Carboxylic Acids and Derivatives

Structure and Nomenclature

Carboxylic acids contain carbonyls with an OH{-}{\rm{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{-}{\rm{COOH}} ) that also contains a hydroxyl bound to the same carbon. Formic acid is the simplest carboxylic acid, and it is planar, with the CO {\rm{C{-}O}} and C=O{\rm{C{=}O}} bond angle close to 120°. A carbonyl group is a functional group containing a C=O{\rm{C{=}O}} double bond.

Structure of Formic Acid

Formic acid is the simplest carboxylic acid. It has planar geometry, with bond angles close to 120°.
Based on the geometry and bond angles, we can conclude that the carbon in formic acid is sp2 hybridized and has a σ+π{\sigma+\pi} carbon-oxygen double bond like aldehydes and ketones. Furthermore, one of the unshared electron pairs of the OH{-}{\rm{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

One of formic acid's unshared electron pairs from the hydroxyl oxygen is delocalized, making the carbonyl group less electrophilic than an aldehyde or a ketone.
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 -oic acid. 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{-}{\rm{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

GABA is referred to by its common name, gamma-aminobutyric acid, but its IUPAC name is 4-aminobutanoic 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{-}{\rm{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 (RCOO{\rm{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

Molecular Structure IUPAC Name Common Name
Formic acid Formic acid
Acetic acid Acetic acid
Pentanoic acid Valeric acid
Octadecanoic acid Stearic acid
2-hydroxypropanoic acid Lactic acid
2-hydroxy-2-phenylethanoic acid Mandelic acid
Prop-2-enoic acid Acrylic acid
(Z)-9-octadecenoic acid Oleic acid
Benzoic acid Benzoic acid
2-hydroxybenzenecarboxylic acid Salicylic acid
Propanedioic acid Malonic acid
Butanedioic acid Succinic acid
Phthalic acid Phthalic acid

Frequently encountered carboxylic acids can be described by their IUPAC names or their common names.