Carbonyl compounds (Aldehydes and Ketones)

The aldehyde may be aliphatic or aromatic, according to whether the –CHO group is attached to an alkyl group (or hydrogen atom) or an aryl group. Similarly, ketones may be aliphatic or aromatic, according to whether the >C=O group is attached to two alkyl groups or two aryl groups or any one of the alkyl or aryl. Reactivity of aldehyde is more than that of ketones due to in aldehydes carbonyl carbon has more partial positive charge than ketones because in ketones two electron-donating alkyl groups are attached while in aldehydes only one electron-donating group is attached.

Carbonyl group structure 

The C=O bond in aldehyde and ketones like the C=C bond in alkenes is made up of one σ- bond and π-bond. In the formation of the carbonyl group, Carbon is sp2-hybridised while Oxygen uses its unhybridized atomic orbitals. Carbonyl carbon, oxygen, and the two atoms directly bonded to the carbonyl carbon lie in one plane.

Carbonyl group nature 

In the carbonyl group oxygen is more electronegative than the carbon atom. As a result, the oxygen atom attracts the electron cloud of the π-bond towards itself. In other words, the π-electron cloud of >C=O bond is unsymmetrical. Hence, Carbonyl carbon acquires a small positive charge and hence acts as an electrophile (Lewis acid) while carbonyl oxygen carries a small negative charge and hence behaves as a nucleophile (Lewis base). This is the reason for the polarity of the carbonyl group.

Similarities between C=O bond and C=C bond

(i) Both are made up of one σ-bond and one π-bond.

(ii) Both use sp2-hybrid orbitals of a Carbon atom for their formation.

(iii) Both are planar in nature.

Dissimilarities between C=O bond and C=C bond 

(i) carbon-oxygen double bond is polar but the carbon-carbon double bond is non-polar.

(ii) Because of the small size of oxygen as compared to carbon, C=O (120pm) bond length is shorter than that of C=C bond length (134pm)

(iii) Carbonyl compounds undergo nucleophilic addition reactions but compounds containing carbon-carbon double bonds undergo electrophilic addition reactions.

Boiling points of aldehyde and ketones are somewhat higher than those of non-polar compounds and weakly polar compounds of comparable molecular masses. This is due to the reason that these aldehydes and ketones contain polar carboxyl groups and undergo weak intermolecular association due to dipole-dipole interactions between the opposite ends of the carbonyl dipoles.

The dipole-dipole interactions are, however, weaker than intermolecular H-bonding between alcohol molecules and carboxylic acids having comparable molecular masses.

Nucleophilic addition reaction

Aldehydes and ketones show nucleophilic addition reactions due to greater electronegativity of oxygen as compared to carbon, the carbon of the carbonyl group behaves as the electrophile. Therefore, a nucleophile readily attacks the electrophilic carbon atom of the polar carbonyl group. During this process, the complete transfer of π-electrons of the carbon-oxygen double bond takes place from carbon to oxygen atom.

Carbonyl compounds are those organic compounds that contain carbon-oxygen double bonds. Aldehyde and ketones also undergo substitution reactions at the carbon next to the carbonyl carbon (α-carbon). This type of reaction is shown by only those carbonyl compounds which have alpha hydrogen or we call it acidic hydrogen.

Conclusion

Lower aldehydes and ketones containing up to four carbon atoms such as methanal, ethanal, etc. are soluble in water due to Hydrogen bonding between the polar carbonyl group and the water molecules. As the size of the alkyl group (hydrocarbon part) increases the solubility of the carbonyl compounds rapidly falls off. Boiling points of aldehyde and ketones are somewhat higher than those of non-polar compounds and weakly polar compounds of comparable molecular masses. As a result, the higher member after C5-onwards is practically insoluble in water. Both aldehyde and ketones are used as synthetic perfumes, ketones have been used in the blending of perfumes and flavouring agents.