Trans-Cinnamic Acid Mechanism

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In this experiment, the addition of bromine to trans-cinnamic acid was carried out to identify the product and mechanism from its melting point. The possible products were erythro-dibromide, threo-dibromide, or a mixture of both. The formation of the product was due to syn-addition, anti-addition, or both. Syn-addition was the addition of two substituents to the same side while anti-addition was the addition of two substituents to opposite sides of a double bond. Syn-addition yielded erythro-dibromide while anti-addition formed threo-dibromide. The following figures show the three mechanisms by which the reaction can occur.

Figure 1.) Mechanism 1 with 100% trans- product ([Ar] represents a benzene)

Figure 2.) Mechanism 2 with 35% syn- product …show more content…
Orange powder
Added 0.8 g of glacial acetic acid to flask
0.8 g of glacial acetic acid
Orange mixture
Added a boiling chip and 1 mL of 1M bromine solution to the flask and attached to a reflux condenser.
The solution in the flask is a dark orange color.
Place the flask in a water bath at 50 o C

Reflux for 10 minutes and cooled an ice bath for 5 minutes.
The orange color fades to a light yellow-orange color
Vacuum filtration. Wash with cold water
After vacuum filtration, crude product is orange powder
Filter paper with crude product back into round-bottom flask with 4 mL of 50% ethanol.
Light orange solution
Flask in hot water bath
Crude solid dissolved, light yellow-orange
Flask in an ice bath
Recrystallization
Vacuum filtration and weigh the final product. Find the melting point.
Final product is light-yellow color …show more content…
This melting point range indicates that the product formed was erythro-2,3-dibromo-3-phenylpropanoic acid since the observed melting point range was close to the literature melting point of 204o C.
Exercise
1.)
(a.)
(b.) The trans-cinnamic acid and the trans-anethole are similar but the trans-cinnamic yields a bromonium ion which produces anti-addition. Meanwhile, the trans-anethole yields syn product because has a more stable carbocation than cinnamic acid so the bromide can attack the same face as the first bromine.
3b.)
4.)
(a.) If the cinnamic acic used was actually a mixture of cis and trans isomers, then the product that would be yielded would a combination of the erythro- and threo- products since there would be both syn addition and anti addition. This would cause a melting point that is lower than the literature melting point of erythro-2,3-dibromo-3-phenylpropanoic acid.
(b.) If only 0.5 mL of the bromine solution was added, then the product would contain a combination of both unreacted trans-cinnamic acid and the dibromide. This would result in a lower yield of the
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