Elimination Reactions The E2 Mechanism
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1 Elimination Reactions The E2 Mechanism
2 The E2 Mechanism X X- B: B-
3 δ- B:- δ+ R 1 δ- R 2 δ+ X δ- The E2 Mechanism R 3 R 4 transition state Free energy (G) Eact B:- B R 1 R 2 X R 1 R 2 R 3 R 4 R 4 R 3 X:- Reaction progress
4 The E2 Mechanism-Why?
5 The E2 Mechanism-Regioselectivity
6 The E2 Mechanism-Regioselectivity Base Base methyl-1-butene methyl-2-butene
7 The E2 Mechanism-Regioselectivity Base Base 3 3 Attack of base Attack of base ofmann product (least substituted, and least stable alkene) Zaitsev product (most substituted, and most stable alkene) Kinetic product Thermodynamic product
8 The E2 Mechanism-Regioselectivity ~30% ~70%
9 The E2 Mechanism-Regioselectivity 2-bromopentane 1-pentene 2-pentene
10 The E2 Mechanism-Stereoselectivity
11 Regioselectivity-The tendency of a reaction to preferentially produce one constitutional isomer. Stereoselectivity-The tendency of a reaction to produce one of two stereoisomers in unequal amounts. Stereospecificity-Used to describe a reaction in which the configuration of the product is dependent on the configuration of the starting material.
12 The E2 Mechanism What if we change the base from methoxide to t-butoxide? 3 O 3 O 3 methoxide 3 t-butoxide A bulky base
13 The E2 Mechanism-Regioselectivity Bulky base attacks preferentially at least hindered position Base 3 Base Bulky base avoids hindered position ofmann product (least substituted) Zaitsev product (most substituted)
14 The E2 Mechanism-Regioselectivity Effect of Steric Bulk of Base
15 The E2 Mechanism-Regioselectivity Examples of Bulky Bases N N N 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN) 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) N
16 The E2 Mechanism-Regioselectivity Effect of Steric Bulk of Base
17 Examples
18 1a. + + cis isomer b.
19 1c. KO, 2O d. K (3)3O, (3)3O (solv) e. heat O 2 3 Substitution!!!
20 The E2 Mechanism Stereoselectivity and Stereospecificity
21 The E2 Mechanism-Stereospecificity X B: X c d a c b d a b B
22 The E2 Mechanism-Stereoselectivity and Stereospecificity
23 The E2 Mechanism-Stereoselectivity and Stereospecificity R1 R2 R1 R2 R4 R1 R4 R3 R4 R 3 R3 R2 R2 R4 R1 R3 R4 R 3 R2 R1 R4 R3 R2 R1
24 The E2 Mechanism-Stereoselectivity Base: Base: S Base: Base: Major product
25 The E2 Mechanism-Stereoselectvity Base: Base: R Base: Base: Major product
26 The E2 Mechanism-Stereoselectvity S Base R Base Note: The two enantiomers yield the same major product from an elimination reaction.
27 The E2 Mechanism-Stereospecificity Base: S R 3 3 3? Base: 3 3 Only product 3
28 The E2 Mechanism-Stereospecificity Base: R R 3 3 3? Base: 3 3 Only product 3
29 The E2 Mechanism-Stereospecificity S R 3 Base 3 R R 3 Base Note: Each of the two diasteromers yields only one product. 3
30 The E2 Mechanism Regioselectivity in yclohexanes
31 The E2 Mechanism Regioselectivity in yclohexanes
32 Na+ O 3 - major product minor product 3 3
33 Na+ O 3 - no trans hydrogen only product not formed 3 3
34 The E2 Mechanism Regioselectivity in yclohexanes Na+OEtheat Only product formed and the reaction is very slow!! no trans hydrogen least stable conformer!!
35 3a. l O - l l enantiomers b. l O - l l enantiomers Is one reaction faster than the other????
36 3c. O - l l l constitutional isomers d. O - l l l constitutional isomers Is one reaction faster than the other????
37 What s Going On ere?? Regioselectivity in yclohexanes 3 O heat 3 O heat Major product for both reactions!!
38 Elimination Reactions The E1 Mechanism
39 The E1 Mechanism
40 R 1 R 3 transition states R 2 δ+ δ- R 4 X B:- The E1 Mechanism R 1 R 2 δ- δ+ R 3 R 4 Free energy (G) Eact R 1 R 3 R 2 R 4 B:- R 1 R 2 R 1 R 2 X B X:- R 3 R 4 R 3 Reaction progress R 4
41 The E1 Mechanism Base:
42 The E1 Mechanism
43 The E1 Mechanism-Regioselectivity alkene product l Reaction progress
44 The E1 Mechanism Regioselectivity Zaitsev Product Favored KO, alcohol % % 3 3 3
45 The E1 Mechanism Regioselectivity Zaitsev Product Favored KO, alcohol % %
46 The E1 Mechanism In the Presence of Acid +
47 Substitution vs Elimination
48 SN1 vs E1 Ionization to form carbocation (rate limiting) SN1: Nucleophilic attack by solvent (fast) E1: Basic attack by solvent (fast)
49 SN1 and E1 Mechanisms Rearranged substitution product SN1 E1 Rearranged elimination product
50 lassification of Nucleophiles (and Bases) Nucleophile (only) Base (only) Strong Nuc/ Strong Base Weak Nuc/ Weak Base alides Sulfur nucleophiles l- S- 2S - RS- RS :- DBN O- MeO- O 2O MeO I- DBU EtO- EtO
51 SN2 vs E2
52 Table 8.2- Regiochemistry and Stereochemstry of Substitution and Elimination Reactions Regiochemistry Stereochemistry SN2 SN1 E2 E1 Nucleophile attacks the position where the leaving group is attached Nucleophile attacks the carbocation where the leaving group was, unless... Zaitsev product is favored over offmann product unless a sterically hindered base is used. Zaitsev product is always favored over offmann product. Inversion Racemization Trans alkenes favored. Anti-periplanar elimination required. Trans alkenes favored.
53 Substitution vs Elimination 1º halide 2º halide more nucleophilic more basic strong bulky base more nucleophilic strong bulky base weak nucleophile weak base SN2 + E2 SN2 + E2 E2 SN2 SN2 + E2 E2 E2 SN1 + E1+ SN2 + E2 SN1 + E1+ SN2 + E2 aprotic solvent protic solvent 3º halide more nucleophilic more basic weak nucleophile weak base SN1 E2 SN1 + E1 SN1 + E1
Elimination Reactions The E2 Mechanism
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