Unit 11

Organic Chemistry

Class 12 Chemistry

Phenols

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NEB/CDC syllabus scope: This 4-teaching-hour chapter covers introduction and nomenclature; preparation of phenol from chlorobenzene, diazonium salt and benzene sulphonic acid; physical properties; acidic nature compared with alcohol and water; action with NH₃, Zn, Na, benzene diazonium chloride and phthalic anhydride; acylation, Kolbe’s reaction and Reimer–Tiemann reaction; electrophilic substitution by nitration, sulphonation, bromination and Friedel–Crafts alkylation; FeCl₃, aqueous bromine and Liebermann tests; and uses of phenol.

1. Introduction to Phenols

Definition Phenols are aromatic hydroxy compounds in which one or more hydroxyl groups (–OH) are bonded directly to carbon atoms of an aromatic ring.

Simplest phenol C₆H₅OH

The simplest member is phenol (hydroxybenzene).

Phenol differs from an alcohol because its –OH group is attached directly to an aromatic sp² carbon. This structural difference changes both its acidity and its substitution chemistry.

Diagram 1: Structure of phenol

Compound	Structure idea	Functional class
Phenol	C₆H₅–OH	Phenol
Benzyl alcohol	C₆H₅–CH₂OH	Alcohol
Ethanol	CH₃CH₂OH	Alcohol

Remember C₆H₅OH is phenol, but C₆H₅CH₂OH is benzyl alcohol. In benzyl alcohol, –OH is attached to a side-chain sp³ carbon rather than directly to the ring.

2. Nomenclature of Phenols

The retained name phenol is accepted as the parent name. The carbon carrying –OH is numbered 1, and other substituents are assigned the lowest possible locants.

Substitution pattern	Name	Common positional term
2-methylphenol	2-Methylphenol	o-Cresol
3-methylphenol	3-Methylphenol	m-Cresol
4-methylphenol	4-Methylphenol	p-Cresol
2-nitrophenol	2-Nitrophenol	o-Nitrophenol
4-bromophenol	4-Bromophenol	p-Bromophenol

Diagram 2: Ortho, meta and para positions in phenol

3. Preparation of Phenol

3.1 From Chlorobenzene — Dow Process

Chlorobenzene is heated with concentrated aqueous sodium hydroxide under severe temperature and pressure. Sodium phenoxide forms first and is then acidified.

C₆H₅Cl + 2NaOH → C₆H₅ONa + NaCl + H₂O

high temperature and pressure

C₆H₅ONa + HCl → C₆H₅OH + NaCl

Important Concept Chlorobenzene is normally resistant to nucleophilic substitution. The harsh Dow-process conditions are needed to replace Cl by –OH.

3.2 From Benzene Diazonium Chloride

On warming an aqueous solution of benzenediazonium chloride, the diazonium group is replaced by –OH with evolution of nitrogen gas.

C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂↑ + HCl

3.3 From Benzene Sulphonic Acid

Benzene sulphonic acid is converted to its sodium salt and fused with sodium hydroxide. Sodium phenoxide is obtained and then acidified.

C₆H₅SO₃Na + 2NaOH → C₆H₅ONa + Na₂SO₃ + H₂O C₆H₅ONa + HCl → C₆H₅OH + NaCl

Diagram 3: Three syllabus preparations of phenol

4. Physical Properties of Phenol

Property	Description
Appearance	Pure phenol is a colourless crystalline solid; it can develop colour on standing because of oxidation impurities.
Melting point	About 40.5 °C
Boiling point	About 181.7 °C
Odour	Characteristic phenolic odour
Water solubility	Moderately/slightly soluble; the –OH group hydrogen-bonds with water, but the aromatic ring is hydrophobic.
Organic solvents	Soluble in many organic solvents.
Hydrogen bonding	Phenol molecules form intermolecular hydrogen bonds, contributing to relatively high boiling point.

Diagram 4: Intermolecular hydrogen bonding in phenol

5. Acidic Nature of Phenol

Phenol is a weak acid, but it is appreciably more acidic than ordinary aliphatic alcohols.

