Alcohols, Phenols, and Ethers are essential organic compounds in Chemistry with vast industrial, pharmaceutical, and biological importance. These compounds contain oxygen atoms and belong to the family of oxygen-containing functional groups. They differ in structure, bonding, and reactivity, but all play a crucial role in chemical synthesis and everyday life.
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Alcohols: Compounds with a hydroxyl group (-OH) attached to a saturated carbon atom.
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Phenols: Compounds with a hydroxyl group attached to an aromatic ring.
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Ethers: Compounds with an oxygen atom bonded to two alkyl or aryl groups (R–O–R’).
In this chapter, we will discuss their classification, nomenclature, physical and chemical properties, preparation methods, reactions, and applications in detail.
Alcohols, Phenols, and Ethers Class 12 Chemistry Notes | Detailed Explanation with Questions
Alcohols
Definition
Alcohols are organic compounds where a hydroxyl group (-OH) is attached to a saturated carbon (sp³ hybridized) atom. The general formula for alcohols is CₙH₂ₙ₊₁OH.
Classification of Alcohols
Based on the number of hydroxyl groups and the type of carbon attached to –OH:
1. Based on the Number of –OH Groups
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Monohydric Alcohols: Contain one –OH group. Example: Ethanol (C₂H₅OH)
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Dihydric Alcohols: Contain two –OH groups. Example: Ethylene glycol (HO–CH₂–CH₂–OH)
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Trihydric Alcohols: Contain three –OH groups. Example: Glycerol (HO–CH₂–CHOH–CH₂–OH)
2. Based on the Type of Carbon Atom
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Primary Alcohol (1°): –OH group attached to a carbon with one alkyl group. Example: CH₃CH₂OH (Ethanol)
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Secondary Alcohol (2°): –OH group attached to a carbon with two alkyl groups. Example: CH₃CHOHCH₃ (Isopropanol)
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Tertiary Alcohol (3°): –OH group attached to a carbon with three alkyl groups. Example: (CH₃)₃COH (Tert-butanol)
Nomenclature of Alcohols
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Common Name: Alkyl group + alcohol. Example: Ethyl alcohol (C₂H₅OH).
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IUPAC Name:
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Longest chain containing –OH group is selected.
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Numbering starts from the carbon nearest to –OH.
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Suffix -ol is added.
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Example: CH₃CH₂OH → Ethanol
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Methods of Preparation of Alcohols
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From Alkenes
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Hydration of alkenes in presence of H₂SO₄ gives alcohols.
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Example: CH₂=CH₂ + H₂O → CH₃CH₂OH (Ethanol)
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From Alkyl Halides
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Alkyl halide reacts with aqueous KOH to form alcohol.
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Example: CH₃CH₂Cl + KOH → CH₃CH₂OH + KCl
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From Carbonyl Compounds
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Reduction of aldehydes gives primary alcohols, while ketones give secondary alcohols.
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Fermentation
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C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (Using enzymes)
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Physical Properties of Alcohols
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Boiling Point: High due to hydrogen bonding.
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Solubility: Lower alcohols are soluble in water; solubility decreases with chain length.
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Odour: Pleasant smell; higher alcohols are oily.
Chemical Properties of Alcohols
1. Reactions Involving the –OH Group
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Reaction with Metals:
2CH3CH2OH+2Na→2CH3CH2ONa+H2↑2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂↑
(Formation of sodium ethoxide) -
Esterification:
Alcohol + Carboxylic acid → Ester + Water
C2H5OH+CH3COOH→CH3COOC2H5+H2OC₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O
2. Oxidation Reactions
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Primary alcohol → Aldehyde → Carboxylic acid
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Secondary alcohol → Ketone
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Tertiary alcohol → No reaction under mild conditions
3. Dehydration
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In presence of conc. H₂SO₄, alcohols form alkenes.
