Ethers, Epoxides and Sulphates

• Ethers


  Compounds with a general formula of R-O-R' where R and R' are alkyl or aryl groups. They may be symmetrical or unsymmetrical.
• Ethers


  • Bent structure like water with sp3 hybridized oxygen atom
  • C-O-C bond angle is about 110°
  • Contain polar C-O bonds
  • Highly polar, even though they lack the polar hydroxyl group of alcohols
• Tetrahydrofuran (THF)


  A strongly polar ether solvent without the reactivity of a hydroxyl group
• Boiling points of ethers


  • Similar to those of alkanes of comparable molecular weight
  • Pure ethers cannot engage in hydrogen bonding as they lack O-H groups
  • But they have large dipole moments which have relatively little effect on their boiling point
• Hydrogen bonding of ethers


  Ethers can engage in hydrogen bonding with compounds containing O-H or N-H groups, but not with other ether molecules
• Solvation of ions by ethers


  • Ionic substances like lithium iodide are moderately soluble in ethers due to solvation of the cation by the ether's lone pairs
  • Unlike alcohols, ethers cannot serve as hydrogen-bond donors so they do not solvate small anions well
• Crown ethers


  • Cyclic polyethers that specifically solvate metal cations by complexing the metal in the center of the ring
  • Different crown ethers solvate different cations depending on their relative sizes and number of binding sites
• Ethers as solvents


  • Dissolve a wide range of polar and nonpolar substances
  • Easily evaporate from reaction products due to low boiling points
  • Nonpolar substances are more soluble in ether than in alcohol
  • Polar substances are nearly as soluble in ether as in alcohol
  • Effectively solvate cations due to large dipole moments and ability to serve as hydrogen bond acceptors
• Why ethers are used as solvents for strong bases


  • Alcohols cannot be used as they are quickly protonated by strong bases, destroying the reagent
  • Ethers are nonhydroxylic and normally unreactive towards strong bases like Grignard reagents
• Naming of simple ethers


  • Name the two alkyl groups attached to the oxygen and add the word "ether"
  • Groups are named in alphabetical order
  • Symmetrical ethers can use "dialkyl" or just "alkyl"
• Naming of ethers with other functional groups


  The ether part is named as an alkoxy substituent
• Cyclic ethers


  • Heterocyclic compounds containing a ring with a non-carbon atom (usually oxygen)
  • The heteroatom is numbered 1 in the ring
  • Generally behave like acyclic ethers
• Epoxides


  Three-membered-ring ethers that are much more reactive than other ethers due to the strain of the ring
• Synthesis of epoxides


  Using peroxyacids, most commonly meta-chloroperoxybenzoic acid (MCPBA)
• Reactions of epoxides


  1. With Grignard and organolithium reagents to give ring-opened alcohols

  2. With lithium aluminium hydride to reduce to alcohols
• Oxetanes


  • Four-membered cyclic ethers that are more reactive than larger cyclic ethers and open-chain ethers, but less reactive than epoxides
• Furans


  Five-membered cyclic ethers, including the saturated tetrahydrofuran (THF) which is an excellent nonhydroxylic organic solvent
• Pyrans


  Six-membered cyclic ethers, also referred to as oxanes
• Dioxanes


  • Heterocyclic ethers with two oxygen atoms in a six-membered ring
  • Most are toxic and carcinogenic
• Synthesis of ethers


  1. Williamson method

  2. Addition of an alcohol across a double bond (alkoxymercuration-demercuration)

  3. Bimolecular condensation of alcohols (industrial method)
• Williamson ether synthesis


  Two-step process involving an SN2 attack of an alkoxide on an unhindered primary alkyl halide or tosylate
• Williamson ether synthesis


  • Examples of the reaction
• Phenyl ethers


  Phenoxide ions are easily produced and can be used in the Williamson synthesis, but phenyl halides or tosylates cannot
• Alkoxymercuration-demercuration


  Adds an alcohol molecule across the double bond of an alkene to produce an ether
• Industrial synthesis of ethers


  1. Acid-catalyzed bimolecular condensation of primary alcohols to make symmetrical ethers

  2. Not a good method for making unsymmetrical ethers
• Cleavage of ethers


  1. Can be cleaved by heating with HBr or HI to give alkyl halides

  2. Hydrogen iodide is most reactive, hydrogen fluoride is not reactive
• Phenyl ether cleavage


  Phenol cannot react further to become a halide because an SN2 reaction cannot occur on a sp2 carbon
• Autoxidation of ethers


  Slow oxidation in the presence of atmospheric oxygen to produce explosive hydroperoxides and dialkyl peroxides
• Sulfides (thioethers)


  Compounds with the general formula R-S-R', analogous to ethers but with sulfur instead of oxygen
• Synthesis of sulfides


  Treating an alkyl halide with a thiolate ion, similar to the Williamson ether synthesis
• Thiols and thiolates


  • Thiols (RSH) are the sulfur analogues of alcohols, with the SH group called a mercapto group
  • Thiolates are easily synthesized using the Williamson ether synthesis with a dithiolate nucleophile