Chem357_Sub_Elim___Part_2__FINAL_Student_

Chem357_Sub_Elim___Part_2__FINAL_Student_ - Substitution...

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1 Substitution & Elimination Substitution & Elimination Unit 1 - E2 Unit 1 - E2 Paula Y. Paula Y. Bruice Bruice , , Chapter 8 & 9 Chapter 8 & 9 2 Nucleophilic Substitution and Elimination Reactions Substitution and Elimination Reactions C C X H + Nuc C C Nuc H X + Substitution Rxn C C + X H-Nuc Elimination Rxn In the ELIMINATION reaction, the nucleophile is acting like a BASE and e x t r a c t i n g a H + f r o m t h e ! c a r b o n rather than attacking the " carbon. # + # + # -
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3 Nucleophilic Nucleophilic Substitution and Elimination Reactions Substitution and Elimination Reactions CH 3 CH 2 CH CH 2 CH 3 Br NaOCH 3 CH 3 OH 2 o alkyl halide ? 4 Nucleophilic Substitution and Elimination Reactions Substitution and Elimination Reactions CH 3 CH 2 CH CH 2 CH 3 Br NaOCH 3 CH 3 OH CH 3 CH 2 CH CH 2 CH 3 OCH 3 2 o alkyl halide substitution product
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5 R CH CH 2 X R' Base C CH 2 R' R + X H-Base + The E2 Reaction RATE = k [alkylX] [Base ] x = 1 y = 1 The reaction is 1 s t o r d e r with respect to alkylX and 1 s t o r d e r with respect to Base . The reaction is 2 n d o r d e r O V E R A L L . . . B I O M O L E C U L A R r . d . s E2 - Kinetics E2 - Kinetics 6 One molecule of alkylX comes together with the base in the r a t e d e t e r m i n i n g s t e p ( r.d.s. ) making this BIMOLECULAR . E2 biomolecular (ie. 2nd order) elimination reaction R CH CH 2 X R' Base C CH 2 R' R + X H-Base + RATE = k [alkylX] [Base ] x = 1 y = 1 E2 - Kinetics E2 - Kinetics
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7 E2 - Mechanism E2 - Mechanism r.d.s. MECHANISM: What does the R e a c t i o n C o o r d i n a t e D i a g r a m look like for an E2 reaction? R C CH 2 X R' Base H 8 S.M. Products ! G Reaction Coordinate Transition state concerted o r one-step no intermediate E2 R CH CH 2 X R' Base C CH 2 R' R X H-Base E2 - Mechanism & Energetics
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9 F a c t o r s t h a t i n f l u e n c e t h e r a t e o f a n E 2 r e a c t i o n : A. Structure of the Electrophile (e.g. Alkyl Halide) B. Leaving group C. Nucleophile/Base - strength - structure D. Regiochemistry E2 - Influencing Factors E2 - Influencing Factors 10 A. Structure of the Electrophile (e.g. Alkyl Halide) CH 3 Br CH 3 CH 2 Br CH 3 CH 2 CH 2 Br Electrophile
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