An aldehyde is either a functional group consisting of a terminal carbonyl group, or a compound containing a terminal carbonyl group. A carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. Aldehydes are named by IUPAC nomenclature by changing the suffix -e of the parent alkane (A non-aromatic saturated hydrocarbon with the general formula CnH2n+2) to -al.
The simplest member of the aldehyde group of organic compounds, formaldehyde, or methanol. Aldehyde has the general formula RCOH where the group R is any hydrocarbon.
Ketone is either the functional group characterized by a carbonyl group linked to two other carbon atoms or a compound that contains this functional group. Ketone is a class of chemical compounds contains the carbonyl group in which the carbon atom is covalently bonded to an oxygen atom.
Ketones are named by appending -one to the stem. Ketone has the general formula RCOR' where the groups R and R' may be the same or different, or incorporated into a ring.
Boiling point of aldehydes and ketones are somewhat higher than those of alkanes of comparable molecular mass due to dipole-dipole interactions between the opposite ends of the carbonyl group. However, their boiling points are lower than those of corresponding alcohols and carboxylic acids due to the absence of intermolecular H- bonding.
Structure of carbonyl group
Carbonyl carbon atom is joined to three atoms by sigma bonds. Since these bonds utilise sp2-orbitals, they lie in the same plane and are 120° apart. The carbon-oxygen double bond is different than carbon-carbon double bond. Since, oxygen is more electronegative, the electrons of the bond are attracted towards oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial positive charge making the bond polar. The high values of dipole moment,
(2.3 – 2.8D) cannot be explained only on the basis of inductive effect and thus, it is proposed that carbonyl group is a resonance hybrid of the following two structures.
Nomenclature of aldehydes and ketones
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.
The following steps are followed:
a) The longest carbon chain containing the carbonyl carbon is taken to decide the name of the parent alkane.
b) The carbon chain is numbered from the end nearer to the carbonyl group and the substituents are prefixed in alphabetical order along with Arabic numerals including their positions in the carbon chain. The carbon of the aldehydic group and the carbonyl carbon in cyclic ketones always get the number 1.
c) When the aldehyde group is attached to a ring, the numbering of the ring carbon atoms starts from the carbon atom attached to the aldehyde group. The suffix carbaldehyde is added after the full name of the hydrocarbon.
The simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzaldehyde.
Other ring substituted aromatic aldehydes are derivatives of benzaldehyde is ortho-hydroxy benzaldehyde.
The trivial names of aldehydes are derived from the trivial names of the corresponding carboxylic acids by replacing the ending ic, for acid with aldehyde. The positions of the substituents in the carbon chain are indicated by Greek letters a, b, g, d etc, the a-carbon being the one directly linked to the aldehyde group, the b-carbon the next and on.
The common names of ketones are derived by adding the names of the alkyl or aryl groups directly linked to the carbonyl group before the word ketone. The simplest ketone is acetone.
The positions of the substituents are indicated by the Greek letters a, a', b', b' and so on. a, a' carbons being the ones directly attached to the carbonyl group.
Ketones with a carbonyl group attached to a benzene ring are named as phenones in the IUPAC.
Other examples of ketones
a) Chain isomerism
Aldehydes with 4 or more carbon atoms and ketones with five or more carbon atoms show chain isomerism.Examples: C4H8O has the isomers.
b) Position isomerism
Aromatic aldehydes and higher ketones give position isomers.
Examples: C5H10O has isomers.