Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Heteroatomic Functional Groups and Organic Nomenclature
3.8 Nomenclature of Carboxylic Acids

Compounds that have the ─COOH functionality are named as alkanoic acids. The -e of the corresponding root alkane name is changed to -oic followed by the addition of the word acid. In naming carboxylic acids, the longest continuous chain that contains the ─COOH functionality forms the root name of the alkanoic acid as shown below.

Since the carboxylic acid functionality is a terminal functionality, the carbon of the COOH is always assigned #1. Therefore, the position of the ─COOH functionality does not have to be specified by a number. The position of other groups (substituents) that are bonded to the root chain must be specified by a number based on the numbering system dictated by the ─COOH position, an example is shown below.

Problem 3.12

i. Give the line-angle structures of the following molecules.

1. 2-Chloro-3-ethylhexanoic acid

2. 2-Chloro-3,3-dimethylpentanoic acid

3. 2-Ethylbutanoic acid

ii. Give the IUPAC name for the compound shown below.

Over the years, trivial names have been used for carboxylic acids; one type of trivial name is based on the use of letters of the Greek alphabet. Letters of the Greek alphabet are used to indicate the position of substituents on the chain of the carboxylic acids. For this system, the first carbon that is next to the carbonyl carbon of the reference carboxylic acid functionality is assigned the Greek letter α and a substituent that is bonded to this carbon is described as being in the α position. The carbon next to that first carbon of the longest chain, or the carbon that is two carbons away from the carbonyl carbon, is assigned the Greek letter β. Likewise, the carbon that is in the third position from the carbonyl carbon of the reference carboxylic acid functionality is assigned the Greek letter γ, as shown below.

Substituents that are bonded to these carbons are assigned the corresponding Greek letter to indicate the position of the substituent as shown in the examples below.

3.8.1 Nomenclature of Difunctional Carboxylic Acids

A large percentage of carboxylic acids contains not only the ─COOH functionality, but other functionalities, such as an alcohol (─OH), alkene, or alkyne functionalities. For these molecules, the carboxylic acid group will always get priority and is assigned #1. In naming such polyfunctional molecules, the position of another functionality is specified based on the numbering system dictated by the location of the carboxylic acid functionality, an example is shown below. Note that the number 2 indicates the position of the alkene, which signifies the letter “e” in “hexenoic” of the name.

Problem 3.13

i. Give the line-angle structure of the following molecules.

1. 3-Hydroxypentenoic acid

2. 4-Hydroxy-2-pentenoic acid

3. 4-Chloro-2-hexynoic acid

4. 5-Chloro-2,2-dimethyl-3-hexynoic acid

ii. Give the IUPAC name for the compounds shown below.

3.8.2 Nomenclature of Cyclic Carboxylic Acids

The carboxylic acid functionality is a terminal functionality and hence cannot be in a ring; that is, this functionality must be bonded to a ring. In naming cyclic carboxylic acids, carboxylic acid becomes the root name and the cyclic structure becomes the substituent. The IUPAC name then becomes cycloalkanecarboxylic acid (note, two words). The general nomenclature system is illustrated below.

Note that number 1 is not included in the name to indicate the position of the carboxylic acid functionality. In numbering cyclic carboxylic acids that have substituents, the carbon that has the ─COOH functionality gets #1 and the numbering proceeds in the direction so that the lowest number combination results. Examples of naming a cyclic carboxylic acid are given below.

Problem 3.14

i. Give the line-angle structures for the following compounds.

1. 2,3-Dichloro-4-methylcyclopentanecarboxylic acid

2. 1-Methylcyclohexanecarboxylic acid

3. 2,2-Dimethylcyclobutanecarboxylic acid

ii. Give the IUPAC names of the molecules shown below.

Note that if the carboxylic acid functionality is not bonded directly to a cyclic structure, but to a carbon of the longest continuous linear region of the molecule, the cyclic structure is then named as a cycloalkyl group, as shown in the example below.

If there are two carboxylic acid functionalities present, then both carboxylic groups are indicated by numbers as shown in the example below.

As usual, always ensure that the number combination is always the lowest. Thus, the IUPAC name in the above example is not 6-methylcyclohexane-1,4-dicarboxylic acid.

If there is a different functionality present in a cyclic carboxylic acid molecule, a system similar to that described in the previous section regarding the nomenclature of molecules with difunctional groups is used, as shown in the example below. Note that the “e” is used to indicate the presence of an alkene functionality and the number that precedes gives the position of the double bond in the molecule.

At this point, it is worth establishing a priority of functional groups in the naming of polyfunctional molecules. The functionality of highest priority will be used to determine the root name of the molecule. The priority trend of some functional groups is shown below.

Hence, if a molecule contains a carboxylic acid functionality and an alcohol functionality, the carboxylic acid functionality is used to form the root name and the molecule is not named as an alkanol, but as a carboxylic acid as shown in the example below.