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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

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Chapter 7 Elimination Reactions

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  1. 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

  2. b) Step 2 is Deprotonation to form the Alkene

  3. 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

  4. Mechanism • Orbital Picture

  5. 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

  6. 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

  7. 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

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