IUPAC Naming Rules and Common Examples of 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In the general structure of an aldehyde, it can be noticed that the two hydrogen atoms of the halogen group have been replaced by a doubly bonded oxygen atom.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In the general structure of a ketone, it can be noticed that the two hydrogen atoms of the halogen group have been replaced by a doubly bonded oxygen atom.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.
FAQs on Aldehyde Nomenclature: Complete Guide for Students
1. What is the functional group of aldehydes and how is it represented in chemical structures?
The functional group characteristic of all aldehydes is the aldehyde group. It consists of a carbonyl group (a carbon atom double-bonded to an oxygen atom, C=O) where the carbon atom is also bonded to at least one hydrogen atom. This group is represented as -CHO and is always located at the end of a carbon chain.
2. What are the basic IUPAC rules for the nomenclature of aldehydes?
According to the IUPAC system, the nomenclature of aliphatic aldehydes follows these key steps:
- Identify the longest continuous carbon chain that includes the aldehyde group's carbon. This chain forms the parent alkane name.
- The carbon of the -CHO group is always numbered as carbon-1 (C1).
- Replace the final '-e' from the parent alkane's name with the suffix '-al'.
- Identify any substituents, assign them the lowest possible number, and list them alphabetically before the parent name. For example, a 4-carbon aldehyde is named butanal.
3. How are common names for simple aldehydes typically derived?
The common names of aldehydes are derived from the common names of the corresponding carboxylic acids by replacing the '-ic acid' suffix with '-aldehyde'. For example, the one-carbon aldehyde HCHO is called formaldehyde (from formic acid), and the two-carbon aldehyde CH₃CHO is called acetaldehyde (from acetic acid).
4. What is the correct IUPAC name for the aldehyde with the structure CH₃-CH(CH₃)-CHO?
To name CH₃-CH(CH₃)-CHO, we follow the IUPAC rules:
- The longest carbon chain containing the -CHO group has 3 carbon atoms, so the parent name is derived from propane.
- Numbering starts from the aldehyde carbon, making it C1. This places the methyl (-CH₃) group on carbon-2.
- We replace the '-e' in propane with '-al'.
Therefore, the correct IUPAC name is 2-methylpropanal.
5. How does the nomenclature change when an aldehyde (-CHO) group is attached directly to a cyclic ring?
When the -CHO group is attached to a cyclic structure, the naming convention changes. The aldehyde group is considered a substituent on the ring. The full name of the cycloalkane is retained, and the suffix '-carbaldehyde' is added. For instance, an aldehyde group attached to a cyclohexane ring is named cyclohexanecarbaldehyde. The carbon atom of the ring to which the aldehyde is attached is numbered as C1.
6. Why is the IUPAC suffix '-al' used for aldehydes but not for ketones?
The suffix '-al' is specific to aldehydes because the aldehyde functional group (-CHO) is by definition at the end of a carbon chain. This terminal position is a defining characteristic. In contrast, the carbonyl group (C=O) in a ketone is always located within the carbon chain, never at the end. To signify this internal position, the suffix '-one' is used for ketones, along with a number to indicate the location of the carbonyl carbon.
7. What are some common aromatic aldehydes and how is their nomenclature handled by IUPAC?
The simplest aromatic aldehyde is benzaldehyde, which consists of a benzene ring with a -CHO group attached. This common name is officially accepted by IUPAC as the parent name. For substituted benzaldehydes, the ring is numbered with the carbon bearing the -CHO group as C1. For example, a bromine atom at the fourth position would be named 4-bromobenzaldehyde.
8. In a molecule with multiple functional groups, how is the aldehyde group prioritised in naming?
The priority of functional groups determines the suffix of the IUPAC name. The aldehyde group has a higher priority than ketones, alcohols, and amines, but a lower priority than carboxylic acids and their derivatives. When a higher-priority group like a carboxylic acid is present, the aldehyde group is treated as a substituent and is named using the prefix 'formyl' or 'oxo'. For example, the compound HOOC-CH₂-CHO is named 3-oxopropanoic acid.
