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Chapter 7 Elimination Reactions. Unimolecular Elimination, E1 Elimination Reactions Removal of L.G. and H, but no substitution Formation of a C==C double bond Solvolysis of 2-bromo-2-methylpropane actually gives 2 products Rate of reaction for both products = k[haloalkane] E1

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chapter 7 elimination reactions
Chapter 7 Elimination Reactions
  • Unimolecular Elimination, E1
    • Elimination Reactions
      • Removal of L.G. and H, but no substitution
      • Formation of a C==C double bond
    • Solvolysis of 2-bromo-2-methylpropane actually gives 2 products
      • Rate of reaction for both products = k[haloalkane] E1
      • E1 Mechanism
        • Step 1 is Dissociation (just like for SN1)

SN1

E1

slide3
E1 can give a mixture of products because any H can be removed
  • Bimolecular Elimination, E2
    • Strong Bases affect nucleophilic substitution reactions (OH- and RO-)
      • Rate = [B-][RX] (second order or bimolecular)
      • Must occur before a carbocation forms, or we would just have SN1, E1
    • E2 Mechanism
      • Occurs in a single, concerted step
      • Must have strong base

SN1 Product

E1 Products

slide4
Mechanism
  • Orbital Picture
slide5
Comparison of E1 and E2
    • E1 is stepwise and involves a carbocation
    • E2 is concerted
  • Experimental Evidence for Concerted E2 Mechanism
    • Second order rate law shows base and RX involved in R.D.S.
    • Better L.G. is faster (Bond breaking is important in R.D.S.)
    • Stereochemistry is important
      • If anti Hydrogen is present, E2 is fast
      • If anti Hydrogen is not present E2 is very slow
slide6
Substitution and Elimination
    • SN2, SN1, E2, E1 all occur from RX + Nu
    • How do we predict what the reaction will produce?
      • Weakly basic strong nucleophiles give substitution products
        • Good Nucleophiles
        • Base weaker than OH-
        • I-, Br-, RS-, N3-, RCO2-, PR3, CN-
        • SN2 for primary and secondary RX
        • SN1 for tertiary RX
      • Weakly basic weak nucleophiles can only do SN1/E1 (H2O, ROH)
      • Strongly basic Nu’s give more E as Bulk increases (OH-, RO-, NH2-)

SN2 mostly

E2 mostly

E2 mostly

slide7
Sterically hindered bases favor Elimination
      • Too bulky to do substitution
      • E2 or E1 depending on RX
  • Reactivity of Haloalkanes
    • Primary Haloalkanes
      • SN2 mostly, can be slowed by bulk at C2
      • E2 only if Bulky base
    • Secondary Haloalkanes
      • SN2 with good nucleophiles, weak base, Polar Aprotic Solvent
      • SN1/E1 with good LG, weak Nu, Polar Protic Solvent
      • E2 with strong base, Polar Protic Solvent
    • Tertiary Haloalkane
      • SN1/E1 with good LG, no base (solvolysis)
      • E2 with strong base