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Ch1b09Lecture20

Ch1b09Lecture20 - 1 Lecture 20 Summary of last lecture the...

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Summary of last lecture: •the last bit of kinetics •organic reaction mechanisms: displacement reactions Preview of coming attractions: •nucleophilic displacement reactions: S N 2 and S N 1 mechanisms Lecture 20 - February 24, 2009 1
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What is the mechanism of a nucleophilic substitution reaction? questions of mechanism involve kinetics. fit reaction velocity to general equation: velocity = " d ROH [ ] dt = " d Br " [ ] dt = + d RBr [ ] dt = + d OH " [ ] dt = k ROH [ ] # Br " [ ] $ consider the following specific reaction at a 1˚ carbon 2 for this reaction, experimentally find α = β = 1 CH 3 C H H OH + Br - CH 3 C H H Br + OH -
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Br - can be replaced with I - , OH - , CH 3 O - , etc. -OH can be replaced with Br, Cl, etc. and the overall reaction may be generalized as: with X - , Y - = nucleophile this is quite a general reaction, independent of X, Y Y - + C X R C R Y + X - H H H H 3
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the experimental rate law provides the composition of the transition state for the reaction; in this case, the TS contains the alkyl group, the leaving group and the attacking nucleophile. while kinetics provides the composition of the TS, structural information must be inferred indirectly - the essence of a mechanistic analysis is to deduce the structure of the TS using both experimental and theoretical approaches. remember - kinetics can never be used to establish a mechanism is correct, but only whether it is consistent with the experimental observations " d CH 3 OH [ ] dt = k CH 3 OH [ ] Br " [ ] 4
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This particular type of displacement reaction is termed S N 2 (nucleophilic substitution - bimolecular) what about the geometry of the attacking group relative to the leaving group? back to our displacement reaction: from the kinetic analyses, the transition state contains the alkyl, attacking and leaving group 5 C H H Br HO H –,‡ ! " ! " ! +
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important question: are X and Y positioned on the same or opposite sides of the attacked carbon? same side vs opposite side these products are indistinguishable for CH 3 OH But, they can be distinguished if the attacked molecule is chiral: 6 C H H OH Br H C H Br HO H H C CH 3 CH 2 OH H CH 3
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an example of configurational isomers: chiral molecules and enantiomers chiral molecules are non-superimposable on their mirror image - this often (but not always) occurs when a carbon has 4 different substituents. The pairs of mirror image related structures are known as enantiomers.
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