Chem357_SN1_E1__FINAL_Student_

Chem357_SN1_E1__FINAL_Student_ - Substitution Elimination...

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1 Substitution & Elimination Substitution & Elimination Unit 3 - S Unit 3 - S N 1 & E1 1 & E1 Paula Y. Paula Y. Bruice Bruice , , Chapter 8 & 9 Chapter 8 & 9 2 Solvolysis Solvolysis Reactions - S Reactions - S N 1 & E1 1 & E1 Let us consider the following empirical result: How can this happen? C Br CH 3 CH 3 CH 3 HO CH 3 CH 3 CH 3 H-Br + H 2 O (solvent) N O T E that H 2 O is the solvent here. H 2 C H 3 C CH 3
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3 2.0 x 10 -3 3.0 x 10 -3 2.0 x 10 -3 1.0 x 10 -3 1.0 x 10 -3 1.0 x 10 -3 Rate of S N 1 (M/s -1 ) 5.0 x 10 -4 0.40 M 0.40 M 7.5 x 10 -4 0.60 M 0.20 M 5.0 x 10 -4 0.40 M 0.20 M 2.5 x 10 -4 0.20 M 0.60 M 2.5 x 10 -4 0.20 M 0.40 M 2.5 x 10 -4 0.20 M 0.20 M Rate E1 (M/s -1 ) [ alkylBr ] (M) [ H 2 O ] (M) + H 2 O (solvent) H 2 C H 3 C CH 3 C Br CH 3 CH 3 CH 3 + HO CH 3 CH 3 CH 3 H-Br + H 2 O nucleophile electrophile ! " ! + RATE = k [alkylBr] [H 2 O] x y Solvolysis Solvolysis Reactions - S Reactions - S N 1 & E1 1 & E1 4 + H 2 O (solvent) H 2 C H 3 C CH 3 C Br CH 3 CH 3 CH 3 + HO CH 3 CH 3 CH 3 H-Br + H 2 O nucleophile electrophile ! " ! + RATE = k [alkylBr] BOTH SN1 and E1 reactions are 1 s t o r d e r with respect to alkylBr and I N D E P E N D E N T o f [ H 2 O ] . The reactions are 1 s t o r d e r O V E R A L L . So what does this mean? Solvolysis Solvolysis Reactions - S Reactions - S N 1 & E1 1 & E1
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5 C Br H 3 C H 3 C CH 3 ! " ! + C CH 3 H 3 C CH 3 Br pure p orbital SLOW r.d.s S N 1 & E1 Mechanisms 1 & E1 Mechanisms 6 C Br H 3 C H 3 C CH 3 O CH 3 CH 3 CH 3 ! " ! + H 2 O C CH 3 H 3 C CH 3 Br O H 3 C H 3 C H 3 C pure p orbital H H H H H 2 O H 2 O OH H 3 C H 3 C H 3 C HO CH 3 CH 3 CH 3 MECHANISM: PLANAR CARBOCATION RESULTS SLOW r.d.s FAST S N 1 & E1 Mechanisms 1 & E1 Mechanisms
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7 S.M. Products ! G Reaction Coordinate T.S. 1 S N 1 T.S. 2 C Br H 3 C H 3 C CH 3 C CH 3 H 3 C CH 3 O H 3 C H 3 C CH 3 H H OH H 3 C CH 3 H 3 C T.S. 3 O H H OH H 3 C CH 3 H 3 C Energetics Energetics for S for S N 1 Reaction 1 Reaction 8 C Br H 3 C H 3 C CH 3 ! " ! + H 2 O C CH 3 H 3 C C Br OH H 3 C H 3 C H 3 C MECHANISM: SLOW r.d.s FAST H H H H 2 O FAST H 3 C H 3 C CH 2 S N 1 E1 O H 3 C H 3 C CH 3 H H S N 1 & E1 Mechanisms 1 & E1 Mechanisms
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9 S.M. ! G Reaction Coordinate T.S. 1 S N 1 and E1 T.S. 2 C Br H 3 C H 3 C CH 3 C CH 3 H 3 C CH 3 O H 3 C H 3 C CH 3 H H OH H 3 C CH 3 H 3 C T.S. 3 O H H (CH 3 ) 3 C OH T.S. 2 (CH 3 ) 2 C=CH 2 H 3 C H 3 C CH 2 S N 1 E1 S N 1 versus E1 1 versus E1 Energetics 10 S N 1 unimolecular (ie. 1st order) substitution reaction nucleophilic N O T E : The "1" D O E S N O T mean that it is "one step"!! + H 2 O (solvent) H 2 C H 3 C CH 3 C Br CH 3 CH 3 CH 3 + HO CH 3 CH 3 CH 3 H-Br + H 2 O nucleophile electrophile ! " ! + E1 elimination S N 1 & E1 Reactions 1 & E1 Reactions
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11 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 S N 1 r e a c t i o n : A. Structure of the Electrophile (e.g. Alkyl Halide) B. Leaving group C. Nucleophile D. Solvent S N 1 - Influencing Factors 1 - Influencing Factors 12 S N 1 - Influencing Factors
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