A carbonyl group is defined as functional group that contains an oxygen bond that has a carbon atom double-bonded to it and is depicted as C=O; this is most often seen in organic chemistry where it exists as part of many larger functional groups and is quite common to several different classes of organic compounds. As a side note, the term carbonyl is sometimes used to describe a ligand, such as carbon monoxide or carbonyl sulfide, that appears in an organometallic or inorganic complex; an example of this would be a metal carbonyl like nickel carbonyl.
There are several chemicals that are considered prime examples of an organic carbonyl group, these include urea and the carbamates. Carbamates make up a group of derivatives that come from the chemicals thioesters, lactones, hydroxamates, isocyanates, lactams, carbonate esters, phosgene and acyl cholorides chloroformates. There is another special group of carbonyl compounds that possess a central methylene unit that has acidic protons. Acetylacetone, diethyl malonate and Meldrum's acid are proper examples of these 1,3-dicarbonyl compounds.
There is yet another class of carbonyl compounds that make up an important class within this group. Unsaturated carbonyl compounds are the direct result of the carbonyl group conjugating with an alkene, thereby producing derivatives with special properties. The principal difference between this set of compounds and simple carbonyls is that the former can be used to form carbon-carbon bonds because they exhibit vinylogous reactivity; an example of this is the simple process of alkylation. Maleic acid, mesityl oxide, acrylic acid, and acrolein, also known as proponol, are some of the compounds that fall under the unsaturated carbonyl compounds list. In order for a scientist to manually create an unsaturated carbonyl compound in the laboratory, the Perkin reaction or an aldol reaction are the methods that must be utilized.