Orgo 6.16-6.17

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Orgo 6.16-6.17
2013-10-30 00:56:14
CHM 201

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  1. Comparison of Sn1 and Sn2:
    Effect of nucleophile
    • Sn1: nucleophile strength is unimportant (usually weak)
    • Sn2: strong nucleophiles are required
  2. The nucleophile takes part in the __ of the Sn2 reaction but in the __ of the Sn1. Therefore, a strong __promotes the Sn2 but not the Sn1.
    Weak __ fail to promote the Sn2 reaction; therefore, the reactions with weak __ often go by the Sn1 mechanism if the substrate is __ or __.
    • slow step x2
    • nucleophile x4
    • Sn1
  3. The structure of the substrate is an important ifactor in determining the sub mechanism. 

    Most __ and __ are poor substrates for Sn1 becuase they cannot do what? 
    However, what makes them good for Sn2?
    • methly halides and primary halides
    • easily ionize to high-energy methyl and primary carbocations
    • they are unhindered
  4. Tertiary halides are too __ to undergo Sn2 reactions, but they can ionize to form __. Tertiary halides undergo substiution through the __ mechanism. Secondary halides can undergo substitution by __.
    • hindered 
    • tertiary carbocations
    • Sn1 
    • either mechanism
  5. The slow step of the Sn1 reaction involves formation of __. Solvation of these ions is crucial to __ and __ for their formation. Very __ such as water adn alcohol are needed for the Sn1. The solvent may be heated to reflux to provide the energy needed for ionization.
    • two ions
    • stabilizing them
    • lowering the activation energy 
    • polar ionizing solvents
  6. Less __ is generated in the __ of the Sn2 reaction. Strong solvation may weaken the strength of the __ because of the energy needed to strip off the solvent molecules .Thus, the Sn2 reaction goes faster in __ if the nucleophile will dissolve. __ may enhance the strength of weak nucleophiles.
    • charge separation
    • transition state
    • nucleophile
    • less polar solvents
    • polar aprotic solvents
  7. Summarize effect of solvent for both mechanisms
    • Sn1: good ionizing solvent required
    • Sn2: may go faster in less polar solvent
  8. The rate of the sn1 is proportional to hat? 
    The rate of Sn2 is proportional to what?
    • the concentration of the alkyl halide but not hte concentration of the nucleophile (first order rate equation)
    • concentrations of both the alkyl halide adn the nucleophile (second order)
  9. The Sn1 reaction involves a __ that cab be attacked from either face, giving a mix of __ and _.
    The Sn2 takes place through a __, which __ the stereochem of hte carbon atom. __ is the result
    • flat carbocation intermediate
    • inversion and retention of configuration
    • back-side attack
    • complete inversion of configuraiton
  10. The Sn1 reaction involves a __, which can __, usually by a __ or an __, to give a more __.
    • carbocation intermediate
    • rearrange
    • hydride or aklyl shift
    • stable carbocation
  11. The Sn2 reaction takes place in __ with __. No __ is possible.

    Generalize rearrangement in both.
    • one step
    • no intermediates
    • no rearragenemt
    • Sn1: common
    • Sn2: impossible
  12. An __ involves the loss of two atoms or groups from the substrate, usually with the formation of a __. Elimination reactions frequently accompany and compete with __.
    • elimination
    • pi bond
    • substituion reactions
  13. Depending on teh reagents and conditions, elims can be __ or __.
    first or second order
  14. The abbreviation E1 stands for __ , __. The mechanism is __ because the __ involves a single molecule rather than a collision between two molecules.
    • elimination, unimolecular
    • unimoleuclar
    • rate-limiting TS
  15. The slow step of an E1 reaction is the same as in the Sn1 reaction: __. In a fast second step, a base abstracts a proton from the carbon atom adjacent the __. The electrons that once formed the c-h bond now form a __ between two carbon atoms.
    • unimolecular ionization to form a carbocation
    • C+
    • pi bond
  16. The E1 reaction requires __, so it follows the same order of reactivity: __

    Mechanism of E1
    • ionization to a carbocation intermediate 
    • tertiary> secondary> prim> methyl
    • 1: unimolecular ionization to give a carbocation (rate-limiting)
    • 2: deprotonation by a weak base (often the solvent) gives the alkene (fast)
  17. Because the __ involves __ of the __, the rate equation is __. The rate depends on __ and not on __.

    The weak base taeks part in the __.
    • rate limiting step
    • unimolecular iionization
    • alkyl halide
    • first order
    • concentration of the alkyl halide only
    • the strength or concentration of hte base
    • fast second step of the reaction
  18. The E1 reaction almost always competes witht eh __. Whenever a carbocation is formed, it can undergo either __ or __, and __ often result.

    __ is an elimination of a hydrogen and a halogen atom. Under the first order conditions (absence of a strong base), __ takes place by the E1 mechanism: Ionization of hte alkyl halide gives a __, which loses a proton to give the alkene. __ results from __ on the carbocation.
    • Sn1 reaction
    • sub or elim
    • mixtures of products
    • dehydrohalogenation x2
    • carbocation intermediate
    • sub
    • nucleophilic attack
  19. Why arent Sn1 and E1 not really used?
    they produce mixture of products
  20. In the second step of the E1 mechanism, the carbon atom next to the C+ must __ to __ as the base attacks the protons and electrons flow intot he new __.

    Ionization step is __, with a __. Teh second step is a __by a base. The base is not invoveld int he reaction until __, so the rate depends only on the __. Weak bases are __ in E1 reactions.
    • rehybridize 
    • sp2
    • pi bond
    • strongly endothermic
    • rate limiting step
    • fast exothermic deproonation
    • after the RLS
    • concentration of the alkyl halide
    • common
  21. Like other carbocation reactions, teh E1 may be accompanied by __. 

    Difference of rearrangement in Sn1 and E1
    • rearrangement
    • the solvent acts like a base in E1 and a nucleophile in Sn1
  22. A carbocation can ?
    • 1) react with its own leaving group to return to the reactant
    • 2) react with a nucleophile to form a substitution product
    • 3) lose a proton to form an elim product
    • 4) rearrange to a more stable carbocation, then react further

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