Ch_322a_6.024

Ch_322a_6.024 - depart. C=C H H H CH 3 CH 3 CH 2 O H + +...

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Mechanisms of Dehydrohalogenation: The E2 Reaction The reaction of isopropyl bromide with sodium ethoxide in ethanol to give propene : CH 3 CHCH 3 Br isopropyl bromide + Na + - OCH 2 CH 3 sodium ethoxide ethanol CH 3 CH=CH 2 + HOCH 2 CH 3 + Na + Br - propene follows the rate expression: Rate = k [CH 3 CHBrCH 3 ][NaOCH 2 CH 3 ] second-order kinetics The rate expression implies that both the alkyl halide and base are involved in the transition state of the rate-determining step. The notation for the bimolecular reaction is E2 .
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A Mechanism for the E2 Reaction The mechanism proposed for the E2 reaction is based on the observed second order rate expression, as well as the stereochemical outcome observed in alkyl halides with multiple stereocenters. The E2 like the S N 2 is a single step mechanism . C C H H CH 3 CH 2 O - Br H The alkyl bromide reacts from a conformation where the leaving groups are anti-coplanar . CH 3 H C C H H H CH 3 CH 2 O Br CH 3 H !" !" As the base remove the H + , the double bond begins to develop and the Br - begins to
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Unformatted text preview: depart. C=C H H H CH 3 CH 3 CH 2 O H + + Br-The E2 Reaction: A Closer Look The stereoelectronic requirements of the transition state for the E2 reaction are more easily seen in a Newman projection. C C H H CH 3 CH 2 O-H Newman projection CH 3 H Br H H H Br CH 3 H CH 3 CH 2 O-The alkyl bromide reacts from a conformation where the leaving groups are anti-coplanar. Overlap of Developing p-Orbitals in the Transition State The anti-coplanar conformation allows immediate overlap of the p-orbitals of the developing double bond in the transition state. C C H H H H CH 3 CH 2 O Br !" !" CH 3 C C H H H CH 3 CH 3 CH 2 O H Br-The required change in hybridization from sp 3 to sp 2 is underway in the TS with the p-orbitals "growing in." The stabilization energy of this developing bonding ! MO drives the stereoelectronic requirements of the E2 transition state....
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Ch_322a_6.024 - depart. C=C H H H CH 3 CH 3 CH 2 O H + +...

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