Mecanismos de Reações Orgânicas

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378 Organic Chemistry Solutions Manual And so the rate expression becomes (substituting for unknown [enolate]) rate = and this matches the given rate expression though the apparently third-order rate constant revealed as an equilibrium constant (K₁) multiplied by a second-order rate constant (k₂). The Hammett p value shows a modest gain of electrons in the transition state. We must not the pre-equilibrium into account in this as ArCHO is not involved in that step. In fact a Hamm value of +2.5 is typical of nucleophilic attack on a carbonyl group conjugated with the arome ring (see p. 1095 for an example). The unexpected products come from variations in this mechanism. is conjugated and unreactive so that the chloroketone ignores it and does a Darzens reaction itself, rather like the self-condensations we met in Chapter 21. o o 0 0 Ph Ph 0 CI Ph Ph Ph Ph Ph H H CI CI CI CI CI With salicylaldehyde, the second example, the OH group will exist as an oxyanion under reaction conditions. Alkylation with the chloroketone allows enolate formation from the leading to an intramolecular aldol reaction. 0 CHO CHO CI 0 H Et0 EtOH 0 EtOH 0 Ph 0 Ph 0 0 0 0 E1cB H 0 Ph 0 Ph 0 Ph Problem 15 If you believed that this reaction went by elimination followed by conjugate addition, what experiment would you carry out to try and prove that the enone is an intermediate? o NaCN Ph CI EtOH Ph CN Purpose of the problem Turning the usual question backwards: what evidence do you want, rather than how to what you are given. Suggested solution The suggested mechanism of elimination followed by conjugate addition might be contrasted with the direct SN2 displacement to see what evidence is needed.