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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 |
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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 |
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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.
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| propanal |
| 3,4-dimethylheptanal |
Answers
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pentanal |
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2-methylbutanal |
| propanal |
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| 3,4-dimethylheptanal |
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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. |
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E-mail instructor:
Eden Francis
Clackamas Community College
©2001, 2003 Clackamas Community College, Hal Bender
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![Structures and names of formaldehyde and acetaldehyde. [63str05.JPG]](images/63str05.JPG)