The carbonylation of olefins with CO and a nucleophilic reaction partner possessing a labile H atom results in the formation of carboxylic acids or their derivatives such as esters, thioesters, amides, anhydrides, etc., in the presence of metal carbonyls:

Thus this type of carbonylation reaction, employing metal carbonyls as catalysts (i. e., carbonyls of Ni, Co, Fe, Rh, Ru, and Pd), belongs to the wide range of Reppe reactions. Analogous to hydroformylation, the conversion of olefins with CO and H₂O to carboxylic acids is designated hydrocarboxylation.

The Koch reaction, is a similar hydrocarboxylation which starts with the same materials (such as olefin, CO and H₂O), but which, by using proton catalysts and milder reaction conditions, leads mainly to tertiary carboxylic acids.

The reaction of ethylene with CO and H₂O to form propionic acid is an example of an industrially operated hydrocarboxylation under Reppe conditions. The reaction takes place at 200-240 bar and 270- 320 °C in a liquid phase consisting of the components and the catalyst Ni propionate dissolved in crude propionic acid:



Ni(CO)₄, the actual catalyst, is formed in situ. Other carbonyls, such as Fe or Co carbonyls, are also catalytically active. After the reaction, the Ni(CO)₄ present in the crude product is reconverted into the propionate by air oxidation in the presence of propionic acid and recycled to the reactor. The propionic acid yield amounts to 95% (based on C₂H₄). The byproducts are CO₂, ethane and higher carboxylic acids, which, due to the synthetic route, possess an odd number of carbon atoms, e.g., valeric acid. If, instead of H₂O, another reactant with an active hydrogen is used, then carboxylic acid derivatives can be formed directly. For example, propionic acid esters can be manufactured directly by replacing water with alcohols.

One process improvement - a more active Reppe catalyst - has been developed by Halcon and also by Eastman. By adding halogen (e.g., iodine) and phosphine ligands to the standard Ni/Mo catalyst, reaction conditions could be reduced to 10-35 bar and 175-225°C without loss of yield.