In chemistry, carbonylation refers to reactions that introduce carbon monoxide (CO) into Organic compound and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry.
Organic chemistry
Several industrially useful organic chemicals are prepared by carbonylations, which can be highly selective reactions. Carbonylations produce organic
, i.e., compounds that contain the
functional group such as
aldehydes (),
carboxylic acids () and
esters ().
[Arpe, .J.: Industrielle organische Chemie: Bedeutende vor- und Zwischenprodukte, 2007, Wiley-VCH-Verlag, ] Carbonylations are the basis of many types of reactions, including hydroformylation and Reppe reactions. These reactions require metal catalysts, which bind and activate the CO.
Hydroformylation
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 that bind CO, forming intermediate . Many of the commodity carboxylic acids, i.e. propionic, butyric, valeric, etc, as well as many of the commodity alcohols, i.e. propanol, butanol, amyl alcohol, are derived from aldehydes produced by hydroformylation. In this way, hydroformylation is a gateway from alkenes to oxygenates.
Decarbonylation
Few organic carbonyls undergo spontaneous decarbonylation, but many can be induced to do so with appropriate catalysts. A common transformation involves the conversion of aldehydes to alkanes, usually catalyzed by metal complexes:[Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010.]
Few catalysts are highly active or exhibit broad scope.[Kreis, M.; Palmelund, A.; Bunch, L.; Madsen, R., "A General and Convenient Method for the Rhodium-Catalyzed Decarbonylation of Aldehydes", Advanced Synthesis & Catalysis 2006, 348, 2148-2154. ]
Acetic acid and acetic anhydride
Large-scale applications of carbonylation are the Monsanto acetic acid process and Cativa process, which convert methanol to acetic acid. In another major industrial process, acetic anhydride is prepared by a related carbonylation of methyl acetate.
Oxidative carbonylation
Dimethyl carbonate and dimethyl oxalate are produced industrially using carbon monoxide and an oxidant, in effect as a source of :[
]
The oxidative carbonylation of methanol is catalyzed by copper(I) salts, which form transient carbonyl complexes. For the oxidative carbonylation of alkenes, palladium complexes are used.
Hydrocarboxylation, hydroxycarbonylation, and hydroesterification
In hydrocarboxylation, alkenes and alkynes are the substrates. This method is used to produce propionic acid from ethylene using nickel carbonyl as the catalyst:
The above reaction is also referred to as hydroxycarbonylation, in which case hydrocarboxylation refers to the same net converstion but using carbon dioxide in place of CO and in place of water:
Acrylic acid was once mainly prepared by the hydrocarboxylation of acetylene.
Hydroesterification is like hydrocarboxylation, but it uses alcohols in place of water.[El Ali, B.; Alper, H. "Hydrocarboxylation and hydroesterification reactions catalyzed by transition metal complexes" In Transition Metals for Organic Synthesis, 2nd ed.; Beller, M., Bolm, C., Eds.; Wiley-VCH:Weinheim, 2004. ] For example, the carbomethoxylation of ethylene to give methyl propionate, a precursor to methyl methacrylate:
The process is catalyzed by Herrmann's catalyst, . Under similar conditions, other Pd-diphosphines catalyze formation of .
Koch carbonylation
The Koch reaction is a special case of hydrocarboxylation reaction that does not rely on metal catalysts. Instead, the process is catalyzed by strong acids such as sulfuric acid or the combination of phosphoric acid and boron trifluoride. The reaction is less applicable to simple alkene. The industrial synthesis of glycolic acid is achieved in this way:[Karlheinz Miltenberger, "Hydroxycarboxylic Acids, Aliphatic" in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH: Weinheim, 2003]
The conversion of isobutene to pivalic acid is also illustrative:
Other reactions
Alkyl, benzyl, vinyl, aryl, and allyl halides can also be carbonylated in the presence carbon monoxide and suitable catalysts such as manganese, iron, or nickel powders.
In the industrial synthesis of ibuprofen, a alcohol is converted to the corresponding carboxylic acid via a Pd-catalyzed carbonylation:[
]
Carbonylation in inorganic chemistry
, compounds with the formula (M = metal; L = other ) are prepared by carbonylation of transition metals. Iron and nickel powder react directly with CO to give and , respectively. Most other metals form carbonyls less directly, such as from their oxides or halides. Metal carbonyls are widely employed as catalysts in the hydroformylation and Reppe processes discussed above.[Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ] Inorganic compounds that contain CO ligands can also undergo decarbonylation, often via a photochemical reaction.
