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Carbonyl
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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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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. For carbonyl metal processing refer to carbonyl metallurgy
Carbonyl compounds
A carbonyl group characterizes the following types of compounds:
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 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.
Reactivity
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. 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:
a,ß-Unsaturated carbonyl compounds
a,ß-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure Cß=Ca-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 a position in an electrophilic addition.
On the other hand, these compounds are activated towards nucleophiles in nucleophilic addition.
Spectroscopy
- 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.
See also
Further reading
- L.G. Wade, Jr. Organic Chemistry, 5th ed. Prentice Hall, 2002. ISBN 0-13-033832-X
- The Frostburg State University Chemistry Department. (2000).
- Advanced Chemistry Development, Inc. (1997).
- William Reusch. tara (2004).
- Purdue Chemistry Department (retrieved Sep 2006). Includes water solubility data.
- William Reusch. (2004) Retrieved 23 May 2005.
- ILPI. (2005) .
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