An aldehyde is a class of organic compound with the general structure


It is a carbonyl group attached to a hydrogen.

According to the IUPAC system, aldehydes are named by the substituent group, with the suffix '-al'. Aldehydes are always the first carbon according to the IUPAC convention1. Here are some examples:

A Simple Example
    H    O
    |   //
    |   \
    H    H

Let's name it!

  1. We've already determined it's an aldehyde, since it has the -COH group, and the whole point of this section is naming aldehydes.
  2. Count the longest carbon chain. Two carbons. Therefore, it's an ethane derivative.
  3. Tack on the -al suffix and form the name: ethane + al = ethanal
The trivial name for this compound is acetaldehyde.

A Quite Complicated Example2
       H--C--H H    O
           \   |   //
    H  H H--C--C--C
    |  |   /   |   \
 H--C--C--C--H H    H
    |  |  |
    H  H  H

Naming this one is complicated, but not too hard.

  1. This is an aldehyde. The aldehyde group is therefore carbon 1.
  2. Now count the longest carbon chain. Starting from the one carbon, there is a branch at the three carbon. The three-carbon (propyl) group is longer than the two-carbon (ethyl) group. Therefore, the numbering is counted on the propyl group. The parent alkane is hexane. Since there is that ethyl group on carbon three, it is 3-ethylhexane.
  3. Now to do something about that pesky aldehyde group. Since the aldehyde group is always on carbon 11, there is no need for a 1- prefix. Now add the suffix -al. The compound is 3-ethylhexanal.

And Just to Clear Things Up

This is an aldehyde without an alkyl group attached. The only carbon in this is the carbon in the carbonyl group. Since the carbon in the aldehyde group is counted in the parent chain, this would be methanal. The trivial name of this compound is formaldehyde.

Other Resources

A good, but not as indepth, resource for naming organic compounds is A guide to naming organic compounds3.

1Unless there are any functional groups of higher priority, of course.

2Previous knowledge in naming branched-chain alkanes is assumed. I've always seen branched-chain alkanes covered before aldehydes in a chem course and I will assume you have.

3I believe that rather than duplicating that node, my writeup complements it.

Sources: myself. (Ain't chemgeekery grand?) Please /msg me with feedback!