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Ethers and Epoxides
Chapter 18
Ethers and Their RelativesAn 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 solventTetrahydrofuran (THF) is a solvent that is a cyclic ether Epoxides contain a C-O-C unit which make-up a three membered ringThiols (R–S–H) and sulfides (R–S–R′) are sulfur (for oxygen) analogs of alcohols and ethers
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18.1 Naming Ethers
Ethers are named two ways:As alkoxy derivatives of alkanesDerived 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 di- precedes the name of the alkyl group(Ethers can be described as symmetrical or unsymmetrical)
Naming Ethers
CH3CH2 O CH2CH3
Diethyl etherEthoxyethane
CH3CH2 O CH3
Ethyl methyl etherMethoxyethane
CH3CH2 O CH2CH2CH2Cl3-Chloropropyl ethyl ether1-Chloro-3-ethoxypropane
Naming Ethers
Epoxides (oxiranes)“epoxy” always preceeds the name of the alkane
O
1,2-Epoxycyclohexane
O2-Methyl-2,3-epoxybutane
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Name the following according to IUPAC rules:
CH3CHOCHCH3
CH3 CH3
Br
OCH3OCH3
O
O
18.2 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)
Structure, Properties, and Sources of Ethers (comparative boiling points)
CH3CH2OCH2CH3Diethyl ether
bp =35oCsolubility in water: 7.5 g/100mL
CH3CH2CH2CH2CH3Pentanebp =36oC
solubility in water: insoluble
CH3CH2CH2CH2OH1-Butanolbp =117oC
solubility in water: 9 g/100mL
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Preparation of Ethers
Acid catalyzed synthesis of ethers:
H2SO4
Dibutyl ether130oC
OH2butanol
O
+ H2O
Limited to symmetrical ethers. WHY?
Preparation of Ethers
Williamson ether synthesis
Metal alkoxides react with primary alkyl halides by an SN2 pathway to yield ethers.
Williamson ether synthesis
Complete the reaction:
CH3CH2CH2CH2CH2OButoxide ion
+ CH3CH2IIodoethane
the alkoxide ion is produced by treating an alcohol with a strong base: NaH
An alternative is the use of silver oxide, AgO2:
CH3CH2CH2CH2CH2OHButanol
CH3CH2IAgO2
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Williamson ether synthesis
Secondary and tertiary substrates react following an E2 mechanism:
O + CH3CH2BrSN2 ?
BrCH3CH2OE1 ?+
Consider:
The treatment of cyclohexanol with NaH gives an alkoxide ion that reacts with iodoethane to yield an ether. Write the reaction showing all steps
How would you prepare the following compounds using a Williamson synthesis?
Methyl propyl etherAnisole (methyl phenyl ether)Benzyl isopropyl etherEthyl 2,2-dimethylpropyl ether
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Predict the product:
+Br
CH3CH2CHCH3
O
18.4 Alkoxymercuration of Alkenes
React alkene with an alcohol and mercuric acetate or trifluoroacetateDemercuration with NaBH4 yields an etherOverall Markovnikov addition of alcohol to alkene
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Predict the product:
CH31. Hg(O2CCF3)2, (CH3)2CHOH
2. NaBH4?
18.5 Reactions of Ethers: Acidic Cleavage
Ethers are generally unreactiveStrong acid will cleave an ether at elevated temperatureHI, HBr produce an alkyl halide from less hindered component by SN2 (tertiary ethers undergo SN1)
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Mechanism:
Note that the halide attacks the protonated ether at the less highly substituted site.
