bromalkenes
DESCRIPTION
Bromalkanes Lab ReportTRANSCRIPT
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Joe Julian
CH:221:12
Tuesday, 1-4 PM
November 16, 2004
Identification of the Stereochemical Configuration of the
Brominated Products of trans-Cinnamic Acid and trans-Stilbene
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Abstract
Alkenes can from vicinal dihalides through the addition of halogens through the
displacement of a π bond. In alkenes where both carbons involved in the π bond have an equal
number of substituents, the halogen can attack either carbon, forming a racemic mixture. In this
experiment, trans-cinnamic acid and trans-stilbene underwent bromination. The goal was to
achieve as close to 100% yield as possible for both products. The configuration of the products
was determined by their melting points. The products were identified to be (±)-erytho-2,3-
dibromo-3-phenylpropanoic acid (from trans-cinnamic acid) and meso-1,2-dibromo-1,2-
diphenylethane (from trans-stilbene). The percent yield was found to be 122% for (±)-erytho-
2,3-dibromo-3-phenylpropanoic acid and 118% for meso-1,2-dibromo-1,2-diphenylethane. This
was attributed to water weight present in the products because they were not given enough time
to dry.
Introduction
Alkenes can form vicinal dihalides through the addition of halogens1. This is
accomplished by the nucleophilic addition of the π bond to a halide1. This reaction displaces a
halide ion and forms a halonium ion that is present in a three-membered ring with both carbons
that were involved in the π bond. Nucleophilic attack from the halide ion causes the highly
strained halonium ion ring to break1. This creates a product where the π bond has been broken to
form a vicinal dihalide product1. This type of reaction is classified as anti-addition because the
halogens must add to opposite sides of the π bond via a backside attack1. Racemization of the
product occurs because the halide ion has an even chance of attacking either carbon involved in
the halonium ion ring.
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In this experiment, trans-cinnamic acid and trans-stilbene underwent bromination to
form vicinal dihalide products (see Equations 1 and 2). The goal of this experiment was to
identify the stereochemistry of the product via the product’s melting point and to achieve as
close to 100% yield as possible for the product. To ensure that synthesis of product occurred, the
product was reacted with silver nitrate and checked for the presence of a precipitate.
(Equation 1)
(Equation 2)
Experimental Procedure
Preparation of trans-Cinnamic Acid2
A reflux apparatus was assembled, and in a 25mL round-bottom flask, 450mg of trans-
cinnamic acid, 6.0mL acetic acid, and 1.152g of pyridinium tribromide were placed. A magnetic
stir bar was added, and the flask was attached to the reflux apparatus.
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Preparation of trans-Stilbene2
A reflux apparatus was assembled, and in a 25mL round-bottom flask, 300mg of trans-
stilbene, 6.0mL of acetic acid, and 600mg of pyridinium tribromide were placed. A magnetic
stir bar was added, and the flask was attached to the reflux apparatus.
Bromination Process2
Both trans-cinnamic acid and trans-stilbene were subject to the same bromination
procedure. After the round-bottom flask was attached, the flow of water was started through the
condenser. The mixture was heated and gently refluxed for 20 minutes while stirred with the stir
bar. The round-bottom flask was removed from the heat source and the water source was turned
off after 20 minutes. The flask was allowed to cool for 5 minutes. The stir bar was removed
with a magnetic wand and 8.0mL of distilled water was added to the flask. An ice-water bath
was prepared by filling a 250mL beaker halfway with equal volumes of ice and water. The flask
was placed in the ice-water bath for 15 minutes.
A vacuum filtration apparatus was assembled using a Buchner funnel. The product was
filtered and the crystals were washed with distilled water. The product was allowed to sit in the
funnel for 15 minutes with the vacuum left on to facilitate drying. The product was then
weighed.
Product Identification2
The melting point of the product was recorded using a Hoover Uni-Melt device. The
product was then tested with silver nitrate. In a small test tube, about 10mg of the product was
dissolved in 0.5mL of 95% ethanol. To the resulting solution, 0.5mL of 2% ethanolic silver
nitrate was added and the tube was allowed to stand for 5 minutes. Any observations were
recorded.
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Results
The following results were tabulated from data acquired from the experiment. Table 1
showed the melting point of each product so it could be positively identified by comparing it to
the melting points of known compounds in Table 2.
Brominated Reactant
Melting Point Product Identification
trans-cinnamic acid
197°C (±)-erytho-2,3-dibromo-3-
phenylpropanoic acid
trans-stilbene
233-236°C
meso-1,2-dibromo-1,2-diphenylethane
Table 1. The melting points of the synthesized products from each reactant that underwent
bromination. The products were able to be identified by the melting point.
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Compound
Melting Point (°°°°C)
2,3-dibromo-3-phenylpropanoic acid
(±)-threo
94
(±)-erythro
203
(±)-1,2-dibromo-1,2-diphenylethane
110
meso-1,2-dibromo-1,2-diphenylethane
238
Table 2. 2 The melting points for possible products after bromination of trans-cinnamic acid and
trans-stilbene.
