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#1 2010-08-29 22:17:59

orgopete
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1-Bromo-1-phenylethane & sodium ethoxide elim rxn to give styrene

Yahoo!Answers in chemistry wrote:

Reaction between 1-bromo-1-phenylethane and sodium ethoxide in ethanol?

This is an inherently ambiguous question. The secondary halide can be involved in several types of reactions, SN1 or SN2-substitution or E1 or E2-elimination reactions. A rule of thumb is that if the pKa of the base (of the conjugate acid) is greater than 12, an elimination reaction will occur. Sodium ethoxide favors elimination.

The kinetics for an E1 reaction are proportional to [RX] and for an E2 reaction are proportional to [RX][NaOEt]. The more dilute this reaction is, the more an E1 reaction is favored. High concentrations favor E2. The concentrations are not mentioned, nor is it easy to anticipate which concentrations would favor an E1 over an E2. Because the products of E1 or SN1 reactions can be difficult to predict, I shall predict this is intended to be an E2-elimination reaction to give styrene (1-phenylethene).

The mechanism for this reaction is shown below (although with a different halide).

       http://www.curvedarrow.com/chem/ZZ406FB940.jpg

This example is excerpted from A Handbook of Organic Chemistry Mechanisms, available at http://www.curvedarrowpress.com/ or Amazon.com

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