Stereochemistry of the Addition of Bromine to Trans-Cinnamic Acid
Once this precipitate is separated by vacuum filtration the melting point helps identify the product. Laboratory notebook reference: Pages 9-10 Chemical Reactions: [pic] Observations: The experiment was begun by mixing trans-cinnamic acid and glacial acetic acid with an added stir bar and placed on the magnetic stirrer the solution mixed up very well. After stirring, bromine/acetic acid was added over five increments and stirred and the solution turned a dark orange color and then began to fade to a lighter color. After vacuum filtration the substance dried and appeared much harder and less liquefied than before.
Once the dried substance was obtained the melting point was found along with the melting point range. Data: |Mass of filter paper |0. 554 g | |Filter paper + dried substance
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. 3 “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 trans acid? (4 points) If it reacts by the same mechanism you would get the same product of 2,3-dibromo-3-phenylpropanoic acid.
You would need to find the melting point to determine if it produced an erythro or threo product. 3. What will be the possible effect on your results of the following experimental errors or variations? (4 points) (a) You added a total of 5 mL of the bromine solution instead of 10 mL. The reaction may not have enough of the reactant Bromine to move to completion, or not enough product would be made. (b) You misread the label on the bottle of cyclohexane and used it in place of cyclohexene.
If you still actually used cyclohexane there would be no effect. 4. Draw structure of the product(s) (show stereochemistry) of bromine addition to fumaric acid assuming that bromine adds to this compound the same way it does to cinnamic acid. (4 points) [pic] Would you expect the product from the reaction in Question 4 to be optically active? Explain. (4 points) No, you would not expect it to be optically active because there are no asymmetrical centers therefore the molecule will be achiral.