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Chapter 13. Ethers

Recommended Article: 【Organic Chemistry】 Organic Chemistry Table of Contents

1. Nomenclature

2. Reactions

3. Synthesis Methods

1. Nomenclature

⑴ Official Nomenclature

① Take an alkane with a higher number of carbon atoms as the main chain.

② Name the remaining portion other than the parent main as alkoxy

③ Number the carbon in the main chain connected to the alkoxy group as 1

④ If there are multiple substituents in the main chain, name them in alphabetical order

○ Example: (3S, 4R)-3-butoxy-4-methylhexane

⑤ Examples

○ 2-methoxybutane

○ 1-ethoxy-3-methylpentane

○ 1-butoxy-2,3-dimethylpentane

○ 1,4-diisopropoxybutane

⑵ Common Nomenclature

① Write the names of substituents on both sides of the ether oxygen in alphabetical order and add the term “-ether”.

② Examples

○ ethyl methyl ether

○ diethyl ether (ethyl ether)

○ diphenyl ether

○ tert-butyl isobutyl ether

○ sec-butyl isopropyl ether

○ cyclohexyl isopentyl ether

2. Reactions

⑴ Low Reactivity

① Ethers are often used as solvents due to their limited reactivity

② Examples of solvents used

○ diethyl ether: also known as ether

○ tetrahydrofuran: also known as THF

○ tetrahydropyran: also known as THP

○ 1,4-dioxane

○ 1,2-dimethoxyethane: also known as DME

○ tert -butyl methyl ether: also known as MTBE

⑵ Reaction 1. Decomposition by HX

① HI and HBr readily react, while HCl reacts less readily

② S_N2 Reaction: Nucleophiles attack ethers that do not contain tertiary carbons, considering steric hindrance

③ S_N1 Reaction: Ethers containing tertiary carbons result in carbocation formation at the tertiary carbon

④ Phenyl Groups Present

○ Phenyl alkyl ethers: Decomposition between oxygen and phenyl does not occur. However, under HBr or BBr3, phenolization can occur

○ Biphenyl ethers: Ether decomposition does not occur

⑶ Reaction 2. Hydrolysis by H₂O, H₂SO₄

① Ether Formation: ROH + R’OH → ROR’

② Under strong acidic conditions, reverse reaction to ROH and R’OH can occur

⑷ Reaction 3. Protection of THP Ether: Ether protection using DHP (dihydropyran)

① Protection Step

② Deprotection Step

⑸ Reaction 4. Claisen Rearrangement Reaction

① Reactants: allyl vinyl ether or allyl phenyl ether

Figure 1. Claisen Rearrangement Reaction using allyl phenyl ether as reactant

⑹ Reaction 5. Crown Ethers

① Also called phase transfer catalysts, ion carriers, quaternary ammonium salts, cryptands

② Named as 3n-crown-n-ether due to the repeating CH₂CH₂O structure

③ Form complexes with different ions based on their pore sizes: Can transport cations after coordination, in nonpolar solvents

○ Li⁺: 12-crown-4-ether (n=4)

○ Na⁺: 15-crown-5-ether (n=5)

○ K⁺: 18-crown-6-ether (n=6)

○ Example: KF doesn’t dissolve in acetonitrile, but crown ether ionizes K⁺ and captures F^- ions

3. Synthesis Methods

⑴ Synthesis of Symmetrical Ethers

① Summary: Elimination reactions dominate at high temperatures, while addition reactions dominate at low temperatures

② (not desirable) H₂SO₄, 180°C: R-CH₂OH + H₂SO₄ → R=CH₂. Alcohol E1 Elimination Reaction

Figure 2. Failed synthesis of symmetrical ethers

③ (desirable) H₂SO₄, 140°C: R-CH₂OH + H₂SO₄ → R-CH₂-O-CH₂-R

Figure 3. Successful synthesis of symmetrical ethers

④ Drawback: Synthesis of symmetrical ethers yields various ethers (ROR, R’OR, R’OR’), making it unsuitable

⑤ Exceptionally, successful synthesis of unsymmetrical ethers is achieved using a tertiary alcohol as one component (∵ S_N1 Reaction)

⑵ Williamson Ether Synthesis: Synthesis of Unsymmetrical Ethers

① Mechanism

○ Step 1. ROH + NaH → RO^-Na⁺ + H₂

○ Step 2. RO^- + R’I → ROR’ + I^- (S_N2): Stereochemistry of R’ is inverted

Figure 4. Mechanism of Williamson Ether Synthesis

② (Formula) Reaction of alcohol under NaH, R’X conditions can be considered as an S_N2 reaction

③ Limitation: If R’X is not a methyl halide or primary halide alkyl, elimination reaction dominates, making it unsuitable

⑶ Alkoxy Mercuration-Demercuration

① Overview

○ Useful when Williamson Ether Synthesis is not applicable

○ Alkoxy mercuration-demercuration is Markovnikov hydration where ROH is the reactant

○ Similar to oxymercuration-demercuration of alkenes

② (Formula) Reaction under 1. Hg(O₂CCF₃)₂, ROH, 2. NaBH₄, HO^- conditions

③ Mechanism

Figure 5. Mechanism of Alkoxy Mercuration-Demercuration

○ Hg exposes inner electrons to create a cyclic intermediate

○ ROH, being a weak nucleophile, performs an S_N1-like reaction, attacking the carbon with multiple substitutions

Input: 2019.01.12 16:45