CBSE Class 12 Chemistry Revision Notes Chapter 7 Alcohols, Phenols and Ethers

Alcohols, phenols and ethers are organic compounds containing oxygen as an important part of their functional group. In CBSE Class 12 Chemistry, this chapter explains nomenclature, preparation, properties, reactions and uses of these compounds.

Alcohols, Phenols and Ethers is an important organic chemistry chapter in Class 12 Chemistry. Alcohols and phenols contain the hydroxyl group, -OH. In alcohols, the -OH group is attached to an aliphatic carbon atom. In phenols, the -OH group is directly attached to an aromatic ring.

Ethers contain an alkoxy or aryloxy group. They may also be seen as compounds formed when the hydrogen atom of the hydroxyl group in alcohol or phenol is replaced by an alkyl or aryl group.

Use these CBSE Class 12 Chemistry Revision Notes Chapter 7 to revise the chapter in a simple way. The notes cover classification, nomenclature, preparation methods, physical properties, acidity, important reactions, commercial alcohols and ether chemistry.

Key Takeaways

• Alcohols: Alcohols contain one or more -OH groups attached to aliphatic carbon atoms.
• Phenols: Phenols contain -OH groups directly attached to aromatic rings.
• Ethers: Ethers contain an oxygen atom attached to two alkyl or aryl groups.
• Important Reactions: Kolbe’s reaction, Reimer-Tiemann reaction, Williamson synthesis, oxidation and dehydration are key exam topics.

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Access 30 Minutes Class 12 Chemistry Chapter 7 Alcohols, Phenols and Ethers Notes

Start with classification and nomenclature. These topics help you identify alcohols, phenols and ethers correctly.

Then revise preparation methods and physical properties. After that, focus on chemical reactions of alcohols, phenols and ethers. Give more time to acidity of phenols, oxidation of alcohols, Kolbe’s reaction, Reimer-Tiemann reaction and Williamson synthesis.

Class 12 Chemistry Chapter 7 Alcohols Phenols and Ethers Notes Overview

Alcohols, phenols and ethers are oxygen-containing organic compounds.

Alcohols are used in solvents, fuels and medicines. Phenols are used in antiseptics, dyes and polymers. Ethers are used as solvents and in organic synthesis.

CBSE Class 12 Chemistry Chapter 7 Notes on Classification

Alcohols and phenols are classified on the basis of the number of hydroxyl groups present.

Classification of Alcohols

Primary, Secondary and Tertiary Alcohols

Monohydric alcohols are further classified based on the carbon atom attached to the -OH group.

Allylic and Benzylic Alcohols

Classification of Phenols

Phenols are classified based on the number of -OH groups attached to the aromatic ring.

Classification of Ethers

Ethers are classified as simple or mixed ethers.

Alcohols Phenols and Ethers Class 12 Notes on Nomenclature

Alcohols, phenols and ethers are named using common and IUPAC systems.

Nomenclature of Alcohols

In common names, alcohols are named by writing the alkyl group followed by the word alcohol.

Example:

CH₃OH = methyl alcohol

In IUPAC names, the suffix -e of alkane is replaced by -ol.

Example:

CH₃OH = methanol

Common and IUPAC Names of Alcohols

Nomenclature of Phenols

The simplest hydroxy derivative of benzene is called phenol.

Phenol is both its common name and accepted IUPAC name.

Dihydroxy Derivatives of Benzene

Nomenclature of Ethers

In common names, the alkyl or aryl groups attached to oxygen are written in alphabetical order, followed by the word ether.

Example:

CH₃OC₂H₅ = ethyl methyl ether

In IUPAC names, ethers are named as alkoxy derivatives of hydrocarbons.

Common and IUPAC Names of Ethers

Structure of Functional Groups in Alcohols, Phenols and Ethers

In alcohols, oxygen of the -OH group is attached to carbon through a sigma bond.

In phenols, the -OH group is attached to sp² hybridised carbon of the aromatic ring.

In ethers, oxygen is attached to two carbon atoms.

The bond angle in alcohols is slightly less than the tetrahedral angle because of lone pair repulsion on oxygen.

