ZLM-chapter 8-9-substitution and elimination reaction

ZLM-chapter 8-9-substitution and elimination reaction -...

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LMZ/CHEM109A/ Winter 2010 Chapter 8-9. Nucleophilic Substitution (S N ) and Elimination (E) of alkyl halides I. Introduction: 1. alkyl halides 1. Substitution and elimination reactions II. reaction modes: C-X bond fragmentation 1. with the assistant of a nucleophile (external or internal) S N 2 E2 2. formation of a carbocation S N 1 or E1 III. Nucleophilic Substitution Reaction (Second Order): S N 2 reaction: A. Mechanism & Stereochemistry (sec. 8.2) rate law: -inversion of configureation (walden inversion) - reversibility (in principle) B. Factors that affect the rate of S N 2 1 The effect of the nature of leaving group on the rate of S N 2 (sec. 6.11A) TsO = H 3 C S O O O tosylate Reasons : 2. The effect of the structure of the substrate on the rate of S N 2 (sec. 6.11B) TsO I Br Cl H 2 O OH NH 2 RO relative reactivity as leaving group: > > > > >>> 60 30 10 0.2 0
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LMZ/CHEM109A/ Winter 2010 reason: 3. Effect of the concentration and strength of the nucleophile (sec. 6.10): - the higher the nucleophilicity of the nucleophile, the faster the rate of S N 2 reaction relative nucleophilicity in protic solvents: >> > > >>> Nu = HS CN I CH 3 OH O Cl NH 3 > > > Br N 3 > H 2 O F CH 3 CO 2 a. Factors that affect the nucleophility (sec. 6.10) i) A strong base may not necessarily a good nucleophile - basicity: measures the affinity of a base for a proton, defined by equilibrium constants for abstracting a proton - nucleophilicity : measures the affinity of an electron rich species for an electrophilic center (usually carbon) in a S N reaction, defined by the rate of attack ii) In general, a species with a negative charge is a stronger nucleophile than a similar neutral species > HO > CH 3 O 3 NH 3 H 2 N > H 2 O ; ; Nucleophilicity: > HS H 2 S ; iii) Nucleophilicity decreases from left to right in the periodic table, following the increase in electronegativity from left to right.
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ZLM-chapter 8-9-substitution and elimination reaction -...

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