Alkanols (Alcohols) – At a glance

 

 Definition

Alkanols (alcohols) are organic compounds derived from alkanes in which one or more hydrogen atoms are replaced by a hydroxyl group (–OH).

They contain the functional group –OH attached to a saturated carbon atom (sp³ carbon).

General Formula

C_nH_2n+1OH  or  C_nH_2n+2O

Functional Group

Hydroxyl group (–OH)

This group:

• makes alcohols polar

• allows hydrogen bonding

• increases boiling point

• increases solubility in water

Homologous Series

Alcohols belong to a homologous series because:

• Same functional group (–OH)

• Similar chemical properties

• Gradual change in physical properties

• Successive members differ by –CH₂

Examples of Alkanols

Name	Molecular Formula	Structural Formula
Methanol	CH₃OH	CH₃–OH
Ethanol	C₂H₅OH	CH₃–CH₂–OH
Propanol	C₃H₇OH	CH₃–CH₂–CH₂–OH
Butanol	C₄H₉OH	CH₃–CH₂–CH₂–CH₂–OH

 Classification of Alcohols

Based on the number of alkyl groups attached to the carbon bearing –OH:

 Primary (1°)

–OH carbon attached to one alkyl group
Example: Ethanol

R–CH₂–OH

 Secondary (2°)

–OH carbon attached to two alkyl groups
Example: Propan-2-ol

R–CH(OH)–R

 Tertiary (3°)

–OH carbon attached to three alkyl groups
Example: 2-methyl-2-propanol

R_3C–OH

Physical Properties

 State

• Lower members → liquids

• Higher members → oily liquids/solids

 Boiling Point

• Higher than alkanes due to hydrogen bonding

• Increases with molecular mass

Solubility

• Methanol & ethanol → completely miscible with water

• Solubility decreases as carbon chain increases

 Odour

• Mild or characteristic smell

Chemical Properties

Combustion

Alcohols burn in air to form CO₂ + H₂O

Example:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

 Reaction with Sodium

Forms sodium alkoxide + hydrogen gas

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

Test for alcohol → bubbles of hydrogen gas

 Oxidation

Depends on type:

Type	Product formed
Primary	Aldehyde → Carboxylic acid
Secondary	Ketone
Tertiary	No reaction easily

Example:

CH₃CH₂OH → CH₃CHO → CH₃COOH

 Dehydration (Removal of water)

Forms alkenes

Using conc. H₂SO₄ or Al₂O₃

C₂H₅OH → C₂H₄ + H₂O

                 (Ethene formed)

 Esterification

Alcohol + Carboxylic acid → Ester + Water

C₂H₅OH + CH₃COOH →  CH₃COOC₂H₅ + H₂O

Produces pleasant fruity smell

Preparation of Alcohols

Laboratory Methods

1. Hydration of alkenes

2. Fermentation of sugars (ethanol)

3. Hydrolysis of haloalkanes

Example (Fermentation)

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Uses of Alcohols

Alcohol	Uses
Methanol	Fuel, solvent, antifreeze
Ethanol	Drinks, fuel, antiseptic
Propanol	Disinfectant
Butanol	Solvent, perfumes

Safety Notes

• Methanol is poisonous

• Alcohols are flammable

• Avoid inhalation and ingestion in the lab

 Quick Summary Table

Property	Key Point
Functional group	–OH
Formula	CₙH₂ₙ₊₁OH
Bonding	Hydrogen bonding
Types	Primary, Secondary and tertiary (1°, 2°, 3°)
Reactions	Combustion, oxidation, esterification, dehydration
Uses	Fuel, solvent, antiseptic