Elimination Reactions The E2 Mechanism

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Elimination Reactions The E2 Mechanism

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Elimination Reactions The E2 Mechanism

The E2 Mechanism X B:

δ- B:- δ+ R 1 C δ- R 2 C δ+ X δ- The E2 Mechanism R 3 R 4 transition state Free energy (G) Eact B:- B R 1 R 2 C C X R 1 R 2 R 3 R 4 R 4 R 3 C C X:- Reaction progress

The E2 Mechanism-Why?

The E2 Mechanism-Regioselectivity

The E2 Mechanism-Regioselectivity Base Base C 3 C C C C 3 Attack of base Attack of base C C C 3 C 2 C 3 ofmann product (least substituted, and least stable alkene) 3 C 3 C C C C 3 Zaitsev product (most substituted, and most stable alkene) Kinetic product Thermodynamic product

The E2 Mechanism-Regioselectivity ~30% ~70%

The E2 Mechanism-Regioselectivity 2-bromopentane 1-pentene 2-pentene

The E2 Mechanism-Stereoselectivity

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.

The E2 Mechanism What if we change the base from methoxide to t-butoxide? C 3 O - methoxide A bulky base

The E2 Mechanism-Regioselectivity Bulky base attacks preferentially at least hindered position Base C 3 Base Bulky base avoids hindered position C C C C 3 C C C 3 C 2 C 3 ofmann product (least substituted) 3 C 3 C C C C 3 Zaitsev product (most substituted)

The E2 Mechanism-Regioselectivity Effect of Steric Bulk of Base

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

The E2 Mechanism-Regioselectivity Effect of Steric Bulk of Base

1a. C 3 O - C 3 O b. DBU c. C 2 C 3 2 O KO, 2O

The E2 Mechanism Stereoselectivity and Stereospecificity

The E2 Mechanism-Stereospecificity X B: X c d a c b d a b B

The E2 Mechanism-Stereoselectivity and Stereospecificity

The E2 Mechanism-Stereoselectivity Base: Base: S Base: Base: Major product

The E2 Mechanism-Stereoselectvity Base: Base: R Base: Base: Major product

The E2 Mechanism-Stereoselectvity S R Note: The two enantiomers yield the same major product from an elimination reaction.

The E2 Mechanism-Stereospecificity Base: S R C 3 C 3 C 3? Base: 3 C 3 C Only product 3 C

The E2 Mechanism-Stereospecificity Base: R R C 3 C 3 C 3? Base: 3 C 3 C Only product 3 C

The E2 Mechanism-Stereospecificity S R C 3 3 C R R C 3 Note: Each of the two diasteromers yields only one product. 3 C

The E2 Mechanism Regioselectivity in Cyclohexanes

The E2 Mechanism Regioselectivity in Cyclohexanes ow do we explain this reaction?? Na + OC 3 - Only this product forms

The E2 Mechanism Regioselectivity in Cyclohexanes First clue: The reaction shows bimolecular kinetics. Second clue: Reactions with labelled compounds. D Na + OC 3 - no deuterium in the product!! D Na + OC 3 - D only the hydrogen is removed!!

The E2 Mechanism Regioselectivity in Cyclohexanes Third clue: Only this product forms Na + OC 3 - Na + OC 3 - Only this product forms

The E2 Mechanism Regioselectivity in Cyclohexanes

Na+ OC 3 - major product minor product C 3 C 3

Na+ OC 3 - no trans hydrogen only product not formed C 3 C 3

The E2 Mechanism Regioselectivity in Cyclohexanes Na+OEtheat Only product formed and the reaction is very slow!! C no trans hydrogen C 3 C 3 C C 3 C 3 3 C C 3 least stable conformer!!

3a. Cl O - Cl Cl enantiomers b. Cl O - Cl Cl enantiomers Is one reaction faster than the other????

3c. O - Cl Cl Cl constitutional isomers d. O - Cl Cl Cl constitutional isomers Is one reaction faster than the other????

What s Going On ere?? Regioselectivity in Cyclohexanes C 3 O heat C 3 O heat Major product for both reactions!!

Elimination Reactions The E1 Mechanism

The E1 Mechanism

The E1 Mechanism Base:

The E1 Mechanism

The E1 Mechanism

The E1 Mechanism Regioselectivity

The E1 Mechanism Regioselectivity Zaitsev Product Favored

The E1 Mechanism In the Presence of Acid +

Substitution vs Elimination

SN1 vs E1 Ionization to form carbocation (rate limiting) SN1: Nucleophilic attack by solvent (fast) E1: Basic attack by solvent (fast)

SN1 and E1 Mechanisms Rearranged substitution product SN1 E1 Rearranged elimination product

SN2 vs E2

Classification of Nucleophiles (and Bases) Nucleophile (only) Base (only) Strong Nuc/ Strong Base Weak Nuc/ Weak Base alides Sulfur nucleophiles Cl- S- 2S - RS- RS :- DBN O- MeO- O 2O MeO I- DBU EtO- EtO

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

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