Ethers: Preparation, Properties, and Reactions

  • Preparation of Ethers:

    1. By Dehydration of Alcohols: Alcohols undergo dehydration in the presence of protic acids (H2​SO4​,H3​PO4​).

      • Reaction outcome (alkene or ether) depends on reaction conditions.
      • Example: Ethanol with H2​SO4​ at 443 K → Ethene. At 413 K → Ethoxyethane (diethyl ether) as main product.
      • Mechanism: Nucleophilic bimolecular reaction (SN​2) involving attack of alcohol molecule on a protonated alcohol.
      • Method suitable for primary alkyl groups only, unhindered alkyl group, low temperature. Otherwise, alkene formation is favored.
      • Follows SN​1 pathway for secondary or tertiary alcohols (not suitable for ether prep).
      • Dehydration of secondary and tertiary alcohols to ethers is unsuccessful as elimination (alkene formation) competes over substitution.
      • Diethyl ether used as inhalation anesthetic (replaced due to slow effect/unpleasant recovery).
    2. Williamson Synthesis: (Important laboratory method for symmetrical and unsymmetrical ethers).

      • Alkyl halide (R−X) reacts with sodium alkoxide \(\mathrm{(R'{-}O^-Na^+).\ R{-}X\ +\ R'{-}O^-Na^+\ \rightarrow\ R{-}O{-}R’\ +\ NaX}\).
      • Ethers with substituted alkyl groups (secondary or tertiary) can be prepared.
      • Reaction involves SN​2 attack of an alkoxide ion on a primary alkyl halide.
      • Limitations: If secondary or tertiary alkyl halides are used, elimination competes over substitution. If a tertiary alkyl halide is used, only alkene is formed (no ether).

        • Example: \(\mathrm{CH_3ONa + (CH_3)_3C{-}Br \rightarrow}\) exclusively 2-methylpropene (alkoxides are strong bases, favoring elimination).
      • Phenols also converted to ethers by this method (using phenoxide moiety).
  • Physical Properties of Ethers:
    1. C-O bonds are polar, resulting in a net dipole moment.
    2. Weak polarity does not significantly affect boiling points.
    3. Boiling points comparable to alkanes of comparable molecular masses.
    4. Much lower boiling points than alcohols of comparable molecular masses due to absence of hydrogen bonding in ethers.

      • Example: n-Pentane (309.1 K), Ethoxyethane (307.6 K), Butan-1-ol (390 K).
    5. Miscibility with Water: Resembles alcohols of same molecular mass.

      • Both ethoxyethane and butan-1-ol are miscible to almost same extent (7.5 and 9 g per 100 mL water), while pentane is immiscible.
      • Reason: Oxygen of ether can also form hydrogen bonds with water molecules.
  • Chemical Reactions of Ethers: Least reactive functional groups.

    1. Cleavage of C-O bond in Ethers:
      • Occurs under drastic conditions with excess hydrogen halides (HX).
      • Dialkyl ether: \(\mathrm{R{-}O{-}R + HX \rightarrow RX + ROH}\). If HX in excess and high temp, alcohol reacts further to RX.
      • Alkyl aryl ethers: Cleaved at alkyl-oxygen bond (more stable aryl-oxygen bond). Yields phenol and alkyl halide.
      • Mixed ethers (different alkyl groups): Cleaved similarly.
      • Reactivity Order of HX: \(\mathrm{HI\ >\ HBr\ >\ HCl}\). Cleavage with conc. HI or HBr at high temp.
      • Mechanism: Starts with protonation of ether.

