Chemistry · Year 13 · Advanced Organic Synthesis · Spring Term

Phenols and Their Reactivity

Exploring the enhanced reactivity of phenols compared to benzene.

• National Curriculum Attainment Targets
• A-Level: Chemistry - Aromatic Chemistry
• A-Level: Chemistry - Organic Chemistry

About This Topic

Phenols contain a hydroxyl group attached directly to a benzene ring, which markedly increases reactivity towards electrophilic aromatic substitution compared to benzene. The oxygen lone pairs donate electrons through resonance, activating the ortho and para positions and stabilising the intermediate carbocation. Students examine mechanisms showing this delocalisation and predict faster reaction rates, often with polysubstitution.

This topic integrates with A-Level aromatic and organic chemistry, reinforcing substituent effects and mechanisms. Phenols exhibit greater acidity than alcohols, as the phenoxide anion benefits from resonance stabilisation across the ring, though less than carboxylic acids with their additional carbonyl group. Practical tests, such as decolourisation of bromine water to form 2,4,6-tribromophenol precipitate, allow students to verify predictions and draw curly arrows accurately.

Active learning excels with this content through model-building and comparative experiments. Students construct physical models of resonance hybrids or perform parallel reactivity tests, which clarify abstract electron effects, encourage peer explanation of observations, and strengthen predictive skills essential for exams.

Key Questions

1. Explain why phenols are more reactive than benzene towards electrophilic substitution.
2. Compare the acidity of phenols with alcohols and carboxylic acids.
3. Predict the products of reactions involving phenols, such as with bromine water.

Learning Objectives

• Explain the mechanism by which the hydroxyl group activates the benzene ring in phenols towards electrophilic aromatic substitution.
• Compare the acidity of phenol, ethanol, and benzoic acid, justifying the differences based on resonance and inductive effects.
• Predict the major products formed when phenol reacts with bromine water, including the reaction conditions.
• Analyze the role of lone pair delocalisation in stabilizing the intermediate sigma complex during electrophilic substitution of phenols.
• Differentiate between the reactivity of phenols and benzene in terms of reaction rates and substitution patterns.

Before You Start

Benzene and Electrophilic Aromatic Substitution

Why: Students must understand the basic mechanism and reactivity of benzene towards electrophiles before exploring the enhanced reactivity of phenols.

Structure and Bonding in Organic Molecules

Why: A solid grasp of electron delocalisation, lone pairs, and inductive effects is necessary to explain the reactivity and acidity of phenols.

Key Vocabulary

Phenol	An organic compound where a hydroxyl group (-OH) is directly attached to a benzene ring. It is distinct from alcohols where -OH is attached to an aliphatic carbon.
Electrophilic Aromatic Substitution	A type of substitution reaction where an electrophile replaces a hydrogen atom on an aromatic ring. Phenols undergo this reaction more readily than benzene.
Resonance	A way of describing delocalised electrons within molecules, where the bonding cannot be expressed by a single Lewis structure. In phenols, lone pairs on oxygen delocalise into the ring.
Phenoxide ion	The anion formed when phenol loses a proton. It is resonance stabilized, contributing to phenol's acidity.
Ortho and Para Directors	Substituents on an aromatic ring that direct incoming electrophiles to the ortho and para positions. The -OH group in phenol is an ortho, para director.

Watch Out for These Misconceptions

Common Misconception

The -OH group withdraws electrons inductively, deactivating the ring like a meta-director.

What to Teach Instead

Resonance donation from oxygen lone pairs overrides inductive withdrawal, activating ortho/para positions. Model-building in pairs lets students visualise and debate electron flow, correcting this by comparing model stability.

Common Misconception

Phenols have acidity similar to carboxylic acids due to the -OH group.

What to Teach Instead

Phenols are weaker acids (pKa ~10) than carboxylic acids (pKa ~5) despite resonance, lacking carbonyl stabilisation. Comparative titrations with indicators in small groups highlight pH differences and reinforce phenoxide resonance discussions.