CH3CH2OH→CH2=CH2+H2OCH₃CH₂OH → CH₂=CH₂ + H₂O
Phenols
Definition
Phenols are aromatic compounds containing one or more hydroxyl groups directly attached to a benzene ring.
Classification of Phenols
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Monohydric Phenols: One –OH group. Example: Phenol (C₆H₅OH)
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Dihydric Phenols: Two –OH groups. Example: Catechol (1,2-position), Resorcinol (1,3-position).
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Trihydric Phenols: Three –OH groups. Example: Phloroglucinol (1,3,5-positions).
Nomenclature
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IUPAC: Phenol as parent name. Substituents are indicated by numbers or ortho (1,2), meta (1,3), para (1,4) positions.
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Example: o-Cresol, m-Cresol, p-Cresol.
Methods of Preparation of Phenols
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From Haloarenes:
C6H5Cl+NaOH→C6H5OH+NaClC₆H₅Cl + NaOH → C₆H₅OH + NaCl (Dow’s process) -
From Benzene Sulphonic Acid:
C6H5SO3H+NaOH→C6H5ONa→C6H5OHC₆H₅SO₃H + NaOH → C₆H₅ONa → C₆H₅OH -
From Diazonium Salts:
C6H5N2+Cl−+H2O→C6H5OH+N2+HClC₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl
Physical Properties of Phenols
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Solid at room temperature.
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Have characteristic phenolic odour.
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Moderately soluble in water due to H-bonding.
Chemical Properties of Phenols
1. Acidic Nature
Phenols are acidic due to the resonance stabilization of phenoxide ion.
C6H5OH→C6H5O−+H+C₆H₅OH → C₆H₅O⁻ + H⁺
2. Reactions with Bases
Phenol reacts with NaOH to form sodium phenoxide.
3. Electrophilic Substitution Reactions
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Nitration: Phenol + HNO₃ → o-nitrophenol + p-nitrophenol
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Halogenation: Phenol + Cl₂ → 2,4,6-trichlorophenol
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Friedel-Crafts reaction: Alkylation and acylation at ortho and para positions.
Ethers
Definition
Ethers are organic compounds with the general formula R–O–R’, where R and R’ can be alkyl or aryl groups.
Classification
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Simple or Symmetrical Ethers: Both R groups are the same. Example: CH₃–O–CH₃ (Dimethyl ether)
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Mixed or Unsymmetrical Ethers: Different R groups. Example: CH₃–O–C₂H₅ (Ethyl methyl ether)
Nomenclature
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Common Name: Alkyl groups in alphabetical order + ether. Example: Ethyl methyl ether.
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IUPAC Name: Alkoxy prefix + alkane. Example: Methoxyethane (CH₃–O–C₂H₅).
Methods of Preparation
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Williamson’s Synthesis
Alkyl halide + Sodium alkoxide → Ether + NaX
C2H5ONa+CH3I→C2H5OCH3+NaIC₂H₅ONa + CH₃I → C₂H₅OCH₃ + NaI -
Dehydration of Alcohols
2C2H5OH→C2H5OC2H5+H2O2C₂H₅OH → C₂H₅OC₂H₅ + H₂O (at 413 K with conc. H₂SO₄)
Physical Properties of Ethers
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Volatile liquids with pleasant smell.
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Insoluble in water but soluble in organic solvents.
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Lower boiling points than alcohols due to absence of hydrogen bonding.
Chemical Properties of Ethers
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Cleavage by Acids:
C2H5OC2H5+HI→C2H5I+C2H5OHC₂H₅OC₂H₅ + HI → C₂H₅I + C₂H₅OH -
Autoxidation:
Ethers form peroxides on exposure to air.
Uses of Alcohols, Phenols, and Ethers
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Alcohols: Used as solvents, fuels (ethanol), disinfectants (isopropanol).
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Phenols: Used in antiseptics (Dettol), resins, dyes, and plastics.
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Ethers: Used as solvents and anesthetics (diethyl ether).