C₆H₅OH ⇌ C₆H₅O⁻ + H⁺

5.1 Reaction with Sodium Hydroxide

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

Ordinary alcohols such as ethanol do not react appreciably with aqueous NaOH in the same acid–base manner.

5.2 Why Phenol Is More Acidic than Alcohol

After phenol loses H⁺, the phenoxide ion is formed. Its negative charge is delocalized by resonance over oxygen and the aromatic ring. This stabilizes the conjugate base.

Diagram 5: Resonance stabilization explains phenol acidity

5.3 Comparison with Water and Alcohol

Compound	Approximate acidity idea	Main reason
Phenol	More acidic	Phenoxide ion is resonance stabilized.
Water	Intermediate in this comparison	Hydroxide has no alkyl electron-donating group.
Ethanol / typical alcohol	Less acidic	Alkyl group donates electron density and destabilizes the alkoxide relative to phenoxide.

Exam Important The best explanation is not “phenol is acidic because it has –OH.” Explain that phenoxide ion is resonance stabilized, whereas an ordinary alkoxide ion does not receive comparable aromatic resonance stabilization.

6. Important Chemical Reactions of Phenol

6.1 Action with Ammonia

On heating phenol with ammonia under suitable catalytic conditions, the –OH group can be replaced by –NH₂ to form aniline.

C₆H₅OH + NH₃ → C₆H₅NH₂ + H₂O

catalyst / heat

6.2 Action with Zinc Dust

Phenol is reduced by heating with zinc dust, giving benzene.

C₆H₅OH + Zn → C₆H₆ + ZnO

Conversion to remember Phenol → Benzene

Reagent: zinc dust and heat.

6.3 Action with Sodium Metal

The acidic hydrogen of the hydroxyl group is replaced by sodium, producing sodium phenoxide and hydrogen.

2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂↑

6.4 Action with Benzene Diazonium Chloride — Azo Coupling

Phenol couples with benzenediazonium chloride in alkaline medium. The para product is generally favoured when the para position is free.

C₆H₅OH + C₆H₅N₂⁺Cl⁻ → p-HOC₆H₄–N=N–C₆H₅ + HCl

The coloured product is p-hydroxyazobenzene (an azo dye).

Diagram 6: Phenol coupling with benzenediazonium chloride

6.5 Action with Phthalic Anhydride

Phenol condenses with phthalic anhydride in the presence of concentrated sulphuric acid to form phenolphthalein.

2C₆H₅OH + phthalic anhydride → phenolphthalein + H₂O

conc. H₂SO₄ / heat

Phenolphthalein is colourless in acidic solution and pink in a suitable alkaline pH range, which is why it is widely used as an acid–base indicator.

Diagram 7: Formation of phenolphthalein

7. Acylation of Phenol

Phenol reacts at oxygen with acylating agents such as ethanoyl chloride or ethanoic anhydride to form esters.

With Ethanoyl Chloride

C₆H₅OH + CH₃COCl → C₆H₅OCOCH₃ + HCl

With Ethanoic Anhydride

C₆H₅OH + (CH₃CO)₂O → C₆H₅OCOCH₃ + CH₃COOH

Product The ester phenyl ethanoate (phenyl acetate) is formed by O-acylation.

8. Kolbe’s Reaction (Kolbe–Schmitt Reaction)

Sodium phenoxide reacts with carbon dioxide under pressure and heat. After acidification, the major product is 2-hydroxybenzoic acid (salicylic acid).

C₆H₅ONa + CO₂ → o-HOC₆H₄COONa o-HOC₆H₄COONa + HCl → o-HOC₆H₄COOH + NaCl

Diagram 8: Kolbe–Schmitt synthesis of salicylic acid

Do not confuse This Kolbe–Schmitt reaction of phenoxide is different from Kolbe electrolysis used in hydrocarbon chemistry.

9. Reimer–Tiemann Reaction

Phenol reacts with chloroform and aqueous sodium hydroxide to introduce a formyl group (–CHO), mainly at the ortho position. Acid work-up gives 2-hydroxybenzaldehyde (salicylaldehyde).

