Orgo 6.13-6.15

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Orgo 6.13-6.15
2013-10-29 18:40:54
CHM 201

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  1. When a solvent is the nucleophile, it is called __. 

    How can you know when something isnt Sn2?
    • a solvolysis
    • because Sn2 requires a strong nucleophile and a substrate that isn't hindered
  2. If the rate does not depend on the concentration of the nucleophile, what is it?

    When does the nucleophile react?
    • Sn1
    • after the slow step
  3. This type of substitution is called an __, for __, __, __. The term __ means there is only one molecule involved int he transition state of the rate limiting step.
    • Sn1 reaction
    • sub. 
    • nucleophilc
    • unimolecular x2
  4. The Sn1 mechanism is what? The first step is a __. The second is a __. 

    The carbocation is a strong __, reacting very fast with nucleophiles, including weak nucleophiles. The __ in Sn1 is usualyl weak ebcause a strong one would be more likely to attack the substrate and force some kind of __. If the __ is an uncharged molecule, the positively charged product msut __ to give the final uncharged product.
    • multistep process
    • slow ionization to form a carbocation
    • fast attack on the carbocation by a nucleophile
    • electrophile
    • nucleophile
    • second order rx
    • nucleophile
    • lose a proton
  5. Key mechanism of Sn1 reactions
    • 1: form carbocation (rate-limiting)
    • 2: nucleophilic attack on the carbocation (fast) (if the nucleophile is water or an alcohol, a third step is needed to deprotonate the product_
  6. The reaction energy diagram of hte Sn1 reaction shows what?
    The ionization (first step) is __, and its large EA determines what? 
    The nucleophilic attack (2nd step) is __, with a __. 

    In effect, a nucleophile reacts with the __ when?
    • why the rate does not depend on the strength or concentration of the nucleophile
    • highly endothermic
    • the overall reaction rate
    • strongly exothermic
    • lower-energy transition state
    • the carbocation
    • almost as soon as it forms
  7. Compare and contrast Sn1 and Sn2 reaction energy diagrams?
    - intermediates?
    the Sn1 has a true intermediate, the carbocation, which appears as a relative min in the diagram
  8. Reagents and conditions that favor formation of the __ do what to the Sn1 reaction?; reagents and conditions that hinder its formationi __.
    • accelerate
    • retard the reaction
  9. The rate-limiting step of the Sn1 reaction is what?
    The first TS resembles the __ adn the rates of teh Sn1 reaction depend strongly on __. 

    __ are more stable.
    ionization to form a carbocation, a strongly endothermic process

    • carbocation
    • the stability of the carbocations
    • highly substituted carbocations
  10. Reactivity toward Sn1 sub mechanisms follows the stability of carbocations in what order?
    The order is __ the Sn2 reaction? Why?
    • tertiary > secondary > primary > methyl
    • opposite because alkyl groups enhance the Sn1 by stabilizing the carbocation intermediate but hinder the Sn2 by blocking attack of teh strong nucleophile
  11. __ of the carbocation can also promote the Sn1 reaction.
    __ and __ generally do not undergo Sn1 or Sn2 reactions. An Sn1 reaction would require __ to form a __ or __, either of which is less stable than most alkyl carbocations. An Sn2 reaction would require __ by the nucleophile, which is made impossibly by the repulsion of the electrons in the double or aromatic rign.
    • resonance stabilization
    • vinyl and aryl halides
    • ionization
    • vinyl
    • aryl cation
    • back-side attack
  12. The leaving group is breaking its bond to carbon in the __ of the Sn1 mechanism. A __ helps stabilize the __ through partial bonding as it leaves. The leaving group should be a __, very stable after it leaves with the pair of electrons that bonded it to carbon.
    • rate-limiting ionization step
    • highly polarizable leaving group
    • rate-limiting transition state
    • weak base
  13. The Sn1 reaction goes much more readily in __ that stabilize ions. The __ forms two ions, and ionization is taking place in the __.

