Transcript Ethers and Epoxides

CH 18: Ethers and Epoxides
Renee Y. Becker
Valencia Community College
CHM 2211
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Ethers and Their Relatives
• An ether has two organic groups (alkyl, aryl, or vinyl)
bonded to the same oxygen atom, R–O–R
• Diethyl ether is used industrially as a solvent
• Tetrahydrofuran (THF) is a solvent that is a cyclic
ether
• Epoxides contain a C-O-C unit which make-up a
three membered ring
• Thiols (R–S–H) and sulfides (R–S–R) are sulfur (for
oxygen) analogs of alcohols and ethers
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Naming Ethers
• Ethers are named two ways:
– As alkoxy derivatives of alkanes
– Derived by listing the two alkyl groups in the
general structure of ROR’ in alphabetical
order as separate words and adding the word
ether
• When both alkyl groups are the same, the prefix diprecedes the name of the alkyl group
• (Ethers can be described as symmetrical or
unsymmetrical)
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Naming Ethers
CH3CH2 O CH2CH3
Diethyl ether
Ethoxyethane
CH3CH2 O CH3
Ethyl methyl ether
Methoxyethane
CH3CH2 O CH2CH2CH2Cl
3-Chloropropyl ethyl ether
1-Chloro-3-ethoxypropane
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Naming Ethers
• Epoxides (oxiranes)
– “epoxy” always preceeds the name of the alkane
O
O
1,2-Epoxycyclohexane
2-Methyl-2,3-epoxybutane
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Example 1: Name
OCH3
1
O
6
O
2
O
7
3
O
Br
4
O
Cl
8
O
Br
5
Br
OCH3
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Example 2: Draw
1. Isopropyl methyl ether
2. 4-t-butoxy-1-cyclohexene
3. Phenyl propyl ether
4. O- nitro anisole
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Structure, Properties, and Sources of Ethers
• R–O–R ~ tetrahedral bond angle (112° in
dimethyl ether)
• Oxygen is sp3-hybridized
• Oxygen atom gives ethers a slight dipole
moment (diethyl ether 1.2 D)
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Structure, Properties, and Sources of Ethers (comparative boiling
points)
CH3CH2OCH2CH3
Diethyl ether
bp =35oC
solubility in water: 7.5 g/100mL
CH3CH2CH2CH2CH3
Pentane
bp =36oC
solubility in water: insoluble
CH3CH2CH2CH2OH
1-Butanol
bp =117oC
solubility in water: 9 g/100mL
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Preparation of Ethers
• Acid catalyzed synthesis of ethers:
2
OH
butanol
H2SO4
O
130oC
Dibutyl ether
+ H2O
Limited to symmetrical ethers. WHY?
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Mechanism 1: Acid catalyzed synthesis of ethers
HCl
2
OH
O
+
130 C
H
OH
H O
H3 O+
+
Cl-
H
O
H
O+
H---Cl
H
+
H3O+
+
H
O+
-
Cl
O
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Preparation of Ethers
• Williamson ether synthesis
•Metal alkoxides react with primary alkyl halides by an
SN2 pathway to yield ethers.
•Secondary and tertiary substrates react following an
E2 mechanism
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Mechanism 2: Williamson Ether Synthesis
OH
O – Na+
NaH
O
THF
+
CH3CH2----I
+
H2
+
NaI
THF
O
H
O– Na+
THF
+
Na+ H-
CH3CH2----I
+
H2
O
NaI
+
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Example 3: Williamson ether synthesis
O
CH3CH2O
+ CH3CH2Br
+
Br
SN2
E2
?
?
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Example 4
How would you prepare the following
compounds using a Williamson synthesis?
• Methyl propyl ether
• Anisole (methyl phenyl ether)
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Example 5: Predict the product:
Br
O
+ CH3CH2CHCH3
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Alkoxymercuration of Alkenes
• React alkene with an alcohol and mercuric
acetate or trifluoroacetate
• Demercuration with NaBH4 yields an ether
• Overall Markovnikov addition of alcohol
to alkene
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Reactions of Ethers: Acidic Cleavage
• Ethers are generally unreactive
• Strong acid will cleave an ether at elevated
temperature
• HI, HBr produce an alkyl halide from less
hindered component by SN2 (tertiary ethers
undergo SN1)
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Mechanism 3: Acidic Cleavage
Note that the halide attacks the protonated ether at
the less highly substituted site.
