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 mainly prepared by the hydrocarboxylation of acetylene (modern technology relies on 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
- C2H4 + CO + MeOH → MeO2CCH2CH3
- 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.
Carboalkoxylation and hydrocarboxylationEdit
In industrial chemistry, carboalkoxylation, also called hydroesterification, is a process for converting alkenes with alcohol specifically to esters. That reaction is a form of carbonylation; a closely related reaction is hydrocarboxylation, which employs water in place of alcohols.
- C2H4 + CO + MeOH → MeO2CC2H5
The process is catalyzed by Pd[C6H4(CH2PBu-t)2]2. Under similar conditions, other Pd-diphosphines catalyze formation of polyethyleneketone.
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
- (CH3)2C=CH2 + H2O + CO → (CH3)3CCO2H
Carbonylation in inorganic chemistryEdit
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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