Ch_322a_6.020

Ch_322a_6.020 - Nature of the Leaving Group Good leaving...

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Nature of the Leaving Group Good leaving groups in both the S N 1 and S N 2 reactions produce stable anions . Stable anions are weak bases----the anions of strong acids. H- A + H 2 O H 3 O + + A - strong acid stable anion R- A S N 1 good leaving group R + + A - stable anion Nu: - + R- A good leaving group S N 2 Nu-R + A - stable anion
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Leaving Group Ability Leaving group ability influences the rates of both the S N 1 and S N 2 reactions through the developing anion character of the leaving group in the transition states. S N 1 C X !" ! + C X RDS C X !" Nu !" S N 2 C X Nu: - + RDS The stability of X - influences the energy of both the S N 1 and S N 2 transition states.
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Example: The Halides as Leaving Groups This order of leaving group ability of the halides in R-X compounds is consistent with the acidity order of the hydrogen halides, and the related base strength of the halide ions. leaving group ability I > Br > Cl > F - - - - order of acid strength H-F << H-Cl < H-Br < H-I weakest strongest pK a
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_6.020 - Nature of the Leaving Group Good leaving...

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