Ch_322a_6.026

Ch_322a_6.026 - The SN2 versus the E2 Reaction Because...

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The S N 2 versus the E2 Reaction Because strong nucleophiles are usually also strong bases, the S N 2 and E2 reactions are usually competing pathways as are the S N 1 and E1. C C X H Nu: (a) (b) - . . (b) (a) C C Nu H S N 2 E2 C=C Rate = k elim [RX] [Nu - ] Rate = k sub [RX][Nu - ] Because the rate expressions for the substitution and elimination reactions both depend on the concentrations of the substrate and the nucleophile/base, the competition between the paths cannot be changed by varying the concentration of either reactant . However, this competition does very much depend on the structure of RX and choice of base .
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Some Examples The reaction of primary halides with unhindered alkoxide bases favors substitution . CH 3 CH 2 Br + CH 3 CH 2 ONa ethanol 55 o C CH 3 CH 2 OCH 2 CH 3 + CH 2 =CH 2 S N 2 E2 (90%) (10%) CH 3 CHCH 3 Br + CH 3 CH 2 ONa ethanol 55 o C CH 3 CHCH 3 OCH 2 CH 3 + CH 2 =CHCH 3 S N 2 E2 (21%) (79%) Secondary alkyl halides react with alkoxide bases to give mostly alkenes. Tertiary halides
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Ch_322a_6.026 - The SN2 versus the E2 Reaction Because...

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