Objective Questions-Alcohols, Phenols, and Ethers
Q1. Which functional group is present in alcohols?
a) –CHO
b) –COOH
c) –OH
d) –COOR
Answer:
The functional group present in alcohols is –OH (Hydroxyl group).
Q2. Phenol is more acidic than alcohol because of
a) Electronegativity
b) Resonance stabilization of phenoxide ion
c) Inductive effect
d) Hyperconjugation
Answer:
Phenol is more acidic due to resonance stabilization of the phenoxide ion, making it more stable after losing a proton.
Q3. Williamson’s synthesis is used for preparation of
a) Alcohols
b) Phenols
c) Ethers
d) Aldehydes
Answer:
Ethers are prepared by Williamson’s synthesis.
Q4. What is the IUPAC name of CH₃CH₂OH?
a) Ethanol
b) Methanol
c) Ethanoic acid
d) Propanol
Answer:
The IUPAC name of CH₃CH₂OH is Ethanol.
Q5. Phenol reacts with bromine water to give
a) Nitrobenzene
b) 2,4,6-tribromophenol
c) Benzene
d) Anisole
Answer:
Phenol reacts with bromine water to give 2,4,6-tribromophenol (white precipitate).
Q6. Diethyl ether on reaction with HI gives
a) Ethanol
b) Ethyl iodide and ethanol
c) Phenol
d) Acetaldehyde
Answer:
Diethyl ether reacts with HI to give ethyl iodide and ethanol.
Q7. The general formula of alcohols is
a) CₙH₂ₙO₂
b) CₙH₂ₙ₊₁OH
c) CₙH₂ₙ
d) CₙH₂ₙ₊₂
Answer:
The general formula is CₙH₂ₙ₊₁OH.
Q8. Which alcohol is known as wood spirit?
a) Ethanol
b) Methanol
c) Glycerol
d) Propanol
Answer:
Methanol is known as wood spirit.
Q9. Phenols on nitration give
a) Nitrobenzene
b) o- and p-nitrophenol
c) Benzaldehyde
d) Benzoic acid
Answer:
Phenols on nitration give o- and p-nitrophenol.
Q10. The process of conversion of sugar to alcohol is called
a) Oxidation
b) Fermentation
c) Hydration
d) Nitration
Answer:
The conversion of sugar to alcohol is called fermentation.
Q11. Anisole is
a) C₆H₅OH
b) C₆H₅OCH₃
c) CH₃OH
d) CH₃CH₂OH
Answer:
Anisole is C₆H₅OCH₃.
Q12. Which one is a primary alcohol?
a) CH₃CH₂OH
b) CH₃CHOHCH₃
c) (CH₃)₃COH
d) C₆H₅OH
Answer:
CH₃CH₂OH (Ethanol) is a primary alcohol.
Q13. Phenols react with NaOH to form
a) Alkoxide
b) Phenoxide
c) Aldehyde
d) Ketone
Answer:
Phenols react with NaOH to form phenoxide.
Q14. Boiling point of alcohols is higher than alkanes due to
a) Hydrogen bonding
b) Ionic bonding
c) Vander Waals forces
d) Weak dipole
Answer:
The boiling point is higher due to hydrogen bonding.
Q15. Which of the following is used as antiseptic?
a) Phenol
b) Methanol
c) Ethanol
d) Ether
Answer:
Phenol is used as an antiseptic.
Q16. Oxidation of primary alcohol gives
a) Ketone
b) Aldehyde
c) Ether
d) Alkene
Answer:
Oxidation of primary alcohol gives aldehyde (and further oxidation gives acid).
Q17. The IUPAC name of isopropanol is
a) Propan-2-ol
b) Propan-1-ol
c) Methanol
d) Butanol
Answer:
The IUPAC name is Propan-2-ol.
