Class 12 Chemistry Chapter 7: Alcohols, Phenols and Ethers | NCERT Notes

Chapter 7: Alcohols, Phenols and Ethers

Syllabus: NCERT Rationalised Syllabus 2025–26 (RBSE / CBSE)

Alcohols, phenols and ethers are vital oxygen-containing organic compounds. In alcohols, the hydroxyl (–OH) group is bonded to an alkyl group, while in phenols, it is bonded to an aromatic ring. Ethers contain an oxygen linkage between two carbon groups.

Table of Contents

1. Classification
2. Nomenclature
3. Preparation of Alcohols
4. Physical Properties
5. Chemical Reactions of Alcohols
6. Phenols (Prep & Reactions)
7. Ethers
8. Uses

1. Classification

1.1 Alcohols

Primary (1°) Alcohol: –OH on primary carbon ($R-CH_2-OH$).
Secondary (2°) Alcohol: –OH on secondary carbon ($R_2CH-OH$).
Tertiary (3°) Alcohol: –OH on tertiary carbon ($R_3C-OH$).
Allylic Alcohols: –OH on $sp^3$ carbon next to a C=C bond.

1.2 Phenols

Phenols contain an –OH group directly attached to the benzene ring ($C_6H_5-OH$).

2. Nomenclature

IUPAC names are derived by replacing the terminal “e” of the alkane with the suffix “ol”. The position of the –OH group is indicated by a number.

CH₃–CH(OH)–CH₃ → Propan-2-ol
HO–CH₂–CH₂–OH → Ethane-1,2-diol (Glycol)

3. Preparation of Alcohols

3.1 From Alkenes

(i) Acid Catalysed Hydration: Follows Markovnikov’s rule.

CH₃–CH=CH₂ + H₂O $\xrightarrow{H^+}$ CH₃–CH(OH)–CH₃

(ii) Hydroboration–Oxidation: Gives Anti-Markovnikov product (Primary Alcohol).

CH₃–CH=CH₂ $\xrightarrow{(i) B_2H_6, (ii) H_2O_2/OH^-}$ CH₃–CH₂–CH₂–OH

3.2 From Carbonyl Compounds

Aldehydes: Reduced to 1° Alcohols (using $NaBH_4$ or $LiAlH_4$).
Ketones: Reduced to 2° Alcohols (using $NaBH_4$).
Carboxylic Acids: Reduced to 1° Alcohols (using $LiAlH_4$).

3.3 From Grignard Reagents

HCHO + RMgX → Primary Alcohol
RCHO + RMgX → Secondary Alcohol
RCOR + RMgX → Tertiary Alcohol

4. Physical Properties

Boiling Point: Higher than hydrocarbons/ethers due to Intermolecular Hydrogen Bonding. BP increases with molar mass and decreases with branching.
Solubility: Lower alcohols are miscible with water due to H-bonding. Solubility decreases as the alkyl chain size increases.

5. Chemical Reactions of Alcohols

5.1 Acidity

Alcohols react with Na metal to release $H_2$. Acidity order: $1^\circ > 2^\circ > 3^\circ$. (+I effect of alkyl groups destabilises the alkoxide ion).

5.2 Esterification

R-OH + R'-COOH $\rightleftharpoons$ R'-COOR + H₂O

Carried out in the presence of conc. $H_2SO_4$ (catalyst & dehydrating agent).

5.3 Lucas Test (Distinction)

Reagent: Conc. $HCl$ + Anhydrous $ZnCl_2$.

• Tertiary (3°): Immediate turbidity.
• Secondary (2°): Turbidity after some time (~5 min).
• Primary (1°): No turbidity at room temperature.

5.4 Oxidation

1° Alcohol: $\xrightarrow{PCC}$ Aldehyde $\xrightarrow{KMnO_4}$ Carboxylic Acid.
2° Alcohol: $\xrightarrow{CrO_3}$ Ketone.
3° Alcohol: Resistant to oxidation.

5.5 Dehydration of Alcohols

Alcohols form alkenes on heating with protic acids. Reactivity: $3^\circ > 2^\circ > 1^\circ$.

Primary (1°): 443 K (Concerted mechanism).
Secondary (2°): 440 K (Carbocation mechanism).
Tertiary (3°): 358 K (Carbocation mechanism).

