Nomenclature
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Nomenclature

IUPAC Names

Naming aldehydes is very much like naming alcohols or ketones, with two important differences. Of course, one is the ending. Instead of -ol for alcohols or -one for ketones, use -al for aldehydes. The other is that since the aldehydo group is always at the end of the carbon chain ( the #1 carbon atom), you don't have to use a number to indicate where it is.

Example

One example is the aldehyde shown here (and also in Example 13 in your workbook as the product of oxidizing 1-pentanol). It is called pentanal. Because it has a five-carbon-atom chain, that gives us pentan- for a root, and the aldehydo functional group gives us the -al ending. That makes it pentanal. Notice that the carbon atom in the aldehydo group (or carbonyl group) is counted as part of the chain. Since there are no side chains or other functional groups, nothing else has to be added to the name.

 

pentanal

 

Of course if other groups were attached to this chain at various points, prefixes to indicate what those groups are and numbers to show where they are attached would be added to the name. If there are groups attached, be sure to start numbering from the aldehydo end of the molecule.

You should also note the similarity of the names of the aldehyde and the alcohol from which it can be made. There are two differences: one is the dropping of the number 1- prefix because it is not necessary, and the other is changing the ending from -ol to -al.

 

pentanal

 

1-pentanol

 

Again notice in the example above, that the alcohol and the aldehyde that is made from it both have the same number of carbon atoms with the same arrangement for those carbon atoms. The only difference, the only change that takes place, is on that end carbon atom where two hydrogen atoms are removed to make the aldehydo group. Everything else stays the same. The position of the functional group is the same. The number and arrangement of the carbon atoms are all the same.

Practice

Now I would like you to come up with the names and structural formulas for the compounds below (which are also shown in Exercise 18 in your workbook). Check your answers below and then continue.

 

 

    
propanal
3,4-dimethylheptanal

 

Answers

pentanal
     2-methylbutanal
propanal   
3,4-dimethylheptanal

 

Common Names

There are two aldehydes which have such widespread usage that they have common names which persist. They are the aldehydes with one and two carbon atoms in their molecules. The IUPAC names for these compounds are methanal and ethanal. The common names are formaldehyde and acetaldehyde. You should learn those two common names. For these two compounds you will run into the common names more frequently than the IUPAC names . When you are working on your lab exercise you will need to draw the structural formulas for these compounds.

Structures and names of formaldehyde and acetaldehyde. [63str05.JPG]

 

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E-mail instructor: Eden Francis

Clackamas Community College
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