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What are Carboxylic Acids?

24th Nov 2022 @ 3 min read

Organic Chemistry

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:

three resonance structure of carboxylic acid

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…

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