Exp 23:: Stereochemistry of the Addition of Bromine to Trans-Cinnamic Acid

717 Words Oct 8th, 2012 3 Pages
Subject: Experiment 23: Stereochemistry of the Addition of Bromine to trans-Cinnamic Acid

Introduction/Abstract: The purpose of this experiment was to carry out the bromination of trans-cinnamic acid, to determine the stereochemistry of the dibromide product of 2,3-dibromo-3-phenylpropanoic acid, and find out whether the reaction proceeds by the usual bromonium ion mechanism or some other mechanism. In this experiment trans-cinnamic acid was mixed with glacial acetic acid and stirred in which then bromine/acetic acid solution was added to the mixture little by little. The dibromide becomes the precipitate of this solution during the reaction. Once this precipitate is separated by vacuum filtration the melting point helps identify
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Observed/theoretical X 100
1.476 g/3.08 g X 100=47.9%

Discussion of Results: There was too much error introduced into the experiment to be able to indicate exactly which product was obtained through the experiment. I believe the results were supposed to include the product being erythro-2,3-dibromo-3-phenypropanoic acid and the mechanism that took place in the experiment was in fact the bromonium ion mechanism. My group’s percent error was 47.9% which is extremely low. This low percent error was gained from the lack of mass of the product obtained after vacuum filtration. There are several sources of error that could have given this result including: the product not being dry enough after vacuum filtration, the solution not mixed enough in the experiment, much loss of the product in transferring, and excess acetic acid contaminating the product which would change the melting point of the product.

Postlab Assignment for Expt. 23 “Stereochemistry of Bromine Addition”
1. Would you expect the product from your reaction to be optically active? Explain. (4 points)
We would not expect for our molecule to be optically active because it has no asymmetrical centers and therefore would be achiral.

2. What product or products would you expect to obtain by the addition of bromine to cis-cinnamic acid, assuming that it reacts by the same mechanism as the

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