Alcohols Phenols and Ethers — Previous Year Questions (Class 12 Chemistry)
Alcohols, phenols, and ethers are important organic compounds with diverse applications. This chapter covers their structure, synthesis, properties, and re
TL;DR: Alcohols, phenols, and ethers are important organic compounds with diverse applications. This chapter covers their structure, synthesis, properties, a…
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Alcohols, phenols, and ethers are important organic compounds with diverse applications. This chapter covers their structure, synthesis, properties, and re
Alcohols Phenols and Ethers — Previous Year Questions with Solutions
Q (2023, 3 marks): What is the difference between alcohols and phenols? Explain with structural formulae.
Answer: Difference Between Alcohols and Phenols:
Alcohols:
1. Structure: R-OH (hydroxyl group attached to alkyl group)
2. General formula: CnH(2n+2)O or CnH(2n)O
3. Examples:
- Methanol: CH3-OH
- Ethanol: CH3-CH2-OH
- 2-Propanol: (CH3)2CH-OH
4. Properties:
- Neutral compounds
- Soluble in water
- Acidic but weaker than water
5. Reaction:
- Dehydration gives alkenes
- Oxidation gives aldehydes/ketones
Phenols:
1. Structure: Ar-OH (hydroxyl group attached to benzene ring)
2. General formula: CnH(2n-6)O
3. Examples:
- Phenol (C6H5-OH)
- o-Cresol, m-Cresol, p-Cresol
4. Properties:
- Weakly acidic (more acidic than alcohols)
- Slightly soluble in water
- Form salts with alkali
5. Reaction:
- Substitution reactions on benzene ring
- Oxidation gives quinones
Key Differences:
- Alcohols: -OH on alkyl carbon
- Phenols: -OH on aromatic carbon
- Phenols more acidic than alcohols
- Different reactivity patterns
Q (2022, 4 marks): Explain the method of preparation of alcohols from alkenes and carbonyl compounds.
Answer: Preparation of Alcohols:
1. From Alkenes (Hydration):
- Addition of water across double bond
- Reaction: C=C + H2O -> C-C (with OH groups)
- Mechanism: Markovnikov's rule
- Reaction: R-CH=CH2 + H2O -> R-CH(OH)-CH3
- Catalyst: H2SO4 (concentrated)
- Conditions: Heat
- Example: Ethene + H2O -> Ethanol
CH2=CH2 + H2O -> CH3-CH2-OH
2. From Carbonyl Compounds (Reduction):
a) Aldehydes to Primary Alcohols:
- Aldehyde + reducing agent -> Primary alcohol
- Reducing agents: NaBH4, LiAlH4
- Example: HCHO + NaBH4 -> CH3-OH (methanol)
b) Ketones to Secondary Alcohols:
- Ketone + reducing agent -> Secondary alcohol
- Example: CH3-CO-CH3 + NaBH4 -> (CH3)2CH-OH (isopropanol)
3. Fermentation:
- Glucose + yeast -> Ethanol + CO2
- C6H12O6 -> 2 C2H5OH + 2 CO2
- Conditions: Anaerobic, 25-37 degrees C
- Produces ethanol commercially
4. From Grignard Reagents:
- R-Mg-X + carbonyl -> Alcohol
- Followed by hydrolysis
Q (2021, 3 marks): What are ethers? Explain their preparation from alcohols.
Answer: Ethers:
Definition - Organic compounds with formula R-O-R' where two alkyl/aryl groups connected by oxygen.
Characteristics:
1. General formula: CnH(2n+2)O or CnH(2n)O
2. Functional group: C-O-C (ether linkage)
3. Examples:
- Dimethyl ether: CH3-O-CH3
- Diethyl ether: CH3-CH2-O-CH2-CH3
- Methyl ethyl ether: CH3-O-CH2-CH3
Preparation of Ethers from Alcohols:
1. Williamson Synthesis (Main method):
- Reaction: R-O-Na+ + R'-X -> R-O-R' + Na+X-
- Alkoxide (from alcohol) + alkyl halide -> Ether
- Mechanism: SN2 nucleophilic substitution
- Example: CH3-O-Na+ + CH3-I -> CH3-O-CH3 + NaI
- Conditions: Organic solvent (DMF, DMSO)
- Better for primary halides
2. Intermolecular Dehydration of Alcohols:
- 2 R-OH -> R-O-R + H2O
- Catalyst: H2SO4 (concentrated)
- Conditions: Heat (140 degrees C)
- Example: 2 CH3-CH2-OH -> CH3-CH2-O-CH2-CH3 + H2O
- Good for primary alcohols
- Produces symmetrical ethers
3. Reaction of Alcohols with Alkyl Halides:
- R-OH + R'-X -> R-O-R' + HX
- Less efficient than Williamson
Q (2023, 5 marks): Explain the reactions of alcohols including oxidation, dehydration, and esterification.