In ethers, the bond angle is slightly greater because of repulsion between bulky alkyl or aryl groups.

Preparation of Alcohols

Alcohols can be prepared from alkenes, carbonyl compounds, carboxylic acids, esters and Grignard reagents.

Preparation of Alcohols from Alkenes

Alcohols are prepared from alkenes by acid-catalysed hydration and hydroboration-oxidation.

Acid-Catalysed Hydration of Alkenes

Alkenes react with water in the presence of acid to form alcohols.

In unsymmetrical alkenes, addition follows Markovnikov’s rule.

Example:

CH₃CH=CH₂ + H₂O → CH₃CHOHCH₃

Propene gives propan-2-ol.

Mechanism of Acid-Catalysed Hydration

Step 1: Alkene is protonated to form carbocation.

Step 2: Water attacks the carbocation.

Step 3: Deprotonation gives alcohol.

Hydroboration-Oxidation of Alkenes

In hydroboration-oxidation, alkene first reacts with diborane. The product is then oxidised using hydrogen peroxide in alkaline medium.

General reaction:

Alkene + BH₃, then H₂O₂/OH⁻ → Alcohol

This reaction gives alcohol in a way opposite to Markovnikov’s rule.

Example:

CH₃CH=CH₂ → CH₃CH₂CH₂OH

Propene gives propan-1-ol.

Preparation of Alcohols from Carbonyl Compounds

Aldehydes and ketones can be reduced to alcohols.

Reduction of Aldehydes

Aldehydes give primary alcohols.

RCHO → RCH₂OH

Example:

CH₃CHO → CH₃CH₂OH

Ethanal gives ethanol.

Reduction of Ketones

Ketones give secondary alcohols.

RCOR’ → RCHOHR’

Example:

CH₃COCH₃ → CH₃CHOHCH₃

Propanone gives propan-2-ol.

Common reducing agents:

• H₂/Ni
• NaBH₄
• LiAlH₄

Preparation of Alcohols from Carboxylic Acids and Esters

Carboxylic acids and esters are reduced to primary alcohols.

Carboxylic acids are reduced using LiAlH₄.

Example:

RCOOH → RCH₂OH

Esters are reduced by catalytic hydrogenation or LiAlH₄.

Example:

RCOOR’ → RCH₂OH + R’OH

Preparation of Alcohols from Grignard Reagents

Grignard reagents react with aldehydes and ketones to form alcohols after hydrolysis.

Examples:

Methanal + Grignard reagent → Primary alcohol

Ethanal + Grignard reagent → Secondary alcohol

Propanone + Grignard reagent → Tertiary alcohol

Preparation of Phenols

Phenols are prepared from haloarenes, benzene sulphonic acid, diazonium salts and cumene.

Preparation of Phenols from Haloarenes

Chlorobenzene is fused with NaOH at high temperature and pressure.

Sodium phenoxide is formed first.

On acidification, phenol is obtained.

C₆H₅Cl + NaOH → C₆H₅ONa

C₆H₅ONa + H⁺ → C₆H₅OH

Preparation of Phenols from Benzene Sulphonic Acid

Benzene is first sulphonated with oleum to form benzene sulphonic acid.

Benzene sulphonic acid is heated with molten NaOH to form sodium phenoxide.

On acidification, phenol is obtained.

Preparation of Phenols from Diazonium Salts

Benzene diazonium chloride gives phenol on warming with water.

C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl

This method is useful because diazonium salts are easily formed from aromatic primary amines.

Preparation of Phenols from Cumene

Phenol is commercially prepared from cumene.

Cumene is oxidised in air to form cumene hydroperoxide.

On treatment with dilute acid, it gives phenol and acetone.

This is one of the most important industrial methods for phenol preparation.

Physical Properties of Alcohols and Phenols

The physical properties of alcohols and phenols are mainly due to the -OH group.

The alkyl or aryl group modifies these properties.

Boiling Points of Alcohols and Phenols

Alcohols and phenols have higher boiling points than hydrocarbons, ethers and haloalkanes of comparable molecular masses.

Reason:

Alcohols and phenols form intermolecular hydrogen bonds.