[attachment_0](attachment)

Fig 7.1: Mechanism of acid-catalysed dehydration.

6. Phenols

6.1 Electrophilic Substitution

Nitration (Dil. $HNO_3$): Mixture of o- and p-nitrophenol.
Nitration (Conc. $HNO_3$): 2,4,6-Trinitrophenol (Picric Acid).
Bromination (Aq. $Br_2$): White precipitate of 2,4,6-Tribromophenol.

6.2 Named Reactions

1. Kolbe’s Reaction:
Phenol + NaOH $\xrightarrow{CO_2, 400K, 4-7atm}$ $\xrightarrow{H^+}$ Salicylic Acid

2. Reimer-Tiemann Reaction:
Phenol + $CHCl_3$ + Aq. NaOH $\xrightarrow{Heat}$ $\xrightarrow{H^+}$ Salicylaldehyde

7. Ethers

7.1 Williamson Synthesis

R-X + R'-ONa → R-O-R' + NaX

Constraint: Alkyl halide must be Primary (1°). Tertiary halides result in elimination (Alkene).

7.2 Cleavage by HI

R-O-R' + HI → R-OH + R'-I

Standard Rule ($S_N2$): Iodide attacks the smaller alkyl group.
Exception ($S_N1$): If tertiary group is present, Iodide attacks tertiary carbon.
Anisole: Cleavage yields Phenol + Methyl Iodide.

8. Uses

Methanol: Solvent, highly toxic (causes blindness).
Ethanol: Solvent, fuel. Denatured using $CuSO_4$ and Pyridine.

🎓 Chapter 7 Completed. Fully aligned with NCERT Rationalised Syllabus 2025–26.

(B) Reduction of Carbonyl Compounds

Alcohols are prepared by the reduction of aldehydes, ketones, carboxylic acids and esters.

Aldehydes: Reduced to Primary (1°) alcohols.
Ketones: Reduced to Secondary (2°) alcohols.
Carboxylic acids: Reduced to Primary alcohols (Requires strong reducing agent like LiAlH₄).

R–CHO + [H] → R–CH₂OH (using NaBH₄ or LiAlH₄)
R–CO–R′ + [H] → R–CH(OH)–R′ (using NaBH₄ or LiAlH₄)
R–COOH + 4[H] → R–CH₂OH + H₂O (using LiAlH₄ only)

Important: NaBH₄ is a mild reducing agent and does not reduce carboxylic acids. LiAlH₄ is used for acids.

(C) From Grignard Reagents

Grignard reagents react with aldehydes and ketones to form an adduct, which on hydrolysis yields alcohols.

HCHO + RMgX → Adduct → Primary Alcohol
RCHO + RMgX → Adduct → Secondary Alcohol
RCOR′ + RMgX → Adduct → Tertiary Alcohol

3.2 Physical Properties of Alcohols

Boiling Point: Alcohols have higher boiling points than hydrocarbons, ethers, and haloalkanes of comparable molecular masses. This is due to the presence of intermolecular hydrogen bonding.
Solubility: Lower alcohols are miscible with water in all proportions due to hydrogen bonding. Solubility decreases with increase in the size of the alkyl group (hydrophobic part).

3.3 Chemical Properties of Alcohols

(A) Acidity of Alcohols

Alcohols react with active metals like sodium to liberate hydrogen gas, indicating their acidic nature.

2R–OH + 2Na → 2R–ONa + H₂↑

Acidity Order: 1° > 2° > 3°

Board Fact: Alcohols are weaker acids than water. This is because the electron-releasing alkyl group (+I effect) increases electron density on oxygen, making the O–H bond less polar.

(B) Esterification

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

R–OH + R′–COOH ⇌ R′–COOR + H₂O

Concentrated H₂SO₄ acts as a protonating agent (catalyst) and dehydrating agent (removes water to shift equilibrium forward).

(C) Oxidation of Alcohols

Oxidation involves the cleavage of O–H and C–H bonds to form a C=O double bond.

Primary Alcohols: Oxidised to aldehydes using PCC (Pyridinium chlorochromate). Strong oxidising agents (KMnO₄) usually convert them directly to carboxylic acids.
Secondary Alcohols: Oxidised to ketones using Chromic anhydride (CrO₃).
Tertiary Alcohols: Resistant to oxidation. In presence of strong oxidising agents at high temperature, C–C bond cleavage occurs.