Answer: Reactions of Alcohols:
1. Oxidation:
a) Primary Alcohols:
- R-CH2-OH -[oxidation]-> R-CHO (aldehyde) -[further oxidation]-> R-COOH (carboxylic acid)
- Oxidizing agents: KMnO4, K2Cr2O7/H2SO4
- Example: Ethanol -> Acetaldehyde -> Acetic acid
- Condition: Mild oxidation stops at aldehyde
b) Secondary Alcohols:
- R2CH-OH -> R2C=O (ketone)
- Oxidation stops at ketone
- Example: Isopropanol -> Acetone
c) Tertiary Alcohols:
- Cannot be oxidized easily
- No hydrogen on carbon bearing OH
2. Dehydration (Elimination):
- R2CH-OH -> R2C=C + H2O
- Catalyst: H2SO4 (concentrated)
- Conditions: Heat (170-180 degrees C)
- Mechanism: E1 elimination
- Example: Ethanol -> Ethene + H2O
CH3-CH2-OH -> CH2=CH2 + H2O
- Follows Zaitsev's rule
3. Esterification:
- R-OH + R'-COOH <-> R'-COO-R + H2O
- Acid catalyst: H2SO4 or HCl
- Conditions: Heat
- Reversible reaction
- Example: Ethanol + Acetic acid -> Ethyl acetate + Water
CH3-COOH + CH3-CH2-OH <-> CH3-COO-CH2-CH3 + H2O
- Fischer esterification
- Increases with stronger acid, heat, and removal of water
4. Substitution:
- R-OH + HX -> R-X + H2O
- HX: HCl, HBr, HI
- Mechanism: SN1 (tertiary), SN2 (primary)
- Example: Isobutanol + HCl -> isobutyl chloride + water
Q (2022, 3 marks): What are the uses of alcohols and phenols in industry and medicine?
Answer: Uses of Alcohols:
1. Ethanol:
- Beverage alcohol
- Industrial solvent
- Fuel (biofuel)
- Synthesis of acetaldehyde, acetic acid
- Disinfectant
- Preservative
2. Methanol:
- Antifreeze in cars
- Solvent for many organic compounds
- Formaldehyde production
- Fuel for some vehicles
3. Glycerol:
- Sweetener in food
- Cosmetics and moisturizers
- Lubricant
- Medicine (laxative)
4. Propylene Glycol:
- Food additive
- Cosmetics base
- Antifreeze
Uses of Phenols:
1. Phenol:
- Disinfectant (antiseptic)
- Production of phenol-formaldehyde resins (Bakelite)
- Dyes and pigments
- Explosives (picric acid)
- Production of caprolactam (nylon)
2. Cresols:
- Disinfectants
- Deodorants
3. Thymo and Salol:
- Antiseptic properties
- Used in pharmaceutical preparations
Medicinal Uses:
1. Antiseptic and Disinfectant:
- Phenol sterilizes surgical instruments
- Alcohol hand sanitizers
2. Anesthetic:
- Local anesthetic properties
3. Anti-inflammatory:
- Salicylic acid derivatives
4. Treatment:
- Throat lozenges contain phenol
- Antiseptic ointments
Q (2021, 3 marks): Explain the relative acidity of alcohols, phenols, and water. Give reasons.
Answer: Relative Acidity:
Order: Phenol > Water > Alcohol
Acid Strength Comparison:
1. Phenol:
- pKa ≈ 10
- Weakly acidic
- Forms phenoxide ion (C6H5-O-)
- More acidic than water and alcohols
2. Water:
- pKa ≈ 15.7
- Amphoteric
- Less acidic than phenol
- More acidic than alcohols
3. Alcohols:
- pKa ≈ 16-17
- Weakly acidic
- Form alkoxide ion (R-O-)
- Least acidic among the three
Reasons for Acidity Differences:
1. Stabilization of Conjugate Base:
- Phenoxide ion more stable than alkoxide
- Negative charge on oxygen
- Benzene ring provides resonance stabilization
- Resonance structures delocalize negative charge
- Alkoxide lacks this stabilization
2. Electron Density on Oxygen:
- Benzene ring (aromatic) withdraws electrons
- Makes OH hydrogen more acidic
- Alkyl groups (electron donating) increase electron density
- Makes OH less acidic
3. Resonance Effect:
- Phenoxide has 4 resonance structures
- Distributes negative charge
- Stabilizes the ion
- Alkoxide has no resonance stabilization
Consequence:
- Phenols react with NaOH (strong base)
- Alcohols and water don't (need stronger base)
- Phenol + NaOH -> Sodium phenoxide + Water
Frequently Asked Questions
What is the difference between primary, secondary, and tertiary alcohols?
Primary alcohols have the OH group on a carbon with one other carbon atom; secondary alcohols have OH on a carbon with two other carbon atoms; tertiary alcohols have OH on a carbon with three other carbon atoms attached.
Why is phenol more acidic than water?
Phenol is more acidic than water because the benzene ring withdraws electron density from the oxygen, making the hydrogen more acidic. Additionally, the resulting phenoxide ion is stabilized by resonance.
More Class 12 Chemistry PYQs
- Electrochemistry
- Solutions
- Chemical Kinetics
- Coordination Compounds
- Haloalkanes and Haloarenes
- Aldehydes Ketones and Carboxylic Acids
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