ethers and epoxides

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ETHERS AND EPOXIDES

STRUCTURE

Ethers have two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom.

R groups are identical = symmetrical etherR groups are different = unsymmetrical etherOxygen is sp3 hybridizedNearly tetrahedral angle, depends on the R groupOxygen atom gives a slight dipole moment

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Cyclic ethers contain oxygen atom(s) incorporated in a ring.Three-membered cyclic ethers are called epoxides.

EpoxidesAngle strain (60°)More reactive than other ethers

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NOMENCLATURE

EthersCommon 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

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NOMENCLATURE

EthersCommon 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.

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NOMENCLATURE

EthersCommon 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

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Cyclic Ethers

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EpoxidesThe 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

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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

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PREPARATION OF ETHERS

A. From Alcohols (internal dehydration)

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.

OH OOH

H2SO 4

heat+ + H2O

The mechanism

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B. From Alkyl Halides via Williamson Reaction

R O Na

+R' X ROR' NaX+

SN2+

1o alkyl halidealkoxide ion

Alkoxides prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH or a metal.

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B. From Alkyl Halides via Williamson Reaction

R O Na

+R' X ROR' NaX+

SN2+

1o alkyl halidealkoxide ion

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.

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PREPARATION OF EPOXIDES

A. From Alkenes via the Halohydrin route

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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

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REACTIONS OF ETHERS AND EPOXIDES

A. Acid-Catalyzed Cleavage of Ethers by Concentrated HX

O HX X OH

O HX X

+ +

+ 2 2

Requires strong acid and good nucleophile; 57% conc aq HI or 48% conc aq HBrHCl 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:

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B. Ring Opening Reactions of Epoxides

Ring strain associated with 3-membered ring causes epoxides to undergo a variety of ring-opening 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:

A. Mass spectrometrycleavage to form oxonium ion or loss of either alkyl group

SPECTROSCOPY

B. Infrared spectroscopyROR stretch, 1050-1150 cm–1

ROAr stretch, 1200-1275 cm–1

SPECTROSCOPY

C. UV/visible spectroscopyneed conjugated system

SPECTROSCOPY

D. 1H NMR spectroscopyCH–O–C, 3-4.5 ppm (downfield due to O prox.)CH–O–C epoxide, 2.5 (upfield due to strain)

E. 13C NMR spectroscopyC attached to O are deshielded (shifted downfield), 50-80 ppm