class 12 notes

Class 12 Chemistry Notes Chapter 11 (Alcohols, Phenols and Ethers) – Examplar Problems Book

Philoid

Philoid

— 9 min read

Examplar Problems
Detailed Notes with MCQs of Chapter 11: Alcohols, Phenols, and Ethers. This is a crucial chapter for your exams, covering compounds containing C-O single bonds. Pay close attention to the structures, preparations, and distinct reactions of each class.

Alcohols, Phenols and Ethers: Detailed Notes

1. Introduction:

  • Alcohols: Organic compounds containing one or more hydroxyl (-OH) groups directly attached to a saturated carbon atom (sp³ hybridized). General formula: R-OH.
  • Phenols: Organic compounds containing one or more hydroxyl (-OH) groups directly attached to a benzene ring (sp² hybridized carbon of an aromatic system). General formula: Ar-OH.
  • Ethers: Organic compounds where an oxygen atom is bonded to two alkyl or aryl groups (or one of each). General formula: R-O-R', Ar-O-Ar', or R-O-Ar.

2. Alcohols:

  • Classification:

    • Based on the number of -OH groups:
      • Monohydric: One -OH group (e.g., Ethanol, CH₃CH₂OH)
      • Dihydric: Two -OH groups (e.g., Ethane-1,2-diol or Ethylene glycol, HOCH₂CH₂OH)
      • Trihydric: Three -OH groups (e.g., Propane-1,2,3-triol or Glycerol, HOCH₂(CHOH)CH₂OH)
    • Based on the nature of the carbon atom bearing the -OH group:
      • Primary (1°): -OH group attached to a primary carbon (e.g., Ethanol, CH₃CH₂OH)
      • Secondary (2°): -OH group attached to a secondary carbon (e.g., Propan-2-ol, CH₃CH(OH)CH₃)
      • Tertiary (3°): -OH group attached to a tertiary carbon (e.g., 2-Methylpropan-2-ol, (CH₃)₃COH)
    • Allylic Alcohols: -OH group attached to an sp³ carbon next to a C=C double bond (e.g., CH₂=CH-CH₂OH).
    • Benzylic Alcohols: -OH group attached to an sp³ carbon next to an aromatic ring (e.g., C₆H₅CH₂OH).

  • Nomenclature (IUPAC):

    • Replace the terminal '-e' of the parent alkane with '-ol'.
    • Number the parent chain to give the carbon bearing the -OH group the lowest possible number.
    • Indicate the position of the -OH group by number.
    • For polyhydric alcohols, use suffixes like -diol, -triol, etc., retaining the '-e' of the alkane name (e.g., Ethane-1,2-diol).

  • Methods of Preparation:

    • From Alkenes:
      • Acid-catalysed hydration: Alkene + H₂O (in presence of acid like H₂SO₄) → Alcohol (Markovnikov's rule applies). Mechanism involves carbocation formation.
      • Hydroboration-oxidation: Alkene + Diborane (B₂H₆ or BH₃·THF) followed by H₂O₂/OH⁻ → Alcohol (Anti-Markovnikov addition, syn-addition).
    • From Carbonyl Compounds:
      • Reduction of Aldehydes and Ketones:
        • Aldehyde + Reducing agent (LiAlH₄, NaBH₄, or H₂/Ni, Pt, Pd) → Primary Alcohol
        • Ketone + Reducing agent (LiAlH₄, NaBH₄, or H₂/Ni, Pt, Pd) → Secondary Alcohol
      • Reduction of Carboxylic Acids and Esters:
        • Carboxylic Acid + LiAlH₄ (strong reducing agent) → Primary Alcohol
        • Ester + LiAlH₄ or Catalytic Hydrogenation → Primary Alcohol(s)
    • From Grignard Reagents (RMgX): (Very Important)
      • Formaldehyde (HCHO) + RMgX → (Addition product) --H₃O⁺--> Primary Alcohol
      • Other Aldehydes (R'CHO) + RMgX → (Addition product) --H₃O⁺--> Secondary Alcohol
      • Ketones (R'COR'') + RMgX → (Addition product) --H₃O⁺--> Tertiary Alcohol
      • Esters (R'COOR'') + RMgX (excess) → Tertiary Alcohol (with two identical R groups from RMgX)

  • Physical Properties:

    • Boiling Points: Higher than corresponding hydrocarbons, haloalkanes, and ethers of comparable molecular mass due to intermolecular hydrogen bonding. Boiling point increases with increasing carbon chain length and decreases with branching (less surface area). Order: 1° > 2° > 3° (for isomers, due to packing efficiency and H-bonding accessibility).
    • Solubility: Lower molecular mass alcohols are soluble in water due to their ability to form hydrogen bonds with water molecules. Solubility decreases as the size of the hydrophobic alkyl group increases.

