In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a carbonyl group is often referred to as a carbonyl compound.
The remainder of this article concerns itself with the organic chemistry definition of carbonyl, where carbon and oxygen share a double bond.
A carbonyl group characterizes the following types of compounds:
|Compound||Enone||Acyl halide||Acid anhydride||Imide|
Note that the most specific labels are usually employed. For example, R(CO)O(CO)R' structures are known as acid anhydride rather than the more generic ester, even though the ester motif is present.
Other organic carbonyls are urea and the 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 draws electron density away from carbon to increase the bond's polarity. Therefore, the carbon becomes electrophilic (slightly positive). It can be attacked by nucleophiles (negatively charged ions, like the cyanide ion) or a negatively charged part of another molecule, like the pair "lone pair" of nitrogen atom in the ammonia molecule. During the reaction, the carbon-oxygen double bond is broken, and the carbonyl group may experience addition reactions. This reaction is known as addition-elimination (because a water molecule is often lost) or condensation.
The alpha hydrogens of a carbonyl compound are much more acidic (roughly 1030 times more acidic) than typical sp3 C-H bonds, such as those in methane. For example, the pKa values of acetaldehyde and acetone are 16.7 and 19 respectively, while the pKa value of methane is extrapolated to be approximately 50. 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 would be alkylated by nucleophilic attack by organometallic reagents such as organolithium reagents and Grignard reagents. Carbonyls also may 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.
In case of multiple carbonyl types in one molecule, one can expect the most electrophilic one to react first. Acyl chlorides and carboxylic anhydrides react fastest, followed by aldehydes and ketones. Esters react much slower and amides barely react due to resonance of the amide nitrogen towards the carbonyl group. This effect is mainly due to the decreasing electrophilic character of the carbonyl carbon atom. This reactivity difference permits to induce chemoselectivity. An instructive example is found in the last part of the total synthesis of monensin by Kishi in 1979:
The left wing of the molecule possesses two potential electrophilic sites: an aldehyde (indicated in blue) and an ester (indicated in green). Only the aldehyde, which is more electrophilic, will react with the enolate of the methyl ketone in the other part of the molecule. The methyl ester remains untouched. Of course, other effects can play a role in this selectivity process: electronic effects, steric effects, kinetic versus thermodynamic effect, ...
Other important reactions include:
- Carbonyl Alpha-Substitution Reactions
- Wittig Reaction a phosphonium ylid is used to create an alkene
- Wolff-Kishner reduction into a hydrazone and further into a saturated alkane
- Clemmensen reduction into a saturated alkane
- Mozingo reduction into a saturated alkane
- Conversion into thioacetals
- Hydration to hemiacetals and hemiketals, and then to acetals and ketals
- Reaction with ammonia and primary amines to form imines
- Reaction with hydroxylamines to form oximes
- Reaction with cyanide anion to form cyanohydrins
- Oxidation with oxaziridines to acyloins
- Reaction with Tebbe's reagent and phosphonium ylides to alkenes.
- Perkin reaction, an aldol reaction variant
- Aldol condensation, a reaction between an enolate and a carbonyl
- Cannizzaro reaction, a disproportionation of aldehydes into alcohols and acids
- Tishchenko reaction, another disproportionation of aldehydes that gives a dimeric ester
- Nucleophilic abstraction is used to produce carbon dioxide
α,β-Unsaturated carbonyl compounds
α,β-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure (O=CR)−Cα=Cβ-R; for example enones and enals. In these compounds the carbonyl group is conjugated with an alkene (hence the adjective unsaturated), from which they derive special properties. Unlike the case for simple carbonyls, α,β-unsaturated carbonyl compounds are often attacked by nucleophiles at the β carbon. This pattern of reactivity is called vinylogous. Examples of unsaturated carbonyls are acrolein (propenal), 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 draws electrons away from the alkene, and the alkene group is, therefore, deactivated towards an electrophile, such as bromine or hydrochloric acid. As a general rule with asymmetric electrophiles, hydrogen attaches itself at the α-position in an electrophilic addition. On the other hand, these compounds are activated towards nucleophiles in nucleophilic conjugate addition.
Since α,β-unsaturated compounds are electrophiles, many α,β-unsaturated carbonyl compounds are toxic, mutagenic and carcinogenic. DNA can attack the β carbon and thus be alkylated. However, the endogenous scavenger compound glutathione naturally protects from toxic electrophiles in the body.
- Infrared spectroscopy: the C=O double bond absorbs infrared light at wavenumbers between approximately 1600–1900 cm−1. The exact location of the absorption is well understood with respect to the geometry of the molecule. This absorption is known as the "carbonyl stretch" when displayed on an infrared absorption spectrum.
- Nuclear magnetic resonance: the C=O double-bond exhibits different resonances depending on surrounding atoms, generally a downfield shift. The 13C NMR of a carbonyl carbon is in the range of 160-220 ppm.
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