Chem 335 - Winter 1999 - Organic Chemistry II
Portland State University - Dr. Carl C. Wamser

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Chapter 11 - Ethers, Sulfides, and Epoxides

Structure and Properties

• C-O-C bond
• bent bond, slightly polar
• no H-bonding (low bp), but H-bond acceptor (slightly water-soluble)

Nomenclature

• IUPAC: alkoxy- substituent
• common: alkyl ether
• diethyl ether = ethyl ether = "ether"
• cyclic ethers have special names (and lots of different names)
  • epoxides (3-membered rings) :
    oxirane, epoxyalkane, alkene oxide, oxacyclopropane
  • oxetane (4), oxoloane (5), oxane (6)
• sulfides ( C-S-C bond )
  • IUPAC uses alkylthio- substituent
  • common uses alkyl sulfide
• disulfides: ( C-S-S-C bond )

Preparations of Ethers

• Williamson synthesis
  • SN2 reaction of alkoxide with alkyl halide
  • alkyl halide should be CH3 or 1°
• dehydration of alcohols
  • for symmetrical ethers only, best with CH3 or 1°
• alkenes + alcohols
  • best for 1° alcohols and alkenes that make stable cations

Preparations of Sulfides

• Williamson synthesis with thiolate anion + alkyl halide
• double displacement with Na2S and alkyl halide (symmetrical sulfides only)

Reactions of Ethers

• generally unreactive towards oxidation, reduction, base, and most acids
  (make good solvents)
  EXCEPT for
• cleavage by conc. HI or HBr
  R-O-R + 2 HX ---> 2 R-X + H2O
  reaction is either SN1 or SN2 (or both), depending on the R groups
• radical autoxidation to form hydroperoxides

Reactions of Sulfides

• similar to ethers except S is a better nucleophile and leaving group
  and S is easily oxidized
• sulfoxide ( S=O bond ) , sulfone ( two S=O bonds )

Protecting Groups

• conversion of a reactive group (e.g., an alcohol) to an unreactive group (e.g., an ether)
• conversion should be readily reversible
  • example :

• other alcohol protecting groups:
  • silyl ethers : ROH + Me3SiCl ---> R-O-SiMe3
  • THP ethers

Preparations of Epoxides

• oxidation of alkenes by peroxyacids
• internal SN2 in halohydrins

Reactions of Epoxides

• unlike most other ethers, epoxides are reactive, because of ring strain
• acid-catalyzed ring opening
  • protonation makes ring opening easier
  • always backside attack by nucleophile (like SN2)
  • but nucleophile attacks more substituted carbon (like SN1)
• nucleophilic ring opening
  • direct attack without protonation requires a strong nucleophile
  • backside attack on less-hindered carbon (like SN2)