Research Notes: Carboxylic Acids

Carboxylic acids have a carboxylic acid (carboxyl) group (-C(=O)OH, usually written -COOH or -CO₂H, and are proton donors. Carboxylic acids are polar and form hydrogen bonds with each other. They are widespread in nature and are typically weak acids, meaning they only partially dissociate into H+ cations and RCOO− anions in aqueous solution.

The simplest series of carboxylic acids are the alkanoic acids, R-COOH, where R is a hydrogen or an alkyl group. Compounds may have two or more carboxylic acid groups per molecule, e.g., dicarboxylic acids which have two functional groups (often written as HOOC-R-COOH, where R is usually an alkyl, alkenyl, or akynyl group).

Salts and anions of carboxylic acids are called carboxylates. The carboxylate anion R-COO– is usually named with the suffix -ate, so acetic acid, for example, becomes acetate. In IUPAC nomenclature, carboxylic acids have an -oic acid suffix (e.g. octadecanoic acid). In common nomenclature, the suffix is usually -ic acid (e.g. stearic acid).

Carboxylic acids include:

• Short chain saturated monocarboxylic acids
  • Formic acid (methanoic acid) – HCOOH, found in insect stings (formic from the Latin word meaning ants)
  • Acetic acid (ethanoic acid) – CH₃COOH, the principal component of vinegar
  • Propionic acid (propanoic acid) – CH₃CH₂COOH
• Medium chain saturated monocarboxylic acids
  • Valeric acid (pentanoic acid) – C₄H₉COOH
  • Enanthic acid (heptanoic acid) – C₆H₁₃COOH
  • Pelargonic acid (nonanoic acid) – C₈H₁₇COOH
• Short chain unsaturated monocarboxylic acids
  • Acrylic acid (2-propenoic acid) – CH₂=CHCOOH, used in polymer synthesis
• Fatty acids – medium to long chain saturated and unsaturated monocarboxylic acids, with even number of carbons
  • Butyric acid (butanoic acid) – CH₃CH₂CH₂COOH, found in rancid butter
  • Lauric acid (dodecanoic acid) – CH₃(CH₂)10COOH, found in coconut oil
  • Docosahexaenoic acid – DHA fish oil
  • Eicosapentaenoic acid – EPA fish oil
• Amino acids
• Keto acids – contain a ketone group
  • Pyruvic acid
  • Acetoacetic acid
• Aromatic carboxylic acids
  • Benzoic acid – C₆H₅COOH. Sodium benzoate, the sodium salt of benzoic acid used as a food preservative.
  • Salicylic acid
• Dicarboxylic acids – contain two carboxyl groups
  • Aldaric acid – sugar acids
  • Oxalic acid – in many foods
  • Malonic acid - propanedioic acid, CH₂(COOH)₂. Inhibits succinate dehydrogenase in Kreb's cycle. Ionised form and its esters and salts are known as malonates.
  • Malic acid – in apples
  • Succinic acid – butanedioic acid, ethane-1,2-dicarboxylic acid, HOOC–CH₂–CH₂–COOH; anion, succinate, is component of Kreb's cycle. Esters of succinic acid are called dialkyl succinates.
  • Fumaric acid - anion, fumarate, is component of Kreb's cycle. Maleic acid is the cis-isomer of fumaric acid. Human skin produces fumaric acid when exposed to sunlight.
  • Glutaric acid - aka pentanedioic acid, 1,3-propanedicarboxylic acid, n-pyrotartaric acid
  • Adipic acid – hexanedioic acid
  • Suberic acid - octanedioic acid, C₆H₁₂(COOH)₂
• Tricarboxylic acids – containing three carboxyl groups
  • Citric acid – in citrus fruits
  • Alpha hydroxy acids – contain a hydroxy group
  • Lactic acid (2-hydroxypropanoic acid) – found in sour milk