Preparation and Physical Properties

  • Preparation of Alcohols:

    • From Alkenes:
      • (i) By Acid-Catalysed Hydration: Alkenes react with water in presence of acid (H+) catalyst. Addition follows Markovnikov’s rule for unsymmetrical alkenes.

        • Mechanism: Step 1 (Fast): Protonation of alkene by \(\mathrm{H_3O^+}\) to form a carbocation (electrophilic attack). Step 2: Nucleophilic attack of water on carbocation. Step 3: Deprotonation to form alcohol.
      • (ii) By Hydroboration-Oxidation: (First reported by H.C. Brown in 1959, shared 1979 Nobel Prize).

        • Diborane \(\mathrm{(BH_3)_2}\)​ reacts with alkenes to form trialkyl boranes (addition product).
        • Trialkyl boranes are then oxidized to alcohol by hydrogen peroxide \(\mathrm{(H_2O_2)}\) in presence of aqueous sodium hydroxide \(\mathrm{(NaOH)}\).
        • Addition of borane to double bond occurs such that boron attaches to sp2 carbon with more hydrogens.
        • Alcohol formed appears as if water added opposite to Markovnikov’s rule. Excellent yield.
    • From Carbonyl Compounds:
      • (i) By Reduction of Aldehydes and Ketones: Reduced to corresponding alcohols.

        • Catalytic Hydrogenation: Addition of H2​ in presence of finely divided catalysts (Pt, Pd, or Ni).
        • Reducing Agents: Sodium borohydride \(\mathrm{(NaBH_4)}\) or lithium aluminium hydride \(\mathrm{(LiAlH_4)}\).
        • Aldehydes yield primary alcohols.
        • Ketones yield secondary alcohols.
      • (ii) By Reduction of Carboxylic Acids and Esters:
        • Carboxylic Acids: Reduced to primary alcohols by strong reducing agent \(\mathrm{LiAlH_4}\)​ (expensive, used for special chemicals).
        • Commercially: Acids converted to esters, then reduced by catalytic hydrogenation.
    • From Grignard Reagents: Alcohols produced by reaction of Grignard reagents (RMgX) with aldehydes and ketones.

      • Step 1: Nucleophilic addition of Grignard reagent to carbonyl group to form an adduct.
      • Step 2: Hydrolysis of adduct yields an alcohol.
      • Specific Products:
        • Methanal \(\mathrm{(HCHO) + RMgX \rightarrow \ Primary\ alcohol}\).
        • Other aldehydes \(\mathrm{(RCHO) + R’MgX \rightarrow\ Secondary\ alcohol}\).
        • Ketones \(\mathrm{(RCOR’) + R”MgX \rightarrow\ Tertiary\ alcohol}\).
  • Preparation of Phenols:

    • Phenol (carbolic acid) first isolated from coal tar. Now commercially produced synthetically.
    • From Haloarenes: Chlorobenzene fused with (\mathrm{NaOH}\) at 623 K and 320 atm. Acidification of sodium phenoxide gives phenol.
    • From Benzenesulphonic Acid: Benzene sulphonated with oleum to form benzenesulphonic acid. Heated with molten \(N_aOH\) to form sodium phenoxide, then acidified to phenol.
    • From Diazonium Salts: Aromatic primary amine treated with nitrous acid \(\mathrm{(NaNO_2 + HCl)}\) at 273-278 K forms diazonium salt. Hydrolysed to phenols by warming with water or dilute acids.
    • From Cumene (Industrial Method): Most worldwide production.

      • Cumene (isopropylbenzene) oxidized with air to cumene hydroperoxide.
      • Cumene hydroperoxide treated with dilute acid to yield phenol and acetone (by-product, obtained in large quantities).
  • Physical Properties (Alcohols and Phenols):

    • Properties mainly due to hydroxyl group; alkyl/aryl groups modify them.
    • Boiling Points:
      • Increase with increasing number of carbon atoms (increased van der Waals forces).
      • Decrease with increased branching in carbon chain (decreased surface area, hence decreased van der Waals forces).
      • Higher than hydrocarbons, ethers, haloalkanes/haloarenes of comparable molecular masses.
      • Reason: Intermolecular hydrogen bonding in alcohols and phenols.

        • Example: Ethanol (46/351 K) vs. Propane (44/231 K). Methoxymethane (46/248 K) is intermediate.
    • Solubility (in water):
      • Due to ability to form hydrogen bonds with water molecules.
      • Decreases with increasing size of alkyl/aryl (hydrophobic) groups.
      • Lower molecular mass alcohols are miscible with water in all proportions.

This MCQ module is based on: Preparation and Physical Properties

This assessment will be based on: Preparation and Physical Properties

  • Real-Life Connections & General Knowledge:
    • The industrial production methods for phenol (from cumene, diazonium salts, haloarenes, benzenesulphonic acid) highlight the synthetic versatility of organic chemistry and the economic significance of specific industrial processes.
    • H.C. Brown’s Nobel Prize for hydroboration-oxidation underscores the importance of reagent development in organic synthesis.
  • Case-based Scenarios & Reasoning:
    • Scenario 1: A chemist needs to synthesize propan-1-ol from propene.
      • Question: What reaction conditions (reagents and steps) should be chosen to achieve this, considering the options for alkene hydration, and why?
      • Reasoning: This requires choosing hydroboration-oxidation, as acid-catalyzed hydration would yield propan-2-ol (Markovnikov’s rule). This tests understanding of regioselectivity.
    • Scenario 2: Two isomeric compounds, one an alcohol and one an ether, have very different boiling points despite identical molecular formulas.
      • Question: Explain the primary reason for this difference in boiling points.
      • Reasoning: This assesses understanding of intermolecular forces, specifically hydrogen bonding in alcohols vs. lack thereof in ethers.
  • Conceptual Application:
    • Markovnikov’s Rule vs. Anti-Markovnikov Addition: Explain how acid-catalyzed hydration follows Markovnikov’s rule, while hydroboration-oxidation effectively achieves anti-Markovnikov addition, linking these to carbocation stability and steric hindrance/electronic effects.
    • Nucleophilic Addition in Grignard Reactions: Detail the mechanism of Grignard reagent addition to carbonyl compounds, explaining why different types of aldehydes/ketones lead to primary, secondary, or tertiary alcohols.
    • Hydrogen Bonding and Physical Properties: Discuss how intermolecular hydrogen bonding significantly affects the boiling points and water solubility of alcohols and phenols compared to other organic compounds of similar molecular mass.
  • Numerical/Data Interpretation (if applicable):
    • Boiling points example: Ethanol (351 K), Methoxymethane (248 K), Propane (231 K).
    • Conditions for haloarene conversion to phenol: 623 K, 320 atm.
    • Conditions for diazonium salt formation: 273-278 K.
  • Comparative & Analytical Points:
    • Compare and Contrast: Differentiate between acid-catalyzed hydration and hydroboration-oxidation as methods for synthesizing alcohols from alkenes, focusing on their regioselectivity (Markovnikov vs. anti-Markovnikov).
    • Distinguish: Explain why \(\mathrm{LiAlH_4}\)​ is used for special chemical preparation and not commercially for carboxylic acid reduction, while catalytic hydrogenation is preferred commercially.
    • Analyze Preparation Methods for Phenols: Compare the various laboratory and industrial methods for preparing phenols, noting any by-products (e.g., acetone from cumene method).
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