Bromination of (E)-Stilbene

• Kaisha Butz

Abstract:

The purpose of this experiment was to synthesize the second intermediate (meso-stilbene dibromide) in the E-Stilbene reaction by Bromination. It was hypothesized that if the reaction was heated at 120°C for five minutes the reaction between E-stilbene and the pyridium bromide perbromide would occur, and meso-stilbene would be created. After the reaction occurred the results were analyzed by IR and by an ignition test. The hypothesis was supported by the employed methods.

Introduction:

This experiment was performed to show how bromination of alkenes reacts, and to be able to successfully synthesize meso-stilbene dibromide. The reaction of bromine with alkenes is an addition reaction where the nucleophilic double bond attacks the electrophilic bromine (Mayo, et. al, 2013). Bromine only becomes electrophilic because of induction due to its ability to be polarizable (Mayo, et. al, 2013). Induction occurs when there is a transmission of charge (Bruice, 2014). Bromine as it approaches the (E)-stilbene’s double bond becomes polarized and takes on a slightly positive charge (Mayo, et. al, 2013). This allows it to form a cyclic bond (cyclic bromonium ion) with both sp², now sp³, carbons. The cyclic bromonium ion takes on a positive charge and by anti-addition the second bromine (negatively charged) attacks from the back of the cyclic compound and bonds to either carbon on the opposite side of the other bromine. This creates a meso-stilbene which is 100% formed. There are no stereoisomers formed (Mayo, et. al, 2013).

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It was hypothesized that (E)-stilbene, in a solution of glacial acetic acid reacted with pyridium bromide perbromide heated to 120°C and cooled in an ice bath, would result in the formation of meso-stilbene. It was expected that only meso-stilbene dibromide would be formed, and that its formation could be tested by using IR. The Bromination reaction was also tested by an ignition test.

Structures/Mechanisms:

Material and Methods:

Please refer to pgs. 444-449 of Microscale Organic Laboratory with Multistep and Multiscale Synthesis by Mayo, Pike, and Forbes.

Deviations:

Procedure was done in microscale:

• 230mg of (E)-stilbene was used instead of 600mg.
• 2.2ml of glacial acetic acid was used instead of 6ml.
• A 10ml round-bottom flask was used instead of a 50ml flask.
• The magnetic spin bar was a baby magnetic spin bar.
• 450mg of pyridium bromide perbromide was used instead of 1.2g.
• 2ml of glacial acetic acid was used to wash down the perbromide instead of 6ml.
• 4.5 ml of distilled water was used instead of 12ml.
• Acetone and distilled water were added drop-wise to the crystals instead of three 2ml of distilled water and two 2ml of acetone.

Results:

IR spectroscopy (E)-Stilbene attached to back

IR spectroscopy meso-stilbene attached to back

Table 1:

Table 2:

Table 3:

Table 4:

Calculations:

Crystals: .2451g – .1045g = .1406g

Limiting Reagent: (E)-Stilbene

.230g (E)-Stilbene * (1 mole / 180.25g MW) = 0.0013 moles

.450g Pyridium Bromide Perbromide * (1 mole / 319.83g MW) = 0.0014 moles

Theoretical Yield:

0.0013 moles * 340.05g MW = .4421g

Percent Yield:

(.1406g/.4421g) * 100 = 31.8%

Discussion:

It was found that after bromination of (E)-stilbene into meso-stilbene dibromide that the IR spectroscopy of both were relatively similar in the fingerprint region (Í‚ 500-1000cm^-1). This should be the case. The only difference in the spectroscopy was the lack of the carbon-carbon double bond in the meso-stilbene dibromide. The IR spectroscopy in the lab does not have the ability to measure the wavelength of carbon-bromine bonds because it is not within the range of the machine. Therefore, the two IR spectroscopies of the two substances were very similar because they both contained aromatic rings with similar wave numbers (cm^-1) (Table 1, Table 2).

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It was expected that (E)-stilbene after undergoing bromination in a solution of acetic acid would produce crystals of meso-stilbene. That was the case! Success! Although the percent yield was low the experiment did produce meso-stilbene dibromide. This was supported by an ignition test. A part of the product was burned, and the flames were green. Green flames were indicative of bromide. Because carbon-bromide bonds were not seen in the IR spectroscopy, the flame test was necessary to show that the (E)-stilbene had, in fact, reacted with the pyridium bromide dibromide and created meso-stilbene dibromide.

The percent yield could have been better. One mistake was that the (E)-stilbene was heated and dissolved at 85ºC instead of 120ºC. The experiment continued regardless, and the pyridium bromide dibromide was also heated and dissolved at 85ºC. Once the temperature was noted to be too low the solution was placed back into the heat until the temperature reached 120ºC. The improper temperatures were most likely the main cause for the low percent yield. The temperature was too low for the reaction to occur completely and effectively.

According to Table 1 the primary peaks were all in the fingerprinting zone and were as follows: at wave number 961.39cm^-1 (indicated a C=C bond), 762.29cm^-1 and 690.00cm^-1 (indicated aromatic ring structures). According to Table 2 the primary peaks were also all in the fingerprinting region and were as follows: 761.88cm^-1, 688.59cm^-1, and 626.87cm^-1 (all of which indicated aromatic ring structures).

The hypothesis was proven because meso-stilbene was synthesized even with the incorrect temperature at first. The (E)-stilbene reacted with the pyridium bromide dibromide to create meso-stilbene.

Conclusion:

It was found that (E)-stilbene could be brominated in order to synthesize the second intermediate in a line of reactions so that meso-stilbene could be obtained. The percent yield was poor yet present. The experiment could have gone more smoothly if the temperature had been monitored better, and the mixture not placed on the heat until it was sufficiently hot. That would have allowed for a higher percent yield then previously achieved.

Bibliography

Bruice, Paula. Organic Chemistry. 7th ed. Pearson, 2014. 1337. Print.

Mayo, Dana, Ranold Pike, and David Forbes. Microscale Organic Laboratory with Multistep and Multiscale Synthesis. 5th ed. John Wiley and Sons, 2011. 751. Print.