  • Chemical Reactions:

    • Reactions involving cleavage of O-H bond (Acidity):
      • Alcohols act as weak acids (Bronsted acids). Acidity order: H₂O > 1° Alcohol > 2° Alcohol > 3° Alcohol (due to +I effect of alkyl groups destabilizing the alkoxide ion).
      • Reaction with active metals (Na, K, Al): 2R-OH + 2Na → 2R-ONa (Sodium alkoxide) + H₂
      • Esterification: Alcohol + Carboxylic Acid (in presence of conc. H₂SO₄) ⇌ Ester + H₂O (Fischer Esterification - reversible).
      • Alcohol + Acid Chloride/Anhydride (in presence of base like pyridine) → Ester + HCl/Carboxylic Acid (Irreversible, better yield).
    • Reactions involving cleavage of C-O bond:
      • Reaction with Hydrogen Halides (HX): R-OH + HX → R-X + H₂O. Reactivity order of HX: HI > HBr > HCl. Reactivity order of alcohols: 3° > 2° > 1° (follows carbocation stability).
      • Lucas Test: Distinguishes 1°, 2°, 3° alcohols using Lucas Reagent (anhydrous ZnCl₂ + conc. HCl).
        • 3° Alcohol: Immediate turbidity.
        • 2° Alcohol: Turbidity after 5-10 minutes.
        • 1° Alcohol: No turbidity at room temperature (requires heating).
      • Reaction with Phosphorus Halides:
        • 3R-OH + PCl₃ → 3R-Cl + H₃PO₃
        • R-OH + PCl₅ → R-Cl + POCl₃ + HCl
      • Reaction with Thionyl Chloride (SOCl₂): R-OH + SOCl₂ --Pyridine--> R-Cl + SO₂↑ + HCl↑ (Darzen's process - preferred method for alkyl chlorides as byproducts are gases).
      • Dehydration: Removal of water molecule.
        • Intramolecular (high temp, conc. H₂SO₄ or Al₂O₃): Alcohol → Alkene (Saytzeff's rule applies - more substituted alkene is major product). Ease of dehydration: 3° > 2° > 1°.
        • Intermolecular (lower temp, acid catalyst): 2 R-OH → R-O-R + H₂O (forms ethers, suitable mainly for 1° alcohols).
    • Oxidation: Product depends on the type of alcohol and oxidizing agent.
      • Strong Oxidizing Agents (KMnO₄/H⁺, K₂Cr₂O₇/H⁺):
        • 1° Alcohol → Carboxylic Acid (via aldehyde)
        • 2° Alcohol → Ketone
        • 3° Alcohol → Resistant to oxidation under normal conditions; undergo C-C bond cleavage at high temp to give a mixture of carboxylic acids with fewer carbons.
      • Mild Oxidizing Agents (PCC - Pyridinium Chlorochromate, CrO₃):
        • 1° Alcohol → Aldehyde
        • 2° Alcohol → Ketone
    • Dehydrogenation (Catalytic): Passing alcohol vapours over heated Copper (Cu at 573 K).
      • 1° Alcohol → Aldehyde + H₂
      • 2° Alcohol → Ketone + H₂
      • 3° Alcohol → Alkene (undergoes dehydration) + H₂O

  • Specific Alcohols:

    • Methanol (CH₃OH): 'Wood spirit'. Prepared by catalytic hydrogenation of carbon monoxide (CO + 2H₂ --ZnO/Cr₂O₃ catalyst, high P&T--> CH₃OH). Highly poisonous. Used as a solvent, antifreeze, and in formaldehyde production.
    • Ethanol (C₂H₅OH): Prepared by fermentation of sugars (e.g., molasses). Used as a solvent, in alcoholic beverages, as a fuel additive (gasohol), and in organic synthesis. Denatured alcohol is ethanol made unfit for drinking by adding poisonous substances like methanol or pyridine.

3. Phenols:

  • Nomenclature: Hydroxy derivatives of benzene. Parent name is 'Phenol'. Substituents are numbered starting from the carbon bearing the -OH group. Common names (Cresol, Catechol, Resorcinol, Hydroquinone) are also used.

  • Methods of Preparation:

    • From Haloarenes (Dow's Process): Chlorobenzene + NaOH (aq) --623 K, 320 atm--> Sodium Phenoxide --H⁺--> Phenol. (Requires drastic conditions). Presence of electron-withdrawing groups (like -NO₂) at ortho/para positions facilitates the reaction.
    • From Benzene Sulphonic Acid: Benzene --Oleum (H₂SO₄+SO₃)--> Benzene Sulphonic Acid --NaOH (fusion), then H⁺--> Phenol.
    • From Diazonium Salts: Aniline --NaNO₂/HCl, 273-278 K--> Benzene Diazonium Chloride --H₂O (warm)--> Phenol + N₂ + HCl.
    • From Cumene (Isopropylbenzene): (Industrial Method) Cumene --O₂ (air oxidation)--> Cumene Hydroperoxide --H⁺/H₂O--> Phenol + Acetone (Propanone). Economical process yielding valuable byproduct acetone.

  • Physical Properties:

    • Usually colourless liquids or low melting solids with a characteristic phenolic odour.
    • Boiling Points: Higher than corresponding aromatic hydrocarbons and haloarenes due to intermolecular hydrogen bonding.
    • Solubility: Sparingly soluble in water (due to larger hydrocarbon part), but soluble in organic solvents and aqueous alkali (due to salt formation).

  • Chemical Reactions:

    • Acidity of Phenols:
      • Phenols are more acidic than alcohols and water. This is due to:
        • Resonance stabilization of the phenoxide ion (negative charge delocalized over the benzene ring).
        • Sp² hybridization of carbon attached to -OH (more electronegative than sp³ carbon), making the O-H bond more polar.
      • Phenols react with active metals (like Na) and also with aqueous alkalis (NaOH, KOH) to form salts (phenoxides). Alcohols do not react with aqueous alkalis.
        • C₆H₅OH + NaOH → C₆H₅ONa (Sodium phenoxide) + H₂O
      • Effect of Substituents: Electron-withdrawing groups (-NO₂, -CN, -X) increase acidity (especially at ortho/para positions), while electron-donating groups (-CH₃, -OCH₃, -NH₂) decrease acidity. Order: Nitrophenols > Phenol > Cresols.
    • Electrophilic Aromatic Substitution: The -OH group is strongly activating and ortho-, para- directing.
      • Nitration:
        • Phenol + Dilute HNO₃ (at low temp) → Mixture of o-Nitrophenol and p-Nitrophenol (separable by steam distillation as o-isomer is steam volatile due to intramolecular H-bonding).
        • Phenol + Conc. HNO₃ → 2,4,6-Trinitrophenol (Picric Acid). Yield is low; better prepared by nitrating sulphonated phenol.
      • Halogenation:
        • Phenol + Br₂ (in CS₂ or CCl₄, low polarity solvent) → Mixture of o-Bromophenol and p-Bromophenol (major).
        • Phenol + Bromine Water (Br₂/H₂O) → 2,4,6-Tribromophenol (white precipitate). This reaction is used as a test for phenol.
      • Kolbe's Reaction: Sodium Phenoxide + CO₂ --Heat, Pressure--> Sodium Salicylate --H⁺--> Salicylic Acid (o-Hydroxybenzoic acid).
      • Reimer-Tiemann Reaction: Phenol + Chloroform (CHCl₃) + aq. NaOH/KOH --Heat--> Intermediate --H⁺--> Salicylaldehyde (o-Hydroxybenzaldehyde). If CCl₄ is used instead of CHCl₃, salicylic acid is formed.
    • Reaction with Zinc Dust: Phenol + Zn (dust) --Heat--> Benzene + ZnO. (Reduction of phenol).
    • Oxidation: Phenol + Strong oxidizing agents (like Na₂Cr₂O₇/H₂SO₄ or air) → Benzoquinone (conjugated diketone). Phenols darken on exposure to air due to slow oxidation.

