Colourless, highly toxic gas used in chemical warfare as well as in industrial processes including the making of dyestuffs and polyurethane resins. Either alone or in combination with chlorine, it was used against troops in World War I. It smells like musty hay. Inhalation causes severe lung injury several hours after exposure. First prepared in 1811, it is manufactured by the reaction of carbon monoxide and chlorine in the presence of a catalyst. Gaseous phosgene is usually stored and transported as a liquid under pressure in steel cylinders or as a solution in toluene. Mixed with water, it forms carbon dioxide and hydrochloric acid.
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The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex (a metal carbonyl, e.g. nickel carbonyl); in this situation, carbon is triple-bonded to oxygen : C≡O.
The remainder of this article concerns itself with the organic chemistry definition of carbonyl.
A carbonyl group characterizes the following types of compounds (where -CO denotes a C=O carbonyl group):
|Compound||Aldehyde||Ketone||Carboxylic acid||Ester||Amide||Enone||Acyl chloride||Acid anhydride|
The alpha hydrogens of a carbonyl compound are much more acidic (~1030 times more acidic) than a typical C-H bond. For example, the pKa values of acetaldehyde and acetone are 16.7 and 19, respectively. This is because a carbonyl is in tautomeric resonance with an enol. The deprotonation of the enol with a strong base produces an enolate, which is a powerful nucleophile and can alkylate electrophiles such as other carbonyls.
Amides are the most stable of the carbonyl couplings due to their high resonance stabilization between the nitrogen-carbon and carbon-oxygen bonds.
Carbonyl groups can be reduced by reaction with hydride reagents such as NaBH4 and LiAlH4, or catalytically by hydrogen and a catalyst such as copper chromite, Raney nickel, rhenium, ruthenium or even rhodium. Ketones give secondary alcohols; aldehydes, esters and carboxylic acids give primary alcohols.
Carbonyls can be alkylated by nucleophilic attack by organometallic reagents such as organolithium reagents and Grignard reagents. Carbonyls also be alkylated by enolates as in aldol reactions. Carbonyls are also the prototypical groups with vinylogous reactivity, e.g. the Michael reaction where an unsaturated carbon in conjugation with the carbonyl is alkylated instead of the carbonyl itself.
Other important reactions include: