Carbonyl group

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.[1]

A ketone compound containing a carbonyl group (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).

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:

CompoundAldehydeKetoneCarboxylic acidCarboxylate esterAmide
Structure
General formulaRCHORCOR'RCOOHRCOOR'RCONR'R''
CompoundEnoneAcyl halideAcid anhydrideImide
Structure
General formulaRC(O)C(R')CR''R'''RCOX(RCO)2ORC(O)N(R')C(O)R''
Carbon dioxide

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.

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.[2]

The carbonyl carbon is typically electrophilic. A qualitative order of electrophilicity is RCHO (aldehydes) > R2CO (ketones) > RCO2R' (esters) > RCONH2 (amides). A variety of nucleophiles attack, breaking the carbon-oxygen double bond.

A carbonyl compound

The polarity of C=O bond also enhances the acidity of any adjacent C-H bonds. The pKa values of acetaldehyde and acetone are 16.7 and 19 respectively,[3]

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.[4] In addition, the ultraviolet-visible spectra of propanone in water gives an absorption of carbonyl at 257 nm.[5]
  • 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

References
  1. Saul Patai, ed. (1966). The Carbonyl Group. PATAI'S Chemistry of Functional Groups. Vol. 1. John Wiley & Sons. doi:10.1002/9780470771051. ISBN 9780470771051.Jacob Zabicky, ed. (1970). The Carbonyl Group. PATAI'S Chemistry of Functional Groups. Vol. 2. John Wiley & Sons. doi:10.1002/9780470771228. ISBN 9780470771228.
  2. G. Berthier, J. Serre (1966). "General and Theoretical Aspects of the Carbonyl Group". In Saul Patai (ed.). The Carbonyl Group. PATAI'S Chemistry of Functional Groups. Vol. 1. John Wiley & Sons. p. 1-77. doi:10.1002/9780470771051.ch1. ISBN 9780470771051.
  3. Ouellette, R.J. and Rawn, J.D. "Organic Chemistry" 1st Ed. Prentice-Hall, Inc., 1996: New Jersey. ISBN 0-02-390171-3
  4. Mayo D.W., Miller F.A and Hannah R.W “Course Notes On The Interpretation of Infrared and Raman Spectra” 1st Ed. John Wiley & Sons Inc, 2004: New Jersey. ISBN 0-471-24823-1.
  5. "Archived copy" (PDF). Archived from the original (PDF) on 2015-08-24. Retrieved 2015-07-11.{{cite web}}: CS1 maint: archived copy as title (link)

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