4. Ethers:

  • Classification:

    • Symmetrical (Simple) Ethers: Both alkyl/aryl groups are the same (e.g., Diethyl ether, C₂H₅-O-C₂H₅).
    • Unsymmetrical (Mixed) Ethers: The two groups are different (e.g., Ethyl methyl ether, CH₃-O-C₂H₅).

  • Nomenclature (IUPAC): Named as 'Alkoxyalkanes'. The smaller alkyl group along with the oxygen atom is considered the alkoxy substituent, and the larger alkyl group forms the parent alkane. (e.g., CH₃-O-C₂H₅ is Methoxyethane). For cyclic ethers, 'oxa-' prefix is used (e.g., Oxacyclopropane or Oxirane for ethylene oxide).

  • Methods of Preparation:

    • By Dehydration of Alcohols:
      • Intermolecular Dehydration: Controlled reaction, usually for primary alcohols at lower temperature (e.g., Ethanol --Conc. H₂SO₄, 413 K--> Diethyl ether). Not suitable for preparing mixed ethers or ethers from 2°/3° alcohols (alkene formation dominates).
    • Williamson Synthesis: (Very Important - for both symmetrical and unsymmetrical ethers)
      • Sodium Alkoxide/Phenoxide + Alkyl Halide → Ether + NaX
      • R-ONa + R'-X → R-O-R' + NaX
      • Mechanism: SN₂ attack of alkoxide/phenoxide ion on the alkyl halide.
      • Limitations: Best results with primary alkyl halides. Secondary and tertiary alkyl halides tend to undergo E2 elimination with strong bases like alkoxides, forming alkenes. Therefore, to prepare an ether like t-butyl ethyl ether, use sodium ethoxide and t-butyl halide (elimination dominates) is wrong. Use sodium t-butoxide and ethyl halide (SN₂ occurs).
      • Aryl halides are unreactive towards SN₂ reaction, so phenoxides can react with alkyl halides, but alkoxides cannot react with aryl halides under these conditions to form aryl ethers.

  • Physical Properties:

    • Boiling Points: Much lower than alcohols of comparable molecular mass (no H-bonding between ether molecules). Similar to alkanes of comparable mass. Polarity is low.
    • Solubility: Slightly soluble in water (oxygen can form H-bonds with water), similar to lower alcohols. Solubility decreases with increasing hydrocarbon part size. Ethers are good solvents for organic compounds.

  • Chemical Reactions:

    • Cleavage of C-O bond by Acids (HX):
      • Ethers are relatively unreactive but are cleaved by strong acids like HI and HBr at high temperatures. HCl cleaves only under drastic conditions.
      • R-O-R' + HX --Heat--> R-X + R'-OH
      • If HX is in excess and heat is applied: R-O-R' + 2HX --Heat--> R-X + R'-X + H₂O
      • Mechanism: Protonation of ether oxygen followed by SN₁ or SN₂ attack by halide ion (X⁻).
      • Cleavage Site (Unsymmetrical Ethers):
        • If one group is primary or secondary alkyl, and the other is tertiary alkyl: Halide attacks the tertiary group (SN₁ mechanism favoured due to stable 3° carbocation). R-O-C(CH₃)₃ + HI → R-OH + (CH₃)₃C-I
        • If both groups are primary or secondary: Halide attacks the smaller alkyl group (SN₂ mechanism favoured due to less steric hindrance). CH₃-O-C₂H₅ + HI → CH₃-I + C₂H₅-OH
        • Alkyl Aryl Ethers (e.g., Anisole): Cleavage always occurs between Oxygen and the alkyl group (O-R bond) because the O-Aryl bond is strong (due to resonance/sp² character). Ar-O-R + HX → Ar-OH (Phenol) + R-X.
    • Electrophilic Substitution (in Aromatic Ethers like Anisole):
      • The alkoxy group (-OR) is activating and ortho-, para- directing due to resonance (+R effect dominates over -I effect).
      • Halogenation: Anisole + Br₂ in Ethanoic acid → Mixture of o-Bromoanisole and p-Bromoanisole (major).
      • Friedel-Crafts Reaction:
        • Alkylation: Anisole + CH₃Cl/Anhyd. AlCl₃ → Mixture of o-Methoxy toluene and p-Methoxy toluene (major).
        • Acylation: Anisole + CH₃COCl/Anhyd. AlCl₃ → Mixture of o-Methoxyacetophenone and p-Methoxyacetophenone (major).
      • Nitration: Anisole + Conc. H₂SO₄/Conc. HNO₃ → Mixture of o-Nitroanisole and p-Nitroanisole (major).

