Carboxylic acids contain carbonyls with an −OH on one side and an R group on the other side. In IUPAC nomenclature, carboxylic acids are named by changing the suffix to -anoic acid.
A carboxylic acid is a compound that contains an alkyl or aryl group (R) attached to a carboxyl group (−COOH) that also contains a hydroxyl bound to the same carbon. Formic acid is the simplest carboxylic acid, and it is planar, with the C−O and C=O bond angle close to 120°. A carbonyl group is a functional group containing a C=O double bond.
Structure of Formic Acid
Based on the geometry and bond angles, we can conclude that the carbon in formic acid is sp2hybridized and has a σ+π carbon-oxygen double bond like aldehydes and ketones. Furthermore, one of the unshared electron pairs of the −OH oxygen is delocalized. This makes the carbonyl group less electrophilic than an aldehyde or ketone.
Delocalization of an Unshared Electron Pair in Formic Acid
More than any other class or compounds, carboxylic acids are frequently identified by their common names rather than their systematic names. For example, butyric acid was so named because it was first obtained from butter. Butyric acid has a noxious odor and is a component of rancid butter and body odor.
IUPAC names for carboxylic acids are constructed by counting the carbons in the longest continuous carbon chain containing the carboxyl group and replacing the -e at the end of the corresponding alkane, alkene, or alkyne with -oicacid. For example, CH3CH2COOH is propanoic acid because the corresponding 3-carbon alkane is propane. When substituent groups are present, always count the C in the −COOH group as number 1. Carboxylic acids can also exist as diacids. A diacid is a molecule containing two acidic functional groups.
When using common names, substituents are indicated by Greek letters rather than numbers. For example, the common name for a common inhibitory neurotransmitter found in the human central nervous system, GABA, is gamma-aminobutyric acid, but its IUPAC name is 4-aminobutanoic acid.
GABA, or 4-Amniobutanoic Acid
Substituent locations are identified by number, with the carbon chain numbering beginning at the carboxyl group. When naming the main carbon chain, carboxylic acids take precedence over other groups. Double bonds in the main chain are labeled based on their lowest-number position on the carbon chain, and they are denoted by adding the ending -enoic acid. When a compound consists of a ring attached to a carboxyl group, the ring is named, and the suffix -carboxylic acid is added. A benzene ring attached to a −COOH group, for example, is named benzenecarboxylic acid, which is commonly shortened to benzoic acid. To name compounds containing two carboxyl groups, add the suffix -dioic acid or -dicarboxylic acid. The carboxylate anion (R−COO−) of a carboxylic acid carries the suffix -ate (-ic acid for the conjugate acid; -ate for the conjugate base). For example, the conjugate base of acetic acid is acetate. A conjugate acid is a molecule or ion formed when a Brønsted-Lowry base has accepted a proton. A conjugate base is a molecule or ion remaining after a Brønsted-Lowry acid donates its proton to another molecule or ion.
Common Carboxylic Acids
Frequently encountered carboxylic acids can be described by their IUPAC names or their common names.