chapter 16 ethers, epoxides, and sulfides copyright © the mcgraw-hill companies, inc. permission...
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Chapter 16Ethers, Epoxides, and Sulfides
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© 2013 Pearson Education, Inc. Chapter 14 2
Ethers
• Formula is R—O—Rwhere R and R are alkyl or aryl.
• Symmetrical or unsymmetrical
Nomenclature of Ethers, Epoxides, and Sulfides
name as alkoxy derivatives of alkanes
CH3OCH2 CH3
methoxyethane
CH3CH2OCH2 CH3
ethoxyethane
CH3CH2OCH2CH2CH2Cl
3-chloro-1-ethoxypropane
Substitutive IUPAC Names of Ethers
Name the groups attached to oxygen in alphabetical order as separate words; "ether" is last word.
CH3OCH2 CH3
ethyl methyl ether
CH3CH2OCH2 CH3
diethyl ether
CH3CH2OCH2CH2CH2Cl
3-chloropropyl ethyl ether
Functional Class IUPAC Names of Ethers
bent geometry at oxygen analogousbent geometry at oxygen analogous
to water and alcohols to water and alcohols
Structure and Bondingin
Ethers and Epoxides
© 2013 Pearson Education, Inc. Chapter 14 7
Structure and Polarity
• Oxygen is sp3 hybridized.• Bent molecular geometry.• Tetrahedral C—O—C angle is 110°.• Polar C—O bonds.
H
OH
(CH3)3CO
C(CH3)3
112°
105° 108.5°
132°
HO
CH3
CH3
OCH3
Bond angles at oxygen are sensitiveto steric effects
Most stable conformation of diethyl ether
resembles that of pentane.
An oxygen atom affects geometry in much thesame way as a CH2 group
Most stable conformation of tetrahydropyran
resembles that of cyclohexane.
An oxygen atom affects geometry in much thesame way as a CH2 group
Physical Properties of Ethers
boiling point
36°C
35°C
117°C
Table 16.1 Ethers resemble alkanes more than alcohols
with respect to boiling point O
OH
Intermolecular hydrogenbonding possible in alcohols; not possible in alkanes or ethers.
solubility in water (g/100 mL)
very small
9
7.5
Table 16.1 Ethers resemble alcohols more than alkanes
with respect to solubility in water O
OH
Hydrogen bonding towater possible for ethersand alcohols; not possible for alkanes.
© 2013 Pearson Education, Inc. Chapter 14 14
Hydrogen Bond Acceptor
• Ethers cannot hydrogen bond with other ether molecules, so they have a lower boiling point than alcohols.
• Ether molecules can hydrogen bond with water and alcohol molecules.
• They are hydrogen bond acceptors.
© 2013 Pearson Education, Inc. Chapter 14 15
Ethers as Solvents • Ethers are widely used as solvents
because they can dissolve nonpolar and polar
substances. they are unreactive toward strong bases.
Ethers are relatively unreactive.
Their low level of reactivity is one reason why
ethers are often used as solvents in chemical
reactions.
© 2013 Pearson Education, Inc. Chapter 14 16
Ether Complexes
• Grignard reagents: Complexation of an ether with a Grignard reagent stabilizes the reagent and helps keep it in solution.
• Electrophiles: The ether’s nonbonding electrons stabilize the borane (BH3).
O B
H
H
H
+ _
BH3 THF
Crown Ethers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
structurecyclic polyethers derived from repeating
—OCH2CH2— units
propertiesform stable complexes with metal ions
applicationssynthetic reactions involving anions
Crown Ethers
© 2013 Pearson Education, Inc. Chapter 14 19
Crown Ether Complexes
• Crown ethers can complex metal cations in the center of the ring.
• The size of the ether ring will determine which cation it can solvate better.
• Complexation by crown ethers often allows polar inorganic salts to dissolve in nonpolar organic solvents.
O
O O
O
O
O
18-Crown-6
forms stable Lewis acid/Lewis base complex with K+
K+
not soluble in benzene
Ion-Complexing and Solubility
K+F–
Ion-Complexing and Solubility
K+F–
add 18-crown-6
benzene
O
O O
O
O
O
Ion-Complexing and Solubility
18-crown-6 complex of K+ dissolves in benzene
benzene
F–
O
O O
O
O
O
O
O O
O
O
O
K+
Ion-Complexing and Solubility
+ F–F– carried into benzene to preserve electroneutrality
benzene
O
O O
O
O
O
O
O O
O
O
O
K+
Application to organic synthesis
Complexation of K+ by 18-crown-6 solubilizes potassium salts in benzene.
Anion of salt is in a relatively unsolvated state in benzene (sometimes referred to as a "naked anion").
