What is the carbonyl group?

Carbonyl group

The carbonyl group , in organic chemistry, is a divalent chemical unit consisting of a carbon atom (C) and an oxygen atom (O) which are connected by a double bond . The group is a constituent of carboxylic acids, esters, anhydrides, acyl halides, amides, and quinones, and is the characteristic functional group of aldehydes and ketones . Carboxylic acids and all of their derivatives, aldehydes, ketones, and quinones are also collectively known as carbonyl compounds . It is a functional group formed then of a carbon atomwhich has a double bond and an oxygen atom. The word carbonyl is sometimes referred to as carbon monoxide .

Carbonyl group

What is the carbonyl group?

In organic chemistry the carbonyl group is a functional group that is made up of a carbon atom that has a double bond to an oxygen atom. It can also refer to carbon monoxide .

  • Characteristics of the carbonyl group
  • Structure of the carbonyl group
  • Reactivity
  • Examples of Carbonyl Group Compounds

Characteristics of the carbonyl group

The most important characteristics of the carbonyl group are the following:

  • It is characterized by having a carbon-oxygen double bond that is present in a variety of functional groups, including aldehydes and ketones .
  • The reactivity of aldehydes and ketones originates essentially in the structure of the carbonyl group, which, being constituted by a polar bond , behaves as an electrophile in the partially positively charged carbon nucleus.
  • The unsaturation in the carbon-oxygen bond together with the aforementioned polarity of the bond allows the addition of a nucleophile.
  • Among carbonyl compounds, aldehydes and ketones behave chemically in a similar way.
  • The carbonyl group can be an aldehyde or ketone . When it is aldehyde, aldoses are formed and in the second case, ketoses are formed .
  • The carbonyl group is planar ; This means that the atoms of oxygen, carbonyl carbon, and the two atoms attached to it are in the same plane.
  • The carbonyl group is polarized because the presence of oxygen, which is more electronegative than carbon, tends to shift the electron density of the molecule and the electrons ¶ of the carbonyl group towards its nucleus, causing it to surround itself with a higher density. electronics, and giving rise to a weaker positive charge on the alpha carbon.
  • They have quite high boiling points in relation to hydrocarbons
  • Their boiling points are also lower than the corresponding alcohols .

Structure of the carbonyl group

In the structure of the carbonyl group, carbon is linked to three other atoms by means of bonds which either use sp2 orbitals or are kept in a plane that separates them from each other at an angle of 120º.  The remaining p orbital of carbon and oxygen is attached through a double bond. The immediate part of the molecule that surrounds the carbonyl carbon is flat ; the oxygen , the carbonyl and the two atoms bonded directly thereto are in a plane.

The electrons that have the carbonyl double bond are held together with two atoms of different electronegativity so that they cannot be shared in the same way. The cloud especially is very mobile and is strongly attracted to the atom that is more electronegative, which in this case is oxygen .

Many spectroscopic studies that have been carried out on carbonyl groups in addition to electron diffraction studies on aldehydes and ketones indicate that oxygen, carbonyl carbon and the other two atoms are all in the same plane. The three different angles of the bond are close to 120 °.


Carbonyl groups can be reduced by hydride reagents such as NaBH4 and LiAlH4 , and also by organometallic reagents such as organolithium reagents and Grignard reagents. The carbonyl group has the following reactivity reactions:

  • Reduction Wolff-Kishner
  • Clemmensen reduction
  • Conversion to thioacetals
  • Hydration to hemiacetals and hemicetals , then to acetals and ketals
  • Reaction with ammonia and primary amines , producing imines
  • Reaction with hydroxylamines , producing oximes
  • Reaction with the cyanide anion , producing cyanohydrins
  • Oxidation with oxaziridines , producing α-hydroxyketones (or α-hydroxyaldehydes).
  • Reaction with Tebbe’s reagent or Wittig’s reagent , producing alkenes.
  • Perkin reaction
  • Reaction Tischenko
  • The aldol condensation , producing β-hydroxycarbonyl compounds and then α, β-unsaturated carbonyl compounds.
  • The Cannizzaro Reaction , producing alcohols and carboxylic acids

Examples of Carbonyl Group Compounds

Some examples of α, β-unsaturated carbonyl compounds are acrolein , mesityl oxide , acrylic acid , and maleic acid . Unsaturated carbonyl compounds can be prepared in the laboratory by the aldol reaction or the Perkin reaction . Organic carbonyl compounds such as urea and carbamates also exist in nature ; as well as some inorganic carbonyl compounds such as carbon dioxide , carbonyl sulfide and phosgene .

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