13.7.2 Functional Groups Containing Carbon, Hydrogen and Oxygen: Alcohols

This topic explains alcohols — organic compounds containing the hydroxyl group (–OH) attached to a hydrocarbon chain.

Definition

Alcohols are characterized by the presence of the hydroxyl group (–OH) attached to a hydrocarbon chain.

Simple explanation: An alcohol is an organic compound with an –OH group attached to a carbon chain.

Main Idea

• General formula: R–OH
• The –OH group is the functional group of alcohols.
• The R represents an alkyl group (a hydrocarbon chain).

Simple explanation: Alcohols have an –OH group attached to a hydrocarbon chain.

Core Concept

General Formula

R–OH

• **R** = alkyl group (e.g., CH₃–, C₂H₅–, C₃H₇–)
• –OH = hydroxyl group (the functional group)

Simple explanation: R–OH is the general formula for alcohols.

Examples from the Book

The book asks: "Which of the following compounds is alcohol?"

Answer: Both 1-propanol (CH₃CH₂CH₂OH) and 1-butanol (CH₃CH₂CH₂CH₂OH) are alcohols because they contain the –OH group.

Simple explanation: Any compound with an –OH group attached to a hydrocarbon chain is an alcohol.

Detailed Explanation

What Makes a Compound an Alcohol?

• A hydrocarbon chain (alkyl group).
• An –OH group attached to one of the carbon atoms.

Examples:

Simple explanation: If –OH is attached to a carbon chain, it's an alcohol.

Naming Alcohols

• The alkane name (with -e changed to -ol).
• A number to indicate the position of the –OH group.

Examples:

Simple explanation: The name ends in -ol and includes a number for the position of –OH.

Homologous Series of Alcohols

Alcohols form a homologous series with the general formula:

C_nH_2n+1OH

Simple explanation: Alcohols form a homologous series — each member differs by –CH₂–.

Types of Alcohols

Simple explanation: Alcohols are classified by how many carbons are attached to the carbon with the –OH group.

Physical Properties of Alcohols

Simple explanation: Alcohols have higher boiling points than alkanes because of hydrogen bonding.

Chemical Properties of Alcohols

Alcohols undergo several important reactions:

1. Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat

2. Reaction with sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

• Primary alcohols → aldehydes → carboxylic acids
• Secondary alcohols → ketones

Simple explanation: Alcohols burn, react with sodium, and can be oxidized.

Uses of Alcohols

Simple explanation: Alcohols are used as solvents, fuels, and antiseptics.

Intermediate Level

Structure of Alcohols

• The carbon chain forms the backbone.
• The –OH group is attached to one of the carbon atoms.
• The –OH group makes the molecule polar.

Example (ethanol, C₂H₅OH):

CH₃–CH₂–OH

Simple explanation: The –OH group is attached to the end or middle of the carbon chain.

Hydrogen Bonding in Alcohols

• The –OH group allows hydrogen bonding between alcohol molecules.
• Hydrogen bonding requires more energy to break.
• So more heat is needed to boil alcohols.

Simple explanation: Hydrogen bonding makes alcohols harder to boil than alkanes.

Solubility of Alcohols

• Small alcohols (methanol, ethanol) are miscible with water (dissolve completely).
• Larger alcohols become less soluble as the hydrocarbon chain gets longer.
• The –OH group is hydrophilic (water-loving); the hydrocarbon chain is hydrophobic (water-hating).

Simple explanation: Small alcohols dissolve in water; large alcohols do not.

Types of Alcohols

Simple explanation: The type depends on how many carbons are attached to the carbon with –OH.

Reactions of Alcohols

Uses of Alcohols in Detail

Important Points

• - Alcohols: organic compounds containing the hydroxyl group (–OH) attached to a hydrocarbon chain.
• - General formula: R–OH.
• - Functional group: –OH (hydroxyl group).
• - Examples:
• - Methanol: CH₃OH
• - Ethanol: C₂H₅OH
• - 1-Propanol: CH₃CH₂CH₂OH
• - 1-Butanol: CH₃CH₂CH₂CH₂OH
• - Naming: ends in -ol (e.g., methanol, ethanol).
• - Homologous series: CₙH₂ₙ₊₁OH.
• - Types: primary, secondary, tertiary.
• - Properties: higher boiling points than alkanes (hydrogen bonding); soluble in water (small alcohols).
• - Reactions: combustion, reaction with sodium, oxidation.
• - Uses: solvents, fuels, antiseptics.

Remember

• - Alcohol = –OH group on a hydrocarbon chain.
• - General formula: R–OH.
• - Naming: ends in -ol.
• - Examples:
• - Methanol: CH₃OH
• - Ethanol: C₂H₅OH
• - Propanol: C₃H₇OH
• - Butanol: C₄H₉OH
• - Higher boiling points than alkanes (hydrogen bonding).
• - Soluble in water (small alcohols).
• - Types: primary, secondary, tertiary.

Common Mistakes

• 1. Confusing alcohols with ethers. Alcohols have –OH; ethers have C–O–C.
• 2. Confusing alcohols with phenols. Alcohols have –OH on an alkyl group; phenols have –OH on a benzene ring.
• 3. Forgetting the general formula. Alcohols: R–OH.
• 4. Thinking all alcohols are soluble in water. Only small alcohols are fully soluble; solubility decreases with chain length.
• 5. Thinking alcohols have low boiling points. Alcohols have higher boiling points than alkanes due to hydrogen bonding.
• 6. Confusing primary, secondary, and tertiary alcohols. The type depends on how many carbons are attached to the carbon with the –OH group.

Quick Revision

• Alcohols are characterized by the presence of the hydroxyl group (–OH) attached to a hydrocarbon chain.
• General formula for alcohols: R–OH.
• Examples:
• 1-Propanol: CH₃CH₂CH₂OH
• 1-Butanol: CH₃CH₂CH₂CH₂OH
• Homologous series: CₙH₂ₙ₊₁OH.
• Naming: ends in -ol (e.g., methanol, ethanol, propanol, butanol).
• Types: primary, secondary, tertiary.
• Properties: higher boiling points than alkanes (hydrogen bonding); soluble in water (small alcohols); less soluble as chain length increases.
• Reactions: combustion, reaction with sodium, oxidation.
• Uses: solvents, fuels, antiseptics.

60-Second Revision

• Alcohol = –OH group on a hydrocarbon chain.
• General formula: R–OH.
• Examples: CH₃OH (methanol), C₂H₅OH (ethanol).
• Naming: ends in -ol.
• Higher boiling points than alkanes.
• Warning: alcohols have –OH on an alkyl group; phenols have –OH on a benzene ring.