Chapter 18 - Ethers, Epoxides, and Sulfides

Nomenclature: IUPAC - oxy-derivatives of alkanes or, for simple systems

- two alkane names followed by ether

Ethoxyethane (diethyl ether)

1-ethoxy-3-chloropropane (3-chloropropyl ethyl ether)

Cyclic ethers:

Ethylene oxide................................. Tetrahydrofuran...... Tetrahydropyran

Bonding and Properties:

Water (100 °C)

Methanol (64.5 °C)

Dimethyl ether (-24.8 °C)

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Ethylene oxide (10 °C)

Low boiling points------ no internal H-bonds

Are water soluble like alcohols----- can make H-bonds with H₂O.

Preparation of Ethers:

i) Williamson Ether Synthesis: Make methyl t-butyl ether

> > > Requirements for S_N2 reaction ---> correct partners

ii) Alkoxymercuration-demercuration:

Reactions of Ethers:

i) HX catalyzed cleavage of ethers: (analogous to reaction of HX with alcohols forming alkyl halides - Chap 17.8)

> > > HI > HBr >> HCl . HF not on scale.

Preparation of Epoxides:

.........i) Peracid Oxidation of olefins. Stereospecific Rxn.

......... ii) Halohydrin formation and closure to epoxides:

Reactions of epoxides:

........1) Acidic ring opening of epoxides.

...................a) Protic solvolysis:

Mechanism is intermediate in Sn2 Sn1 character, attacks from the back but adds to a tertiary over a primary carbon.

...................b) Cleavage by H-X

........2)Basic ring opening, These go by simple Sn2 mechanisms

....................a) ROH/ROM; RSK/ROH; H₂O, HONa

.....................b) Grignard

Overnight problem: Predict the product of the following reaction.

Crown ethers: useful for solvolysis of metal ions: 18-crown-6, 12-crown-4

Spectroscopy:

¹H NMR: ethers d= 3.2 - 4.0 ppm; epoxides d= 2.2 - 3.0 ppm

Why the difference? Due to hybridization in epoxides, bent bonds, the O-C-C bonds have more p character in them therefore the C-H bonds have more s character and are more tightly bound (more shielded) ----> move upfield.
>

¹³C NMR: same as for alcohols 60 - 75 ppm; similar upfield shift for epoxides.

IR: C-O stretches at 1000 - 1150 cm^-1.