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9-SubElimHLSF07(full)

9-SubElimHLSF07(full) - What is a substitution reaction The...

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What is a substitution reaction? The atom or group that is substituted or eliminated in these reactions is called a leaving group
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A closer look at the players: + NaOCH 3 CH 3 Cl Br SPh I HOH Br NaCN CN + Br + Cl CH 3 H 3 CO SPh Br OH + H 3 O + + I
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Two potential substitution mechanisms: Unimolecular Nucleophilic Substitution (S N 1) Bimolecular Nucleophilic Substitution (S N 2) I HOH + OCH 3 H 3 C Cl
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1. Rate laws Tobacco, S. MIT, 2003 Experimentally - how do we differentiate between the two substitution mechanisms?
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2. Stereochemistry of Substitution Reactions
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3. Rearrangement?
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Factors that affect which mechanism predominates: 1. The structure of the alkyl halide 2.The nucleophile 3. the solvent of the reaction Take a close look at S N 2 first - then come back to S N 1.
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1a. Effect of alkyl halide structure on S N 2 ( α -branching)
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Reaction coordinate diagrams for (a) the S N 2 reaction of methyl bromide and (b) an S N 2 reaction of a sterically hindered alkyl bromide Relative Rates of Reactivity towards S N 2:
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1b. Effect of alkyl halide structure on S N 2 ( β -branching) R Br :I acetone R I :Br RBr Relative Rate Br Br Br Br ! " 1 0.82 0.36 0.000012
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1c. Effect of Leaving Group on S N 2 Other great leaving groups: Leaving Group Abilities:
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2. Effect of Nucleophile on S N 2 Nucleophilicity is a measure of how readily (fast) a nucleophile attacks an electron-deficient atom Nucleophilicity is measured by a rate constant ( k ) UNLIKE Basicity is a measure of how well a compound (a base) shares its lone pair with a proton Basicity is measured by the acid dissociation constant ( K a ) - an equilibrium expression
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2a. Effect of sterics on nucleophilicity.
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