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Kinetics Notes 2- Semester 2

Kinetics Notes 2- Semester 2 - Applications of Mechanisms...

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1 SH S Applications of Mechanisms in Organic Chemistry
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Nucleophilic Substitution R X Y R Y X H O CH 3 Br CH 3 OH Br + + substrate nucleophile product leaving group + e.g. , substitution reaction
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Nucleophilic Substitution Question. Identify the substrate, nucleophile, leaving group and product for each. CH 3 Br + NaOH Br + CH 3 S - Br + CN -
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Nucleophilic Substitution Two mechanisms general: Rate = k 1 [RX] + k 2 [RX][Y ] RX = CH 3 X k 1 increases k 2 increases k 1 ~ 0 Rate = k 2 [RX][Y ] (bimolecular) S N 2 k 2 ~ 0 Rate = k 1 [RX] (unimolecular) S N 1
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S N 2 Mechanism Kinetics e.g. , CH 3 I + OH CH 3 OH + I find: Rate = k [CH 3 I][OH ], i.e. , bimolecular both CH 3 I and OH involved in RLS and recall, reactivity: R-I > R-Br > R-Cl >> R-F C-X bond breaking involved in RLS concerted, single-step mechanism: CH 3 I + OH CH 3 OH + I [HO---CH 3 ---I]
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S N 2 Mechanism
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S N 2 Mechanism Steric effects e.g. , R–Br + I R–I + Br 1. branching at the α carbon ( X– C –C–C .... ) α β γ Compound Rel. Rate methyl C H 3 Br 150 1º RX CH 3 C H 2 Br 1 2º RX (CH 3 ) 2 C HBr 0.008 3º RX (CH 3 ) 3 C Br ~0 increasing steric hindrance
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