Presentation is loading. Please wait.

Presentation is loading. Please wait.

Chapter 7 Elimination Reactions I.Unimolecular Elimination, E1 A.Elimination Reactions 1)Removal of L.G. and H, but no substitution 2)Formation of a C==C.

Similar presentations


Presentation on theme: "Chapter 7 Elimination Reactions I.Unimolecular Elimination, E1 A.Elimination Reactions 1)Removal of L.G. and H, but no substitution 2)Formation of a C==C."— Presentation transcript:

1 Chapter 7 Elimination Reactions I.Unimolecular Elimination, E1 A.Elimination Reactions 1)Removal of L.G. and H, but no substitution 2)Formation of a C==C double bond B.Solvolysis of 2-bromo-2-methylpropane actually gives 2 products 1)Rate of reaction for both products = k[haloalkane] E1 2)E1 Mechanism a)Step 1 is Dissociation (just like for S N 1) SN1SN1 E1

2 b)Step 2 is Deprotonation to form the Alkene

3 C.E1 can give a mixture of products because any H can be removed II.Bimolecular Elimination, E2 A.Strong Bases affect nucleophilic substitution reactions (OH- and RO-) 1)Rate = [B-][RX] (second order or bimolecular) 2)Must occur before a carbocation forms, or we would just have S N 1, E1 B.E2 Mechanism 1)Occurs in a single, concerted step 2)Must have strong base S N 1 Product E1 Products

4 3)Mechanism 4)Orbital Picture

5 C.Comparison of E1 and E2 1)E1 is stepwise and involves a carbocation 2)E2 is concerted D.Experimental Evidence for Concerted E2 Mechanism 1)Second order rate law shows base and RX involved in R.D.S. 2)Better L.G. is faster (Bond breaking is important in R.D.S.) 3)Stereochemistry is important a)If anti Hydrogen is present, E2 is fast b)If anti Hydrogen is not present E2 is very slow

6 III.Substitution and Elimination A.SN2, SN1, E2, E1 all occur from RX + Nu B.How do we predict what the reaction will produce? 1)Weakly basic strong nucleophiles give substitution products a)Good Nucleophiles b)Base weaker than OH- c)I -, Br -, RS -, N 3 -, RCO 2 -, PR 3, CN - d)S N 2 for primary and secondary RX e)S N 1 for tertiary RX 2)Weakly basic weak nucleophiles can only do S N 1/E1 (H 2 O, ROH) 3)Strongly basic Nu’s give more E as Bulk increases (OH -, RO -, NH 2 - ) E2 mostly S N 2 mostly E2 mostly

7 4)Sterically hindered bases favor Elimination a)Too bulky to do substitution b)E2 or E1 depending on RX C.Reactivity of Haloalkanes 1)Primary Haloalkanes a)S N 2 mostly, can be slowed by bulk at C2 b)E2 only if Bulky base 2)Secondary Haloalkanes a)S N 2 with good nucleophiles, weak base, Polar Aprotic Solvent b)S N 1/E1 with good LG, weak Nu, Polar Protic Solvent c)E2 with strong base, Polar Protic Solvent 3)Tertiary Haloalkane a)S N 1/E1 with good LG, no base (solvolysis) b)E2 with strong base

8


Download ppt "Chapter 7 Elimination Reactions I.Unimolecular Elimination, E1 A.Elimination Reactions 1)Removal of L.G. and H, but no substitution 2)Formation of a C==C."

Similar presentations


Ads by Google