Amides are a class of organic compounds derived from carboxylic acids, where the hydroxyl (-OH) group is replaced by a nitrogen-containing group (-NH2, -NHR, or -NR2). This structural feature results in the characteristic amide functional group: a carbonyl carbon atom (C=O) bonded directly to a nitrogen atom. This bond is known as an amide linkage, or peptide bond when found in biological systems. Understanding the systematic naming convention, or IUPAC nomenclature, is necessary for accurately identifying these molecules.
Identifying the Amide Functional Group and Parent Chain
The amide functional group is represented by the general structure R-CO-N. Amides are broadly classified based on the substituents attached to the nitrogen atom. A primary amide has two hydrogen atoms on the nitrogen (R-CONH2), a secondary amide has one hydrogen and one alkyl or aryl group (R-CONHR’), and a tertiary amide has two non-hydrogen groups (R-CONR’R”).
To establish the parent chain, the longest continuous carbon chain that includes the carbonyl carbon atom must be selected. The carbon atom of the carbonyl group is automatically assigned the position number 1. All other alkyl groups or substituents along this parent chain are then named and numbered accordingly, based on their position relative to the C1 carbonyl carbon.
Applying Nomenclature Rules to Unsubstituted Amides
The nomenclature for primary amides, which are the unsubstituted forms, is derived directly from the name of the corresponding carboxylic acid. To form the IUPAC name, the suffix “-oic acid” is removed from the parent carboxylic acid name. This ending is then replaced with the suffix “-amide” to denote the presence of the functional group.
For example, the simplest carboxylic acid, methanoic acid (which has one carbon), becomes methanamide. Similarly, the two-carbon carboxylic acid, ethanoic acid, is transformed into ethanamide. The common names are also derived this way, where acetic acid becomes acetamide. When the parent chain is three carbons long, the name is derived from propanoic acid, resulting in propanamide.
Since the amide functional group always occupies the number 1 position in the parent chain, a locant number is not necessary to indicate its position. If an unsubstituted amide has a four-carbon chain, the parent acid is butanoic acid, and the amide is named butanamide. Any substituents present on the carbon chain itself are named and numbered as prefixes to the parent amide name. For instance, a chlorine atom on the third carbon of a propanamide chain would be named 3-chloropropanamide.
Naming Amides with Alkyl Groups on Nitrogen
Secondary and tertiary amides contain hydrocarbon groups attached directly to the nitrogen atom instead of hydrogen atoms. This substitution requires a specific labeling convention to distinguish these groups from substituents on the main carbon chain. The capital letter ‘N’ is used as a locant, or position indicator, to show that a substituent is bonded to the nitrogen atom. The nitrogen atom uses this letter to specify the site of attachment.
To name a secondary amide, the alkyl group on the nitrogen is cited as a prefix, immediately preceded by the locant ‘N-‘. For example, if a methyl group is attached to the nitrogen of propanamide, the name is N-methylpropanamide. The N- clearly indicates that the methyl group is on the nitrogen, preventing confusion with a molecule like 2-methylpropanamide.
In the case of a tertiary amide, two alkyl groups are attached to the nitrogen atom. If both groups are the same, the locant ‘N,N-‘ is used, followed by the multiplying prefix ‘di-‘ and the name of the alkyl group. For example, a two-carbon amide with two methyl groups on the nitrogen is named N,N-dimethylethanamide. If the two alkyl groups on the nitrogen are different, both are cited alphabetically, each preceded by its own ‘N’ locant. For instance, a compound with an ethyl group and a methyl group on the nitrogen of ethanamide would be named N-ethyl-N-methylethanamide.