Orgo 14.12-14.16

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DesLee26
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255086
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Orgo 14.12-14.16
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2014-01-04 10:01:23
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CHM 202
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  1. __ are much more reactive than common _ because of the __ associated with the __. Unlike other ethers __ react under both acidic and basic conditions. The products of acid-catalyzed opening depend primarily on the solvent used. 

    Acid-catalyzed hydrolysis of epoxides gives __ with __. The mechanism of this hydrolysis involves __, followed by __ by water. __ results from the __ of water on the protonated epoxide.
    • epoxides
    • dialkyl ethers
    • large strain energy
    • three-membered ring
    • epoxides

    • glycols
    • anti stereochemistry
    • protonation of oxygen (forming a good LG)
    • Sn2 attack
    • anti stereochemistry
    • back-side attack
  2. Acid-Catalyzed opening of epoxides iin Water
    Epoxides open in acidic solutions to form __.

    Steps?
    glycols

    Step 1: protonation of the epoxide to form a strong electrophile

    Step 2: water attacks and opens the ring

    Step 3: Deprotonation to give the diol
  3. Direct __ of an alkene (without isolation of the epoxide intermediate) is possible by using an acidic aqueous solution of a __. As soon as the epoxide is formed, it hydrolyzes to the glycol. __ and __ are often used for the __ of alkenes.
    • anti hydroxylation
    • peroxyacid
    • peroxyacetic acid
    • peroxyformic acid
    • hydroxylation
  4. When the acid-catalyzed opening of an epoxide takes place with an alcohol as the solvent, a molecule of alcohol acts as the __. This reaction produces an __ with __. This is an excelent method for making compounds iwht ether and alcohol functional groups on adjacent carbon atoms.
    • nucleophile
    • alkoxy alcohol
    • anti stereochemistry
  5. Acid-Catalyzed Opening of an Epoxide in an Alcohol Solution
    Epoxides open in __ to form __ alcohols. 

    Steps?
    • acidic alcohol solutions
    • 2-alkoxy

    • Step 1: Protonationof the epoxide to form a strong nucleophile
    • Step 2: The alcohol (solvent) attacks and opens the ring
    • Step 3: Deprotonation to give the product, a 2-alkoxy alcohol
  6. When an epoxide reacts with a __, a halide ion attacks the protonated epoxide. This reaction is analogous to the cleavage of ethers by HBr and HI. The __ initially formed reacts further with HXto give a 1,2-dihalide. This is rarely a useful synthetic reaction because the 1,2-dihalide can be made directly from the alkene by electrophilic addition of X2.
    • hydrohalic acid (HCl, etc)
    • halohydrin
  7. __ are tetracyclic compounds that serve a wide variety of biological functions, including __, __, and__. The biosynthesis of steroids is believed to involve an acid-catalyzed opening of __. __ is a member of the class of natural products called __. The enzyme __ oxidizes __ to the epoxide, which opens and forms a carbocation that cyclizes under the control of another enzyme. The cyclized intermediate rearranges to __, which is converted to cholesterol and other steroids.
    • steroids
    • hormones
    • emulsifiers
    • membrane components
    • squalene-2,3-epoxide
    • squalene
    • terpenes
    • squalene epoxidase
    • squalene
    • lanosterol
  8. Although cyclizatioin of __ is controlled by an enzyme, its mechanism is similar to the acid-catalyzed opening of other epoxides. The epoxide oxygen becomes protnated and is attacked by a __. In this case, the __ is a __. The initial result is a __.
    • squalene-2,3-epoxide
    • nucleophile
    • nucleophie
    • pi bond
    • carbocation
  9. This initial carbocation is attacked by another double bond, leading to the formation of another ring and another tertiary carbocation. A repetition of htis process leads to the __. This sequence of steps converts an __ into a compound with __ and __.
    • cyclized intermediate
    • achiral, acyclic starting material (squalene)
    • four rings
    • seven asymmetric carbon atoms
  10. Most ethers do not undergo __ or __ under bassic conditions, because an __ is a poor LG. __ have about 105 kJ/mol of __ that is released upon ring opening, however and this strain is enough to compensate for the poor __. 

