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    Chapter 8. Ethers and Epoxides

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    hanna liu
    (1) Ethers have the general formula R-O-R' and are named based on the alkyl or aryl groups in alphabetical order followed by "ether". (2) Ethers have lower boiling points than similar alcohols due to an inability to form hydrogen bonds. They act as good solvents due to low reactivity and inertness to acids and bases. (3) Grignard reagents are organometallic compounds containing a carbon-magnesium bond that react as strong nucleophiles. Ethers stabilize Grignard reagents and are commonly used as reaction solvents.

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    Chapter 8. Ethers and Epoxides

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    hanna liu
    108 views10 pages
    (1) Ethers have the general formula R-O-R' and are named based on the alkyl or aryl groups in alphabetical order followed by "ether". (2) Ethers have lower boiling points than similar alcohols due to an inability to form hydrogen bonds. They act as good solvents due to low reactivity and inertness to acids and bases. (3) Grignard reagents are organometallic compounds containing a carbon-magnesium bond that react as strong nucleophiles. Ethers stabilize Grignard reagents and are commonly used as reaction solvents.

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    Chapter 8

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    (1) Ethers have the general formula R-O-R' and are named based on the alkyl or aryl groups in alphabetical order followed by "ether". (2) Ethers have lower boiling points than similar alcohols due to an inability to form hydrogen bonds. They act as good solvents due to low reactivity and inertness to acids and bases. (3) Grignard reagents are organometallic compounds containing a carbon-magnesium bond that react as strong nucleophiles. Ethers stabilize Grignard reagents and are commonly used as reaction solvents.

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    Download as ppt, pdf, or txt
    108 views10 pages

    Chapter 8. Ethers and Epoxides

    Uploaded by

    hanna liu
    (1) Ethers have the general formula R-O-R' and are named based on the alkyl or aryl groups in alphabetical order followed by "ether". (2) Ethers have lower boiling points than similar alcohols due to an inability to form hydrogen bonds. They act as good solvents due to low reactivity and inertness to acids and bases. (3) Grignard reagents are organometallic compounds containing a carbon-magnesium bond that react as strong nucleophiles. Ethers stabilize Grignard reagents and are commonly used as reaction solvents.

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    © All Rights Reserved
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    Download as ppt, pdf, or txt
    of 10

    Chapter 8.

    Ethers and Epoxides


    General formula: Ether --- R–O–R’
    Here R and R’ can be identical or different; alkyl or aryl.
    8.1 Nomenclature of Ethers
    (1) Ethers are usually named by giving the name of each alkyl or aryl group, in
    alphabetical order, followed by the word “ether”.
    CH3
    O O CH2CH3
    CH3 C O CH3
    CH3 dicyclopropyl ether ethyl phenyl ether
    t-butyl methyl ether
    (2) Complex ethers, the –OR group is named as alkoxy group.
    5 CH3
    OCH3 1
    OH 3 2
    4 1 OCH2CH3
    2 4
    OCH3 OCH3
    3 CH3
    5 6
    1,2-dimethyoxycyclpentane cis-2-methyoxycyclopentanol
    (S)-1-ethyoxy-1,3-dimethylcyclohexane
    (3) In IUPAC system, the smaller alkoxy group is named as a substituent.
    9 8 7 6 5 4 3 2 1
    CH3CH2CH2CH2CH2CH CH2CHCH3 2-ethyoxy-4-methylnonane
    CH3 OCH2CH3
    1
    8.2 Physical Properties of Ethers
    Phenomena: see information on pg 232 in the textbook
    (1) colorless, pleasant smell,
    (2) having low boiling point (compared with alcohols with similar MW,
    close to hydrocarbons with similar MW)
    (3) Ethers and alcohols are miscible.
    Low MW ethers --- rather soluble in water
    Explanation: Ethers can’t form hydrogen bonds with one another. So the
    intermolecular interaction between ethers is weaker than that

    between their isomeric alcohols.


    Ethers can form hydrogen bonds with alcohols.
    Low MW ethers can also form hydrogen bonds with R
    water.
       
    R   R   R H O
    O H O R
    O H O R
    a hydrogen bond
    R
    hydrogen bonds

    8.3 Ethers as Solvents


    Ethers ------ relative inert or stable to dilute acids, dilute bases and
    oxidizing/reducing agents.
    2
    Applications: (1) used as excellent solvents to carry out organic reactions.
    (2) used for extracting organic compounds from the natural sources.
    Problems for use: If exposed to air for a long time in the lab, ether may have
    organic peroxides as a result of oxidation.
    CH3CH2OCH2CH3 O2 CH3CH2OCHCH3
    OOH
    peroxide (explosive)
    Solution: before using the ether, shake with aqueous FeSO4 solution to destroy the
    peroxides by reduction.
    8.4 The Grignard Reagent; an Organometallic Compound
    Grignard reagents --- alkyl- or arylmagnesium halides.
    dry ether R' R'
    R X Mg R MgX O ethers acting
    as Lewis bases
    solvent R Mg X to stablize a
    not soluble soluble
    in ether in ether O Grignard reagent
    R' R'
    Here, two commonly used ethers --- diethyl ether and THF (tetrahydrofuran, O )
    the ether used --- must be scrupulously dry free of traces of H2O and alcohols.
    ether CH3CH2 MgBr ethylmagnisum bromide
    CH3CH2 Br Mg

    ether
    Cl Mg MgCl phenylmagnium chloride
    3
    Organometallic compounds --- organic compounds containing a carbon-metal bond.
    Grignard reagents --- organometallic compounds (containing C-Mg bond).
     
