Inorganic chemistry, acarbonyl group is afunctional group with the formulaC=O, composed of acarbonatomdouble-bonded to anoxygen atom, and it isdivalent at the C atom. It is common to several classes of organic compounds (such asaldehydes,ketones andcarboxylic acid), as part of many larger functional groups. A compound containing a carbonyl group is often referred to as a carbonyl compound.[1]
The term carbonyl can also refer tocarbon monoxide as aligand in aninorganic ororganometallic complex (ametal carbonyl, e.g.nickel carbonyl).
The remainder of this article concerns itself with the organic chemistry definition of carbonyl, such that carbon and oxygen share a double bond.
In organic chemistry, a carbonyl group characterizes the following types of compounds:
| Compound | Aldehyde | Ketone | Carboxylic acid | Carboxylate ester | Amide |
|---|---|---|---|---|---|
| Structure |  | |  |  | .svg) |
| General formula | RCHO | RCOR' | RCOOH | RCOOR' | RCONR'R'' |
| Compound | Enone | Acyl halide | Acid anhydride | Imide |
|---|---|---|---|---|
| Structure |  |  |  |  |
| General formula | RC(O)C(R')CR''R''' | RCOX | (RCO)2O | RC(O)N(R')C(O)R'' |
Other organic carbonyls areurea and thecarbamates, the derivatives ofacyl chlorides,chloroformates andphosgene,carbonate esters,thioesters,lactones,lactams,hydroxamates, andisocyanates. Examples of inorganic carbonyl compounds arecarbon dioxide andcarbonyl sulfide.[citation needed]
A special group of carbonyl compounds aredicarbonyl compounds, which can exhibit special properties.
For organic compounds, the length of the C-O bond does not vary widely from 120picometers. Inorganic carbonyls have shorter C-O distances:CO, 113;CO2, 116; andCOCl2, 116 pm.[2]
The carbonyl carbon is typicallyelectrophilic. A qualitative order of electrophilicity is RCHO (aldehydes) > R2CO (ketones) > RCO2R' (esters) > RCONH2 (amides). A variety of nucleophiles attack, breaking the carbon-oxygendouble bond.
Interactions between carbonyl groups and other substituents were found in a study ofcollagen.[3] Substituents can affect carbonyl groups by addition or subtraction of electron density by means of asigma bond.[4] ΔHσ values are much greater when the substituents on the carbonyl group are more electronegative than carbon.[4]
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-oxygendouble bond, and leading toaddition-elimination reactions. Nucleophilic reactivity is often proportional to the basicity of the nucleophile and as nucleophilicity increases, the stability within a carbonyl compound decreases.[5] ThepKa values ofacetaldehyde andacetone are 16.7 and 19 respectively,[6]
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