Aldehyde has one alkyl group, whereas ketone has two alkyl groups.
Aldehydes and ketones are both homologous series containing the carbonyl group. Both organic families have the general formula CnH2nO. The carbonyl functional group (C = O) consists of a carbon atom double–bonded to an oxygen atom. The position of the C=O functional group in the carbon chain marks the difference between aldehydes and ketones. The aldehydes have the carbonyl group at the end of the carbon chain and have the general structure RCHO. The carbonyl group of the ketones is not at the end of the carbon chain. Its position within the molecule varies making ketones to exhibit positional isomerism.
Bond angle of methanol
The carbonyl group plays a significant role in the reactivity of aldehydes and ketones, it provides a site for nucleophilic addition and it is the cause of the acidity of the H atom alpha to the carbonyl carbon. These effects are largely due to the polarity of the carbonyl group created by the electron–withdrawing power of oxygen. Note that the aldehydic H is not acidic.
Geometry:
The carbonyl carbon is sp2 hybridized. The molecule is arranged in a trigonal planar structure with a bond angle of
approximately 120° between substituents.
Nucleophilic attack on carbonyl group
Chemical properties:
i. Reactivity of carbonyl group:
The main reactions of the carbonyl group are nucleophilic additions to the carbon – oxygen double bond. As shown in the
figure, this addition consists of adding a nucleophile and a hydrogen across the carbon – oxygen double bond. Due to
differences in electronegativities, the carbonyl group is polarized. The carbon atom has a partial positive charge, and the
oxygen atom has a partially negative charge. Aldehydes are usually more reactive towards nucleophilic substitutions than ketones
because of both steric and electronic effects.
ii. Electronic effect:
This electronic effect is due to positive charge(+) on the carbonyl group. Greater the +ve charge then greater is the reactivity.
In ketones the +ve charge on the carbonyl group is reduced because of the two alkyl groups, when compared with aldehydes.
Therefore, ketones are less reactive than aldehydes.
iii. Steric effect:
This steric effect is due to the number and size of alkyl groups. As the number and size of alkyl group increases the attack
of a nucleophile decreases and the reactivity decreases. In aldehydes only one alkyl group is present, whereas in ketones
two alkyl groups are present.
Uses:
In chemical industry aldehydes and ketones are used as solvents, starting materials and reagents for the synthesis of other
products.