Nomenclature of Aldehydes

What Are Aldehydes?

In aldehydes, the carbonyl carbon has one hydrogen atom attached to it in addition to the alkyl or aryl chain attached on the other side.


For example, given below are a few commonly known aldehydes.


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What are Ketones?

In ketones, the carbonyl carbon is surrounded with alkyl or aryl chains on both sides.


Following are a few common examples of ketones:


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General Structure of an Aldehyde

In the general structure of an aldehyde, it can be noticed that the two hydrogen atoms of the halogen group have been replaced by a doubly bonded oxygen atom. This -CHO is the part that forms an aldehyde group when attached to an alkyl chain. The carbonyl carbon has a hydrogen attached to it on one side and an alkyl or aryl chain attached on the other side.


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Nomenclature of Aldehydes 

For naming an aldehyde according to the norms set by the International Union of Pure and Applied Chemistry, the following steps should be followed.

  • Determine the longest carbon chain that contains the aldehyde group that is CHO.

  • The aldehyde group is generally attached at the end of the main alkyl chain and therefore, the first position is assigned to the aldehyde group itself, though it is not mandatory to include the numbering in the naming always.

  • The ‘e’ at the end of the name of an alkane is replaced with ‘al’ while naming an aldehyde. 

  • Often Greek letters such as α, β etc. are used in the naming of the aldehydes. For example, α-carbon is used to refer to the carbon that is directly attached to the aldehyde group, β-carbon is the adjacent carbon to the α-carbon and so on.


General Structure of a Ketone

In the general structure of a ketone, it can be noticed that the two hydrogen atoms of the halogen group have been replaced by a doubly bonded oxygen atom. The carbonyl carbon in ketones is surrounded by alkyl or aryl chains on both sides. The common notation of a ketone is R-CO-R.


Nomenclature of Ketones 

For naming a ketone according to the norms set by the International Union of Pure and Applied Chemistry, the following steps should be followed.

  • Determine the longest carbon chain that contains the carbonyl group that is CO.

  • The ‘e’ in the end of the name of an alkane is replaced with ‘one’ while naming a ketone. 

  • The carbonyl group is usually located in the longest carbon chain and the numbering starts from the side from where the carbonyl group will get the lowest number. 

  • Often Greek letters such as α, β etc. are used in the naming of the aldehydes. For example, α-carbon is used to refer to the carbon that is directly attached to the aldehyde group, β-carbon is the adjacent carbon to the α-carbon and so on.


Solved Examples

Question 1. Name the following structure, according to IUPAC. 

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Answer: The name of the given structure according to IUPAC using aldehyde nomenclature is 3-hydroxybutanal, and the common name of the element is aldol.

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Question 2. Name the following structure, according to IUPAC.

Answer: The name of the given structure according to IUPAC using ketone nomenclature is 2-bromo-4,4-dimethylcyclohexanone.

FAQ (Frequently Asked Questions)

Question 1. Why do Ketones have Higher Boiling Points than Aldehydes?

Answer: If there are ketones and aldehydes having a similar molecular mass, then ketones will have a higher boiling point as compared to aldehydes because of the fact that the carbonyl group in ketones is more polarized than aldehydes. The carbonyl carbon in ketones is surrounded by two electron-donating R-groups that makes them more polar than aldehydes; therefore, the dipole moments created by this polarity makes the boiling points of ketones higher than those of aldehydes. As such, the interaction between the molecules of ketones is significantly stronger as compared to the interaction of molecules in Aldehydes, resulting in a higher boiling point.

Question 2. Why are Aldehydes More Reactive than Ketones in Nucleophilic Substitutions?

Answer: As compared to ketones, aldehydes are highly reactive towards nucleophilic situations due to a combination of the electronic and steric effects. The carbonyl carbon in the ketones have two alkyl or aryl chains on both sides that offer steric hindrance, but on the other hand the hydrogen atoms in the aldehydes are relatively smaller and do not provide any steric hindrance. Also, the partial positive charge on the carbonyl carbon in ketones is stabilized by the two R groups, and so the aldehydes are more reactive towards the nucleophilic reactions as compared to the ketones.