + 1/2 O2 + CO → (CH3O)2CO + H2O
The oxidative carbonylation of methanol is catalyzed by copper(I) salts, which form transient carbonyl complexes. For the oxidative carbonylation of alkenes, palladium complexes are used.
Hydrocarboxylation and hydroesterification
- RCH=CH2 + H2O + CO → RCH2CH2CO2H
- ArCH(CH3)OH + CO → ArCH(CH3)CO2H
- C2H4 + CO + MeOH → CH3CH2CO2Me
The Koch reaction is a special case of hydrocarboxylation reaction that does not rely on metal catalysts. Instead, the process is catalyzed by strong acids such as sulfuric acid or the combination of phosphoric acid and boron trifluoride. The reaction is less applicable to simple alkene. The industrial synthesis of glycolic acid is achieved in this way:
- CH2O + CO + H2O → HOCH2CO2H
- Me2C=CH2 + H2O + CO → Me3CCO2H
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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