Q18. Williamson’s synthesis involves
a) Alkene hydration
b) Alkyl halide and sodium alkoxide reaction
c) Nitration of benzene
d) Hydrogenation of alkyne
Answer:
It involves alkyl halide and sodium alkoxide reaction.
Q19. Which of the following is trihydric alcohol?
a) Glycerol
b) Ethanol
c) Methanol
d) Phenol
Answer:
Glycerol is a trihydric alcohol.
Q20. Which of the following is a mixed ether?
a) C₂H₅OC₂H₅
b) CH₃OCH₃
c) C₂H₅OCH₃
d) C₆H₅OCH₃
Answer:
C₂H₅OCH₃ (Ethyl methyl ether) is a mixed ether.
Short Answer Questions
Q1. Explain why phenol is more acidic than alcohol.
Answer:
Phenol is more acidic because the phenoxide ion formed after losing a proton is stabilized by resonance. In alcohols, the alkoxide ion does not have such resonance stabilization, making alcohols weaker acids.
Q2. Write two methods for the preparation of ethers.
Answer:
Ethers can be prepared by:
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Williamson’s Synthesis: Alkyl halide reacts with sodium alkoxide to form ether.
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Dehydration of Alcohols: Two molecules of alcohol react in the presence of conc. H₂SO₄ at 413 K to form ether.
Q3. Differentiate between primary, secondary, and tertiary alcohols with examples.
Answer:
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Primary Alcohol: –OH attached to carbon with one alkyl group. Example: Ethanol.
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Secondary Alcohol: –OH attached to carbon with two alkyl groups. Example: Isopropanol.
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Tertiary Alcohol: –OH attached to carbon with three alkyl groups. Example: Tert-butanol.
Q4. Write the reaction of phenol with bromine water.
Answer:
Phenol reacts with bromine water to form 2,4,6-tribromophenol as a white precipitate:
C6H5OH+3Br2→C6H2Br3OH+3HBrC₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr
Q5. What is fermentation? Give one example.
Answer:
Fermentation is a biochemical process in which sugars like glucose are converted into alcohol using enzymes. Example:
C6H12O6→2C2H5OH+2CO2C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Long Answer Questions
Q1. Explain the physical properties of alcohols and phenols.
Answer:
Alcohols and phenols have high boiling points due to intermolecular hydrogen bonding. Lower members are soluble in water, but solubility decreases with chain length. Phenols are moderately soluble and have characteristic antiseptic odour.
Q2. Discuss the electrophilic substitution reactions of phenol.
Answer:
Phenols undergo nitration, halogenation, sulphonation, and Friedel–Crafts reactions at the ortho and para positions due to the activating effect of the –OH group. For example, nitration gives o- and p-nitrophenol.
Q3. Describe the preparation of alcohols from alkenes.
Answer:
Alkenes react with water in the presence of concentrated H₂SO₄ to form alcohols (hydration). Example: Ethene + Water → Ethanol.
Q4. Explain Williamson’s synthesis of ethers with mechanism.
Answer:
In Williamson’s synthesis, an alkyl halide reacts with sodium alkoxide to form an ether via nucleophilic substitution (SN2 mechanism). Example: CH₃I + C₂H₅ONa → CH₃OC₂H₅ + NaI.
Q5. Give a detailed classification of alcohols, phenols, and ethers with examples.
Answer:
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Alcohols: Monohydric (Ethanol), Dihydric (Ethylene glycol), Trihydric (Glycerol).
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Phenols: Monohydric (Phenol), Dihydric (Catechol), Trihydric (Phloroglucinol).
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Ethers: Simple (Diethyl ether), Mixed (Ethyl methyl ether).
Conclusion
The chapter on Alcohols, Phenols, and Ethers forms the foundation for understanding oxygen-containing organic compounds. From industrial applications to laboratory synthesis, these compounds play a vital role in Chemistry and everyday life. A thorough understanding of their preparation methods, properties, and reactions is essential for Class 12 board exams and competitive examinations like JEE and NEET.