C₆H₅OH + CHCl₃ + 3NaOH → o-HOC₆H₄CHO + 3NaCl + 2H₂O

Diagram 9: Reimer–Tiemann formylation

Kolbe–Schmitt

Introduces –COOH mainly at ortho position using CO₂ and sodium phenoxide.

Reimer–Tiemann

Introduces –CHO mainly at ortho position using CHCl₃ and NaOH.

10. Electrophilic Substitution Reactions of Phenol

The –OH group strongly activates the aromatic ring by resonance donation of an oxygen lone pair. As a result, electrophilic substitution occurs readily, especially at the ortho and para positions.

Directing effect –OH = strongly activating, ortho/para directing

10.1 Nitration

With Dilute Nitric Acid

C₆H₅OH + HNO₃ → o-HOC₆H₄NO₂ + p-HOC₆H₄NO₂ + H₂O

A mixture of ortho- and para-nitrophenols is formed.

With Concentrated Nitric Acid

C₆H₅OH + 3HNO₃ → 2,4,6-(NO₂)₃C₆H₂OH + 3H₂O

The product is 2,4,6-trinitrophenol (picric acid).

10.2 Sulphonation

Phenol reacts with concentrated sulphuric acid. Product distribution depends on temperature.

C₆H₅OH + H₂SO₄ → HOC₆H₄SO₃H + H₂O

Lower temperature favours ortho-phenolsulphonic acid.
Higher temperature favours para-phenolsulphonic acid.

10.3 Bromination

Phenol rapidly decolourizes bromine water and forms a white precipitate of 2,4,6-tribromophenol.

C₆H₅OH + 3Br₂ → 2,4,6-C₆H₂Br₃OH↓ + 3HBr

10.4 Friedel–Crafts Alkylation

At school level, alkylation of the activated phenolic ring is represented as giving mainly ortho- and para-alkyl phenols under suitable Lewis-acid/catalytic conditions.

C₆H₅OH + CH₃Cl → o-CH₃C₆H₄OH + p-CH₃C₆H₄OH + HCl

Mechanistic caution Real Friedel–Crafts chemistry of free phenol can be complicated because the oxygen atom interacts strongly with Lewis acids. For NEB exam purposes, learn the syllabus-level orientation: the –OH group directs alkyl substitution mainly to ortho and para positions.

Diagram 10: Major electrophilic substitutions of phenol

11. Tests of Phenol

11.1 Ferric Chloride Test

Phenol gives a characteristic violet/purple coloration with neutral ferric chloride due to formation of a coloured iron(III)–phenolate complex.

Observation Violet or purple colour with neutral FeCl₃ solution.

11.2 Aqueous Bromine Test

Phenol decolourizes bromine water and gives a white precipitate of 2,4,6-tribromophenol.

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH↓ + 3HBr

Observation Bromine water is decolourized and a white precipitate forms.

11.3 Liebermann’s Nitroso Test

Phenol is treated with sodium nitrite and concentrated sulphuric acid. A deep blue/green coloration develops. On dilution with water it changes toward red, and on making the solution alkaline the blue/green coloration returns.

Diagram 11: FeCl₃, bromine-water and Liebermann tests

Test	Reagent	Positive observation
Ferric chloride test	Neutral FeCl₃	Violet/purple coloration
Bromine-water test	Aqueous Br₂	Decolourization + white 2,4,6-tribromophenol precipitate
Liebermann test	NaNO₂ + conc. H₂SO₄, then water/alkali	Blue/green → red on dilution → blue/green in alkali

12. Uses of Phenol

Manufacture of phenolic resins such as phenol–formaldehyde resins (e.g., Bakelite-type materials).
Starting material for salicylic acid and other pharmaceutical intermediates.
Manufacture of dyes, indicators and specialty organic chemicals.
Used in the production of bisphenol and other industrial intermediates.
Phenolic compounds have historically been used as antiseptics/disinfectants, but concentrated phenol is corrosive and toxic.

Safety Phenol can cause severe chemical burns and systemic toxicity. Its historical antiseptic use does not make concentrated phenol suitable for direct personal use.

