L20a - Nucleophilic Substitution (NS) + Characteristics of...

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1 1 © Kay Sandberg Good trade in charge? Characteristics of the S N 2 reaction ppp O δ + δ - Na + Br Br NaOCH 2 CH 2 CH 3 HOCH 2 CH 2 CH 3 Na + Nucleophilic Substitution (NS) MOP 2 © Kay Sandberg Section 8.3 Dissect Nucleophilic Substitution (NS) I. LG characteristics III. Nucleophile characteristics II. Substrate characteristics IV. Solvent characteristics For S N 2 rxns to be fast! C-X easy to break skinny negative aprotic 3 ©Dr. Kay Sandberg Mechanism comparison How to predict which of the 4 competing mechanisms will dominate E1 vs E2 S N 1 S N 2 4 © Kay Sandberg Section 8.9 Nucleophilic Substitution (NS) Methyl and simple 1 o alkyl halides undergo nucleophilic substitution reactions that exhibit second-order kinetics: Rate = k [nucleophile][alkyl halide] Interpreted as indicating a bimolecular rate-determining step OH 1- , CH 3 Br CH 3 OH, Br 1- TS HO Br C δ - δ - H H H E Reaction Coordinate S N 2 mechanism Substitution bimolecular nucleophilic 5 © Kay Sandberg Section 8.9 Nucleophilic Substitution (NS) First-order kinetics: interpreted as evidence for a unimolecular rate-determining step. C Br CH 3 H 3 C CH 3 + H 2 O C O CH 3 H 3 C CH 3 H + H Br 3 o alkyl halides Rate = [alkyl halide] Even adding the stronger nucleophile hydroxide ion did not increase the substitution rate. S N 1 mechanism 6 © Kay Sandberg Section 8.9 Nucleophilic Substitution (NS) C Br CH 3 H 3 C CH 3 C CH 3 H 3 C CH 3 Br + slow O HH C H 3 C H 3 C CH 3 O H H O fast fast C H 3 C H 3 C CH 3 O H H O H H +
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2 7 Br (CH 3 ) 3 C Section 8.9 S N 1 mechanism O H H Br - (CH 3 ) 3 C E Reaction Coordinate O H H O H H Br - (CH 3 ) 3 C (CH 3 ) 3 C Br © Kay Sandberg 2A 2B 2C 2D 3A 3B 3C 3D 3E 3F 8 Br (CH 3 ) 3 C δ δ + (CH 3 ) 3 C + ,Br - Section 8.9 S N 1 mechanism O H H Br - (CH 3 ) 3 C E Reaction Coordinate O H H O H H - (CH 3 ) 3 C (CH 3 ) 3 C Br δ + δ + δ + δ + © Kay Sandberg H 2 O (CH 3 ) 3 C OH 2 , Br - + (CH 3 ) 3 C OH , Br - + H 3 O, E act , H 2 O 9 Section 8.10 S N 1 mechanism © Kay Sandberg R —X + H 2 O R —OH + H —X formic acid CH 3 Br CH 3 CH 2 Br (CH 3 ) 2 CH Br (CH 3 ) 3 C Br H 3 C Br Br Br Br methyl Br 1 o Br 2 o Br 3 o Br Relative rate 1 2 43 100000000 S N 2 10 © Kay Sandberg Section 8.10 Nucleophilic Substitution (NS) Steric effects R 3 C X < R 2 CH X < RCH 2 X < CH 3 X Increasing rate of substitution by S N 2 mechanism methyl 3 o Most crowded Least crowded 2 o 1 o Increasing rate of substitution by S N 1 mechanism Electronic effects Most stable carbocation Least stable carbocation Stereochemistry of S N 1 vs S N 2? no S N 1 too crowded no S N 2 carbocation too unstable yes both 11 C Z Y X X C Z Y LG chiral Section 8.11 Stereochemistry of S N 1 reactions © Kay Sandberg inversion LG : - Nu: - Nu: - retention X C Nu Z Y X C Z Y Nu 50% 50% Racemic mixture achiral 12 C Z Y X X C Z Y LG chiral Section 8.11 Stereochemistry of S N 1 reactions © Kay Sandberg LG : - inversion retention X C Nu Z Y X C Z Y Nu > 50% < 50% Incomplete racemization Nu: - Nu: - Ion pair: LG-carbocation
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3 13 C C CH 3 Br H H 3 C CH 3 H Section 8.12 Rearrangements in S N 1 reactions © Kay Sandberg Br C C CH 3 H H 3 C CH 3 H C C CH 3 H H 3 C CH 3 H OH slow C C CH 3 H H 3 C CH 3 H fast -H +
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L20a - Nucleophilic Substitution (NS) + Characteristics of...

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