Table 3 showed the percent yield of both products that were identified and Table 4 was the
results of the silver nitrate test, which tested for the presence of bromide.
Theoretical Yield Actual Yield Percent Yield
(±±±±)-erytho-2,3-dibromo-3-phenylpropanoic acid
0.935g
1.14g 122%
meso-1,2-dibromo-1,2-diphenylethane
0.566g
0.67g 118%
Table 3. The percent yield of 2,3-dibromo-3-phenylpropanoic acid and meso-1,2-dibromo-1,2-
diphenylethane based on the calculated theoretical yields for each compound.
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Product
Observations
(±)-erytho-2,3-dibromo-3-
phenylpropanoic acid
Solution turned milky white with pale yellow precipitate
meso-1,2-dibromo-1,2-
diphenylethane
Solution turned milky white with white preciptitate
Table 4. The observations for each product were recorded upon the addition of 2% ethanolic
silver nitrate.
The following two figures were detailed mechanisms for the bromination of trans-
cinnamic acid and trans-stilbene.
Figure 1. Detailed mechanism for the bromination of trans-cinnamic acid. The configuration of
the product was dependent upon which carbon the bromide ion attacked.
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Figure 2. Detailed mechanism for the bromination of trans-stilbene. The same product resulted
regardless of which carbon was attacked by the bromide ion.
Discussion
The bromination of trans-cinnamic acid resulted in a product of (±)-erytho-2,3-dibromo-
3-phenylpropanoic acid. This was determined by the comparison of the melting point of the
product in Table 1 to the known melting point values for the possible products of the
bromination of trans-cinnamic acid in Table 2. Based on the melting point of 197°C, the
configuration of the products was determined to be (±)-erythro2. The stereochemical
configuration for 2,3-dibromo-3-phenylpropanoic acid was either (2R,3S) or (2S,3R) (see Figure
1). The two possible configurations were the result of the bromide ion attacking either carbon 2
or carbon 3. An attack on carbon 2 resulted in a configuration of (2S,3R) and an attack on
carbon 3 resulted in a configuration of (2R,3S). These two different attacks resulted in a racemic
mixture since both attacks were equally likely to happen.
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The bromination of trans-stilbene resulted in a product of meso-1,2-dibromo-1,2-
diphenylethane. This was determined by the comparison of the melting point of the product in
Table 1 to the known melting point values for the possible products of the bromination of trans-
stilbene in Table 2. Based on the melting point of 233-236°C, the configuration of the products
was determined to be (±)-erythro2. The stereochemical configuration for meso-1,2-dibromo-1,2-
diphenylethane was determined to be either (2R,3S) or (2S,3R), but either configuration resulted
in the same product. It did not matter which carbon was attacked by the bromide ion because
both carbons were attached to identical substituents. This gave an internally symmetric product
and a meso configuration (see Figure 2).
As Table 3 shows, the percent yield for both products was over 100%. The cause of this
error was not giving the product enough time to dry. The additional mass was most likely the
result of water weight. This could have prevented by giving the product more time to dry while
it was sitting in the Buchner funnel with the vacuum on. Another source of error for the
synthesis of (±)-erytho 2,3-dibromo-3-phenylpropanoic acid could have been the result of a
harsh reflux. Too harsh of a reflux would have caused the bromide to vaporize and lowered the
percent yield. However, since the product was very damp, it is hard to say how much of an
effect this had on the percent yield of the reaction.
The silver nitrate test was performed to test for the presence of halides3. A precipitate
formation showed that the bromide ion was indeed present and came from the product (see Table
4) 3. The product reacted with the nitrate either by elimination or substitution, freeing the
bromide from the product. The bromide was then able to form the precipitate silver bromide3.
Silver bromide was found to have a pale yellow or white color, which was consistent with the
results in this experiment3.
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Conclusion
The percent yield for both (±)-erytho-2,3-dibromo-3-phenylpropanoic acid and meso-1,2-
dibromo-1,2-diphenylethane was over 100%, which was the result of not giving the products
sufficient time to dry. The synthesis of these products was supported by the formation of a
precipitate when silver nitrate was added.
References
1. Wade, Jr., L.G. Organic Chemistry – 5th Edition. Pearson Education, Inc. New Jersey,
2003.
2. Rowland, A.T.; Allen K. Clark; Carl T. Wigal; Charles E. Bell, Jr.; Douglass F. Taber;
Frederick A. Bettelheim; Jan William Simek; Jerry Manion; Joe Jeffers; Joseph M.
Landesberg; Joseph W. LeFevre; L.G. Wade, Jr.; Louis J. Liotta; Moses Lee; Ronald
J. Wikholm; and William M. Loffredo. Organic Chemistry Laboratory Manual:
Susquehanna University. Thomson Learning: Ohio, 2003.
3. Glagovich, Neil. Central Connecticut State University. 2001.
http://www.chemistry.ccsu.edu/glagovich/teaching/472/qualanal/tests/agno3.html.
Accessed 23, Nov. 2004.