(D) Identification of Alcohols – Lucas Test

Used to distinguish between primary, secondary and tertiary alcohols.

Lucas Reagent: Concentrated HCl + Anhydrous ZnCl₂

Observation:
• Tertiary (3°): Turbidity appears immediately.
• Secondary (2°): Turbidity appears after some time (approx. 5 min).
• Primary (1°): Turbidity does not appear at room temperature.

(E) Dehydration of Alcohols

Alcohols undergo dehydration (removal of a water molecule) to form alkenes on heating with a protic acid (conc. H₂SO₄ or H₃PO₄).

Condition for Ethanol (1°): 443 K (Conc. H₂SO₄)
Condition for 2° Alcohol: 440 K (85% H₃PO₄)
Condition for 3° Alcohol: 358 K (20% H₃PO₄)

Reactivity Order: Tertiary > Secondary > Primary

⬇️ STEP–3 will continue from: Dehydration Mechanism (3-step NCERT)

(B) Reduction of Carbonyl Compounds

Alcohols are prepared by the reduction of aldehydes, ketones, carboxylic acids and esters.

Aldehydes: Reduced to Primary (1°) alcohols.
Ketones: Reduced to Secondary (2°) alcohols.
Carboxylic acids: Reduced to Primary alcohols (Requires strong reducing agent like LiAlH₄).

R–CHO + [H] → R–CH₂OH (using NaBH₄ or LiAlH₄)
R–CO–R′ + [H] → R–CH(OH)–R′ (using NaBH₄ or LiAlH₄)
R–COOH + 4[H] → R–CH₂OH + H₂O (using LiAlH₄ only)

Important: NaBH₄ is a mild reducing agent and does not reduce carboxylic acids. LiAlH₄ is used for acids.

(C) From Grignard Reagents

Grignard reagents react with aldehydes and ketones to form an adduct, which on hydrolysis yields alcohols.

HCHO + RMgX → Adduct → Primary Alcohol
RCHO + RMgX → Adduct → Secondary Alcohol
RCOR′ + RMgX → Adduct → Tertiary Alcohol

3.2 Physical Properties of Alcohols

Boiling Point: Alcohols have higher boiling points than hydrocarbons, ethers, and haloalkanes of comparable molecular masses. This is due to the presence of intermolecular hydrogen bonding.
Solubility: Lower alcohols are miscible with water in all proportions due to hydrogen bonding. Solubility decreases with increase in the size of the alkyl group (hydrophobic part).

3.3 Chemical Properties of Alcohols

(A) Acidity of Alcohols

Alcohols react with active metals like sodium to liberate hydrogen gas, indicating their acidic nature.

2R–OH + 2Na → 2R–ONa + H₂↑

Acidity Order: 1° > 2° > 3°

Board Fact: Alcohols are weaker acids than water. This is because the electron-releasing alkyl group (+I effect) increases electron density on oxygen, making the O–H bond less polar.

(B) Esterification

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

R–OH + R′–COOH ⇌ R′–COOR + H₂O

Concentrated H₂SO₄ acts as a protonating agent (catalyst) and dehydrating agent (removes water to shift equilibrium forward).

(C) Oxidation of Alcohols

Oxidation involves the cleavage of O–H and C–H bonds to form a C=O double bond.

Primary Alcohols: Oxidised to aldehydes using PCC (Pyridinium chlorochromate). Strong oxidising agents (Acidified KMnO₄) usually convert them directly to carboxylic acids.
Secondary Alcohols: Oxidised to ketones using Chromic anhydride (CrO₃).
Tertiary Alcohols: Resistant to oxidation. In presence of strong oxidising agents at high temperature, C–C bond cleavage occurs.

(D) Identification of Alcohols – Lucas Test

Used to distinguish between primary, secondary and tertiary alcohols.

Lucas Reagent: Concentrated HCl + Anhydrous ZnCl₂

(E) Dehydration of Alcohols

Alcohols undergo dehydration (removal of a water molecule) to form alkenes on heating with a protic acid (conc. H₂SO₄ or H₃PO₄).

