Ch_322b_17.03

Ch_322b_17.03 - Chapter 17.3 lecture note

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Reactions Involving Carbanions: Enolate Anions Base-Promoted Halogenation of Ketones Acetone reacts with bromine (Br 2 ) to yield bromoacetone. The rate of the reaction is accelerated by bases such as hydroxide or acetate ion. Enolate anions are important reaction intermediates in many organic reactions. While the enolate anion has partial negative charge at carbon and oxygen centers, reactions typically involve the carbon center acting as a nucleophile. CH 3 CCH 3 + Br 2 + B: - :O: = CH 3 CCH 2 Br :O: + Br - + H-B C : C :O: - C C :O: - C C :O: ! - ! - enolate anion
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A Mechanism for Base-Promoted Halogenation Therefore, the Br 2 is not involved in the rate-determining (slow) step. A mechanism consistent with this and other observations follows. (2) CH 3 C CH 2 O - + Br 2 fast CH 3 CCH 2 Br O = + Br - In the presence of Br 2 , the enolate anion acts as a nucleophile in a fast reaction forming bromoacetone: CH 3 C-CH 2 O : - + Br-Br (1) CH 3 CCH 3 + B: - O slow CH 3 C CH 2 O - enolate anion + B-H fast ketone fast slow CH 3 C=CH 2 OH enol + B: - The added base promotes formation of the enolate anion , the key intermediate, in a slow step. The ketone-enol equilibrium takes place in the background. Fast reprotonation steps of the enolate anion reforms the ketone or enol, depending on the site of protonation. The rate of the reaction depends on the [B: - ], but not on [Br 2 ]: Rate = k [acetone] [B: - ]
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A Free Energy Diagram of the Two-Step Mechanism Free Energy Progress of Reaction CH 3 CCH 3 + B: - O = CH 3 CCH 2 Br O + Br - CH 3 C CH 2 O - + B-H key intermediate Consistent with the mechanism is the observation that the rates of bromination and iodination are the same under identical reaction conditions of solvent, base and temperature. RDS slow step fast step (bromination)
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Stereochemistry as a Mechanistic Probe A solution of R-(+)-sec-butyl phenyl ketone in aqueous ethanol loses its optical activity. Some Additional Observations
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Ch_322b_17.03 - Chapter 17.3 lecture note

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