Classification of Alcohols, Phenols and Ethers

NCERT Reference: Chapter 7 – Alcohols, Phenols and Ethers – Pages 171–173

Quick Notes

Alcohols: –OH group bonded to sp³ carbon of an alkyl group.

Can be monohydric, dihydric, trihydric, or polyhydric.
Also classified as 1°, 2°, or 3° based on the carbon atom bonded to –OH.

Phenols: –OH group directly attached to an aromatic ring.

Can also be mono-, di-, or trihydric based on number of –OH groups.

Ethers: Contain R–O–R′ linkages.

Classified as symmetrical (same alkyl/aryl groups) or unsymmetrical.
Aromatic ethers contain at least one aryl group (e.g. anisole).

Full Notes

Alcohols

Alcohols are compounds in which one or more hydroxyl (–OH) groups are attached to sp³-hybridised carbon atoms of alkyl chains (not aromatic rings).

Their classification is based on the number of –OH groups and the type of carbon attached to the –OH.

Based on Number of Hydroxyl Groups

NCERT Chemistry classification diagram showing monohydric, dihydric and trihydric alcohols with examples such as ethanol, ethane-1,2-diol and glycerol for NCERT Class 12 Chapter 7.

Monohydric alcohols: Contain one –OH group.
Example Ethanol (CH₃CH₂OH)
Dihydric alcohols (Glycols): Contain two –OH groups.
Example Ethane-1,2-diol (HO–CH₂–CH₂–OH)
Trihydric alcohols: Contain three –OH groups.
Example Propane-1,2,3-triol (Glycerol)

Based on the Type of Carbon Attached to –OH Group

NCERT Chemistry diagram distinguishing primary, secondary and tertiary alcohols based on the carbon bearing the hydroxyl group for NCERT Class 12 Chapter 7.

Primary (1°) Alcohols: –OH attached to a primary carbon (bonded to one other C).
Example CH₃CH₂OH
Secondary (2°) Alcohols: –OH attached to a secondary carbon (bonded to two other C atoms).
Example CH₃CHOHCH₃
Tertiary (3°) Alcohols: –OH attached to a tertiary carbon (bonded to three C atoms).
Example (CH₃)₃COH

Allylic Alcohols

Allylic alcohols are when the OH group is bonded to an sp³ carbon atom, which is adjacent to a C=C double bond.

NCERT Chemistry schematic highlighting the allylic position adjacent to a C=C and an OH on the allylic carbon for NCERT Class 12 Chapter 7.

Example Prop-2-en-1-ol

CH₂=CH–CH₂–OH

Vinylic Alcohols

Vinylic alcohols are when OH group is bonded to an sp² carbon atom (OH bonded to a C from a C=C double bond).

NCERT Chemistry diagram showing a vinylic alcohol where the hydroxyl is directly bonded to an sp2 carbon of a C=C for NCERT Class 12 Chapter 7.

Example Ethenol

CH₂=CH–OH

Phenols — Mono-, Di-, and Trihydric Phenols

NCERT Chemistry chart classifying phenols into mono-, di- and trihydric categories with examples like phenol and catechol for NCERT Class 12 Chapter 7.

Phenols are compounds in which –OH groups are directly attached to the aromatic ring (sp² carbon). Their classification also depends on the number of hydroxyl groups present on the ring.

Monohydric Phenols: Contain one –OH group on the aromatic ring.
Example Phenol (C₆H₅OH)
Dihydric Phenols: Contain two –OH groups.
Examples Catechol (1,2-dihydroxybenzene)
Trihydric Phenols: Contain three –OH groups on the benzene ring.
Example Pyrogallol (1,2,5-trihydroxybenzene)

Ethers

Ethers are compounds in which two alkyl or aryl groups are connected via an oxygen atom. They can be symmetrical (same alkyl/aryl groups) or unsymmetrical (different groups).

General Formula: R–O–R′, where R and R′ may be alkyl or aryl groups.

Example Diethyl ether

NCERT Chemistry structure of diethyl ether illustrating a symmetrical ether R–O–R for NCERT Class 12 Chapter 7.

Simple (Symmetrical) Ethers: Both groups are the same.

Example Methyl ethyl ether

NCERT Chemistry structure of methyl ethyl ether illustrating an unsymmetrical ether R–O–R′ for NCERT Class 12 Chapter 7.

Mixed (Unsymmetrical) Ethers: Different alkyl/aryl groups.

Summary

Alcohols have the hydroxyl group on an sp³ carbon and classify by number of –OH groups and by 1°, 2°, 3°.
Allylic alcohols have –OH on the carbon next to a C=C and vinylic alcohols have –OH directly on an sp² carbon of a C=C.
Phenols have –OH directly on an aromatic ring and can be mono-, di- or trihydric.
Ethers contain an R–O–R′ linkage and are symmetrical or unsymmetrical; aromatic ethers include at least one aryl group.