Multiple Choice Questions (MCQs):

  1. Which of the following alcohols will give immediate turbidity with Lucas reagent (Anhyd. ZnCl₂ + Conc. HCl)?
    (a) Ethanol
    (b) Propan-2-ol
    (c) 2-Methylpropan-2-ol
    (d) Propan-1-ol

  2. The most acidic compound among the following is:
    (a) Phenol
    (b) Ethanol
    (c) p-Cresol
    (d) p-Nitrophenol

  3. Williamson synthesis is used for the preparation of:
    (a) Alcohols
    (b) Aldehydes
    (c) Ethers
    (d) Ketones

  4. The reaction of phenol with chloroform in the presence of aqueous alkali followed by hydrolysis gives:
    (a) Salicylic acid
    (b) Salicylaldehyde
    (c) Benzoic acid
    (d) Benzaldehyde

  5. Which reagent is used to convert a primary alcohol directly to an aldehyde?
    (a) KMnO₄/H⁺
    (b) K₂Cr₂O₇/H⁺
    (c) Pyridinium Chlorochromate (PCC)
    (d) LiAlH₄

  6. Hydroboration-oxidation of propene yields:
    (a) Propan-2-ol
    (b) Propan-1-ol
    (c) Propanal
    (d) Propane

  7. Cleavage of Anisole (Methoxybenzene) with hot concentrated HI gives:
    (a) Iodobenzene and Methanol
    (b) Phenol and Iodomethane
    (c) Iodobenzene and Iodomethane
    (d) Phenol and Methanol

  8. Which of the following methods is suitable for preparing t-butyl ethyl ether?
    (a) Treating sodium ethoxide with t-butyl bromide
    (b) Treating sodium t-butoxide with ethyl bromide
    (c) Treating ethanol with t-butyl alcohol in presence of H₂SO₄
    (d) Treating t-butyl alcohol with diethyl sulphate

  9. Identify the product Z in the following reaction sequence:
    Cumene --(i) O₂, (ii) H₃O⁺--> X + Y
    X --Zn dust--> Z
    (a) Benzene
    (b) Toluene
    (c) Phenol
    (d) Acetone

  10. Intermolecular hydrogen bonding is strongest in:
    (a) Diethyl ether
    (b) Ethane
    (c) Ethanol
    (d) Dimethyl ether

Answers to MCQs:

  1. (c) 2-Methylpropan-2-ol (Tertiary alcohol)
  2. (d) p-Nitrophenol (Electron-withdrawing -NO₂ group increases acidity)
  3. (c) Ethers
  4. (b) Salicylaldehyde (Reimer-Tiemann reaction)
  5. (c) Pyridinium Chlorochromate (PCC) (Mild oxidizing agent)
  6. (b) Propan-1-ol (Anti-Markovnikov addition)
  7. (b) Phenol and Iodomethane (Cleavage occurs at Alkyl-Oxygen bond)
  8. (b) Treating sodium t-butoxide with ethyl bromide (SN₂ on primary halide, avoids elimination)
  9. (a) Benzene (Cumene process gives Phenol (X) and Acetone (Y). Phenol with Zn dust gives Benzene (Z))
  10. (c) Ethanol (Alcohols exhibit strong intermolecular H-bonding)

Make sure you understand the mechanisms, especially for name reactions like Kolbe's, Reimer-Tiemann, Williamson Synthesis, and the reactions involving Grignard reagents. Also, focus on the comparative acidity of alcohols and phenols, and the methods to distinguish between primary, secondary, and tertiary alcohols. Good luck with your preparation!

Read more