lecture 9 - Nucleophilic Substitution Nucleophilic...

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1 Nucleophilic Nucleophilic Substitution and Substitution and β -Elimination Elimination Chapter 9 Nucleophilic Substitution Nucleophilic Substitution ± Nucleophilic substitution: any reaction in which one nucleophile substitutes for another at a tetravalent carbon Nucleophilic Substitution ± Nucleophile Nucleophile : a molecule or ion that donates a pair of electrons to another molecule or ion to form a new covalent bond; a Lewis base Example β -elimination is a reaction in which a halide and a hydrogen on the neighboring ( β ) carbon are removed Some nucleophilic substitution reactions
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2 Practice Practice Mechanisms Mechanisms ± Chemists propose two limiting mechanisms for nucleophilic substitution a fundamental difference between them is the timing of bond-breaking and bond-forming steps S N 1 and S N 2 Mechanisms ± At one extreme, the two processes take place simultaneously; designated S N 2 S = substitution N = nucleophilic 2 = bimolecular (two species are involved in the rate- determining step) Mechanism - S N 2 (second order step) (second order step) both reactants are involved in the transition state of the rate-determining step
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3 Mechanism Mechanism - S N 2 Mechanism Mechanism - S N 1 ± Bond breaking between carbon and the leaving group is entirely completed before bond forming with the nucleophile begins ± This mechanism is designated S N 1 where S = substitution N = nucleophilic 1 = unimolecular (only one species is involved in the rate-determining step) Mechanism - S N 1 (first order step) Step 1: ionization of the C-X bond gives a carbocation intermediate Mechanism Mechanism - S N 1 Step 2: reaction of the carbocation (an electrophile) with methanol (a nucleophile) gives an oxonium ion Attack occurs with equal probability from either face of the planar carbocation intermediate Mechanism Mechanism - S N 1 Step 3: proton transfer completes the reaction
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4 Energy Diagram Energy Diagram - S N 1 Evidence of S Evidence of S N reactions reactions 1. What is relationship between the rate of an S N reaction and: the structure of Nu? the structure of RLv? the structure of the leaving group? the solvent? Evidence of S N reactions 2. What is the stereochemical outcome if the leaving group is displaced from a chiral center? 3. Under what conditions are skeletal rearrangements observed? Kinetics Kinetics ± For an S N 1 reaction reaction occurs in two steps the reaction leading to formation transition state for the carbocation intermediate involves only the haloalkane and not the nucleophile the result is a first-order reaction Kinetics (S (S N 1) 1) Kinetics Kinetics ± For an S N 2 reaction, reaction occurs in one step the reaction leading to the transition state involves the haloalkane and the nucleophile the result is a second-order reaction; first order in haloalkane and first order in nucleophile
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5 Kinetics (S N 2) 2) Stereochemistry Stereochemistry ± For an S N 1 reaction at a chiral center, the R and S
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This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

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lecture 9 - Nucleophilic Substitution Nucleophilic...

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