Unsolvated anion is very reactive.
Only catalytic quantities of 18-crown-6 are needed.
Example
CH3(CH2)6CH2BrKF
18-crown-6benzene
CH3(CH2)6CH2F
(92%)
Preparation of EthersPreparation of Ethers
Acid-Catalyzed Condensation of Alcohols*
2 CH3CH2CH2CH2OH
H2SO4, 130°C
CH3CH2CH2CH2OCH2CH2CH2CH3
(60%)
Method is good for primary alcohols. Diethyl ether is made on Method is good for primary alcohols. Diethyl ether is made on
industrial scale using this method. Ethylene will form at higher industrial scale using this method. Ethylene will form at higher
temperatures. Secondary and tertiary alcohols give alkenes as temperatures. Secondary and tertiary alcohols give alkenes as
main product. main product.
H+
(CH3)2C=CH2 + CH3OH (CH3)3COCH3
tert-Butyl methyl ether
tert-Butyl methyl ether (MTBE) was produced on a
scale exceeding 15 billion pounds per year in the U.S.
during the 1990s. It is an effective octane booster in
gasoline, but contaminates ground water if allowed to
leak from storage tanks. Further use of MTBE is unlikely.
Addition of Alcohols to Alkenes
The Williamson Ether SynthesisThe Williamson Ether Synthesis
© 2013 Pearson Education, Inc. Chapter 14 31
Williamson Ether Synthesis
• This method involves an SN2 attack of the alkoxide on an unhindered primary halide or tosylate.
• The alkoxide is commonly made by adding Na, K, or NaH to the alcohol
(71%)
CH3CH2CH2CH2ONa + CH3CH2I
CH3CH2CH2CH2OCH2CH3 + NaI
Example
Another Example
+ CH3CHCH3
ONa
CH2Cl
(84%)CH2OCHCH3
CH3
Alkyl halide must be
primary or methyl
Alkoxide ion can be derived
from methyl, primary,
secondary, or tertiary alcohol.
CH3CHCH3
OH
Na
CH2OH
HCl
CH2OCHCH3
CH3
CH2Cl + CH3CHCH3
ONa
(84%)
Origin of Reactants
What happens if the alkyl halide is not primary?
CH2ONa + CH3CHCH3
Br
CH2OH + H2C CHCH3
Elimination by the E2 mechanism becomes
the major reaction pathway.
Reactions of Ethers:Reactions of Ethers:
Acid-Catalyzed Cleavage of Ethers
Ethers can be cleaved by heating with concentrated HBr and HI.
Reactivity: HI > HBr
HBrCH3CHCH2CH3
OCH3
CH3Br+
(81%)
CH3CHCH2CH3
Brheat
Example
© 2013 Pearson Education, Inc. Chapter 14 39
Mechanism of Ether Cleavage • Step 1: Protonation of the oxygen.
• Step 2: The halide will attack the carbon and displace the alcohol (SN2).
© 2013 Pearson Education, Inc. Chapter 14 40
Mechanism of Ether Cleavage
•This does not occur with aromatic alcohols (phenols).
Step 3: The alcohol reacts further with the acid Step 3: The alcohol reacts further with the acid to produce another mole of alkyl halide.to produce another mole of alkyl halide.
HI
150°CICH2CH2CH2CH2I
(65%)
O
Cleavage of Cyclic Ethers
O••
••
HI
H
O••
+
••
••
••I ••–
ICH2CH2CH2CH2I
HI H
O
••
••
••I ••••
Mechanism
© 2013 Pearson Education, Inc. Chapter 14 43
Autoxidation of Ethers
• In the presence of atmospheric oxygen, ethers slowly oxidize to hydroperoxides and dialkyl peroxides.
• Both are highly explosive.
• Precautions: Do not distill to dryness. Store in full bottles with tight caps.
© 2013 Pearson Education, Inc. Chapter 14 44
Autoxidation of Ethers
© 2013 Pearson Education, Inc. Chapter 14 45
Preparation of Epoxides:Preparation of Epoxides:
A Review and a PreviewA Review and a Preview
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© 2013 Pearson Education, Inc. Chapter 14 46
Epoxides are prepared by two major methods.Both begin with alkenes.
Reaction of alkenes with peroxy acids(Section 6.19)
Conversion of alkenes to vicinalhalohydrins, followed by treatmentwith base (Section 16.10, this chapter)
Preparation of Epoxides
© 2013 Pearson Education, Inc. Chapter 14 47
Synthesis of Epoxides
• Peroxyacids are used to convert alkenes to epoxides.• Most commonly used peroxyacid is meta-
chloroperoxybenzoic acid (MCPBA).• The reaction is carried out in an aprotic acid to
prevent the opening of the epoxide.
