Ethers and Epoxides: Structures, Properties, Reactions

Ether Structure

• Oxygen atom is polarized and can H bond with other molecules as an acceptor

  • Can NOT H-bond with other ethers

• Bond angle of 112°

Ether Boiling Points

• Lack of an H-bonding donor lowers the boiling point

• Have weaker LDFs because the oxygen breaks up the alkyl chain

• Ether bp is slightly lower than an alkane (35°C compared to 36°C) and significantly lower than an alcohol bp

Cyclic Ethers

• Epoxide

  • highly reactive due to small bond angles (60°C) creating ring strain

• Furan (5-membered with 1 O and two double bonds)

• Tetrahydrofuran (5-membered with one O; common organic solvent)

• Tetrahydropyran (6-membered with one O)

• 1,4-dioxane (6-membered with two O’s; common organic solvent)

Ether-Cation Complexes

Ethers are good Lewis bases (good electron pair donors)

• w/ metals (M+): the partial negative on O makes ethers good at complexing to metal ions → helps solubility

• w/ Grignard’s: ether solvents are crucial to Grignard reagent stability → makes them useful in synthesis

• w/ boranes: BH₃THF has an empty p-orbital; the lone pare on the O in an ether can donate to that empty orbital and stabilize it

Crown Ethers

Cyclic multi-ether compounds that can be sized to fit specific cations

• The partial negative on O can stabilize/trap the cations based on their size (Li < Na < K)

• Can help to dissolve certain ionic species in organic reactions

Biomolecular Condensation

• Where ROH reacts with another ROH in the presence of a strong acid

• Requires symmetric eithers

• The alcohol must be small to reduce the potential of elimination rxn competition

  • Ideally use a primary alcohol (R-C-OH)

mCPBA

• Concerted reaction (bond breaking and forming occurs in one step)

• Conserves the stereochemistry of the alkene

  • A trans reactant will result in an anti product (dashed and wedged, ±)

  • A cis reactant will result in a syn product (both wedged or both dashed, ±)

BHT

Butylated hydroxytoluene

• An oxidation inhibitor that is in some ether bottles

• Always check the age of ether bottles

• Look to see if they have inhibitors

• If old and uninhibited → do not touch + alert safety organization

Acid-Catalyzed Epoxide Ring Opening

• Can occur with water (H₃O⁺) or an alcohol with a dry H+ source (HCl, TsOH, H₂SO₄) as the reagents

• The epoxide deprotonates an H from H₃O⁺ or the H+ source first

• Then the H-OH opens the ring by attacking the most substituted end

Base-Catalyzed Epoxide Ring Opening

• Can occur with water (OH^-/H₂O) or alkoxide (R-O^- and ROH) as the reagents

• No deprotonating occurs; the first step is the oxygen from the OH^- or R-O^- attacking the least substituted end

• Epoxide reactions with organometallics occurs similarly