        • Step 2 (SN​2): Iodide (good nucleophile) attacks least substituted carbon of oxonium ion, displacing alcohol. Lower alkyl group forms alkyl iodide.
        • Exception (Tertiary Alkyl Group): If one alkyl group is tertiary, tertiary halide is formed. Reaction follows SN​1 mechanism, as leaving group creates a more stable tertiary carbocation.
        • Anisole: O-CH3​ bond breaks (weaker than O-C6​H5​ due to sp2 hybridised phenyl carbon and partial double bond character). Forms CH3​I and phenol. Phenols don’t react further to halides.
    2. Electrophilic Substitution:
      • Alkoxy group (-OR) is ortho, para directing and activates the aromatic ring (similar to phenol via resonance).
      • (i) Halogenation (Bromination of Anisole): Anisole (phenylalkyl ether) undergoes bromination with Br2​ in ethanoic acid, even without FeBr3​. Due to activating methoxy group. Para isomer is major product (~90% yield).
      • (ii) Friedel-Crafts Reaction: Anisole undergoes alkylation and acylation at ortho and para positions with alkyl halide and acyl halide respectively, in presence of anhydrous AlCl3​.

(iii) Nitration: Anisole reacts with conc. \(\mathrm{H_2SO_4\ /\ HNO_3}\)​ mixture to yield ortho and para nitroanisole (major para)

This MCQ module is based on: Ethers: Preparation, Properties, and Reactions

This assessment will be based on: Ethers: Preparation, Properties, and Reactions

  • Real-Life Connections & General Knowledge:
    • The historical use of diethyl ether as an anesthetic highlights its significant role in early medicine, while its replacement signifies advancements in drug safety and efficacy.
    • The limitations of Williamson synthesis (e.g., formation of alkenes with tertiary alkyl halides) exemplify how side reactions and competing mechanisms dictate synthetic strategies in organic chemistry.
  • Case-based Scenarios & Reasoning:
    • Scenario 1: A student attempts to synthesize t-butyl ethyl ether using sodium ethoxide and t-butyl chloride via Williamson synthesis. The major product observed is 2-methylpropene.
      • Question: Explain why the desired ether is not formed and the alkene is the predominant product. Write the chemical equation for the observed reaction.
      • Reasoning: This tests the understanding of the limitation of Williamson synthesis with tertiary alkyl halides due to alkoxides acting as strong bases, favoring E2 elimination over SN​2 substitution.
    • Scenario 2: An organic chemist tries to cleave an ether using a dilute acid, but the reaction is very slow. When concentrated HI is used at high temperature, the reaction proceeds readily.
      • Question: Explain why drastic conditions and strong hydrogen halides are required for ether cleavage. What is the order of reactivity of hydrogen halides in this reaction?
      • Reasoning: This assesses knowledge of ether’s inertness and the strength of HI/HBr as nucleophiles and acids.
  • Conceptual Application:
    • Nucleophilic Substitution vs. Elimination (SN2 vs E2): Explain the competition between SN​2 and E2 reactions in Williamson synthesis, and how steric hindrance (primary vs. secondary/tertiary alkyl halides) and basicity of alkoxides influence the major product.
    • Mechanism of Ether Cleavage: Detail the step-by-step mechanism of ether cleavage by hydrogen halides, distinguishing between SN​1 (for tertiary carbocation formation) and SN​2 pathways (for less substituted carbons).
    • Resonance and Aromatic Reactivity of Ethers: Explain how the alkoxy group activates the aromatic ring and directs incoming electrophiles to ortho and para positions through resonance, similar to the -OH group in phenols.
  • Numerical/Data Interpretation (if applicable):
    • Boiling points comparison (n-Pentane 309.1 K, Ethoxyethane 307.6 K, Butan-1-ol 390 K).
    • Miscibility of ethoxyethane (7.5 g/100 mL) and butan-1-ol (9 g/100 mL) in water.
  • Comparative & Analytical Points:
    • Compare Ether Preparation Methods: Discuss the advantages and disadvantages of alcohol dehydration vs. Williamson synthesis for preparing different types of ethers.
    • Analyze Ether Cleavage Patterns: Explain why alkyl aryl ethers are cleaved at the alkyl-oxygen bond and not the aryl-oxygen bond when reacting with HX, linking it to the stability of the aryl-oxygen bond due to partial double bond character 
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