13.7.3 Phenols

This topic explains phenols — organic compounds in which an –OH group is attached directly to a benzene ring.

Definition

When an –OH group is attached to a benzene ring, the compound is called a phenol.

Simple explanation: A phenol is an alcohol-like compound where the –OH group is attached to a benzene ring, not a hydrocarbon chain.

Main Idea

• Phenols contain an –OH group directly attached to an aromatic benzene ring.
• The simplest phenol is C₆H₅–OH (also written as C₆H₅OH).
• Phenol was the first antiseptic used in an operation theatre.

Simple explanation: Phenols are like alcohols, but the –OH is on a benzene ring.

Core Concept

General Structure

C₆H₅–OH

Or simply:

Ar–OH

• Ar = aryl group (benzene ring)
• **–OH** = hydroxyl group

Simple explanation: Phenol = benzene ring + –OH group.

The Book's Structure

The book shows the structure of phenol as:

• A hexagon with a circle inside (representing the benzene ring).
• An –OH group attached to one corner.

Or simply:

C₆H₅–OH

Simple explanation: Phenol is a benzene ring with an –OH group attached.

Detailed Explanation

What Makes a Compound a Phenol?

• It has a benzene ring (aromatic ring).
• An –OH group is attached directly to the ring.

Examples:

Simple explanation: If –OH is directly on the benzene ring, it's a phenol. If –OH is on a side chain, it's an alcohol.

Phenol — The Simplest Member

• Formula: C₆H₅OH
• Structure: Benzene ring with one –OH group
• Also called: Carbolic acid, hydroxybenzene

Simple explanation: Phenol is the simplest and most important member of the phenol family.

Historical Importance

The book states:

> "Phenol was the first antiseptic used in an operation theatre."

• Joseph Lister introduced phenol (carbolic acid) as an antiseptic in surgery in the 1860s.
• This dramatically reduced post-surgical infections.
• It revolutionized surgery and medicine.

Simple explanation: Phenol was the first antiseptic used in surgery, saving countless lives.

Difference Between Alcohols and Phenols

Simple explanation: The key difference is whether –OH is attached to a hydrocarbon chain (alcohol) or a benzene ring (phenol).

Physical Properties of Phenol

Simple explanation: Phenol is a crystalline solid with a distinctive smell.

Chemical Properties of Phenol

Phenol is weakly acidic — more acidic than alcohols but less acidic than carboxylic acids.

Reaction with sodium hydroxide:

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

• Phenol reacts with NaOH to form sodium phenoxide (C₆H₅ONa).
• Alcohols do not react with NaOH — this is a key difference.

Reaction with sodium:

2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂

Reaction with bromine water:

Phenol reacts with bromine water to form a white precipitate of 2,4,6-tribromophenol.

Simple explanation: Phenol reacts with NaOH and bromine water — reactions that alcohols do not undergo.

Uses of Phenol

Simple explanation: Phenol is used as an antiseptic, disinfectant, and in making plastics and medicines.

Intermediate Level

Structure of Phenol

• A benzene ring (6 carbons in a ring, with alternating double bonds).
• An –OH group attached directly to one of the ring carbons.

Structural formula:

C₆H₅–OH

Simple explanation: Phenol is a benzene ring with an –OH group sticking out.

Why Phenol Is More Acidic Than Alcohols

• The benzene ring stabilizes the phenoxide ion (C₆H₅O⁻) formed when phenol loses a proton.
• The negative charge on the oxygen is delocalized into the ring.
• This makes it easier for phenol to lose a proton.
• So phenol reacts with NaOH, while alcohols do not.

Simple explanation: The benzene ring helps phenol lose its H⁺, making it more acidic than alcohols.

Reactions of Phenol

Simple explanation: Phenol reacts with NaOH, sodium, and bromine water — reactions that distinguish it from alcohols.

Physical Properties

Simple explanation: Phenol is a solid at room temperature with a distinctive smell.

Uses of Phenol

Formula

C₆H₅–OH

Key Facts

Easy Way to Remember

• Phenol = benzene ring + –OH.
• Alcohol = hydrocarbon chain + –OH.
• Phenol = first antiseptic.
• Formula: C₆H₅OH.

How to Identify

• Look for a benzene ring (hexagon with circle).
• Look for an –OH group attached directly to the ring.
• If both → phenol.

Important Points

• - Phenols: organic compounds in which an –OH group is attached directly to a benzene ring.
• - General formula: Ar–OH (Ar = aryl group).
• - Simplest phenol: C₆H₅OH (also called carbolic acid).
• - Historical importance: first antiseptic used in an operation theatre.
• - Difference from alcohols: In alcohols, –OH is on an alkyl group; in phenols, –OH is on a benzene ring.
• - Acidity: Phenols are weakly acidic (more acidic than alcohols).
• - Reactions:
• - With NaOH → sodium phenoxide
• - With Na → sodium phenoxide + H₂
• - With Br₂ water → white precipitate
• - Uses: antiseptic, disinfectant, production of plastics (bakelite), drugs (aspirin), preservative.

Remember

• - Phenol = benzene ring + –OH.
• - General formula: Ar–OH.
• - Simplest phenol: C₆H₅OH.
• - First antiseptic used in surgery.
• - Weakly acidic (more acidic than alcohols).
• - Reacts with NaOH (alcohols do not).
• - Uses: antiseptic, disinfectant, bakelite, aspirin.

Common Mistakes

• 1. Confusing phenols with alcohols. Alcohols have –OH on an alkyl group; phenols have –OH on a benzene ring.
• 2. Thinking phenol is an alcohol. Phenol is a separate class of organic compounds.
• 3. Forgetting the historical importance. Phenol was the first antiseptic used in surgery.
• 4. Thinking phenol is neutral like alcohols. Phenol is weakly acidic — it reacts with NaOH.
• 5. Confusing phenol with benzyl alcohol. Benzyl alcohol (C₆H₅CH₂OH) is an alcohol (because –OH is on the side chain), not a phenol.
• 6. Thinking all phenols are liquids. Phenol is a solid at room temperature.

Quick Revision

• Phenols: when an –OH group is attached to a benzene ring, the compound is called a phenol.
• Example: Phenol (C₆H₅–OH) — also written as C₆H₅OH.
• Historical importance: Phenol was the first antiseptic used in an operation theatre.
• Difference from alcohols: In alcohols, the –OH group is attached to an alkyl group; in phenols, the –OH group is attached directly to a benzene ring.
• Acidity: Phenols are weakly acidic — more acidic than alcohols but less acidic than carboxylic acids.
• Reactions:
• With NaOH → sodium phenoxide
• With Na → sodium phenoxide + H₂
• With Br₂ water → white precipitate (2,4,6-tribromophenol)
• Uses: antiseptic, disinfectant, production of plastics (bakelite), drugs (aspirin), preservative.

60-Second Revision

• Phenol = benzene ring + –OH.
• Formula: C₆H₅OH.
• First antiseptic used in surgery.
• Weakly acidic — reacts with NaOH.
• Warning: benzyl alcohol (C₆H₅CH₂OH) is an alcohol, not a phenol.