ETHERS AND EPOXIDES
STRUCTURE
Ethers have two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom.
R groups are identical = symmetrical ether
R groups are different = unsymmetrical ether
Oxygen is sp3 hybridized
Nearly tetrahedral angle, depends on the R group
Oxygen atom gives a slight dipole moment
1
Cyclic ethers contain oxygen atom(s) incorporated in a ring.
Three-membered cyclic ethers are called epoxides.
Epoxides
Angle strain (60°)
More reactive than other ethers
2
NOMENCLATURE
Ethers
Common names – simple ethers are named by identifying the two organic substituents
(alphabetically) and adding the word ether.
Ethyl methyl ether
Diethyl ether
Cyclopentyl isopentyl ether
3
NOMENCLATURE
Ethers
Common names – simple ethers are named by identifying the two organic substituents
(alphabetically) and adding the word ether.
IUPAC – use the more complex alkyl group as root name, and the rest of the ether as an
alkoxy group.
4
NOMENCLATURE
Ethers
Common names – simple ethers are named by identifying the two organic substituents
(alphabetically) and adding the word ether.
IUPAC – use the more complex alkyl group as root name, and the rest of the ether as an
alkoxy group.
2-methoxybutane
1-ethoxy-3-methylpentane
1,4-diisopropoxybutane
5
Cyclic Ethers
6
Epoxides
The oxygen is a substituent on the parent chain, its position is identified with 2 numbers
followed by the word epoxy.
The parent is the oxirane, groups connected to the epoxide are substituents
7
PHYSICAL PROPERTIES
Ethers have generally low BP due to small polarities and lack of intermolecular hydrogen
bonding. Somewhat similar to alkanes of comparable molecular weight.
Ethers generally dissolve well in water up to 4-5 carbons, they can participate in H-bonding
forces with water
Nonpolar solutes dissolve better in ether than in alcohol
8
PREPARATION OF ETHERS
A. From Alcohols (internal dehydration)
OH
+
H2SO 4
HO
heat
O
+
H 2O
Called condensation reaction: 2 molecules are combined into a larger molecule while, at the
same time, giving a smaller molecule.
Best for making symmetrical ethers formed from unbranched primary alcohols.
Industrial method, not good lab synthesis.
If temp. is too high, alkene forms, elimination predominates with 2° and 3° alcohols.
Diols can cyclize to form 5- or 6-membered rings.
The mechanism
9
B. From Alkyl Halides via Williamson Reaction
+
R O Na
alkoxide ion
+
R'
X
SN2
ROR'
+
NaX
1o alkyl halide
Alkoxides prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH or
a metal.
10
B. From Alkyl Halides via Williamson Reaction
+
R O Na
alkoxide ion
+
R'
X
SN2
ROR'
+
NaX
1o alkyl halide
Alkoxides prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH or
a metal.
Williamson reaction, the best method for the preparation of ethers, is the reaction of metal
alkoxides and primary alkyl halides.
2° and 3° alkyl halides are not suitable, the alkoxide ion is strong enough base to bring about
eliminations.
Phenyl halides do not generally undergo substitutions.
11
12
PREPARATION OF EPOXIDES
A. From Alkenes via the Halohydrin route
13
B. From Alkenes via Peroxyacids
The peroxycarboxylic acid is reduced to a carboxylic acid.
The alkene is oxidized to an epoxide.
3 commonly used oxidizing agents: meta-chloroperoxybenzoic acid (MCPBA), magnesium salt of
monoperoxyphthalic acid (MMPP), peroxyacetic acid
14
REACTIONS OF ETHERS AND EPOXIDES
A. Acid-Catalyzed Cleavage of Ethers by Concentrated HX
O
+
HX
O
+
2 HX
X
2
+
HO
X
Requires strong acid and good nucleophile; 57% conc aq HI or 48% conc aq HBr
HCl less effective because weaker nucleophile in water than I– or Br–
Cleavage of 1° or 2° alkyl ethers is by an SN2 pathway:
Cleavage of 3° alkyl ether is by an SN1 pathway:
15
B. Ring Opening Reactions of Epoxides
Ring strain associated with 3-membered ring causes epoxides to undergo a variety of ringopening reactions. Nucleophilic substitution at one carbon atom with oxygen as leaving group.
Regioselectivity depends on the pH conditions.
Cleavage under acidic conditions, a mechanism:
Cleavage under basic conditions, a mechanism:
16
SPECTROSCOPY
A. Mass spectrometry
cleavage to form oxonium ion or loss of either alkyl group
SPECTROSCOPY
B. Infrared spectroscopy
ROR stretch, 1050-1150 cm–1
ROAr stretch, 1200-1275 cm–1
SPECTROSCOPY
C. UV/visible spectroscopy
need conjugated system
D. 1H NMR spectroscopy
CH–O–C, 3-4.5 ppm (downfield due to O prox.)
CH–O–C epoxide, 2.5 (upfield due to strain)
E. 13C NMR spectroscopy
C attached to O are deshielded (shifted downfield), 50-80 ppm