Aldehyde

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

    O
    ||
 R--C--H

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:

    H    O
    |   //
 H--C--C
    |   \
    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

          H
          |
       H--C--H
          |
       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.

    O
    ||
 H--C--H

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.

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.

A colorless, mobile, and very volatile liquid obtained from alcohol by certain of oxidation.

The aldehydes are intermediate between the alcohols and acids, and differ from the alcohols in having two less hydrogen atoms in the molecule, as common aldehyde (called also acetic aldehyde or ethyl aldehyde), C2H4O; methyl aldehyde, CH2O.

Aldehyde ammonia Chem., a compound formed by the union of aldehyde with ammonia.

© Webster 1913.