Hydrogen bonding increases intermolecular attraction, so more energy is needed for boiling.

Effect of Chain Length and Branching

Boiling point increases with increase in carbon chain length.

Boiling point decreases with increase in branching.

Example:

Butan-1-ol has a higher boiling point than butan-2-ol because butan-1-ol has less branching.

Solubility of Alcohols and Phenols

Lower alcohols are soluble in water because they form hydrogen bonds with water.

Solubility decreases as the size of alkyl or aryl group increases.

Reason:

The hydrocarbon part is hydrophobic.

So, as the non-polar part increases, water solubility decreases.

Acidity of Alcohols and Phenols

Alcohols and phenols are acidic because they can donate a proton from the -OH group.

But phenols are much more acidic than alcohols.

Acidity of Alcohols

Alcohols react with active metals like sodium to form alkoxides and hydrogen gas.

2ROH + 2Na → 2RONa + H₂

Alcohols are weaker acids than water.

The electron-releasing alkyl group increases electron density on oxygen and reduces the polarity of the O-H bond.

Acid strength of alcohols:

Methanol > Primary alcohol > Secondary alcohol > Tertiary alcohol

Acidity of Phenols

Phenols react with sodium and sodium hydroxide.

Phenol + NaOH → Sodium phenoxide + H₂O

Phenols are more acidic than alcohols because phenoxide ion is resonance stabilised.

In alcohols, the alkoxide ion has negative charge localised on oxygen.

In phenoxide ion, the negative charge is delocalised over the benzene ring.

That is why phenol is more acidic than ethanol.

Effect of Substituents on Acidity of Phenols

Electron-withdrawing groups increase the acidity of phenol.

Electron-releasing groups decrease the acidity of phenol.

Ortho and para nitrophenols are more acidic than phenol because the nitro group stabilises phenoxide ion by resonance.

Class 12 Alcohols Phenols and Ethers Revision Notes on Chemical Reactions

The chemical reactions of alcohols, phenols and ethers depend on their functional groups.

Alcohols show reactions involving O-H bond cleavage and C-O bond cleavage.

Phenols mainly show acidic reactions and electrophilic substitution.

Ethers show cleavage with hydrogen halides and electrophilic substitution if an aryl group is present.

Chemical Reactions of Alcohols

Alcohols react as nucleophiles and electrophiles.

They undergo reaction with metals, esterification, conversion into alkyl halides, dehydration and oxidation.

Reaction of Alcohols with Metals

Alcohols react with sodium to form sodium alkoxides and hydrogen gas.

2ROH + 2Na → 2RONa + H₂

Example:

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

This reaction shows the acidic nature of alcohols.

Esterification of Alcohols

Alcohols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

Example:

R-OH + R’COOH → R’COOR + H₂O

This reaction is carried out in the presence of concentrated sulphuric acid.

Acetylation

The introduction of the acetyl group, CH₃CO-, into alcohols or phenols is called acetylation.

Example:

Salicylic acid on acetylation gives aspirin.

Reaction of Alcohols with Hydrogen Halides

Alcohols react with hydrogen halides to form alkyl halides.

ROH + HX → R-X + H₂O

Reactivity order of alcohols:

Tertiary > Secondary > Primary

Reactivity order of hydrogen halides:

HI > HBr > HCl

Lucas Test

Lucas test is used to distinguish primary, secondary and tertiary alcohols.

Lucas reagent is concentrated HCl and anhydrous ZnCl₂.

Turbidity appears because alkyl halide is formed, which is insoluble in the reaction mixture.

Reaction of Alcohols with Phosphorus Trihalides

Alcohols react with phosphorus trihalides to form alkyl halides.

Example:

3ROH + PBr₃ → 3RBr + H₃PO₃

This method is used for preparing alkyl bromides.

Dehydration of Alcohols

Alcohols undergo dehydration to form alkenes.

This reaction takes place in the presence of concentrated H₂SO₄, H₃PO₄, anhydrous ZnCl₂ or alumina.

Example:

CH₃CH₂OH → CH₂=CH₂ + H₂O

Ethanol gives ethene when heated with concentrated H₂SO₄ at 443 K.