Condition for Ethanol (1°): 443 K (Conc. H₂SO₄)
Condition for 2° Alcohol: 440 K (85% H₃PO₄)
Condition for 3° Alcohol: 358 K (20% H₃PO₄)

Reactivity Order: Tertiary > Secondary > Primary

⬇️ STEP–3 will continue from: Dehydration Mechanism (3-step NCERT)

4. Phenols

Phenols are aromatic compounds in which the hydroxyl (–OH) group is directly attached to an sp² hybridised carbon of a benzene ring.

4.1 Nomenclature of Phenols

The parent compound is named as Phenol.
Position of substituents is indicated by numbers or prefixes ortho (o-), meta (m-) and para (p-).

C₆H₅OH → Phenol
o-CH₃–C₆H₄–OH → o-Cresol (2-Methylphenol)
1,2-Benzenediol → Catechol
1,4-Benzenediol → Hydroquinone

4.2 Methods of Preparation of Phenols

(A) From Haloarenes (Dow’s Process)

C₆H₅Cl + NaOH \xrightarrow{623\,K,\;300\,atm} C₆H₅ONa \xrightarrow{H^+} C₆H₅OH

(B) From Benzene Sulphonic Acid

Benzene \xrightarrow{Oleum} Benzene Sulphonic Acid \xrightarrow{NaOH,\;Fusion} Sodium Phenoxide \xrightarrow{H^+} Phenol

(C) From Diazonium Salts

C₆H₅N₂Cl + H₂O \xrightarrow{Warm} C₆H₅OH + N₂ + HCl

(D) From Cumene (Isopropylbenzene)

Cumene \xrightarrow{O_2} Cumene Hydroperoxide \xrightarrow{H^+} Phenol + Acetone

This is the most important commercial method as it produces acetone as a valuable by-product.

4.3 Physical Properties of Phenols

Phenols are colourless liquids or crystalline solids but turn pink due to slow oxidation in air.
They have higher boiling points than haloarenes due to intermolecular hydrogen bonding.
Phenols are sparingly soluble in water due to the large hydrophobic aryl group.

4.4 Acidic Nature of Phenol

Phenols are weak acids. They turn blue litmus red and react with aqueous NaOH.

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

Why Phenol is more acidic than Alcohol?
The phenoxide ion formed after losing a proton is stabilised by resonance (delocalisation of negative charge), whereas the alkoxide ion in alcohols is not stabilised.

Effect of Substituents:
• Electron Withdrawing Groups (e.g., –NO₂) increase acidity (o/p effect).
• Electron Releasing Groups (e.g., –CH₃) decrease acidity.

4.5 Chemical Reactions of Phenols

(A) Electrophilic Substitution Reactions

The –OH group activates the benzene ring towards electrophilic substitution and directs the incoming group to ortho and para positions.

1. Nitration:

With Dilute HNO₃: Mixture of o- and p-nitrophenol.
With Conc. HNO₃: 2,4,6-Trinitrophenol (Picric Acid).

2. Bromination (Distinction Test):

With Br₂ in CS₂ (Low polarity):
Monobromination occurs. Major product is p-Bromophenol.
With Bromine Water (High polarity):
Polybromination occurs. Forms white precipitate of 2,4,6-Tribromophenol.

(B) Kolbe’s Reaction

Sodium Phenoxide \xrightarrow{CO_2,\;400\,K,\;4–7\,atm} \xrightarrow{H^+} 2-Hydroxybenzoic acid (Salicylic acid)

(C) Reimer–Tiemann Reaction

Phenol + CHCl₃ + Aq. NaOH \xrightarrow{Heat} \xrightarrow{H^+} Salicylaldehyde (2-Hydroxybenzaldehyde)

Intermediate: Dichlorocarbene (:CCl₂) acts as the electrophile.

(D) Reaction with Zinc Dust

Phenol + Zn \xrightarrow{\Delta} Benzene + ZnO

(E) Oxidation

Phenol \xrightarrow{Na_2Cr_2O_7,\;H_2SO_4} Benzoquinone

4.6 Uses of Phenols

Used in the manufacture of Bakelite (polymer).
Used as an antiseptic (in dilute solution) and disinfectant.
Used in the synthesis of drugs like Aspirin and Salol.
Used to manufacture dyes and explosives (Picric acid).

