Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Heteroatomic Functional Groups and Organic Nomenclature
3.5 Nomenclature of Aldehydes
The systematic IUPAC naming of aldehydes is similar to the naming of organic molecules discussed previously in that the root names are based on the longest chain that contains the functional group, the aldehyde functionality in this case. Compounds that have the aldehyde functionality, ─CHO, are named as alkanals. The e of the corresponding alkane is changed to al. Thus, a compound that has five carbons and has the aldehyde functionality, the IUPAC name is pentanal. In naming aldehydes, the longest continuous chain that contains the ─CHO functionality forms the root name of the alkanal. Since the ─CHO functionality is a terminal functionality, the carbon of the carbonyl group is assigned number 1. Thus, the position of the ─CHO functionality is not specified since it is always #1. Shown below is an example for the correct numbering of aldehydes.
Note that for the above incorrect example, even though the branch is in position 2 and the ─CHO functionality is in position 6, the branch does not take priority over the ─CHO functionality. That is, the ─CHO functionality must always be assigned the #1 position.
Problem 3.8
i. Give the IUPAC names for the compounds shown below.
ii. Give structures for each of the following molecules.
1. 2,3-Dimethylhexanal
2. 2-Chloro-3-methylbutanal
3. 2,2-Dimethylbutanal
3.5.1 Nomenclature of Difunctional Aldehydes
It is possible to have the ─CHO functionality present in a molecule that has another functionality, such as an ─OH, alkene, or alkyne functionalities. For such a molecule, the aldehyde group will always get priority and gets assigned position #1. If there is only one other functionality present, the position of the other functionality is specified based on the numbering system of the root name given to the alkanal. An example is shown below for the nomenclature of a molecule that contains the aldehyde and alkene functionalities.
Note that the “e” is used to specify the alkene functionality and that the number that directly precedes gives the position of the alkene functionality. If there are other functionalities, such as an ─OH, it is specified as a hydroxyl group.
Problem 3.9
i. Give the IUPAC names for the compounds shown below.
ii. Give the structures of the compounds shown below.
1. 3-Ethyl-2-hydroxyhexanal
2. 2-Propenal
3. 3,7-Dimethyl-2,6-octadienal (Geranial)
4. 3-Phenyl-2-propenal (Cinnamaldehyde) [note that the phenyl substituent is ─C6H5]