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=Catalytic Carbonyl Hydrogenation= In chemistry, carbonyl hydrogenation is the addition of hydrogen atoms to a carbonyl group via heterogenous catalysis in order to produce an alcohol. This type of hydrogenation can be performed on aldehydes, ketones , and esters , and employs transition metal catalysts. The identity of the transition metal used varies depending on the substrate being hydrogenated, but typical catalysts used include nickel, palladium, rhodium, copper, or platinum.

Aldehydes
The hydrogenation of aldehydes will afford primary alcohols. Aldehydes can be effectively hydrogenated using a Raney nickel catalyst, high hydrogen pressure, and a temperature of 80-100 °C. This method has been shown to reduce an aliphatic aldehyde to its primary alcohol with >90% yield, as shown below: For higher temperature hydrogenations, from 100-125 °C, copper-chromium oxide is a more suitable catalyst.

Ketones
Carbonyl hydrogenation can be performed on a variety of different ketones, including cyclic, acyclic, aromatic, and aliphatic ketones. These reactions will produce secondary alcohols. The preferred catalyst for aliphatic and acyclic ketones is Raney nickel, along with the promoters triethylamine, chloroplatinic acid, and sodium hydroxide. Copper-chromium oxide can also be used. The hydrogenation of cylic ketones, such as cyclohexanone, widley uses platinum or rhodium catalysts, with rhodium catalysts yielding alcohol products with high stereoselectivities. Specifically, when using a rhodium catalyst in THF and hydrochloric acid to hydrogenate differently substituted cyclohexanones, it was found that the axial alcohol product was favoured over the equatorial alcohol product in a ratio of over 10:1.

Esters
Ester hydrogenation to the corresponding alcohol is usually performed at high temperature and hydrogen pressure, with the most suitable catalyst being copper-chromium oxide. The hydrogenation of β-hydroxy or β-keto esters generates diols, and can be successfully carried out using this copper-chromium oxide catalyst. A Raney nickel catalyst can also be used for these reactions however the yields are generally lower with this catalyst, and it cannot be used for phenyl-substituted esters as it will hydrogenate the phenyl group to cyclohexane.

The catalytic hydrogenation of esters has significant industrial applications: the hydrogenation of fatty esters to fatty alcohols is useful as fatty alcohols are widely used in detergents and cosmetics.

Homogenous catalysis
In recent years, more development has been made towards acheiving the hydrogenation of carbonyl compounds using homogenous catalysts. The advantage of using these catalysts over the traditional heterogenous ones is that they can be used under milder conditions, and therefore may offer higher selectivity. Many of these newly proposed catalysts are organometallic complexes of ruthenium, osmium, or iridium.