18.1 Classification of Monohydric Alcohols | Alcohol | Chemistry 11

Monohydric alcohols contain only one hydroxyl (-OH) group per molecule. They are classified into three main types based on the number of alkyl groups attached to the carbinol carbon — the carbon atom directly bonded to the -OH group.

Classification of Alcohols

Monohydric alcohols are categorized into:

(a) Primary (1°) alcohols
(b) Secondary (2°) alcohols
(c) Tertiary (3°) alcohols

The general formula for saturated monohydric alcohols is $C_{n} H_{2 n + 1} O H$ (or $R - O H$ ).

(a) Primary Alcohols

In primary (1°) alcohols, the carbinol carbon is bonded to only one other alkyl group (or no alkyl groups, as in methanol).

General Formula: $R - C H_{2} - O H$
Examples:
- Methanol: $C H_{3} O H$
- Ethanol: $C H_{3} C H_{2} O H$
- 1-Propanol: $C H_{3} C H_{2} C H_{2} O H$

Ethyl alcohol (Primary alcohol)

(b) Secondary Alcohols

In secondary (2°) alcohols, the carbinol carbon is bonded to two alkyl groups (same or different).

General Formula: $R_{2} C H O H$
Example:
- 2-Propanol (Isopropyl alcohol): $C H_{3} C H (O H) C H_{3}$

Isopropyl alcohol (Secondary alcohol)

(c) Tertiary Alcohols

In tertiary (3°) alcohols, the carbinol carbon is bonded to three alkyl groups (same or different).

General Formula: $R_{3} C O H$
Example:
- 2-Methyl-2-propanol (Tert-butyl alcohol): $(C H_{3})_{3} C O H$

Tert-butyl alcohol (Tertiary alcohol)

Summary Table

Alcohol Class	General Structure	Carbinol Carbon bonded to	Example
Primary (1°)	$R - C H_{2} - O H$	One alkyl group	Ethanol ( $C H_{3} C H_{2} O H$ )
Secondary (2°)	$R_{2} C H O H$	Two alkyl groups	2-Propanol ( $C H_{3} C H (O H) C H_{3}$ )
Tertiary (3°)	$R_{3} C O H$	Three alkyl groups	2-Methyl-2-propanol ( $(C H_{3})_{3} C O H$ )

Note: This classification is important because the reactivity of alcohols — including oxidation behaviour and reaction with Lucas reagent — depends on whether they are primary, secondary, or tertiary.

Alcohol Class

General Structure

Carbinol Carbon bonded to

Example

Primary (1°)

R - C H_{2} - O H

One alkyl group

Ethanol (

C H_{3} C H_{2} O H

)

Secondary (2°)

R_{2} C H O H

Two alkyl groups

2-Propanol (

C H_{3} C H (O H) C H_{3}

)

Tertiary (3°)

R_{3} C O H

Three alkyl groups

2-Methyl-2-propanol (

(C H_{3})_{3} C O H

)