Nomenclature and Structure of Carboxyl Group

NCERT Reference: Class 12 Chemistry, Chapter 8 – Aldehydes, Ketones and Carboxylic and Carboxylic Acids – Page 205

Quick Notes

Carboxylic acids contain the –COOH (carboxyl) functional group.
They are named by replacing the ‘-e’ in the parent alkane with ‘-oic acid’ (IUPAC).
In common names, acids are often named after their natural sources (e.g., formic acid from ants).
The carboxyl carbon is sp² hybridised and exhibits planar geometry.

Full Notes

Carboxylic acids are organic compounds that contain the carboxyl functional group (–COOH).

NCERT Class 12 Chemistry structural diagram of a carboxylic acid showing the COOH functional group with carbonyl and hydroxyl units.

They are essential organic compounds known for their acidity and wide uses in biology and industry.

Nomenclature

IUPAC Naming:

Carboxylic acids are named by replacing the ‘-e’ of the corresponding alkane with ‘-oic acid’. The carboxyl carbon is always numbered as carbon-1, so no number is needed to locate it.

HCOOH = Methanoic acid
CH₃COOH = Ethanoic acid
CH₃CH₂COOH = Propanoic acid

Common Names:

Many carboxylic acids are also known by their traditional (common) names, which are often derived from their natural sources.

Greek letters (α, β, γ…) are used to denote positions of substituents relative to the carboxyl group.

Structure	Common Name	Source
HCOOH	Formic acid	Ants (formica)
CH₃COOH	Acetic acid	Vinegar (acetum)
CH₃CH₂COOH	Propionic acid	Milk
CH₃(CH₂)₂COOH	Butyric acid	Butter
C₆H₅COOH	Benzoic acid	Benzoin resin

Structure of Carboxyl Group

The carboxyl group (–COOH) is made up of a carbonyl (C=O) and a hydroxyl (–OH) group attached to the same carbon atom.

NCERT Class 12 Chemistry molecular structure of carboxylic acid highlighting the carbonyl and hydroxyl components of the carboxyl group.

Hybridisation and Geometry

The carbon atom is sp² hybridised.
It forms three sigma bonds:
- One with the hydroxyl oxygen
- One with the carbonyl oxygen
- One with the adjacent carbon (or hydrogen)
The geometry around the carbon is trigonal planar, with bond angles close to 120°.

Resonance Structure

The carboxyl group exhibits resonance between two structures:

One where the double bond is between C=O
One where the double bond is between C–OH

This delocalisation contributes to increased acidity (due to stable conjugate base –COO⁻) and equalised bond lengths in the carboxylate ion.

Bonding Features

The C=O bond is polar, with oxygen pulling electron density away from carbon.
The O–H bond is also polar, contributing to the acid’s ability to donate protons.
Due to resonance, the C–O bond in –OH and C=O are partially delocalised, making both bonds shorter and stabilising the structure.

Summary

Carboxylic acids contain the –COOH functional group composed of a carbonyl and hydroxyl unit.
Named by replacing the ‘-e’ of alkanes with ‘-oic acid’; common names often reflect natural sources.
The carbon is sp² hybridised with planar geometry and ~120° bond angles.
Resonance stabilisation delocalises charge, enhancing acidity and equalising bond lengths.