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2444-092311 - Nucleophiles Electron rich Electron donor HO...

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Nucleophiles Electron rich Electron donor Electron rich Electron donor Nucleophiles I I HO HO H Cl H OH Cl I C X ! + ! C I X I H 3 C Br ! + ! H 3 C I Br ! + ! Polarized covalent bond
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Substitution I Me NaCN I CN Me H CN Backside attack by nucleophile ( CN) C is more electron rich than N No intermediate Pentacoordinate Transition state Bimolecular (collision) - 2nd order 100% inversion of configuration S N 2 S N 1 1° and 2° halides Cl NaI H 2 O S N 2 attack must be slow due to steric hindrance Water is required for facile ionization to a carbocation Slow ionization - fast coupling - first order reaction Carbocation intermediate I I Usually requires a polar aprotic solvent
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Williamson Ether Synthesis I Me NaOMe THF I OMe Me H OMe Alkoxides (RO ) such as methoxide are nucleophiles S N 2 reaction with 1° and 2° halides lead to formation of the corresponding ether Remember the common polar, aprotic solvents O O H 3 C CH 3 O CH 2 Cl 2 H N(CH 3 ) 2 O H 3 C S CH 3 O diethyl ether (ether) tetrahydrofuran (THF) acetone dichloromethane (methylene chloride) N,N-dimethylformamide (DMF) dimethyl sulfoxide (DMSO)
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Alcohols are Bases OH H–Cl Cl Cl O Of OH donates 2 electrons to H to form oxonium ion Loss of water leads to cation OH 2 Cl S N 1 S N 2 Inorganic halides OH SOCl 2 Cl PBr 3 OH Br P red , I 2 OH I
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Rearrangements Br Br H H AlCl 3 Br H H Br Br H H AlCl 3 + Br—AlCl 3 1,2-hydride shift Driving force: 2° cation more stable than 1° by about 12-15 kcal mol –1 Br I aq. THF 84 85 86 87 I - NaI + NaBr
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Another acid-base reaction BASE: H X H C C X BASE: BASE: H BASE: H :X :X C C (1) (2) Br H 1A 2 Br H CH 3 CH 3 1B OEt ! + !" ! + substitution - too sterically hindered for collision to occur acid-base reaction !" NaOEt , EtOH reflux + EtOH + NaBr
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Elimination E2 Cl KOH H H Cl HO diastereospecific " -H removed by strong base No intermediate E2 transition state Alkene product (S) (R) Me Cl KOH H Me (R) (S) H Cl Me HO Me (Z) Me Me Note: in water, with 3°, ionization to cation, then removal of " -
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The 4 Assumptions 1. Is water present? Yes - then 1st order, No - then 2nd order S N 2, S N 1, E2, E1 ?
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