Common Misconception

Electrophilic substitution on phenols occurs equally at all ring positions.

What to Teach Instead

Ortho/para directionality arises from resonance stabilisation of those sigma complexes. Station rotations with bromine tests show selective precipitation patterns, prompting students to revise mechanisms collaboratively.

Active Learning Ideas

Pairs Modeling: Phenol Resonance Structures

Provide molecular model kits for students to build benzene and phenol molecules. Pairs manipulate oxygen lone pairs to demonstrate resonance donation to the ring, then sketch mechanisms for ortho attack. Compare models side-by-side to note activation differences.

25 min·Pairs

Small Groups: Bromination Reactivity Stations

Prepare stations with phenol, benzene, and anisole solutions plus bromine water. Groups add reagent dropwise, time colour changes, and note precipitate formation. Rotate stations, pooling data to discuss substituent effects.

35 min·Small Groups

Whole Class: Acidity Comparison Demo

Display phenol, ethanol, and ethanoic acid with universal indicator or pH probes. Add dilute NaOH slowly to each, observing colour shifts and recording approximate pKa values. Class discusses resonance stabilisation via shared annotations.

20 min·Whole Class

Individual: Product Prediction Cards

Distribute cards with phenol reactions (Br2, NaOH, FeCl3). Students predict products and mechanisms alone, then reveal via projector. Self-check against mark scheme and note errors for group debrief.

15 min·Individual

Real-World Connections

• Phenol derivatives are crucial in the pharmaceutical industry. For example, paracetamol (acetaminophen) is synthesized using reactions involving phenolic compounds, impacting global healthcare.
• Phenol itself is a precursor for producing Bakelite, one of the first synthetic plastics, used historically in electrical insulators and early consumer goods. Its properties continue to influence polymer science.
• Antiseptics like carbolic acid, an early form of phenol, were pioneered by Joseph Lister in the 19th century, revolutionizing surgical practices and significantly reducing infection rates in hospitals.

Assessment Ideas

Quick Check

Present students with a diagram of phenol reacting with bromine water. Ask them to draw the curly arrows for the mechanism and identify the product. Then, ask them to write a sentence comparing the reaction rate to benzene's bromination.

Discussion Prompt

Pose the question: 'Why is phenol more acidic than ethanol, but less acidic than carboxylic acid?' Have students discuss in pairs, referencing resonance structures and inductive effects, then share their reasoning with the class.

Exit Ticket

Give students three compounds: benzene, phenol, and ethanol. Ask them to rank these compounds in order of reactivity towards electrophilic substitution and briefly explain their ranking for phenol versus benzene.

Frequently Asked Questions

Why are phenols more reactive than benzene in electrophilic substitution?

The -OH group's oxygen donates electrons via resonance, increasing ring electron density at ortho and para sites. This stabilises the Wheland intermediate more than in benzene. Students master this by drawing mechanisms and confirming via faster bromine decolourisation rates in practicals.

How does phenol acidity compare to alcohols and carboxylic acids?

Phenols (pKa 10) exceed alcohols (pKa 16) due to resonance in phenoxide but trail carboxylic acids (pKa 5) without carbonyl aid. Indicator tests or titrations reveal this trend clearly, helping students link structure to pKa values.

What happens when phenols react with bromine water?

Phenol rapidly forms a white precipitate of 2,4,6-tribromophenol due to high reactivity and ortho/para activation. No catalyst needed, unlike benzene. This diagnostic test confirms predictions and demonstrates polysubstitution in under a minute.

How can active learning improve understanding of phenols reactivity?

Activities like resonance model-building and reactivity stations provide concrete evidence of electron donation effects. Pairs or groups test predictions with real reagents, discuss anomalies, and refine mechanisms together. This builds confidence in abstract concepts through observation and peer teaching, outperforming lectures alone.