⬇️ STEP–5 will start from: ETHERS (Nomenclature → Preparation → Properties → Reactions)

5. Ethers

Ethers are organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups. Their general formula is R–O–R′.

5.1 Nomenclature of Ethers

Common System: Names of alkyl groups are written in alphabetical order followed by the word ether.
IUPAC System: Ethers are named as alkoxyalkanes. The larger alkyl group is taken as the parent alkane, while the smaller group constitutes the alkoxy prefix.

CH₃–O–CH₃ → Methoxymethane (Dimethyl ether)
CH₃–O–C₂H₅ → Methoxyethane (Ethyl methyl ether)
C₆H₅–O–CH₃ → Methoxybenzene (Anisole)

5.2 Methods of Preparation of Ethers

(A) Dehydration of Alcohols

Alcohols undergo dehydration in the presence of protic acids (H₂SO₄, H₃PO₄). The product depends on the reaction temperature.

2C₂H₅OH \xrightarrow{H_2SO_4,\;413\,K} C₂H₅OC₂H₅ (Ether)

Temperature Criticality:
• At 413 K: Ether is the major product (S_N2 mechanism).
• At 443 K: Alkene is the major product (Elimination).
• This method is suitable only for preparing symmetrical ethers from primary alcohols.

(B) Williamson Synthesis (Important)

R–X + R′–ONa → R–O–R′ + NaX

This is an S_N2 reaction involving the attack of an alkoxide ion on a primary alkyl halide.

5.3 Physical Properties of Ethers

The C–O bonds are polar, giving ethers a net dipole moment.
Boiling Points: Much lower than alcohols (no intermolecular H-bonding) and comparable to alkanes of similar mass.
Solubility: Lower ethers are soluble in water because the oxygen atom can form hydrogen bonds with water molecules. Solubility decreases as the alkyl chain size increases.

5.4 Chemical Reactions of Ethers

(A) Cleavage of C–O Bond by Hydrogen Halides

Ethers are the least reactive functional groups. They are cleaved by strong acids like HI or HBr at high temperatures.

R–O–R + HX \xrightarrow{\Delta} R–X + R–OH

Rules for Mechanism:

Case 1 (Primary/Secondary Groups): Reaction follows S_N2. The halide ion (I^–) attacks the smaller alkyl group due to less steric hindrance.
Example: CH₃–O–C₂H₅ + HI → CH₃I + C₂H₅OH
Case 2 (Tertiary Group): Reaction follows S_N1. The cleavage occurs to form the stable tertiary carbocation.
Example: (CH₃)₃C–O–CH₃ + HI → (CH₃)₃C–I + CH₃OH

(B) Cleavage of Aryl Ethers (Anisole)

C₆H₅–O–CH₃ + HI → C₆H₅OH (Phenol) + CH₃I

Reasoning: The bond between oxygen and the phenyl ring has partial double bond character (resonance) and is stronger. Hence, it does not break. Iodide always attacks the methyl group.

(C) Electrophilic Substitution Reactions

The alkoxy group (–OR) is ortho, para directing and activates the benzene ring.

Halogenation: Anisole reacts with bromine in ethanoic acid (without FeBr₃ catalyst).
Product: p-Bromoanisole (Major) + o-Bromoanisole (Minor).
Friedel-Crafts Alkylation: Anisole + CH₃Cl (anhyd. AlCl₃).
Product: 2-Methoxytoluene + 4-Methoxytoluene (Major).
Nitration: Anisole + Conc. HNO₃ + Conc. H₂SO₄.
Product: 2-Nitroanisole + 4-Nitroanisole (Major).

5.5 Uses of Ethers

Diethyl ether: Used as an industrial solvent and earlier as an anaesthetic.
Used as a medium for preparing Grignard reagents.

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6. Uses of Alcohols

6.1 Uses of Methanol (CH₃OH)

Used as an industrial solvent.
Used in the manufacture of formaldehyde.
Used as a fuel and antifreeze.
Note: Methanol is highly toxic and may cause blindness.

6.2 Uses of Ethanol (C₂H₅OH)

Used as a solvent in medicines, perfumes and paints.
Used as a fuel (power alcohol).
Used in alcoholic beverages.
Denatured Alcohol: Ethanol made unfit for drinking by adding copper sulphate and pyridine.

