
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 term carbonyl can also refer to
carbon monoxide as a
ligand in an
inorganic or
organometallic
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
complex (a
metal carbonyl, e.g.
nickel carbonyl).
The remainder of this article concerns itself with the organic chemistry definition of carbonyl, where carbon and oxygen share a double bond.
Carbonyl compounds
In organic chemistry, a carbonyl group characterizes the following types of compounds:

Other organic carbonyls are
urea and the
carbamate
In organic chemistry, a carbamate is a category of organic compounds with the general formula and structure , which are formally derived from carbamic acid (). The term includes organic compounds (e.g., the ester ethyl carbamate), formally o ...
s, the derivatives of
acyl chloride
In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...
s
chloroformates and
phosgene
Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, espe ...
,
carbonate esters,
thioesters,
lactones,
lactam
A lactam is a cyclic amide, formally derived from an amino alkanoic acid. The term is a portmanteau of the words ''lactone'' + ''amide''.
Nomenclature
Greek prefixes in alphabetical order indicate ring size:
* α-Lactam (3-atom rings)
* β-Lacta ...
s,
hydroxamates, and
isocyanates. Examples of inorganic carbonyl compounds are
carbon dioxide and
carbonyl sulfide.
A special group of carbonyl compounds are
dicarbonyl compounds, which can exhibit special properties.
Structure and reactivity
For organic compounds, the length of the C-O bond does not vary widely from 120
picometers. Inorganic carbonyls have shorter C-O distances:
CO, 113;
CO2, 116; and
COCl2, 116 pm.
The carbonyl carbon is typically
electrophilic
In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...
. A qualitative order of electrophilicity is RCHO (aldehydes) > R
2CO (ketones) > RCO
2R' (esters) > RCONH
2 (amides). A variety of nucleophiles attack, breaking the carbon-oxygen
double bond.
Interactions between carbonyl groups and other substituents were found in a study of a protein called
collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
. Substituents can affect carbonyl groups by addition or subtraction of electron density by means of a
sigma bond.
Δ''H''σ values are much higher when the substituents on the carbonyl group are more electronegative than carbon.
[
]
The polarity of C=O bond also enhances the acidity of any adjacent C-H bonds. Due to the positive charge on carbon and the negative charge on oxygen, carbonyl groups are subject to additions and/or nucleophilic attacks. A variety of nucleophiles attack, breaking the carbon-oxygen double bond, and leading to addition-elimination
Elimination may refer to:
Science and medicine
* Elimination reaction, an organic reaction in which two functional groups split to form an organic product
*Bodily waste elimination, discharging feces, urine, or foreign substances from the bo ...
reactions. Nucleophiliic reactivity is often proportional to the basicity of the nucleophile and as nucleophilicity increases, the stability within a carbonyl compound decreases. The pKa values of acetaldehyde
Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the mos ...
and acetone are 16.7 and 19 respectively,
Spectroscopy
* Infrared spectroscopy: the C=O double bond absorbs infrared light at wavenumbers between approximately 1600–1900 cm−1(5263 nm to 6250 nm). 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. In addition, the ultraviolet-visible spectra of propanone in water gives an absorption of carbonyl at 257 nm.
* 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
* Carbon–oxygen bond
* Organic chemistry
* Functional group
* Bridging carbonyl
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe ch ...
* Electrophilic addition
References
Further reading
* L.G. Wade, Jr. ''Organic Chemistry, 5th ed.'' Prentice Hall
Prentice Hall was an American major educational publisher owned by Savvas Learning Company. Prentice Hall publishes print and digital content for the 6–12 and higher-education market, and distributes its technical titles through the Safari B ...
, 2002.
* The Frostburg State University Chemistry Department
Organic Chemistry Help
(2000).
* Advanced Chemistry Development, Inc
IUPAC Nomenclature of Organic Chemistry
(1997).
* William Reusch. tar
(2004).
* Purdue Chemistry Departmen
(retrieved Sep 2006). Includes water solubility data.
* William Reusch. (2004
Retrieved 23 May 2005.
* ILPI. (2005
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Functional groups