Naming aldehydes and ketones
In naming aldehydes using the IUPAC system we use the suffix -al. The end e is replaced by al.
So the common names of the first four aldehydes become:
Methanal
Ethanal
Propanal
Butanal

Now this is all pretty simple, but what if there are side chains off the main aldehyde chain. For example, take the molecule pictured on the right.

Step 1 The aldehyde forms the root name
Step 2 Number the carbons so that the C=O is always carbon number 1.

Step 3 Name all the substituents in alphabetical order.

3-methylbutanal

  
Consider the example shown on the right. It has two substituent groups, the methyl and hydroxy. The IUPAC name is 4-hydroxy-3-methylbutanal.
Name the following molecules  
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Solution
Naming ketones  
A ketone contains a carbonyl (C=O) functional group connected to two carbon atoms. When naming a ketone we remove the end "e" from the name and replace it with the suffix "one" with a locator number. For example the molecule CH3COCH3 becomes propanone. Here a locator number for the functional group is not needed as there is no ambiguity as to where the carbonyl group is located. It can only be on carbon number 2 for the molecule to be classified as a ketone. There is no ambiguity as to the location of the carbonyl group in propanone (acetone) and butanone. In the literature, however, you will find that propanone and butanone are written as propan-2-one and butan-2-one respectively. Since there is no ambiguity and structural isomers can not exist for the two simplest ketones, locator numbers will not be required.

Now lets look at the steps to derive a IUPAC name.

Step 1) Identify the longest carbon chain that contains the carbonyl group and number the carbons so that the carbonyl group is on the lowest carbon.

Step 2) Identify the various substituent groups

Step 3) Place the substituent groups in the name in alphabetical order and use position numbers to locate the substituent groups and the carbonyl group.

Step 4) Change the end e to a one.

Step 5) Write the name with the locator number, if ambiguity exists, for the carbonyl group.
In this case the name is propanone, as no ambiguity exists.

Lets take the molecule shown on the right as an example.
Step 1 The longest carbon chain is pentane
Step 2 No substituent groups
Step 3 and 4 The carbonyl group is found at carbon number 2 .
Hence the name is 2-pentanone or pentan-2-one. Both names are correct and commonly used interchangeably.

What is the IUPAC name of this molecule?

Solution

What is the IUPAC name of this molecule?

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What is the IUPAC name of this molecule?

Solution

Name the molecule on the right

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