nucleophilicsubstitution-091015205723-phpapp01

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Nucleophilic Nucleophilic Substitution Substitution
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Nucleophilic Substitutions (S N 1, S N 2) of Halides
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The Unimolecular (S N 1) Reaction: 2 steps R X R+ X (1) ¾ R-X dissociates, forming a carbocation intermediate , R+, and the leaving group, X-. ¾ Slow step = rate determining step (r.d.s.). ¾ Entails only one species, R-X . R+N u R-Nu (2) ¾ Carbocation and nucleophile combine. ¾ Fast step. ¾ Overall reaction: R-X + Nu - Æ R-Nu + X - . Rate of the reaction = k[R-X]
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S N 1 Reaction ¾ Carbocation stability is important factor: 3 º > 2º> 1º . ¾ Polar, protic solvents such as ethanol support carbocation formation (stabilize intermediate). ¾ Aprotic solvents such as acetone do NOT support carbocation formation. ¾ Weak nucleophile is OK. CX R R R C + R R R rate determining step Nu CN u R R R + C Nu R R R x
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The Bimolecular (S N 2) Reaction: 1 step (1) ¾ Nucleophile combines with alkyl halide to form pentacoordinate transition state. ¾ Slow step = rate determining step (r.d.s.).
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This note was uploaded on 11/25/2011 for the course CHEM 52705 taught by Professor Santon during the Spring '11 term at University of Texas.

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