What are Carboxylic Acids?

Carboxylic acids are organic acids with the carboxyl group (–COOH). They are represented by the following chemical formula: R–COOH. Here R can be alkyl (e.g., –CH3), alkenyl (e.g., –CH2=CH2), or aryl (e.g., –C6H5).

The carboxyl group is characterized by the presence of the carboxyl group, which consists of carbon-oxygen double bond (–C=O), carbon-oxygen single bond (–C–O), and oxygen-hydrogen (–O–H). A carboxylic acid may contain other functional groups, like alkenyl (–C=C–), hydroxyl (–OH) etc. 

The resonance structure of carboxylic acid is illustrated below:

The example above is for acetic acid. Because of the methyl (–CH3) (a nucleophilic group) and electronegative oxygen atom, the –C=O– bond can be easily dissociated, creating a partial negative charge on oxygen and a partial positive charge on carbon. The oxygen of the –OH group offers its lone pair of electrons to electron-deprived carbon—see the above figure. Electrons here are not restricted to a particular bond, rather they associate themselves with multiple atoms. We term this phenomenon as delocalization in this context.

The delocalization of electrons increases the stability of the carboxyl group and further makes them acidic. The oxygen of –OH in the carboxyl group carries a partial positive charge, which makes it much easier to lose a proton (H+ ion). After deprotonation, a carboxylic acid turns into a carboxylate anion.

Carboxylic acids with a small number of carbons are highly soluble in a polar solvent, like water. As explained earlier, the resonance stabilizes the charge separation and enhances the polar characteristics of these organic acids. For acids with a large carbon chain, solubility in water decreases but solubility in nonpolar solvent, like ethers, increases.

The polar nature of the carboxyl group also affects other physical properties, especially boiling point.

Carboxylic acids have much higher boiling point than predicted based on the molar mass. For example, Ethanol (Molar mass = 46 g/mol) boils at 78 °C, while formic acid (Molar mass = 46 g/mol) turns into the gaseous state at 101 °C; we can see the difference. The explanation behind this anomalous behavior is given by hydrogen bonding. In short, neighboring molecules are attracted to each other because of positive and negative charge centers existing within a molecule. The presence of attractive forces between neighbors increases the amount of energy required to separate them, which in turn increases the temperature (or boiling point).

Like most acidic substances, carboxylic acids have sourness. They smell like vinegar; in fact, vinegar is nothing very aqueous acetic acid.

Most organic acids, including carboxylic acids are weaker relative to inorganic acids, such as, HCl, H2SO4, or H3PO4, when compared in equal concentration. However, precaution is a must when dealing with them. Concentrated carboxylic acids are fatal.

IUPAC nomenclature uses the suffix "-oic" to identify them. For example, one-carbon acid is methanoic acid, two carbon is ethanoic acid, three carbon is propanoic acid, and so on.

Carboxylic acids are found in abundance in the natural world and play a vital role in the chemical and biological system. They have significant applications in the food industry. Some popular names are mentioned in the table below.

Common name	IUPAC name	Chemical Formula	Comments
Formic acid	Methanoic acid	CH2O2	Simplest carboxylic acid
Acetic acid	Ethanoic acid	C2H4O2	Vinegar is a dilute acetic acid, mainly used in cooking
Butyric acid	Butanoic acid	C4H8O2	Found in butter
Acrylic acid	Prop-2-enoic acid	C3H4O2	Simplest unsaturated carboxylic acid
Glycine	2-Aminoethanoic acid	C2H5NO2	Simplest amino acid; amino acids are building blocks of proteins
Citric acid	2-Hydroxypropane-1,2,3-tricarboxylic acid	C6H8O7	Found in citrus fruits and have application in food industry as flavoring agent and preservative
Stearic acid	Octadecanoic acid	C18H36O2	Saturated fatty acid; has applications in soap, cosmetics, lubrication industry
Palmitic acid	Hexadecanoic acid	C16H32O2	Found in plant extract (palm oil, coconut oil, soybean oil etc)
Alpha-Linolenic acid	Octadeca-9,12,15-trienoic acid	C18H30O2	An essential omega-3 fatty acid; play important role in human health and physiology

Note: All saturated monocarboxylic acid other than formic acid follows the formula CnH2nO2, where n = 1, 2, 3…