    The reaction of an __ with __ leads to the same product as the acid-catalyzed opening of the epoxide: __, with __. In fact, either the __ or __ reaction may be used to open an epoxide, but the acid-catalyzed reaction takes place under milder conditions. Unless there is an acid-sensitive functional group present, the acid-catalyzed hydrolysis is preferred.
    • nucleophilic substitution and elimination
    • alkode ion
    • epoxides
    • ring strain
    • alkoxide leaving group

    • epoxide
    • hydroxide ion
    • 1,2 diol (glycol)
    • anti stereochemistry
    • acid-catalyzed 
    • base-catalyzed reaction
  11. Base-Catalyzed Opening of Epoxides

    - __ and __ do not attack and cleave most ethers. Epoxides are more __, however, because opening the epoxide relieves the strain of the __. __ can attack and open epoxides, even though the leaving group is an __.

    Steps
    • Strong bases and nucleophiles
    • reactive
    • three-membered ring
    • Strong bases
    • alkoxide

    • Step 1: a strong base attacks and opens the ring to an alkoxide
    • Step 2: protonation of the alkoxide gives the diol
  12. Like __, __ react with epoxides to form ring-opened products. 

    __ can also open epoxides. __ reacts with aqueous ammonia to give __, an imporrtant industrial reagent. The __ in __ is still nucleophilic, and __ can react further to give __ and __. Good yields of __ are achieved by using excess ammonia.
    • hydroxide
    • alkoxide ions

    • amines
    • ethylene oxide
    • ethanolamine
    • nitrogen atom
    • ethanolamine
    • ethanolamine
    • diethanolamine
    • triethanolamine
    • ethanolamine
  13. __ give the same product in both the acid-catalyzed and base-catalyzed ring openings. An __ may produce different products under acid-catalyzed and base-catalyzed conditions, however. 

    Under basic conditions, the __ simply attacks the less hindered carbon atom in an __displacement.
    • Symmetrically substituted epoxides
    • unsymmetrical epoxide
    • alkoxide ion
    • Sn2
  14. Under acidic conditions, the alcohol attacks the __. In the __ there is a balancing act between __ and __. We can represent this sharing of __ by drawing resonance forms that suggest what the cations would look like if the ring started to open. These __ help us visualize the charge distribution in the __.
    • protonated epoxide x2
    • ring strain
    • energy it costs to put some of the positive charge on the carbon atoms
    • positive charge
    • no-bond resonance forms
    • protonated epoxide
  15. Most base-catalyzed openings involve what?

    Like other strong nucleophiles, __ and __ attack epoxides to give (after protonation) ring-opened alcohols.
    attack by a strong nucleophile at the less hindered carbon atom

    • Grignard
    • organolithium reagents
  16. Substituted __ can be used in this reaction, with the __ usually attacking hte less hindered epoxide carbon atom. This reaction works best if one of the __ is unsubbed, to allow an unhindered nucleophilic attack. __ are more selective than __ in attacking hte less hindered epoxide carbon atom. Unless one carbon atom is very strongly hindered, __ may give mixtures of products.
    • epoxides
    • carbanion
    • oxirane carbons
    • Organolithium reagents
    • Grignard reagents x2
  17. __ polymerize in place, so they match the shape of the joint perfectly and adhere to microscopic irregularities in the surfaces. There is no solvent to evaporate, so there is no shrinkage. __ are bonded by __, so they are unaffected by water. Epoxies use a prepolymer that can be made as runny or as gummy as desired, and they use a __ that can be modified to control the curing time. In the absence of a __, they have a long shelf life.
    • epolxies x2
    • ether linkages
    • hardening agent x2

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