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Example 6
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Reactions of Ethers: Claisen Rearrangement
• Specific to allyl aryl ethers, ArOCH2CH=CH2
• Heating to 200–250°C leads to an o-allylphenol
• Result is alkylation of the phenol in an ortho position
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Mechanism 4: Claisen Rearrangement
• Concerted pericyclic 6-electron, 6-membered
ring transition state
• Mechanism consistent with 14C labeling
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Example 7
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Cyclic Ethers: Epoxides
• Cyclic ethers behave like acyclic ethers, except
if ring is 3-membered
• Dioxane and tetrahydrofuran are used as
solvents
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Epoxides (Oxiranes)
• Three membered ring ether is called an oxirane
(root “ir” from “tri” for 3-membered; prefix “ox” for
oxygen; “ane” for saturated)
• Also called epoxides
• Ethylene oxide (oxirane; 1,2-epoxyethane) is
industrially important as an intermediate
• Prepared by reaction of ethylene with oxygen at
300 °C and silver oxide catalyst
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Preparation of Epoxides Using a Peroxyacid
• Treat an alkene with a peroxyacid
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Mechanism 5: Preparation of Epoxides Using a Peroxyacid
peroxyacetic acid
H
C6H5
H
O
O
+
H5C6
H
(E)-1,2-diphenylethene
O
H
CH3
C6H5
O
H5C6
OH
+
O
H
trans-1,2-diphenyloxirane
CH3
acetic acid
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Epoxides from Halohydrins
• Addition of HO-X to an alkene gives a halohydrin
• Treatment of a halohydrin with base gives an epoxide
• Intramolecular Williamson ether synthesis
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Ring-Opening Reactions of Epoxides
• Water adds to epoxides with dilute acid at room
temperature
• Product is a 1,2-diol (on adjacent C’s: vicinal)
• Mechanism: acid protonates oxygen and water
adds to opposite side (anti-addition)
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Mechanism 6: Acid catalyzed ring-openings
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Ethylene Glycol
• 1,2-ethanediol from acid catalyzed hydration of ethylene
• Widely used as automobile antifreeze (lowers freezing
point of water solutions)
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Halohydrins from Epoxides
• Anhydrous HF, HBr, HCl, or HI combines with an
epoxide
• Gives trans product
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Base-Catalyzed Epoxide Opening
• Strain of the three-membered ring is relieved on
ring-opening
• Hydroxide cleaves epoxides at elevated
temperatures to give trans 1,2-diols
– Complete the reaction
O
CH2
OH-
H2O, 100oC
Methylenecyclohexane
oxide
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Mechanism 7: Base-Catalyzed Epoxide Opening
O
-
CH2
OH
O–
CH 2OH
H
O
H
OH
CH 2OH
+
-
OH
100 C
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Mechanism 7: Base-Catalyzed Epoxide Opening
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Addition of Grignards to Ethylene Oxide
• Adds –CH2CH2OH to the Grignard reagent’s
hydrocarbon chain
• Acyclic and other larger ring ethers do not react
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Thiols
• Thiols (RSH), are sulfur analogs of alcohols
– Named with the suffix -thiol
– SH group is called “mercapto group”
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Example 8: Name
1
SH
2
SH
Br
3
SH
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Example 9: Draw
1. Cyclopentanethiol
2. 3-methyl-4-heptanethiol
3. 4-ethyl-4-isopropyl-2-methyl-3-hexanethiol
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Sulfides
• Sulfides (RSR), are sulfur analogs of ethers
– Named by rules used for ethers, with sulfide in
place of ether for simple compounds and alkylthio
in place of alkoxy
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Example 10: Name or Draw
1
2
3
4
S
S
ethyl phenyl sulfide
sec-butyl isopropyl sulfide
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Thiols: Formation and Reaction
• From alkyl halides by displacement with a sulfur
nucleophile such as SH
– The alkylthiol product can undergo further
reaction with the alkyl halide to give a
symmetrical sulfide, giving a poorer yield of the
thiol
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Sulfides
• Thiolates (RS) are formed by the reaction of a thiol
with a base
• Thiolates react with primary or secondary alkyl halide to
give sulfides (RSR’)
• Thiolates are excellent nucleophiles and react with many
electrophiles
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