Naming of Carboxylic acids & Its Derivatives

INTRODUCTION

Carboxylic acids are characterized by the presence of carboxyl group. The $-$COOH group which itself is made up of a carbonyl group ($\text{ > }C=O$) and a hydroxyl group ($-$OH) is called carboxyl group (carb from carbonyl and oxyl from hydroxyl)

Carboxylic acids may be aliphatic or aromatic

Comparison of resonating structures of carboxylic group and carbonyl group

Carbonyl group has two resonance structures (I and II)

However, for a carboxyl group, three resonance structures (A, B and C) can be written.

In both structures (A) and (C), the C – atom and the two O – atoms have eight electrons in their respective valence shells while in structure (B), C – atom has only six electrons. Therefore, structure (B) is less stable than structure (C), in other words the two important resonance structures of carboxyl group are structures (A) and (C). In both these structures, carboxyl carbon is electrically neutral. However in case of aldehydes and ketones, only one structure i.e. I is electrically neutral. As a result, the carboxyl carbon of the resonance hybrid is less positive and hence less electrophilic than the carbonyl carbon of aldehydes and ketones. However, it may be noted that like carbonyl group, carboxyl group is also polar due to resonance structures (B)

and (C).

NOMENCLATURE

The aliphatic carboxylic acids are commonly known by their initial names, which have been derived from the source of the particular acid.

Examples:

$\bullet$ HCOOH Formic acid [Latin: Fermica = ant]

$\bullet$ CH₃COOH Acetic acid [Latin: acetum = Vinegar]

$\bullet$ CH₃–CH₂–COOH Propionic acid [Greek: Proton = First; Pion = Fat]

$\bullet$ CH₃(CH₂)₂COOH Butyric acid [Latin: Butyrum = Butter]

$\bullet$ CH₃(CH₂)₃COOH Valeric acid

$\bullet$ CH₃(CH₂)₁₄COOH Palmitic acid

$\bullet$ CH₃(CH₂)₁₆COOH Stearic acid

Alternative system of nomenclature is naming the acids as the derivatives of acetic acids. The only exception being formic acid.

Example:

CH₃ – CH₂ – COOH Methyl acetic acid

(CH₃)₃C – COOH Trimethyl acetic acid

According to the IUPAC system of nomenclature, the suffix of the monocarboxylic acid is 'oic acid', which is added to the name of the alkane corresponding to the longest carbon chain containing the carboxyl group, e.g.

HCOOH methanoic acid

CH₃ – CH₂ – CH₂ – COOH butanoic acid

The positions of side-chains (or substituents) are indicated by numbers, the numbering to be started from the side of the carboxyl group.

Naming of Acyl Groups, Acid Chlorides and Anhydrides:

The group obtained from a carboxylic acid by the removal of the hydroxyl portion is known as an acyl group. The name of an acyl group is created by changing the - ic acid at the end of the name of the carboxylic acid to –yl, examples:

Acid chlorides are named systematically as acyl chlorides.

An acid anhydride is named by substituting anhydride for acid in the name of the acid from which it is derived.

Naming of Amides and Imides:

The names of amides are formed by replacing –oic acid (or –ic acid for common names) by amide or –carboxylic acid by carboxamide.

If the nitrogen atom of the amide has any alkyl groups as substitutents, the name of the amide is prefixed by the capital letter N; to indicate substitution on nitrogen, followed by the name(s) of alkyl group(s).

If the substituent on the nitrogen atom of an amide is a phenyl group, the ending for the name of the carboxylic acid is changed to anilide

Some dicarboxylic acids form cylic amides in which two acyl groups are bonded to the nitrogen atom. The suffix imide is given to such compounds.

Illustration 1. Give the IUPAC names and common names of the following compounds

Solution: IUPAC Name General name

(i) Ethanedioic acid Oxalic acid

(ii) Butanedioic acid Succinic acid

(iii) 2-Hydroxy propanoic acid Lactic acid

(iv) 2, 3 Dihydroxybutanedioic acid Tartaric acid