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18.6 Reactions of Ethers: Claisen Rearrangement
Specific to allyl aryl ethers, ArOCH2CH=CH2
Heating to 200–250°C leads to an o-allylphenolResult is alkylation of the phenol in an ortho position
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Claisen Rearrangement Mechanism
Concerted pericyclic 6-electron, 6-membered ring transition stateMechanism consistent with 14C labeling
18.7 Cyclic Ethers: Epoxides
Cyclic ethers behave like acyclic ethers, except if ring is 3-memberedDioxane 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 epoxidesEthylene oxide (oxirane; 1,2-epoxyethane) is industrially important as an intermediatePrepared by reaction of ethylene with oxygen at 300 °C and silver oxide catalyst
Preparation of Epoxides Using a Peroxyacid
Treat an alkene with a peroxyacid
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+H
H C6H5
C6H5
E-1,2-Diphenyletheneperoxyacetic
acid
CH3COOHO
+
H
H C6H5
C6H5
trans-2.3-DiphenyloxiraneO
acetic acid
CH3COHO
Epoxides from Halohydrins
Addition of HO-X to an alkene gives a halohydrinTreatment of a halohydrin with base gives an epoxideIntramolecular Williamson ether synthesis
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NaOH
H2O?
OHBr
18.8 Ring-Opening Reactions of Epoxides
Water adds to epoxides with dilute acid at room temperatureProduct is a 1,2-diol (on adjacent C’s: vicinal)Mechanism: acid protonates oxygen and water adds to opposite side (trans addition)
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Mechanism of acid catalyzed ring-openings:
Ethylene Glycol1,2-ethanediol from acid catalyzed hydration of ethyleneWidely used as automobile antifreeze (lowers freezing point of water solutions)
Halohydrins from EpoxidesAnhydrous HF, HBr, HCl, or HI combines with an epoxideGives trans product
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Regiochemistry of Acid-Catalyzed Opening of Epoxides
Nucleophile preferably adds to less hindered site if primary and secondary C’sAlso at tertiary because of carbocation character (See Figure 18.2)
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Base-Catalyzed Epoxide Opening
Strain of the three-membered ring is relieved on ring-openingHydroxide cleaves epoxides at elevated temperatures to give trans 1,2-diols
Complete the reaction and diagram the mechanism:
CH2
O
OH-
H2O, 100oC
Methylenecyclohexane oxide
Complete the reactions:
H3CH2CHC CH2
ONH2
NH3
H3CH2CHC CH2
OH3
18O+
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Other strong bases:
Addition of Grignards to Ethylene Oxide
Adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chainAcyclic and other larger ring ethers do not react
18.9 Crown Ethers Large rings consisting repeating (-OCH2CH2-) or similar unitsNamed as x-crown-yx is the total number of atoms in the ringy is the number of oxygen atoms18-crown-6 ether: 18-membered ring containing 6 oxygens atoms
Central cavity is electronegative and attracts cations
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Uses of Crown EthersComplexes between crown ethers and ionic salts are soluble in nonpolar organic solventsCreates reagents that are free of water that have useful propertiesInorganic salts dissolve in organic solvents leaving the anion unassociated, enhancing reactivity
Other Cylic Ethers
OO
O
O
O
Oxetane Oxolane(Tetrahydrofuran)
Oxane(Tetrahydropyran)
1,4-Dioxane
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18.10 Thiols and Sulfides
Thiols (RSH), are sulfur analogs of alcoholsNamed with the suffix -thiolSH group is called “mercapto group” (“capturer of mercury”)
SulfidesSulfides (RSR′), are sulfur analogs of ethers
Named by rules used for ethers, with sulfide in place of ether for simple compounds and alkylthioin place of alkoxy
Thiols: Formation and ReactionFrom 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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SulfidesThiolates (RS−) are formed by the reaction of a thiol with a baseThiolates react with primary or secondary alkyl halide to give sulfides (RSR’)Thiolates are excellent nucleophiles and react with many electrophiles
18.11 Spectroscopy of Ethers Infrared: C–O single-bond stretching 1050 to 1150 cm−1 overlaps many other absorptions.Proton NMR: H on a C next to ether O are shifted downfield to δ 3.4 to δ 4.5
The 1H NMR spectrum of dipropyl ether shows the these signals at δ 3.4In epoxides, these H’s absorb at δ 2.5 to δ 3.5 d in their 1H NMR spectra
Carbon NMR: C’s in ethers exhibit a downfield shift to δ 50 to δ 80
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End fo Chapter 18