2402 Topic 4b Ethers Part 2 2025 - annotatedqsd

Topic 4 focuses on reactions of alcohols, ethers, and epoxides, continuing the discussion from Topic 3.
Primary aim: To learn about sp3 hybridized oxygen-containing compounds.
Specific reactions studied include:
- Dehydration of alcohols under strong acid conditions.
- Reaction of ethers with hydroiodic acid (HI).
- Reactions of epoxides under both acidic and basic conditions.

Ethers are generally unreactive with common reagents, making them frequent solvents in reactions.
They are resistant to strong oxidizing agents and most acids/bases at moderate temperatures.
Key features of ethers:
- They cleave under strong acids (e.g., HI, HBr).
- Structure resembles hydrocarbons in their reaction resistance.
- Common solvents used include diethyl ether and tetrahydrofuran.

The Williamson ether synthesis is classified as an SN2 reaction.
Mechanism steps:
- Step 1: Deprotonation of an alcohol using NaH to generate an alkoxide ion.
- Step 2: The generated alkoxide ion acts as a nucleophile, reacting with an alkyl bromide to yield an ether.
- Alkoxide ion can be formed by:
  - Reaction of alcohol with alkali metals (Na or Li).
  - Reaction with strong bases like sodium hydride (NaH).
Optimal conditions for SN2 require primary alkyl halides to avoid competing E2 elimination reactions.

When preparing unsymmetrical ethers:
- Choose a less hindered alkyl halide for improved SN2 substitution.
- Reaction of a more hindered alkoxide with a less hindered alkyl halide minimizes ethylene elimination.
Example: Tertiary alkyl halides may lead to undesired elimination products.

Ethers react with concentrated hydroiodic acid (HI) to yield:
- Alkyl iodide and alcohol products.
Mechanisms:
- SN1 Mechanism: Occurs with ethers containing tertiary alkyl groups, leading to the formation of stable carbocation intermediates.
- SN2 Mechanism: Preferred with primary alkyl groups due to absence of stable carbocation.
- Importance of protonation to activate ether for subsequent reaction.

Tertiary ethers favor an SN1 mechanism resulting in:
- Formation of a stable tertiary carbocation, followed by nucleophilic attack from iodide ion.
Primary ethers will usually proceed via SN2 due to difficult carbocation stabilization.
Strength of nucleophiles in polar protic solvents:
- Relative strength: I– > Br– > Cl–.

Benzyl ethers can cleave via SN1 mechanisms due to the formation of stabilized benzylic carbocation.
Reactions are rapid at room temperature.
Vinyl Ethers Reaction with HI:
- Vinyl ether involves an sp2 carbon, which complicates reactions as substitution is not favorable.
- Reaction leads to enol and iodo compounds that tautomerize to more stable products (e.g., aldehydes).

Cyclic ethers (e.g., tetrahydropyran) react with HI under heat:
- Formation of oxonium ions allows ring-opening to yield products such as 5-iodo-1-pentanol.