    R MgX here, the R (alkyl or aryl) group --- negatively charged --- carbanion
    Carbanions --- alkyl or aryl groups with negatively charged carbon atoms.
    they are strong bases ( conjugate bases of hydrocarbons).
    Carbanions react with very weak acids: water, compounds with OH, SH or NH bonds.
     
     
    R MgX H OH R H Mg(OH)X
    e.g. we can’t make a Grignard reagent from HSCH2CH2Br. But we can make a
    Grignard reagent from CH3SCH2CH2Br since no acidic proton in this compound.
    CH3SCH2CH2Br Mg ether CH3SCH2CH2MgBr
     
     
    R MgX D OD R D Mg(OD)X
    heavy water isotope labeled organic compound

    e.g. Prepare (1) (problem 8.6) CH3CHDCH3 from (CH3)2CHOH.


    (2) D from
    SOCl2 Mg D2O
    (1) (CH3)2CHOH (CH3)2CHCl
    ether (dry)
    (CH3)2CHMgCl (CH3)2CHD

    (2) HBr Mg D2O D


    Br ether (dry) MgBr
    4
    Other organometallic compounds besides Grignard reagents:
    (1) acetylides (in chapter 3): RC CNa (contain C-Na bond)
    NaNH2 D2O
    RC CH RC CNa RC CNa RC CD
    NH3

    (2) Organolithium compounds: R-Li (contain C-Li bond)


    dry ether D2O
    R X 2 Li R Li LiX R Li R D

    8.5 Preparation of Ethers


    (1) alcohol-sulfuric acid method --- make symmetric ethers from primary alcohols
    H2SO4
    CH3CH2O H HO CH2CH3 o CH3CH2OCH2CH3 H 2O
    140 C
    This reaction is a SN2 mechanism, the E2 Elimination reaction may also occur. But the
    condition of the reactions (e.g. reaction temperature) are different.
    (2) acid-catalyzed addition of alcohol to alkene --- make unsymmetric ethers
    CH3
    H+
    CH3OH CH2 C(CH3)2 CH3O C CH3 Octane number enhancer in
    CH3 unleaded gasoline

    (3) Williamson Synthesis --- make unsymmetric ethers (common methods in labs)
    2 ROH + 2 Na 2 RO Na+ + H2
    SN2
    2 RO Na+ + R' X ROR' + Na+X
    5
    e.g. Synthesize (CH3)3COCH3
    Na CH3Br
    (CH3)3COH (CH3)3CO-Na+ (CH3)3COCH3

    Can't use CH3OH Na (CH3)3CBr


    CH3O-Na+ (CH3)2C CH2

    8.6 Cleavage of Ethers


    Ethers can react with strong proton acids and Lewis acids
    H
    R O R'
    R O R' H+ R O R' R O R' Br B Br
    Br B Br
    Br
    Br
    If the groups R and/or R’ are primary or secondary, a strong nucleophile (Iˉ or Brˉ)
    can break the C-O bond of the ether by a SN2 process.
    heat
    CH3OCH(CH3)2 HI CH3I HOCH(CH3)2
    not CH3OH ICH(CH3)2

    1. heat
    OCH2CH3 BBr3 OH CH3CH2Br
    2. H2O

    not CH3CH2OH Br
    6
    If the R or R’ is tertiary, even weak nucleophiles will cleave the C-O bond of the
    ether by a SN1 process.
    H+
    OC(CH3)3 OH (CH3)3COH
    H2O

    HI (CH3)3CI
    OC(CH3)3 OH

    8.7 Epoxides (Oxiranes)


    Epoxides --- cyclic ethers with a three-membered ring containing one oxygen atom.
    H2 C CH2 H H H CH3
    C C C C
    O H3C O CH3 H 3C O H

    Ethylene oxide cis-2-butene oxide trans-2-butene oxide


    (oxirane) (cis-2,3-dimethyloxirane) (trans-2,3-dimethyloxirane)
    H
    O cyclohexene oxide in general: name = alkene + oxide

    H
    7
    Preparation of epoxides from alkene compounds
    silver catalyst CH2 CH2
    CH2 CH2 O2 o
    250 C, pressure O
    R1 R2 O R1 R2 O
    R C O O H O R C O H
    R3 R4 organic peroxy acid R3 R4
    alkene
    Cl
    O O
    CH3 C O O H and C O OH are commonly used.

    8.8 Reactions of Epoxides


    (1) React with water OH
    +
    H
    O H OH
    OH trans
    (See example 5.6 in book pg 242, this is a SN2 mechanism)
    (2) React with alcohol CH3OH
    HOCH2CH2OCH3
    CH2 CH2 H+

    O HOCH2CH2OH
    8 HOCH2CH2OCH2CH2OH
    (3) React with organometallic compounds
    R MgX H 2O
    RCH2CH2OMgX RCH2CH2OH Mg(OH)X

    R Li H2O
    CH2 CH2 RCH2CH2OLi RCH2CH2OH LiOH
    O

    RC C Na+ H2O
    RC C CH2CH2ONa RC C CH2CH2OH NaOH

    8.9 Cyclic Ethers (read on your own in details)


    Tetrahydrofuran (THF) and 1,4-dioxane --- common and important cyclic ethers,
    they can dissolve in both water and
    O O
    O organic solvents.

    Crown ethers ---- macrocyclic polyethers, different size crown ethers (“host’) form
    O complexes with different size positive ions (“guest”).
    O O

    O O
    O

    [18]crown-6
    9
    Homework:
    Problems: See the Blackboard assignments

    10

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