In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom : C=O.
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.
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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 halide | Acid anhydride |
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| General formula | RCHO | RCOR' | RCOOH | RCOOR' | RCONR'R'' | RC(O)C(R')CR''R''' | RCOX | (RCO)2O |
Other organic carbonyls are urea and carbamates, the derivatives of acyl chlorides chloroformates and phosgene, carbonate esters, thioesters, lactones, lactams, hydroxamates, and isocyanates. Examples of inorganic carbonyl compounds are carbon dioxide and carbonyl sulfide.
Oxygen is more electronegative than carbon, and thus pulls electron density away from carbon to increase the bond's polarity. Therefore, the carbonyl carbon becomes electrophilic, and thus more reactive with nucleophiles. Also, the electronegative oxygen can react with an electrophile; for example a proton in an acidic solution or other Lewis Acid.
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.[1] 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:
α,β-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure Cβ=Cα−C=O. In these compounds the carbonyl group is conjugated with an alkene (hence the adjective unsaturated), from which they derive special properties. Examples of unsaturated carbonyls are acrolein, mesityl oxide, acrylic acid and maleic acid. Unsaturated carbonyls can be prepared in the laboratory in an aldol reaction and in the Perkin reaction. The carbonyl group, be it an aldehyde or acid or a ketone, draws electrons away from the alkene and the alkene group in unsaturated carbonyls are therefore deactived towards an electrophile such as bromine or hydrochloric acid. As a general rule with unsymmetric electrophiles hydrogen attaches itself at the α position in an electrophilic addition. On the other hand, these compounds are activated towards nucleophiles in nucleophilic addition.