    __ solvate these ions by an interaction of the solvent's __ with the charge of the ion.
    • polar solvents
    • rate-limiting step
    • transition state
    • polar solvents
    • dipole moment
  14. __ such as alcohols and water are even more effictive solvents because __ form H bonds with the -OH hydrogen atom, and cations complex with the nonbonding electrons of the -OH oxygen atom.
    • protic solvents
    • anions
  15. __ of an alkyl halide  requires __ and __ of positive and negative charges, similar to what happens when NaCl dissolves in water. Therefore, Sn1 reactions require __ that strongly solvate ions.
    • ionization
    • formation and separation
    • highly polar solvents
  16. One measure of a solvent's ability to solvate ions is its __, a measure of the solvent's polarity.
    dielectric constant
  17. The Sn2 reaction is __: the nucleophile attacks from the back side of the electrophilic carbon atom, giving __ of configuration.

    In contrast, the Sn1 reaction is not __. In the Sn1 mechanism, the __ is sp2 hybridized and planar.
    • stereospecific
    • inversion
    • stereospecific
    • carbocation intermediate
  18. A nucleophile can attack the __ from either face. 
    Attack from both faces gives __ of the product. Such a process, giving both __ of the product, is called __. The product is either __ or at least less __ than the starting material.
    • the carbocation
    • both enantionmers
    • enantiomers
    • racemization
    • racemic
    • optically active
  19. If a __ attacks the carbocation from the front side (the side the leaving group left), the product molecule shows __. Attack from the back side gives a product molecule showing __.
    • nucleophile
    • retention of configuration
    • inversion of configuration
  20. __ is simply a combo of retention and inversion. When __ occurs, the product is rarely completely __, however; there is often more __ than __ of configuration. As the leaving group leaves, it partially blocks the front side of the carbocation. The back side is unhindered, so attack is more likely there.
    • racemization x2
    • racemic
    • inversion
    • retention
  21. Racemization techniques.
    • 1: ionization of a tetrahedral carbon gives a flat carbocation
    • 2: a nucleophile may attack either side of the carbocation.
  22. Carbocations frequently undergo structural changes, called __, to form more stable ions. A __ may occur after a carbocation has formed or it may occur as the leaving group is leaving. __ are not seen in Sn2 reactions, where no __ is formed and the one-step mechanism allows no opportunity for __.
    • rearragements x3
    • carbocation
    • rearrangement
  23. The rearranged product results from a __, the movement of a hydrogen atom with its bonding pair of electrons. A __ is represented by the symbol ~H. In this case, the __ converts the initially formed secondary carbocation to a more stable tertiary carbocation. Attack by the __ gives the rearranged product.
    • hydride shift x3
    • solvent
  24. Hydride shift mechanism
    • 1: unimolecular ionization gives a carbocation
    • 2: a hydride shift forms a more stable carbocation (this rearrangment involves movement of a H atom with its bonding pair of electrons over to the empty p orbital of the carbocation. 
    • 3: Solvent (a weak nucleophile) attacks the rearranged carbocation
    • 4: Deprotonation gives the rearranged product
  25. When neopentyl bromide is boiled in ethanol, it gives only a rearranged substitution product. This products results from a __, the migration of amethyl group together with its pair of electrons. Without rearrangement, ionization of neopentyl bromide wiould give a very unstable __.
    • methyl shift
    • carbocation
  26. Steps of methyl shift in an Sn1 reaction.
    • 1: ionization occurs with a methyl shift
    • 2: Attack by ethanol gives a protonated version of the rearranged product
    • 3: deprotonation gives the rearranged product
  27. Because rearrangement is required for __, only rearranged products are observed. In general, we should expect __ inr eactions involving carbocations whenever a __ or an __ can form a more stable carbocation. Most rearrangements convert __ to __ or __.
    • ionizaition
    • rearrangements
    • hydride shift
    • alkyl shift
    • secondary carbocations
    • tertiary or resonance-stabilized carbocations