Orgo 7.8-7.9

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  1. Explain the trend in BP of alkenes?
    • increases with molecualr weight
    • increased branching leads to greater volatility and lower boiling points
  2. Like alkanes, alkenes are relatively __. They are __ in water but __ in nonpolar solvennts. They tend to be slightly more __ than alkanes for two reasons: what are these reasons?
    • nonpolar
    • insoluble
    • soluble
    • polar
    • 1) the more weakly held electrons in the pi bond are more polarizable (instantaneous dipole moment)
    • 2) the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment)
  3. Alkyl groups are slightly __ toward a double bond, helping to stabilize it. This donation polarizes the __, with a small partial positive charge on the allkyl group and a small negative charge on the C=C atom.
    • electron donating
    • vinylic bond
  4. In a cis-disubbed alkene, the vector sum of hte two dipole moments is directed __. In a trans-disubbed alkene, the two dipole moments __.
    If an alkene is symmetrically trans disubbed, teh dipole moment is __.
    • perpendicular to the double
    • tend to cancel out
    • zero
  5. Compounds with permanent dipole moments engage in __ while those without permanent dipole moments engage in __. cis-2-butene and trans-2-butene have similar __ but only cis has __. Because of its increased, intermolecular attractions, cis must be heated to a slighly higher temp to boil.
    • dipole dipole attractions
    • van der Waals attractions
    • van der Waals attractions
    • dipole dipole attractions
  6. __ is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene.
  7. The __ give excellent yields with bulky secondary and teritiary alkyl halides. A strong base forces __ by __. The molecule's bulkiness hinders __, and a relatively pure elimination proudct results. Tertiary halides are the best __ because they are prone to elimination and cannot under Sn2 sub.
    • E2 dehydrohalogenation
    • second order elimination
    • abstracting a proton
    • sn2
    • E2 substrates
  8. If the substrate is probe to substitution, what can a bulky base do?

    large alykl groups on a bulky base do what?
    • minimize the amount of substitution
    • hinder its approach to attack a carbon atom (sub), but can easily abstract a proton
  9. Bulky bases can also accomplish __ that don't follow __. __ often prevents a bulky base from abstracting a proton that leads to the most highly subbed alkene. In these cases, it abstracts a less hindered proton, leading to the formation of the __.
    • dehydrohalogenation
    • Zaitsev rule
    • Steric hindrance
    • hoffman product
  10. Hoffman vs. Zaitsev
    Zaitsev product will occur on an unhindered molecule, but Hofmann will on a bulky ion
  11. Like Sn2, E2 is__: Different stereoisomers of the reactant give different stereoisomers of the product. The E2 is stereospecific because it __.. The products are alkenes, and different __ of starting materials commonly give different __.
    • stereospecific
    • normally goes through an anti and coplanar transition state
    • diastereomers
    • diastereomers of alkenes
  12. Nearly all cyclohexanes are most stable in __, which places the C-C bonds as __ and any two adjacent C atoms with __ in an __, ideally oriented for E2 reaction.
    • chair conformations
    • staggered
    • axial bonds
    • anti-coplanar conformation
  13. On any two adjacent carbon atoms, one has its __ pointing up and the other _ pointing down. These two bonds are trans to each other, and they are __.
    • axial bond x2
    • trans-diaxial
  14. An E2 elim can take place on a chair conformation only if what?
    the proton and the leaving group can get into a trans-diaxial arrangement
  15. __ are converted to alkenes by __ with iodide ion in acetone. This __ is rarely an important synthetic reaction, because the most likely origin of a __ is from __.
    • vicinal dibromides
    • reduction
    • debromination
    • vicinal dibromide
    • bromination of an alkene
  16. __ is formally a __ because a molecule of Br2 )an oxidizing agent) is removed. The reaction with iodide takes place by the E2 mechanism, with the same geonmetric constraints as the ___.
    • debromination
    • reduction
    • E2 dehydrohalogenation
  17. Elimination usually takes place through an __.
    anti-coplanar arrangement
  18. __ usually takes place in __, without a __ or base to force __. The substrate is usually a __ or __.
    • First order dehydrohalogenation
    • a good ionizing solvent
    • strong nucleophile or base
    • second order kinetics
    • secondary or tertiary alkyl halide
  19. First order elimination requires __ to form a __, which loses a proton to a weak base. __ is generally accompanied by Sn1 sub, because the nucleophilc solvent can also attack the __ directly to form the sub product
    • ionization
    • carbocation
    • E1 dehydrohalogenation
    • carbocation
  20. Dehydrohalogenation mechanism of E1
    • elim by the E1 mechanism
    • accompanied by Sn1
  21. E2 debromination of a vicinal dibromide
    E2 debromination takes place by a concerted, stereospecific mechanism. I- removes one Br atom and the other Br leaves as bromide ion
  22. E1 dehydrohalogenatioins are prone to __.
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Orgo 7.8-7.9
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