© 2013 Pearson Education, Inc. Chapter 14 48
Halohydrin Cyclization
• If an alkoxide and a halogen are located in the same molecule, the alkoxide may displace a halide ion and form a ring.
• Treatment of a halohydrin with a base leads to an epoxide through this internal SN2 attack.
© 2013 Pearson Education, Inc. Chapter 14 49
HOH
BrH
NaOH
H2O
(81%)
H
H
O
Another look
O Br
HH
••
••••
•• ••••–
via:
© 2013 Pearson Education, Inc. Chapter 14 50
anti
additioninversion
Epoxidation via Vicinal Halohydrins
Br2
H2O
OH
Corresponds to overall syn addition ofoxygen to the double bond.
Br NaOH
O
Reactions of Epoxides:Reactions of Epoxides:A Review and a PreviewA Review and a Preview
Reactions of epoxides involve attack by anucleophile and proceed with ring-opening.For ethylene oxide:
Nu—H +
Nu—CH2CH2O—H
H2C CH2
O
In General...
NaOCH2CH3
CH3CH2OH
(50%)
Example
H2C
O
CH2
CH3CH2O CH2CH2OH
••••O
H2C CH2
CH3CH2 O••
•• ••–
••
•
•CH3CH2 O
••CH2CH2 O
••H ••
•
••
•
O CH2CH3
–
Mechanism
–
••
•• •
•CH3CH2 O ••CH2CH2 O
••
O CH2CH3
•••
•
H
Example
O
H2C CH2
KSCH2CH2CH2CH3
ethanol-water, 0°C
(99%)
CH2CH2OHCH3CH2CH2CH2S
For epoxides where the two carbons of thering are differently substituted:
In General...
CH2
O
C
R
H
Nucleophiles attack herewhen the reaction iscatalyzed by acids:
Anionic nucleophilesattack here:
Anionic Nucleophile Attacks Less-crowded Carbon
1. diethyl ether2. H3O+
MgBr
+
O
H2C CHCH3
CH2CHCH3
OH
(60%)
Hydride attacksless-crowded
carbon.
Lithium Aluminum Hydride Reduces Epoxides
O
H2C CH(CH2)7CH3
1. LiAlH4, diethyl ether2. H2O
(90%)OH
H3C CH(CH2)7CH3
Acid-Catalyzed Ring-OpeningReactions of Epoxides
Example
O
H2C CH2 CH3CH2OCH2CH2OH
(87-92%)
CH3CH2OCH2CH2OCH2CH3 formed only on heating and/or longer reaction times.
CH3CH2OH
H2SO4, 25°C
Example
O
H2C CH2 HBr
10°CBrCH2CH2OH
(87-92%)
BrCH2CH2Br formed only on heating and/or longer reaction times.
Mechanism Br••
••••
–••
••
•
•O••
Br
CH2CH2 H
••••
••O
H2C CH2
••HBr
••••
••
••O
H2C CH2+
H
Acid-Catalyzed Hydrolysis of Ethylene Oxide
••O
H2C CH2
••
O••
H
H
H+
••O
H2C CH2+
HO••
H
H
••
Step 1
••O
H2C CH2+
H
O
••••
H
H
Step 2
+ ••
•
•O
O
CH2CH2
H
H
H
•
•
Acid-Catalyzed Hydrolysis of Ethylene Oxide
•
•
Step 3
+ ••
•
•O
O
CH2CH2
H
H
H
O••
••
H
H •
•
O••
H
H+
H
••
•
•O
O
CH2CH2
H
H
••
Acid-Catalyzed Hydrolysis of Ethylene Oxide
Acid-Catalyzed Ring Opening of Epoxides
Nucleophile attacks more substituted carbon of protonated epoxide.
Inversion of configuration at site of nucleophilic attack.
Characteristics:
CH3OH
CC
H
H3C CH3
CH3O
Nucleophile Attacks More-substituted Carbon
H2SO4
CH3CH CCH3
CH3OH
OCH3
(76%)
Stereochemistry
Inversion of configuration at carbon being attacked by nucleophile
(73%)
H
H
O HBr
HOH
BrH
O
R
R
(57%)
R
S
Stereochemistry
H3C CH3
H3C CH3
H
HH
H OHCH3O
Inversion of configuration at carbon being attacked by nucleophile
CH3OH
H2SO4
R O
R
S
R
Stereochemistry
H3C CH3
H3C CH3
H
HH
H OHCH3OCH3OH
H2SO4
+ +CH3O O
H3CH
H3CH
H+
H
anti-Hydroxylation of Alkenes H
H
CH3COOH
O
H
H
O
H2O
HClO4
(80%)
HOH
OHH
+ enantiomer