13. High-Yield Reaction Summary

Reaction	Reagent / condition	Main product / observation
Chlorobenzene → phenol	NaOH, high T/P; then acid	Phenol
Diazonium salt → phenol	Warm water	Phenol + N₂
Benzene sulphonate → phenol	Fused NaOH; acidify	Phenol
Acid–base	NaOH	Sodium phenoxide
With Na	Sodium metal	Sodium phenoxide + H₂
With Zn dust	Heat	Benzene
With NH₃	Heat/catalyst	Aniline
Azo coupling	Benzenediazonium chloride / alkaline	p-Hydroxyazobenzene
Phthalic anhydride	Conc. H₂SO₄ / heat	Phenolphthalein
Acylation	CH₃COCl or (CH₃CO)₂O	Phenyl ethanoate
Kolbe–Schmitt	Na phenoxide + CO₂; acidify	Salicylic acid
Reimer–Tiemann	CHCl₃ / NaOH; acidify	Salicylaldehyde
Nitration	Dilute HNO₃	o-/p-nitrophenol
Strong nitration	Conc. HNO₃	Picric acid
Bromination	Bromine water	2,4,6-tribromophenol, white ppt.

14. Common Exam Mistakes

Calling benzyl alcohol a phenol. In phenol, –OH must be directly bonded to the aromatic ring.
Saying phenol is more acidic only because oxygen is electronegative. The key reason is resonance stabilization of phenoxide ion.
Writing that ordinary ethanol reacts strongly with NaOH like phenol. It does not under normal aqueous acid–base conditions.
Forgetting acidification after forming sodium phenoxide in preparation reactions.
Confusing Kolbe–Schmitt with Kolbe electrolysis.
Confusing Kolbe–Schmitt and Reimer–Tiemann products: Kolbe introduces –COOH; Reimer–Tiemann introduces –CHO.
Writing meta substitution as the main orientation of phenol. –OH strongly directs electrophilic attack to ortho and para.
Forgetting that bromine water gives 2,4,6-tribromophenol, not just monobromophenol.
Writing picric acid as mononitrophenol. Picric acid is 2,4,6-trinitrophenol.
Confusing FeCl₃ test with bromine-water test. FeCl₃ gives violet colour; bromine water gives decolourization plus white precipitate.
Writing phenolphthalein formation without phthalic anhydride and acid catalyst.
Claiming phenol is a strong acid. It is a weak acid, though more acidic than ordinary alcohols.

15. Worked Examples

Worked Example 1: Phenol or alcohol?

C₆H₅OH: –OH directly on aromatic ring → phenol.

C₆H₅CH₂OH: –OH on sp³ side-chain carbon → alcohol.

Worked Example 2: Why does phenol react with NaOH? C₆H₅OH + NaOH → C₆H₅ONa + H₂O

The phenoxide ion is stabilized by resonance. This makes phenol acidic enough to be deprotonated by hydroxide.

Worked Example 3: Identify Kolbe product

Sodium phenoxide + CO₂ under pressure followed by acidification gives mainly 2-hydroxybenzoic acid (salicylic acid).

Worked Example 4: Identify Reimer–Tiemann product

Phenol + CHCl₃ + NaOH followed by acidification gives mainly 2-hydroxybenzaldehyde (salicylaldehyde).

Worked Example 5: Phenol with bromine water C₆H₅OH + 3Br₂ → C₆H₂Br₃OH↓ + 3HBr

Observation: bromine water decolourizes and a white precipitate of 2,4,6-tribromophenol forms.

16. Important Exam Questions

Short-Answer Questions

Define phenol and distinguish it from an alcohol.
Give the IUPAC/common names of ortho-, meta- and para-cresol.
How is phenol prepared from chlorobenzene?
How is phenol prepared from benzenediazonium chloride?
How is phenol prepared from benzene sulphonic acid?
Why is phenol more acidic than ethanol?
Compare the acidity of phenol, water and ethanol.
Write the reaction of phenol with sodium hydroxide.
What happens when phenol is heated with zinc dust?
Write the reaction of phenol with sodium metal.
What product forms when phenol couples with benzenediazonium chloride?
How is phenolphthalein prepared from phenol?
What is acylation of phenol?
State Kolbe’s reaction of phenol.
State Reimer–Tiemann reaction.
Why is –OH an ortho/para-directing group?
What products are obtained by nitration of phenol with dilute and concentrated HNO₃?
What happens when phenol is treated with bromine water?
Describe the FeCl₃ test for phenol.
Describe Liebermann’s test for phenol.
State any three uses of phenol.

