The carbonyl group is a divalent chemical unit consisting of a carbon (C) and an oxygen (O) atom, connected by a double bond. The ketones (RCOR’) and the aldehydes (RCHO) are usually referred to as carbonyl compounds. The carbonyl group is also found in quinones, amides, acyl halides, anhydrides, esters and carboxylic acids.
In this short post, we shall focus on the IUPAC nomenclature of ketones and aldehydes.
General molecular formula: RCHO/ ArCHO.
Aldehydes have a carbon atom surrounded by oxygen, hydrogen and an alkyl group. Its position is always terminal (i.e, always at the end of the chain), hence it takes no. 1 when numbering.
The aldehydes originated from the name of the alcohol that was dehydrogenated to obtain the aldehyde. Hence the term ‘aldehyde’ meaning ‘alcohol dehydrogenated’
- Saturated aliphatic aldehydes are referred to as alkanals
IUPAC Nomenclature of aldehydes
There are certain rules to take note of when naming aldehydes. For example, what is the name the following organic compound?
The following rules apply:
- Name the longest chain containing the carbonyl group, replacing the “e” in the corresponding alkane by “al”.
- Do not indicate the position of carbonyl carbon since it is always terminal (i.e taking no. 1).
- Indicate the position of any branching (attached substituent) with respect to the carbonyl carbon.
- Perhaps there is more than one substituent along the chain, indicate their positions with respect to the carbonyl carbon and name alphabetically.
- If the substituents are the same, indicate their positions and apply appropriate prefixes such as “di”, “tri”, etc.
- You can also apply Greek letters in naming. The numbering starts from the carbon adjacent the carbonyl carbon. Hence C-2 is α-C, C-3 is β-C, etc.
From the rules above, we’d both agree that the name of the compound is “2-methylpropanal”.
Now, how do you name this:
- NB: For safety, write out the substituents with their positons first and the parent chain, then name the compound.
Therefore, we have – “2- ethyl”, “3-chloro”, “3-ene”, “4-methyl”, “pentenal”.
Applying the given rules, we have: 3-chloro-2-ethyl-4-methyl-3-pentenal
NB: The “3” before the pentenal indicates the position of the attached double bond.
- The aldehyde of benzene is called ‘benzaldehyde’.
Now, name the following:
Present the structures of the following aldehydes:
(iii)pentanal (iv)3-chlorobutanal (v)phenylethanal
Drop the answers to these in the comment section:
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