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Hydroformylation, also known as oxo synthesis, is a chemical process that adds a formyl group and a hydrogen atom to an alkene to form an aldehyde. An alkene is a molecule which contains only carbon and hydrogen atoms with at least one double bond between carbon atoms. The aldehyde that results from the hydroformylation of a specific alkene is a compound in which at least one of the carbon-carbon double bonds has been replaced with a carbon-carbon single bond and a carbon-oxygen double bond.
The aldehydes which are produced through hydroformylation are a mixture of those with linear carbon chains and those with branched carbon chains. Depending on the eventual use of the aldehydes, one form may be more desirable than another. The ratio between the two forms can be shifted by altering the conditions used in the process.
Hydroformylation is achieved by heating hydrogen gas (H2), carbon monoxide gas (CO) and an alkene under pressure. The mixture is stable under these conditions until the addition of a catalyst, a substance that causes or accelerates the chemical reaction of two or more compounds without itself being consumed or modified in the process. Varying the pressure and ratio of the gases, the temperature of the components, the catalyst used or any combination of factors can affect the ratios between the different forms of the aldehydes produced.
When Otto Roelen discovered hydroformylation in 1938, he was using a cobalt complex which acted as the catalyst in the reaction. For over 30 years, various cobalt complexes were the dominant catalysts in the industrial use of this process. Cobalt complexes which use phosphines, or hydrides of Phophorous, as the source of electrons in the reaction allow it to occur under lower pressures and higher temperatures. This increased the ability to vary conditions, making it easier to push a reaction toward the desired form of the aldehyde produced.
By the 1960s, researchers began looking for catalysts that would give them even more control over the products of hydroformylation. One tack they took was to investigate the use of other elements in the same group of transition metals as cobalt, especially rhodium and iridium. Rhodium complexes using phosphines allow the use of both lower temperatures and lower pressures while producing a high ratio of linear to branched aldehydes.
In the 1970s, rhodium complexes began to replace cobalt complexes as catalysts in commercial processes. By 2004, 75% of commercial production of aldehydes used rhodium catalysts. This widespread use of rhodium complexes in hydroformylation allows the large-scale production of aldehydes that are then modified to form compounds used in making products such as plastics, detergents, solvents, and lubricants.