MCAT Organic Chemistry Review
1.4 Carboxylic Acids and Derivatives
Carboxylic acids contain both a carbonyl group (C=O) and a hydroxyl group (–OH) on a terminal carbon. Carboxylic acids, like aldehydes, are terminal functional groups; therefore, their associated carbon is usually numbered 1. This is the most oxidized functional group that appears on the MCAT, with three bonds to oxygen; only carbon dioxide, with four bonds to oxygen, contains a more oxidized carbon. Carboxylic acids are thus the highest-priority functional group in MCAT-tested nomenclature, and all other groups are named as substituents via prefixes. Carboxylic acids are named by replacing –e in the name of the parent alkane with the suffix –oic acid.
Once again, the common names for carboxylic acids show up fairly often on the MCAT. Formic acid is the common name for methanoic acid; acetic acid is ethanoic acid; and propionic acid is propanoic acid. These compounds are shown in Figure 1.10. Be sure that you know both these common names and IUPAC names for Test Day.
Figure 1.10. Naming Carboxylic Acids The carboxylic acid group does not receive a number in the name because it is a terminal functional group; common names are included in parentheses.
Did you notice that some of the common names are similar between aldehydes and carboxylic acids? Remembering that formaldehyde and formic acid both refer to molecules with methane as a parent alkane—and that acetaldehyde and acetic acid contain ethane as a parent alkane—will help consolidate this information.
The carboxylic acid derivatives are the final category of functional groups. These include esters, amides, and anhydrides.
Esters are common carboxylic acid derivatives. In these compounds, the hydroxyl group (–OH) is replaced with an alkoxy group (–OR, where R is a hydrocarbon chain). Ester nomenclature is based on the naming conventions for carboxylic acids. The first term is the alkyl name of the esterifying group. Think of this first term as an adjective describing the ester, based on the identity of the alkyl (R) group. The second term is the name of the parent acid, with –oate replacing the –oic acid suffix. For example, methanoic acid (formic acid) would form butyl methanoate with exposure to butanol under appropriate reaction conditions. Examples of ester nomenclature are shown in Figure 1.11.
Figure 1.11. Naming Esters Groups bound directly to the ester oxygen are named as substituents, and are not numbered.
Another group of carboxylic acid derivatives includes amides. In an amide, the hydroxyl group is replaced by an amino group (nitrogen-containing group). These compounds can be more complex—the amino nitrogen can be bound to zero, one, or two alkyl groups. Amides are named similarly to esters, except that the suffix becomes –amide. Substituents attached to the nitrogen atom are labeled with a capital N–, indicating that this group is bound to the parent molecule via a nitrogen. These substituents are included as prefixes in the compound name and are not numbered. Several examples of amide nomenclature are included in Figure 1.12.
Figure 1.12. Naming Amides Groups bound directly to the amide nitrogen are named as substituents with the prefix N–, and are not numbered.
One final group of carboxylic acid derivatives is the anhydrides. In the formation of an anhydride, one water molecule is removed, and a bond is formed between two carboxylic acid molecules. Many anhydrides are cyclic, which may result from the intramolecular dehydration of a dicarboxylic acid. Anhydrides are named by replacing acid with anhydride in the name of the corresponding carboxylic acid if the anhydride is formed from only one type of carboxylic acid. If the anhydride is not symmetrical, both carboxylic acids are named (without the suffix acid) before anhydride is added to the name. Some examples of anhydrides are shown in Figure 1.13.
Figure 1.13. Naming Anhydrides Phthalic anhydride and succinic anhydride are given as examples of cyclic anhydrides; their names need not be memorized.
Because hydro– is a prefix meaning water and an– is a prefix meaning not or without, we can remember that anhydrides have had water molecules removed during formation.
MCAT Concept Check 1.4:
Before you move on, assess your understanding of the material with these questions.
1. What would be the names of the ester, amide, and anhydride derivatives of pentanoic acid? Assume that the R group on the ester is –CH3, and that the amide is unsubstituted.
2. Name the following compound: