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. 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: 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; CO, 116; and COCl, 116 pm . The carbonyl carbon is typically electrophilic. A qualitative order of electrophilicity is RCHO (aldehydes) > RCO (ketones) > RCOR' (esters) > RCONH (amides). A variety of nucleophiles attack, breaking the carbon-oxygen double bond. The polarity of C=O bond also enhances the acidity of any adjacent C-H bonds. The pK values of acetaldehyde and acetone are 16.7 and 19 respectively,