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Chapter 7 Elimination Reactions

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Presentation on theme: "Chapter 7 Elimination Reactions"— Presentation transcript:

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 Bimolecular Elimination, E2
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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