Reppe synthesis
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.
Hydroformylation entails the addition of both carbon monoxide and hydrogen to unsaturated organic compounds, usually alkenes. The usual products are aldehydes:
The reaction requires metal catalysts that bind the CO, the H2, and the alkene, allowing these substrates to combine within its coordination sphere.
Many organic carbonyls undergo decarbonylation. A common transformation involves the conversion of aldehydes to alkanes, usually catalyzed by metal complexes:
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:
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