2.Nomenclature and Structure of Carbonyl Group

I. Aldehydes and ketones

• Aldehydes and ketones are the simplest and most important carbonyl

compounds.
There are two systems of nomenclature of aldehydes and ketones.

(a) Common names

• Aldehydes and ketones are often called by their common names instead of IUPAC names.

• The common names of most aldehydes are derived from the common names of the corresponding carboxylic acids by replacing the ending –ic of acid with aldehyde.

• At the same time, the names reflect the Latin or Greek term for the original source of the acid or aldehyde.

• The location of the substituent in the carbon chain is indicated by Greek letters a, b, g, d, etc.

• The a-carbon being the one directly linked to the aldehyde group, b- carbon the next, and so on. For example

• the common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group.

• The locations of substituents are indicated by Greek letters, a a¢, b b¢ and so on beginning with the carbon atoms next to the carbonyl group, indicated as aa¢.

• Some ketones have historical common names, the simplest dimethyl ketone is called acetone.

• Alkyl phenyl ketones are usually named by adding the name of acyl group as prefix to the word phenone. For example(b) IUPAC names

• The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending –e with –al and –one respectively.

• In case of aldehydes the longest carbon chain is numbered starting from the carbon of the aldehyde group

• in case of ketones the numbering begins from the end nearer to the carbonyl group.

• The substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain.

• The same applies to cyclic ketones, where the carbonyl carbon is numbered one.

• When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane.

• The numbering of the ring carbon atoms start from the carbon atom attached to the aldehyde group.

• The name of the simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzenecarbaldehyde.

• However, the common name benzaldehyde is also accepted by IUPAC. Other aromatic aldehydes are hence named as substituted benzaldehydes.

• Structure of the Carbonyl Group

• The carbonyl carbon atom is sp2-hybridised and forms three sigma (s) bonds.

• The fourth valence electron of carbon remains in its p-orbital and forms a p-bond with oxygen by overlap with p-orbital of an oxygen.

• In addition, the oxygen atom also has two non bonding electron pairs.

• Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the p-electron cloud is above and below this plane.

• The bond angles are approximately 120° as expected of a trigonal coplanar structure (Figure 8.1).

• The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon.

• Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre.

• Carbonyl compounds have substantial dipole moments and are polar than ethers. The high polarity of the carbonyl group is explained on the basis of resonance involving a neutral

(A) and a dipolar (B) structures as shown.