Ease of Dehydration

Tertiary alcohols dehydrate most easily.

Order:

Tertiary alcohol > Secondary alcohol > Primary alcohol

Reason:

Tertiary carbocation is more stable than secondary and primary carbocations.

Oxidation of Alcohols

Oxidation of alcohols involves breaking O-H and C-H bonds to form carbon-oxygen double bond.

Oxidation of Primary Alcohols

Primary alcohols first oxidise to aldehydes.

With strong oxidising agents, aldehydes further oxidise to carboxylic acids.

RCH₂OH → RCHO → RCOOH

Mild oxidising agent:

PCC

Strong oxidising agents:

• Acidified KMnO₄
• K₂Cr₂O₇/H⁺

Oxidation of Secondary Alcohols

Secondary alcohols oxidise to ketones.

R₂CHOH → R₂CO

Example:

Propan-2-ol → Propanone

Oxidation of Tertiary Alcohols

Tertiary alcohols do not oxidise under normal conditions.

This is because they do not have hydrogen attached to the carbon bearing the -OH group.

Under strong conditions, C-C bond cleavage may occur.

Dehydrogenation of Alcohols

When vapours of primary or secondary alcohols are passed over heated copper at 573 K, dehydrogenation occurs.

Chemical Reactions of Phenols

Phenols show acidic reactions and electrophilic aromatic substitution reactions.

The -OH group activates the benzene ring and directs incoming groups to ortho and para positions.

Reaction of Phenol with Sodium and Sodium Hydroxide

Phenol reacts with sodium to form sodium phenoxide and hydrogen gas.

2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂

Phenol also reacts with NaOH.

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

This shows that phenol is more acidic than alcohols.

Electrophilic Substitution Reactions of Phenol

The -OH group increases electron density at ortho and para positions of the benzene ring.

So, phenol easily undergoes electrophilic substitution.

Nitration of Phenol

With dilute nitric acid at low temperature, phenol gives a mixture of ortho-nitrophenol and para-nitrophenol.

With concentrated nitric acid, phenol gives 2,4,6-trinitrophenol.

2,4,6-trinitrophenol is also called picric acid.

Ortho and Para Nitrophenol Difference

Halogenation of Phenol

Phenol reacts with bromine easily because the -OH group strongly activates the benzene ring.

Bromination in Non-Polar Solvent

When phenol reacts with bromine in solvents like CHCl₃ or CS₂ at low temperature, ortho and para bromophenols are formed.

Bromination with Bromine Water

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

This reaction is also used as a test for phenol.

Kolbe’s Reaction

In Kolbe’s reaction, sodium phenoxide reacts with carbon dioxide under pressure.

After acidification, salicylic acid is formed.

Main product:

o-Hydroxybenzoic acid

This reaction is important for preparing salicylic acid.

Reimer-Tiemann Reaction

In Reimer-Tiemann reaction, phenol reacts with chloroform in the presence of sodium hydroxide.

A -CHO group is introduced at the ortho position.

Main product:

Salicylaldehyde

This is an important reaction of phenol.

Reaction of Phenol with Zinc Dust

Phenol is converted into benzene when heated with zinc dust.

C₆H₅OH + Zn → C₆H₆ + ZnO

This reaction removes the -OH group from phenol.

Oxidation of Phenol

Phenol is oxidised by chromic acid to benzoquinone.

In air, phenols slowly oxidise and form dark coloured products containing quinones.

Some Commercially Important Alcohols

Methanol and ethanol are two important alcohols.

Methanol

Methanol is also called wood spirit.

Formula:

CH₃OH

It was earlier obtained by destructive distillation of wood.

Today, methanol is prepared by catalytic hydrogenation of carbon monoxide.

CO + 2H₂ → CH₃OH

Catalyst:

ZnO-Cr₂O₃

Properties and Uses of Methanol

Methanol is dangerous. Even small quantities can cause blindness, and large quantities may cause death.

Ethanol

Ethanol is also called ethyl alcohol.

Formula:

C₂H₅OH

It is commercially prepared by fermentation of sugars.

