Hydroformylation
[1][2] This chemical reaction entails the net addition of a formyl group (−CHO) and a hydrogen atom to a carbon-carbon double bond.
The process entails treatment of an alkene typically with high pressures (between 10 and 100 atmospheres) of carbon monoxide and hydrogen at temperatures between 40 and 200 °C.
[5][6] The term oxo synthesis was coined by the Ruhrchemie patent department, who expected the process to be applicable to the preparation of both aldehydes and ketones.
For example, the hydroformylation of propylene can afford two isomeric products, butyraldehyde or isobutyraldehyde: These isomers reflect the regiochemistry of the insertion of the alkene into the M–H bond.
Hence, the mixed carbonyl/phosphine complexes offer a greater selectivity for anti-Markovnikov addition, thus favoring straight chain products (n-) aldehydes.
To suppress competing isomerization of the alkene, the rate of migratory insertion of the carbonyl into the carbon-metal bond of the alkyl must be relatively fast.
The original Ruhrchemie process produced propanal from ethene and syngas using cobalt tetracarbonyl hydride.
In the hydroformylation of higher molecular weight olefins the separation of the catalyst from the produced aldehydes is difficult.
[16] By conducting the reaction at low temperatures, one observes increased selectivity favoring the linear product.
The cobalt is recovered from the liquid product by oxidation to water-soluble Co2 +, followed by the addition of aqueous formic or acetic acids.
By extraction with olefin and neutralization by addition of sulfuric acid solution under carbon monoxide pressure the metal carbonyl hydride can recovered.
The resulting aldehydes are directly hydrogenated to the fatty alcohols, which are separated by distillation, which allows the catalyst to be recycled.
[17] The Union Carbide (UCC) process, also known as low-pressure oxo process (LPO), relies on a rhodium catalyst dissolved in high-boiling thick oil, a higher molecular weight condensation product of the primary aldehydes, for the hydroformylation of propene.
The catalyst complex carries nine sulfonate-groups and is highly soluble in water (about 1 kg L−1), but not in the emerging product phase.
A mixture of butyraldehyde and isobutyraldehyde in the ratio 96:4 is generated with few by-products such as alcohols, esters and higher boiling fractions.
[21] The excess olefin and syngas is separated from the aldehyde phase in a stripper and fed back to the reactor.
[21] Potential catalyst poisons coming from the synthesis gas migrate into the organic phase and removed from the reaction with the aldehyde.
A high reaction temperature and low carbon monoxide pressure favors the isomerization of the Markovnikov product to the thermodynamically more stable β-isomer, which leads to the n-aldehyde.
Low temperatures and high carbon monoxide pressure and an excess of phosphine, which blocks free coordination sites, can lead to faster hydroformylation in the α-position to the ester group and suppress the isomerization.
The insertion of carbon monoxide in an intermediate metal-phenyl bond can lead to the formation of benzaldehyde or by subsequent hydrogenation to benzyl alcohol.
[30] Although the original hydroformylation catalysts were based on cobalt, most modern processes rely on rhodium, which is expensive.