7. Uses of Phenols

Used as antiseptics and disinfectants.
Used in manufacture of phenolic resins (Bakelite).
Used in manufacture of dyes and explosives (Picric acid).
Used in pharmaceutical preparations.

8. Uses of Ethers

Used as solvents for fats, oils, waxes and resins.
Diethyl ether is used as a general anaesthetic.
Used as reaction medium in Grignard reactions.
Used as starting fluids for engines.

9. NCERT Chapter Summary

Alcohols, phenols and ethers are oxygen-containing organic compounds.
Alcohols show hydrogen bonding and undergo dehydration, oxidation, esterification and substitution reactions.
Phenols are more acidic than alcohols due to resonance stabilisation of phenoxide ion.
Ethers are relatively unreactive but undergo cleavage with hydrogen halides.
Lucas test is used for distinguishing primary, secondary and tertiary alcohols.

⬆️ End of Chapter 7: Alcohols, Phenols and Ethers

10. Multiple Choice Questions (MCQs)

The most acidic among the following is:
(a) Ethanol (b) Phenol (c) Methanol (d) tert-Butanol
Ans: (b) Phenol
Lucas test is used for the distinction of:
(a) Alcohols (b) Phenols (c) Ethers (d) Haloalkanes
Ans: (a) Alcohols
Which reagent converts a primary alcohol into an aldehyde (controlled oxidation)?
(a) Acidified KMnO₄ (b) PCC (c) K₂Cr₂O₇ (d) Heat with Cu
Ans: (b) PCC
Kolbe’s reaction results in the formation of:
(a) Phenol (b) Benzoic acid (c) Salicylic acid (d) Salicylaldehyde
Ans: (c) Salicylic acid
Which ether yields phenol upon cleavage with HI?
(a) Diethyl ether (b) Anisole (c) Dimethyl ether (d) Ethyl methyl ether
Ans: (b) Anisole

11. Very Short Answer Questions (VSA)

Why are phenols more acidic than alcohols?
Ans: Due to the resonance stabilisation of the phenoxide ion, whereas the alkoxide ion in alcohols is destabilised by the +I effect of alkyl groups.
Name the reagent used in Lucas test.
Ans: Concentrated HCl and anhydrous ZnCl₂.
Which class of alcohols is resistant to oxidation?
Ans: Tertiary alcohols (3°).
Name the substances added to denature ethanol.
Ans: Copper sulphate (for colour) and Pyridine (for foul smell).
Which bond in anisole is difficult to cleave by HI?
Ans: The Oxygen–Phenyl ($O-C_{aryl}$) bond is difficult to cleave due to partial double bond character.

12. Short Answer Questions (SA)

Explain why the boiling points of alcohols are higher than those of corresponding ethers.
Why does tertiary alcohol react immediately with Lucas reagent while primary alcohol does not?
Write the equation for Williamson ether synthesis. What is the limitation regarding the alkyl halide used?
Explain the mechanism of acid-catalysed dehydration of ethanol to ethene.
Write the chemical equations for Reimer-Tiemann reaction and Kolbe’s reaction.

13. Long Answer Questions (LA)

(a) Arrange the following in increasing order of acidity: Ethanol, Phenol, 4-Nitrophenol.
(b) Explain the mechanism of dehydration of a secondary alcohol to alkene.
Describe the preparation of Phenol from:
(i) Cumene
(ii) Chlorobenzene (Dow's Process)
(iii) Benzene diazonium chloride

14. Self-Test (Click to Reveal Answer)

Q1. Why does phenol not give protonation reaction easily like alcohols?

Ans: Because the lone pair on oxygen in phenol is involved in resonance with the benzene ring, making it less available for protonation.

Q2. Which alcohol dehydrates fastest: 1°, 2° or 3°?

Ans: Tertiary (3°) alcohol dehydrates fastest due to the formation of the most stable tertiary carbocation.

Q3. Why do ethers have much lower boiling points than isomeric alcohols?

Ans: Because ethers cannot form intermolecular hydrogen bonds with each other, whereas alcohols can.

Q4. What is the role of conc. H₂SO₄ in the esterification reaction?

Ans: It acts as a catalyst and a dehydrating agent (removes water) to shift the equilibrium in the forward direction.

