Aldehydes and ketones

ALDEHYDE AND KETONE

Aldehydes and ketones are two classes of organic compounds that contain the carbonyl functional group, which is a carbon atom double-bonded to an oxygen atom.

The general formula for aldehydes is RCHO, where R represents any alkyl or aryl group. The general formula for ketones is RCOR', where R and R' can be any alkyl or aryl group.

 Aldehydes are named by replacing the -e ending of the parent alkane with -al. For example, CH3CHO is named as ethanal. Ketones are named by replacing the -e ending of the parent alkane with -one. For example, CH3COCH3 is named as propanone.

 Aldehydes and ketones are polar molecules due to the presence of the carbonyl group. They can form dipole-dipole interactions and can participate in hydrogen bonding with water and other polar solvents.

 Aldehydes are prepared by the oxidation of primary alcohols, while ketones are prepared by the oxidation of secondary alcohols. They can also be prepared by various other methods such as ozonolysis of alkenes, Friedel-Crafts acylation, and the Cannizzaro reaction.

 Aldehydes and ketones undergo a variety of reactions such as nucleophilic addition reactions, oxidation reactions, and reduction reactions. They are important intermediates in many organic reactions and are used in the synthesis of a variety of organic compounds.

 Aldehydes and ketones have important applications in industry and in everyday life. They are used in the production of plastics, pharmaceuticals, perfumes, and solvents. Some of them are also used as food additives and preservatives.

 

Common names of aldehyde and ketone

•    The common names of aldehydes are derived from the common names of the corresponding carboxylic acids in which the ending ‘-ic’  is replaced with ‘-aldehyde’ of acid with aldehyde.

For example:

     HCHO                                           CH3CHO

Formaldehyde                                  Acetaldehyde

Common naming system of ketone:

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

For example:

•    Alkyl phenyl ketones are usually named by adding the acyl group as prefix to phenone. For example:

 

IUPAC names of aldehyde and ketone

Nomenclature of Aldehydes

IUPAC naming system:

•    In IUPAC system, the suffix ‘e’ of alkane is replaced by the ‘al’.

For example:

  HCHO                                   CH3CHO

Methanal                                 Ethanal

Nomenclature of Ketones

IUPAC naming system:

•    In IUPAC system, the suffix ‘e’ of alkane is replaced by the ‘one’.

For example:

Structure of the Carbonyl Group

Aldehydes, ketones and carboxylic acids are the compounds containing carbon-oxygen double bond (>C=O) called carbonyl group. The carbon and oxygen of the carbonyl group are sp2 hybridised and the carbonyl double bond is comprised of one σ-bond and one π-bond. The π electron cloud is present above and below the plane and is present between carbonyl carbon and oxygen.


Due to high electronegativity of oxygen compared to carbon, the carbonyl group is polar and the dipole is formed.

Preparation of Aldehydes and Ketones

By oxidation of alcohols

Oxidation of primary and secondary alcohols in presence or oxidizing agent like K2Cr2O7/H2SO4, KMnO4. CrO3 gives aldehydes and ketones respectively.

By dehydrogenation of alcohols

 

From hydrocarbons

By ozonolysis of alkenes:

Ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones or a mixture of both depending on the substitution pattern of the alkene.

 

By hydration of alkynes:

This is a commercial method to prepare ethanal.

All other alkynes gives ketones.