Definition
Carboxylic acids are organic acids with the general structure R-COOH (less commonly: R-CO2H). It is composed of a residue, a carbonyl group, and a hydroxyl group. The functional group is called the carboxy group (carboxyl group). Molecules with two or three carboxy groups are called dicarboxylic acids or tricarboxylic acids. An example of a dicarboxylic acid is oxalic acid. In addition to aliphatic carboxylic acids, aromatic ones also exist. The simplest is benzoic acid: Fatty acids also belong to the carboxylic acids. They consist of a carboxy group on a hydrocarbon chain, which is usually unbranched and may contain double bonds. The salts of carboxylic acids are called carboxylates. Salts of fatty acids are active as soaps and emulsifiers. Carboxylic acids can be synthesized, for example, by oxidation of alcohols (R-OH) and aldehydes (R-CHO).
Nomenclature
Many simple carboxylic acids have familiar trivial names, such as acetic acid, butyric acid, and citric acid. The systematic name is formed by combining the name of the carbon skeleton with the suffix -acid. Thus, the IUPAC name for acetic acid is ethanoic acid. In English, by the way, the suffix is -ic acid, meaning something like “acetic acid“.
Representative
A small selection of carboxylic acids:
- Malic acid
- Formic acid
- Amino acids
- Butyric acid
- Acetic acid
- Fatty acids such as lauric acid or palmitic acid
- Fumaric acid
- (carbonic acid)
- Lactic acid
- Propionic acid
- Oxalic acid
- Citric acid
Properties
- Carboxylic acids are acids (proton donors). The pKa is typically around 5, but depending on the structure of the molecule, carboxylic acids can be much more acidic. Thus, the trichloroacetic acid has a pKa of 0.66.
- Carboxylic acids are both hydrogen bond acceptors and as -donors.
- Carboxylic acids with a low molecular mass are soluble in water.
- Carboxylic acids have an acidic taste (eg acetic acid in vinegar).
- The boiling point of carboxylic acids is higher than that of comparable alcohols, aldehydes and ketones.
Reactions
Carboxylic acids react with bases in an acid-base reaction:
- R-COOH (carboxylic acid) + NaOH (sodium hydroxide, base) R-COO–Na+ (sodium carboxylate) + H2O (water)
With alcohols, carboxylic acids form esters: with amines, amides are formed:
In pharmacy
Countless active ingredients with a carboxy group exist. One important group, for example, is the non-steroidal anti-inflammatory drugs (NSAIDs), which are used as painkillers. Well-known examples are acetylsalicylic acid, ibuprofen and diclofenac. Carboxylic acids are also used, among other things, for chemical syntheses, as preservatives, as disinfectants, acidity regulators and for the production of active ingredient salts.