Long-Answer Questions

Describe three methods of preparation of phenol with balanced equations.
Explain the acidic nature of phenol by resonance stabilization of the phenoxide ion.
Compare the acidic strength of phenol, water and alcohol.
Describe the reactions of phenol with NH₃, Zn, Na, benzenediazonium chloride and phthalic anhydride.
Explain acylation, Kolbe–Schmitt and Reimer–Tiemann reactions of phenol.
Explain electrophilic substitution reactions of phenol: nitration, sulphonation, bromination and alkylation.
Describe three qualitative tests for phenol with observations.

Conversion / Reaction Questions

Chlorobenzene → phenol.
Benzenediazonium chloride → phenol.
Benzene sulphonic acid → phenol.
Phenol → benzene.
Phenol → aniline.
Phenol → sodium phenoxide.
Phenol → salicylic acid.
Phenol → salicylaldehyde.
Phenol → phenyl ethanoate.
Phenol → picric acid.
Phenol → 2,4,6-tribromophenol.
Phenol + phthalic anhydride → phenolphthalein.

Diagram Questions

Draw the structure of phenol and distinguish it from benzyl alcohol.
Show ortho, meta and para positions in phenol.
Draw the three preparation routes to phenol.
Draw resonance structures of phenoxide ion.
Draw the azo coupling reaction of phenol.
Draw phenolphthalein-formation flow diagram.
Draw Kolbe–Schmitt reaction flow diagram.
Draw Reimer–Tiemann reaction flow diagram.
Draw an electrophilic substitution reaction map for phenol.
Draw a summary diagram of the three qualitative tests of phenol.

Exam Strategy This is a short but reaction-heavy chapter. Memorize each reagent → condition → product pair, then separately learn the two conceptual explanations most often asked: phenol acidity and ortho/para activation by –OH.

17. One-Minute Revision

Phenol is C₆H₅OH; –OH is directly attached to an aromatic sp² carbon.
Phenol is different from benzyl alcohol, C₆H₅CH₂OH.
Phenol can be prepared from chlorobenzene, diazonium salt and benzene sulphonic acid.
Phenol is a weak acid but more acidic than ordinary aliphatic alcohols.
Phenoxide ion is resonance stabilized.
Phenol reacts with NaOH to form sodium phenoxide.
Phenol + Na gives sodium phenoxide + H₂.
Phenol + Zn dust/heat gives benzene.
Phenol + NH₃ under suitable catalytic conditions gives aniline.
Phenol couples with benzenediazonium chloride to form an azo dye.
Phenol + phthalic anhydride gives phenolphthalein.
Acylation gives a phenyl ester such as phenyl ethanoate.
Kolbe–Schmitt introduces –COOH mainly ortho to –OH.
Reimer–Tiemann introduces –CHO mainly ortho to –OH.
–OH strongly activates the ring and is ortho/para directing.
Dilute HNO₃ gives o-/p-nitrophenol; concentrated HNO₃ gives picric acid.
Bromine water gives white 2,4,6-tribromophenol and is decolourized.
FeCl₃ test gives violet/purple colour.
Liebermann test shows characteristic blue/green → red → blue/green colour changes.
Phenol is used as an important industrial intermediate, but concentrated phenol is corrosive and toxic.

18. Diagram Practice

Students should practice these diagrams for the NEB examination:

Structure of phenol showing –OH directly bonded to the benzene ring.
Ortho, meta and para positions relative to –OH.
Preparation routes from chlorobenzene, diazonium salt and benzene sulphonic acid.
Hydrogen bonding in phenol.
Resonance stabilization of phenoxide ion.
Azo coupling with benzenediazonium chloride.
Formation of phenolphthalein.
Kolbe–Schmitt reaction.
Reimer–Tiemann reaction.
Electrophilic substitution reaction map.
FeCl₃, bromine-water and Liebermann test summary.

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