Chapter_11_-_Reactions_Of_Alkyl_Halides_-_Nucleophilic_Substitutions_&_Eliminations

Chapter_11_-_Reactions_Of_Alkyl_Halides_-_Nucleophilic_Substitutions_&_Eliminations

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CHM 138 H Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations 1
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Reactions of Alkyl Halides Elimination (E 2 , E 1 ) H Br elimination addition (ch 5) + HBr Substitution (S N 2 , S N 1 ) Br H 2 O OH + HBr 2
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S N 2 : Substitution nucleophilic bimolecular 1) MECHANISM 2) EVIDENCE 3) FACTORS + Br C Br H H H C OH H H H Example: + OH 3
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E.g. H 3 C–Br + OH H 3 C OH + Br C Br H H H HO C HO H H H Br 1) MECHANISM 4
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Energy Progress of reaction C HO H H H Br C Br H H H HO Energetics of the S N 2 mechanism 5
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Rate = C HO H H H C Br H H H HO Br 2) EVIDENCE: (a) Reaction kinetics 6
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S N 2 reactions at a stereocenter occur with C CH 2 CH 3 HO H 3 C H + Br C HO H H H C Br H H H HO Br 2) EVIDENCE: (b) Stereochemistry C CH 2 CH 3 Br CH 3 H 7
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C H Br H H C H 3 C Br H H 3 C C H 3 C Br H H C H 3 C Br CH 3 H 3 C Steric effects 3) FACTORS: (a) Structure of the substrate 8
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Relative S N 2 rate C H H H Br > methyl C H H 3 C H Br > C CH 3 H 3 C H Br C CH 3 H 3 C CH 3 Br > 9
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(b) The nature of the nucleophile Nucleophilicity : e.g. Nu + CH 3 Br H 2 O : - example: HO + CH 3 Br HOCH 3 + Br HS + CH 3 Br HSCH 3 + Br H 2 O H 2 O vs. 10
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- anions are better nucleophiles than - within a group Notable trends : - in common S N 2 reactions RS > CN > I > RO > HO > Br > Cl > NH 3 > CH 3 CO 2 > H 2 O Nucleophilicity - stronger bases are 11
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within a period: - stronger bases are generally better nucleophiles e.g. same attacking atom: Basicity : Nucleophilicity : Nu + CH 3 Br H 2 O Nu + H + H 2 O NuH : : 12
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13
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(c) The nature of the leaving group The better leaving groups are those which C HO H H H C Br H H H HO Br 14
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e.g. NH 2 OH < F < Cl < Br < I S O O O tosylate < The better leaving groups are those which 15
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(d) The solvent Protic solvent: Aprotic solvent: Aprotic solvents can be classified as C O H N(CH 3 ) 2 H 3 C C N S O H 3 C CH 3 C O H 3 C CH 3 DMSO acetonitrile DMF acetone 16
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This note was uploaded on 04/25/2010 for the course CHM CHM138H1 taught by Professor Browning during the Fall '08 term at University of Toronto- Toronto.

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Chapter_11_-_Reactions_Of_Alkyl_Halides_-_Nucleophilic_Substitutions_&_Eliminations

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