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Chemistry > Aldehydes, Ketones and Carboxylic Acids > Nomenclature and Structure of Carbonyl Group
Aldehydes, Ketones and Carboxylic Acids

Nomenclature and Structure of Carbonyl Group

Do you know that the Carbonyl group is present in all type of organic compounds such as carbohydrates, nucleic acids, fats, proteins, vitamins, and hormones? These organic compounds are essential to every living organism. Structure of aldehydes and ketones are responsible for the smell and taste of many different aromatic compounds found in nature.

But do you know what is a carbonyl group, to begin with? If you don’t, let us help you out. We will learn about the carbonyl group and its nomenclature and structure below. Let’s start.

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What is Carbonyl Group?

Aldehydes and Ketones fall into the category of simplest compounds containing the carbonyl group. They are also essential types of carbonyl compounds. A carbonyl group consists of carbon and oxygen joined together by a double bond. The joining of carbonyl carbon is with hydrogen on one side in aldehydes whereas the joining of two carbon atoms on both the side of carbonyl carbon in the case of ketones.

In a carbonyl group, the carbon and oxygen have sp2 hybridization and is planar. Carbonyl group structure is “C=O” and members of this group are carbonyl compounds (X-C=O).

Carbonyl Compound (Source: Wikipedia)

The joining of the same carbonyl group to an oxygen atom on one side forms carboxylic acid compounds. The derivatives of this class are anhydrides, esters, etc.  If the attachment of the carbonyl group is to nitrogen then the class of compounds is amides.  Similarly, the joining of the carbonyl group to the members of halogen group forms acyl halide compounds. Refer to the table below to study the structure and general formula of different members of the carbonyl groups.

Carbonyl Group

Members of Carbonyl Group (Source: Wikipedia)

Nomenclature of Aldehydes and Ketones

Common Name

Common names are often used for referring to an aldehyde or ketone instead of their respective IUPAC names. The common names of many aldehydes are taken from the common names of the respective carboxylic acids by exchanging the end letters “–ic” with an aldehyde. Mostly these names reflect corresponding Latin or Greek term of the original source of the aldehyde compound or an acid compound.

We use letters such as α, β, γ, and δ, and so on to indicate the location of the substituent present in the carbon chain. The α-carbon is the one to which the aldehyde group is attached. β- carbon is the carbon next to the α-carbon, and so on.

The proper common naming of the ketone requires the naming of the two alkyl or the aryl groups joined to the carbonyl group.  Again, we use α α′, β β′, etc to indicate the substituents location.  The naming of α α′ begins with the carbon atoms present next to the carbonyl group.

However, there are certain ketones that have historical common names. These common names are in use to date such as we call the simplest ketone “dimethyl ketone” as acetone. We name the Alkyl phenyl ketones by the addition of acyl group as the prefix to the word “phenone”.

Carbonyl compound

Examples of Ketones


The IUPAC naming of aliphatic aldehydes and aliphatic ketones is comparatively easy. It is derived by taking the corresponding alkane and replacing the end letter –e of the alkane with either –al or –one. In case of aldehyde –al is written whereas in case of ketone –one is written.

The numbering of the longest carbon chain in the case of aldehyde begins from the carbon-containing the aldehyde group. However, in case of ketones, the numbering starts from the side of carbon-containing the carbonyl group.

Adding prefix in alphabetical order along with the numerals help in indicating the positions of the substituents in the carbon chain. This is same for cyclic ketones as well. In the case of cyclic ketones, the numbering begins with the carbonyl carbon.

We have to add the suffix carbaldehyde after the complete name of the cycloalkane in case the attachment is between an aldehyde group and a ring. The numbering of the ring begins from the carbon atom that is attached to the aldehyde group.

The nomenclature of the simplest aromatic aldehyde containing a benzene ring along with an aldehyde group is benzenecarbaldehyde. Moreover, IUPAC has also accepted the common name benzaldehyde. Additionally, the naming of aromatic aldehyde is done as substituted benzaldehydes.

Carbonyl group

Examples of Aldehydes

carbonyl group

Examples of Ketones

Structure of Carbonyl Group

The carbon atom present in the carbonyl group has an sp2 hybridization and it develops three sigma(σ) bonds. However, the fourth valence electron of the carbon atom remains in the p-orbital.  Thus, it develops π-bond by overlapping with the oxygen from the p-orbital of the oxygen atom. Additionally, the oxygen atom also contains two non-bonded pairs of electrons.

Therefore, the carbonyl carbon along with its three attached atom lies in the same plane. The cloud of π-electron is present below and above the plane. The structure is a trigonal coplanar structure and the bond angles are nearly 120°.

Polarization occurs in the carbon-oxygen bond due to the higher electronegativity of the oxygen atom in comparison to the carbon atom. Thus, the carbonyl carbon demonstrates the electrophilic property of a Lewis acid whereas the carbonyl oxygen demonstrates the electrophilic property of a Lewis base.

Carbonyl compounds contain significant dipole moments. Therefore, it demonstrates more polarity than ethers. The high polarity of the carbonyl group is mainly due to the resonance with respect to neutral and dipolar structures.

carbonyl Group

Orbital Diagram for the Formation of Carbonyl Group

A Solved Question for You

Q. Give the structures of the compounds given below.

  1. 3-Hydroxybutanal
  2. α-Methoxypropionaldehyde
  3. 2-Hydroxycyclopentane carbaldehyde
  4. Di-sec-butyl ketone
  5. 4-Oxopentanal
  6. 4-Fluoroacetophenone


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