Classification of Carboxylic Acids:  The citric acid in lemon, tartaric acid in tamarind, acetic acid in vinegar, malic acid in apples, lactic acid in sour milk all contains the same functional group, i.e. the carboxylic group– an organic acid that consists of a carbonyl group with an \(\text {OH}\) group attached to it. These compounds are generally more acidic than other organic compounds containing hydroxyl groups but are generally weaker than common mineral acids such as hydrochloric acid, sulphuric acid. While naming carboxylic acids, the longest carbon chain contains the carboxyl group at locant \(1\). The name is written by dropping the final -e from the name of the parent alkane and adding the suffix -oic followed by the word “acid.”Let’s understand how this organic functional group is classified.

What is a Carboxylic Group?

It is a class of organic compounds that comprises a carbon atom \((\text {C})\), oxygen atom \((\text {O})\) and a hydroxyl \((-\text {OH})\) group, represented as \((-\text {COOH})\). In \(-\text {COOH}\), the carbon \((\text {C})\) atom is bonded to an oxygen \((\text {O})\) atom by a double bond and a hydroxyl group \((―\text {OH})\) by a single bond. The fourth bond of the carbon atom is linked to a hydrogen \((\text {H})\) atom or to some other univalent combining group.

The \(-\text {COOH}\) group is named the carboxyl group because it comprises the carbonyl group \((\text {C}=\text {O})\) and a hydroxyl group \((-\text {OH})\). The general formula of the carboxylic acid homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n}}}}{{\rm{O}}_{\rm{2}}}\), where \(\text {n} = 1,2,3….\) Carboxylic acids are represented as \(\text {R}-\text {COOH}\), where \(\text {COOH}\) refers to the carboxyl group, and \(\text {R}\) refers to the rest of the molecule to which this group is attached.

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Structure of Carboxyl Group

In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about \({\rm{120}}^\circ \). The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure shown below:

Classification of Carboxylic Acids

Carboxylic acids can be primarily subdivided into two groups. These are-

Aliphatic Carboxylic Acids

Aliphatic carboxylic acids are the carboxyl compounds in which the \(-\text {COOH}\) group is attached to the aliphatic carbon atom, i.e. the \(-\text {COOH}\) group is attached to the carbon atom that is non-aromatic. For example- 

 The aliphatic carboxylic acid can be further subdivided as follows-

Saturated Carboxylic Acid

Saturated carboxylic acids are the carboxyl compounds in which the \(-\text {COOH}\) group is attached to the side group that comprises single-bonded carbon atoms, i.e., \(-\text {R}\) represents a single bonded alkyl group. For example- 

In the above example, acetic acid is a saturated carboxylic acid because the \(-\text {R}\) part is a single bonded alkyl group (methyl) group.

The saturated carboxylic acid can be further divided into monocarboxylic and dicarboxylic acid.

Monocarboxylic acids – Compounds with one \(-\text {COOH}\) group is monocarboxylic acid. For example-

In the above example, acetic acid consists of only one \(-\text {COOH}\) group; hence it is a monocarboxylic acid.

Dicarboxylic acids – Compounds with two \(-\text {COOH}\) groups is a dicarboxylic acid. For example-

In the above example, succinic acid consists of two \(-\text {COOH}\) groups; hence it is a dicarboxylic acid.

Unsaturated Carboxylic Acid

Unsaturated carboxylic acids are the carboxyl compounds in which the \(-\text {COOH}\) group is attached to the side group that comprises double or triple bonded carbon atoms, i.e., \(\text {R}\) represents an alkyl group made up of double and triple bonds. For example-\(2\)-methyl-\(2\)-pentenoic acid

In the above example, \(2\)-methyl-\(2\)-pentenoic acid consists of only a \(-\text {COOH}\) group and a double bond at \(\text {C}-2\); hence it is an unsaturated carboxylic acid.

Functional Group Carboxylic Acid

When other functional groups are present in addition to the carboxyl functional group, then the compound is a functional group of carboxylic acid. For example-Bromoacetic acid is a functional group of carboxylic acid.

Aromatic Carboxylic Acids

Aromatic carboxylic acids are the carboxyl compounds in which the \(-\text {COOH}\) group is attached to the carbon atom which is a part of the aromatic ring. The general structure of aromatic carboxylic acid is \(\text {Ar}-\text {COOH}\). 

The aromatic carboxylic acid can be further divided into two categories. These are-

Homocyclic Aromatic Carboxylic Acid

These carboxylic acids contain only carbon atoms in the ring to which the \(-\text {COOH}\) group is attached. For example- Benzoic acid.

In the above example, benzoic acid consists of a \(-\text {COOH}\) group attached to a ring made up of only carbon atoms.

Heterocyclic Aromatic Carboxylic Acid

These carboxylic acids contain other atoms such as \((\text {N}, \text {O})\) in addition to carbon atoms in the aromatic ring to which the \(-\text {COOH}\) group is attached. For example- Furoic acid

Furoic acid has a hetero-atom \((\text {O})\) in the ring to which the \(-\text {COOH}\) group is attached. Hence, it is a heterocyclic aromatic carboxylic acid

Summary

Carboxylic acids are an important class of organic compounds found in nature as well as artificially synthesised. Higher members of aliphatic carboxylic acids \(\left( {{{\rm{C}}_{12}} – {{\rm{C}}_{18}}} \right)\) known as fatty acids occur naturally in fats. These compounds serve as starting material for several other important organic compounds such as esters, anhydrides, acid chlorides, amides, etc. This article explains the definition, structure of carboxylic acids, classification of carboxylic acids. We also learnt to differentiate between the various isomers along with suitable examples.

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