Q5. Identify the electrophile in the Reimer–Tiemann reaction.

Ans: Dichlorocarbene ($:CCl_2$).

🎯 Chapter 7 Fully Completed & Locked

15. Previous Years Questions (PYQs: 2016–2025)

15.1 Very Short Answer PYQs (1 Mark)

(CBSE 2016) Why is phenol more acidic than ethanol?
Ans: The phenoxide ion formed from phenol is stabilised by resonance, whereas the ethoxide ion from ethanol is destabilised by the +I effect of the ethyl group.
(RBSE 2017) Name the reagent used to distinguish primary, secondary and tertiary alcohols.
Ans: Lucas reagent (Concentrated HCl + Anhydrous ZnCl₂).
(CBSE 2018) Write the IUPAC name of CH₃–O–CH₂CH₃.
Ans: Methoxyethane.
(CBSE 2020) Which class of alcohols does not undergo oxidation to form carbonyl compounds easily?
Ans: Tertiary (3°) alcohols.
(RBSE 2022) Name the electrophile formed in the Reimer–Tiemann reaction.
Ans: Dichlorocarbene (:CCl₂).

15.2 Short Answer PYQs (2–3 Marks)

(CBSE 2016) Write the chemical equation for the dehydration of ethanol to ethene.
Ans:
CH₃CH₂OH $\xrightarrow{Conc.\;H_2SO_4,\;443\,K}$ CH₂=CH₂ + H₂O
(RBSE 2017) Give the order of reactivity of alcohols towards Lucas reagent. Justify your answer.
Ans:
Order: 3° > 2° > 1°
Justification: The reaction proceeds via a carbocation intermediate. Since the stability of carbocations is 3° > 2° > 1°, tertiary alcohols react fastest (immediately).
(CBSE 2019) Why does anisole not undergo cleavage at the aryl–oxygen bond with HI?
Ans: 1. Resonance: The oxygen lone pair participates in resonance with the benzene ring, giving the aryl–oxygen bond a partial double bond character.
2. Hybridisation: The carbon of the aromatic ring attached to oxygen is sp² hybridised, making the bond shorter and stronger than the alkyl–oxygen bond.
(CBSE 2021) Write the equation for Williamson synthesis. Mention one limitation.
Ans:
R–X + R′–ONa → R–O–R′ + NaX
Limitation: The alkyl halide used must be primary (1°). If secondary or tertiary halides are used, elimination occurs to form alkenes instead of ethers.

15.3 Long Answer PYQs (4–5 Marks)

(CBSE 2016) Describe the Lucas test for distinguishing between primary, secondary and tertiary alcohols. Mention the observations.
(CBSE 2018) Explain the mechanism of acid-catalysed dehydration of ethanol to yield ethene. (Draw all 3 steps).
(RBSE 2019) Describe Kolbe’s reaction with chemical equation, reaction conditions and product formed.
(CBSE 2020) Discuss the preparation of ethanol by hydration of ethene. Give its physical properties and two uses.
(CBSE 2024) Explain the cleavage of ethers with HI. Discuss the products formed when (i) Anisole and (ii) tert-Butyl methyl ether react with HI.

15.4 Competency-Based PYQs (New Pattern)

(CBSE 2023)
An alcohol 'A' reacts immediately with Lucas reagent to give turbidity. On heating with conc. H₂SO₄, 'A' gives a major alkene 'B' which on ozonolysis gives propanone and methanal.
(a) Identify A and B.
(b) Write the dehydration reaction.
Ans:
A: 2-Methylpropan-2-ol (tert-Butyl alcohol)
B: 2-Methylpropene (Isobutylene)
(CBSE 2025 Sample)
Phenol reacts with bromine water to give a white precipitate.
(a) Write the chemical equation.
(b) Why does monobromination not occur in this case?
Ans:
Phenol + 3Br₂ (aq) → 2,4,6-Tribromophenol (White ppt) + 3HBr
Reason: In aqueous medium, phenol ionises to form phenoxide ion, which highly activates the benzene ring, leading to trisubstitution. For monobromination, a non-polar solvent like CS₂ is required.

🎯 Chapter 7 Fully Completed & Locked

Class 12 Chemistry Test Paper - Chapter 7

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