Glucose and fructose are converted into ethanol in the presence of the enzyme zymase.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Fermentation takes place in the absence of air.

Properties and Uses of Ethanol

Commercial alcohol is made unfit for drinking by adding copper sulphate and pyridine. This is called denaturation of alcohol.

Preparation of Ethers

Ethers are prepared by dehydration of alcohols and Williamson synthesis.

Preparation of Ethers by Dehydration of Alcohols

Alcohols undergo dehydration in the presence of acid.

At 413 K, ethanol gives ethoxyethane.

2C₂H₅OH → C₂H₅OC₂H₅ + H₂O

At 443 K, ethanol gives ethene.

C₂H₅OH → CH₂=CH₂ + H₂O

So, temperature controls whether ether or alkene is formed.

This method works best for primary alcohols.

Secondary and tertiary alcohols usually give alkenes because elimination competes with substitution.

Williamson Synthesis

Williamson synthesis is an important method for preparing ethers.

In this reaction, sodium alkoxide reacts with alkyl halide.

General reaction:

R-X + R’O⁻Na⁺ → R-O-R’ + NaX

Example:

CH₃ONa + C₂H₅Br → CH₃OC₂H₅ + NaBr

This reaction follows SN2 mechanism.

Limitation of Williamson Synthesis

Williamson synthesis works best with primary alkyl halides.

With tertiary alkyl halides, elimination takes place instead of substitution.

Example:

(CH₃)₃CBr + CH₃ONa → 2-Methylpropene

So, tertiary alkyl halides are not suitable for preparing ethers by Williamson synthesis.

Physical Properties of Ethers

Ethers have polar C-O bonds and a net dipole moment.

But they do not form hydrogen bonds with themselves because they do not have O-H bond.

Boiling Points of Ethers

Boiling points of ethers are much lower than alcohols of comparable molecular mass.

Reason:

Alcohols form intermolecular hydrogen bonds.

Ethers do not form intermolecular hydrogen bonds with themselves.

Solubility of Ethers

Ethers are soluble in water to some extent because oxygen of ether can form hydrogen bonds with water molecules.

Their solubility is comparable to alcohols of similar molecular mass.

Chemical Reactions of Ethers

Ethers are less reactive than alcohols and phenols.

They undergo cleavage with hydrogen halides under strong conditions.

Aryl alkyl ethers also undergo electrophilic substitution reactions.

Cleavage of Ethers with Hydrogen Halides

Ethers react with concentrated HI or HBr at high temperature.

General reaction:

R-O-R + HI → R-I + R-OH

If HI is in excess, the alcohol formed may also convert into alkyl iodide.

Reactivity order of hydrogen halides:

HI > HBr > HCl

Cleavage of Mixed Ethers

In mixed ethers, the product depends on the nature of alkyl groups.

If both groups are primary or secondary, the iodide ion attacks the smaller or less hindered alkyl group through SN2 mechanism.

If one group is tertiary, tertiary halide is formed through SN1 mechanism.

Cleavage of Anisole

Anisole reacts with HI to give phenol and methyl iodide.

C₆H₅OCH₃ + HI → C₆H₅OH + CH₃I

The O-CH₃ bond breaks because the O-C₆H₅ bond has partial double bond character.

Electrophilic Substitution in Aryl Alkyl Ethers

The alkoxy group, -OR, activates the benzene ring.

It directs incoming electrophiles to ortho and para positions.

Halogenation of Anisole

Anisole reacts with bromine in ethanoic acid to give mainly para-bromoanisole.

The para product is major because it is less sterically hindered.

Friedel-Crafts Reaction of Anisole

Anisole undergoes Friedel-Crafts alkylation and acylation.

The alkyl or acyl group enters mainly at ortho and para positions.

Catalyst:

Anhydrous AlCl₃

Nitration of Anisole

Anisole reacts with a mixture of concentrated HNO₃ and H₂SO₄.

It gives a mixture of ortho-nitroanisole and para-nitroanisole.

Para product is usually major.

Important Named Reactions in Alcohols, Phenols and Ethers

Quick Revision Table for Alcohols, Phenols and Ethers

Important Terms in Alcohols, Phenols and Ethers

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