Isomerism in Aldehydes and Ketones: Aldehydes and ketones are commonly known as carbonyl compounds. This is because the functional group in these compounds is the carbonyl group \(\left[ { > {\rm{C}} = {\rm{O}}} \right]\). Hence, aldehydes and ketones are also known as carbonyl compounds. Aldehydes and ketones have a lot of similarities due to functional group similarity. The carbonyl group \(\left( { > {\rm{C}} = {\rm{O}}} \right)\) in these compounds is polarised due to the electronegativity difference between the carbon atom and oxygen atom. Oxygen pulls the shared pair of electrons towards itself that leads to the development of \({\rm{\delta }} + \) charge on the carbon atom and a \({\rm{\delta }} – \) charge on the oxygen atom. In this lesson, we will be exploring these carbonyl compounds by looking at their definition and structure. Let’s get started!

Structure and Nature of Carbonyl Functional Group

The presence of the carbonyl functional group imparts structural similarity to aldehydes and ketones. A carbonyl group is made up of carbon and oxygen in which the carbon atom is double-bonded to an oxygen atom. The carbon atom of the carbonyl group is \({\rm{s}}{{\rm{p}}^2}\) hybridised, which is bonded to three other atoms through sigma bonds. Out of these bonds, one of the sigma bonds is formed with an oxygen atom, while the other two sigma bonds are with hydrogen and/or carbon atoms.

Thus, the carbonyl carbon and the three bonds are arranged in a trigonal planar geometry with bond angles at nearly \(120^\circ \). The fourth valence electron of the carbon atom remains in its unhybridised \(2\text {p}\)-orbital, which overlaps with the \(2\text {p}\)-orbital of the oxygen atom, forming a \({\rm{\pi }}\)-bond. The oxygen atom also contains two lone pairs of electrons.

The \(>\text {C}=\text {O}\) bond is polarised due to the high electronegativity difference between carbon and oxygen atoms. The electronegativity difference between oxygen and carbon leads to the development of a slight positive charge over the carbon atom and a slight negative charge over the oxygen atom. Hence, due to carbon’s positive charge \(>\text {C}=\text {O}\) is prone to nucleophilic attack due to oxygen’s negative charge, it is prone to electrophilic attack.

Aldehydes

The carbonyl group in aldehydes is attached to a hydrogen atom together with either a hydrocarbon group or a second hydrogen atom. The hydrocarbon part might be an alkyl group or a benzene ring. For example-

In aldehydes, the carbonyl group with the hydrogen atom is always present at the end of the chain. It is written as \(-\text {CHO}-\) never as \(\text {COH}\). While naming aldehydes, the longest carbon chain includes the carbonyl carbon atom also. If side groups are attached to the chain, then the carbon atom in the carbonyl group occupies locant \(1\).

Ketones

In ketones, the carbonyl group is attached to two hydrocarbon groups: alkyl or benzene rings. For example-

Unlike aldehydes, the carbonyl group in ketones is always present between the chain and never at the terminals.

Where do Aldehydes and Ketones Differ?

In ketones, no hydrogen atom is directly attached to the carbonyl group, whereas in aldehydes, a hydrogen atom is directly attached to the carbonyl group.

Isomerism in Aldehydes and Ketones

Molecules with the same molecular formula but different arrangements of the atoms in space are known as isomers, and the phenomenon is known as isomerism.

Aldehydes and ketones have the same general formula \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{\rm{O}}\) and exhibit three types of isomerism, these are-

Chain or Structural Isomerism

Aldehydes exhibit chain or structural isomerism with \(4\) or more carbon atoms, and ketones exhibit chain or structural isomerism with \(5\) or more carbon atoms. In this isomerism, the isomers of aldehyde and ketones differ in the chain of carbon atoms. For example, aldehyde with the molecular formula \({{\rm{C}}_4}{{\rm{H}}_8}{\rm{O}}\) has two chain isomers. These are-

Similarly, ketones with the molecular formula \({{\rm{C}}_5}{{\rm{H}}_{10}}{\rm{O}}\) has two chain isomers. These are-

Position or Constitutional Isomerism

In this isomerism, the isomers differ in the position of the functional group. Aliphatic aldehydes do not exhibit position isomerism because the \(-\text {CHO}\) group is always present at the end of the carbon chain. However, aromatic aldehydes and higher ketone exhibit positional isomerism.

For example, \({{\rm{C}}_5}{{\rm{H}}_{10}}{\rm{O}}\) has two positional isomers. These are-

Functional Isomerism

In this type of isomerism, compounds have the same molecular formula but have different functional groups. Aldehydes and ketones have the same structural formula, \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{\rm{O}}\) and can exhibit functional isomerism.
For example, The molecular formula \({{\rm{C}}_3}{{\rm{H}}_6}{\rm{O}}\) corresponds to an aldehyde as well as a ketone.

Summary

Aldehydes and ketones contain the carbonyl functional group, hence are known as carbonyl compounds. They exhibit similar properties as they have the same general formula \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{\rm{O}}\); hence, in ketones, no hydrogen atom is directly attached to the carbonyl group, whereas in aldehydes, a hydrogen atom is directly attached to the carbonyl group.

In this article, we explained the isomerism in these carbonyl compounds. We also learned the definition, structure of aldehyde and ketones. We also learned the different types of isomerism in aldehydes and ketones.

Frequently Asked Questions

Q.1. Are aldehydes and ketones functional group isomers?
Ans: Compounds that bear the same molecular formula but have different functional groups attached exhibit functional isomerism. Aldehydes and ketones have the same structural formula, \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}}}}{\rm{O}}\) and can exhibit functional isomerism.
For example, the molecular formula \({{\rm{C}}_3}{{\rm{H}}_6}{\rm{O}}\) corresponds to an aldehyde as well as a ketone.

Q.2. What is the difference between metamerism and position isomerism?
Ans: Position isomerism describes the different locations of a functional group, while metamerism describes the attachment of different alkyl groups attached to the same functional group.

Q.3. Which type of isomerism is not shown by aliphatic aldehydes?
Ans: Aliphatic aldehydes do not exhibit position isomerism because the \(-\text {CHO}\) group is always present at the end of the carbon chain. However, aromatic aldehydes exhibit position isomerism.

Q.4. Which compounds do not undergo functional group isomerism?
Ans: Alkyl halides do not undergo functional group isomerism as there is only one way through which the halogen atom gets attached to the carbon atom via a single bond.

Q.5. What will be the hybridisation of C and O in aldehyde and ketone?
Ans: Both aldehydes and ketones contain the carbonyl group \(\left[ { > {\rm{C}} = {\rm{O}}} \right]\), in which the carbon atom is double-bonded to the oxygen atom. The carbon atom of the carbonyl group is \({\rm{s}}{{\rm{p}}^2}\) hybridised. It is bonded to three other atoms through a sigma bond in which one of the sigma bonds is formed with an oxygen atom. Thus, the carbonyl carbon and the three bonds adopt a trigonal planar geometry with bond angles at nearly \(120^\circ \). Nevertheless, the fourth valence electron of the carbon atom remains in its unhybridised \(2\text {p}\)-orbital, which overlaps with the \(2\text {p}\)-orbital of the oxygen atom, forming a \({\rm{\pi }}\)-bond. The oxygen atom also contains two lone pairs of electrons.

Learn About Ethanol Formula Here

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