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18.S: Ethers and Epoxides; Thiols and Sulfides (Summary)

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  • Concepts & Vocabulary

    18.0 Introduction

    • Ethers are molecules containing oxygen which is bonded to two carbon groups.
    • Thiols are sulfur analogues of alcohols with an SH group instead of OH.
    • Sulfides are sulfur analogues of ethers with sulfur bonded to two carbon groups instead of oxygen.

    18.1 Names and Properties of Ethers

    • Ether groups are named as alkoxy groups.
    • Ethers do not have intramolecular hydrogen bonding (unlike alcohols), therefore ethers have significantly reduced intermolecular forces causing boiling points that are much lower than similar sized alcohols.

    18.2 Preparing Ethers

    • Alkoxymercuration can be used to prepare an ether from an alkene.

    18.3 Reactions of Ethers: Acidic Cleavage

    • The carbon-oxygen bonds of ethers can be cleaved with strong acids through either nucleophilic substitution or elimination reactions.

    18.4 Reactions of Ethers: Claisen Rearrangement

    • The Claisen rearrangement is a [3, 3] sigmatropic rearrangement reaction that converts aryl or enol ethers into carbonyl compounds (though the aromatic version rearranges into a phenol to re-establish aromaticity.

    18.5 Cyclic Ethers: Epoxides

    • Epoxides, also called oxiranes, have a three-membered ring structure with one oxygen and two carbon atoms.
    • Epoxides can be formed from alkenes by reaction with peroxy acids (MCPBA for example).
    • Epoxides can be formed from halohydrin molecules by reaction with a base, which causes an intramolecular Williamson ether synthesis.

    18.6 Reactions of Epoxides: Ring Opening

    • When epoxides are ring opened under basic conditions, they follow SN2 mechanism leading to the nucleophile adding to the less substituted side of the epoxide.
    • When epoxides are ring opened under acidic conditions, they follow SN1 mechanism leading to the nucleophile adding to the more substituted side of the epoxide.
    • When epoxides are ring opened in aqueous reactions, the result is an anti-diol.
    • Halo acids can be added to epoxides to form anti-halohydrins.

    18.7 Crown Ethers

    • Crown ethers are cyclic ethers containing several oxygen atoms.
    • Crown ethers are named by the number of total atoms in the ring, followed by the word crown and finally the number of oxygen atoms (18-crown-6 for example).

    18.8 Thiols and Sulfides

    • Thiols can be prepared from alkyl halides through reaction with hydrosulfide ion (SH-) or through a more complicated series of reactions including thiourea.
    • Thiols can be oxidized with mild oxidizing agents to form disulfides.
    • Disulfide bridges link cysteine residues in protein structures.
    • Sulfides are sulfur analogues of ether, though are much better nucleophiles with sulfur in place of oxygen.

    18.9 Spectroscopy of Ethers

    • Ethers show standard C-H stretches and bends in IR along with a strong C-O stretch around 1000 cm-1.
    • In 1H NMR, hydrogens on carbons adjacent to the oxygen typically appear between 3.4-4.5 ppm.
    • Hydrogens on carbons of an epoxide ring typically appear between 2.5-3.5 ppm in 1H NMR.

    18.10 Interchapter: A Preview of Carbonyl Chemistry

    • Carbonyl groups are one of the most important features in organic chemistry and consist of a sp2 carbon double-bonded to oxygen.
    • Carbonyl groups are present in ~10 different functional groups.
    • Carbonyl groups are polarized with a partial positive charge on carbon and partial negative charge on oxygen. This makes the carbon atom an electrophile, while the oxygen can act as a nucleophile.
    • Carbonyl groups can react through several mechanisms including nucleophilic addition and nucleophilic acyl substitution, alpha substitution and condensation.

    Skills to Master

    • Skill 18.1 Name ethers using common naming and IUPAC.
    • Skill 18.2 Write reaction equations for preparation of ethers.
    • Skill 18.3 Write mechanisms for reactions of ethers with strong halogen acids.
    • Skill 18.4 Draw mechanisms for Cope and Claisen rearrangements.
    • Skill 18.5 Draw mechanisms for ring-opening epoxides under acidic and basic conditions.
    • Skill 18.6 Draw and name crown ethers.
    • Skill 18.7 Explain how disulfide bridges contribute to protein structure.
    • Skill 18.8 Give an example of S-adenosyl methionine activity in biological systems.
    • Skill 18.9 Use IR and NMR spectra to identify ethers.

    Summary of Reactions

    Ether and Epoxide Preparation

    Ether Preps.png

    Ether Reactions

    Ether Cleave.png


    Epoxide Reactions

    Epoxide Reactions.png

    Sulfur Compound Reactions

    Sulfur Reactions.png