BROMINATION OF TRANS – STILBENE

AIM:

The aim of this experiment is to carry out an addition reaction using trans-stilbene and bromine reagent to produce 1,2-dibromo-1,2-diphenylethane. As most chemicals used in this experiment are toxic / harmful, PPE must be used throughout and the experiment should be carried out inside a fume hood.

THEORY:

Bromine and chlorine readily undergo addition reactions with alkenes. This practical involves the bromination of trans-stilbene using a 10% bromine dichloromethane solution. Hence, an addition reaction takes place and 1,2-dibromo-1,2-diphenylethane is formed.

MECHANISM: The alkene, trans-stilbene, acts as a nucleophile and the bromine acts as an electrophile. Hence, the nucleophilic double bond undergoes an electrophilic addition reaction by the bromine reagent. The Br–Br bond becomes polarized and the more positively charged Br atom is transferred to the alkene to yield a bromonium ion. The addition of bromine begins at one side of the double bond (either side is equally likely, but only one option is drawn) and is followed by attack of bromide ion on the bromonium ion (again, attack could occur at either carbon since the ion is symmetric, but only one option is drawn). The result is a trans dibromide, as shown above.

The product of this reaction, 1,2-dibromo-1,2-diphenylethane, has two stereogenic

centres which normally leads to predict four (2ⁿ = 2² in this case) possible

stereoisomers (i.e. four optical isomers). However, there are only three possible isomers in the case of 1,2-dibromo-1,2-diphenylethane since one, the so-called meso isomer, is superimposable on its mirror image.

PROCEDURE:

• 5g of trans-stilbene were added to a 250cm³ conical flask, followed by 40cm³ of dichloromethane. The flask was swirled to dissolve all the stilbene.
• Approximately 15cm³ of 10% bromine-dichloromethane solution were carefully measured.
• 5cm³ of the bromine solution were added to the solution of stilbene and contents were swirled until the reddish brown colour of the bromine solution disappeared.
• A further 5 cm³ of the bromine solution were added to the solution of stilbene and the contents were swirled again until the reddish brown colour of the bromine solution disappeared for a second time.
• The remaining bromine solution was added and the mixture was swirled for several minutes to ensure bromine’s colour persisted.
• Cyclohexene was added drop wise to the flask until the bromine colour disappeared.
• The flask was then cooled in an ice bath and the crystals were collected after carrying out filtration using a Buchner funnel and flask.
• The crystals were washed with 15cm³ cold dichloromethane and were allowed to dry.
• The melting point of the product was determined.

RESULTS AND CALCULATIONS:

The yield follows from the 5.00 g of trans-stilbene used in the calculation below (the data table records 5.01 g weighed, which gives 60.0%). 5.00 g ÷ 180.24 g mol⁻¹ = 0.0277 mol of alkene, and since each molecule takes up one molecule of bromine the theoretical yield of 1,2-dibromo-1,2-diphenylethane (C₁₄H₁₂Br₂, 340.05 g mol⁻¹) is 0.0277 mol × 340.05 g mol⁻¹ = 9.43 g. Dividing the 5.67 g actually isolated by that maximum gives 5.67 g ÷ 9.43 g × 100 = 60.1%, the same figure as the 60.07% quoted in the conclusion once it is rounded to three significant figures. The product melted over 240.2–241.7 °C.

The calculation treats trans-stilbene as the limiting reagent, which is what the procedure implies: bromine solution was added in three portions and swirled until its colour persisted after the last one, then the excess was discharged with cyclohexene. A persistent colour means bromine was still present after all the stilbene had been consumed, so the alkene set the ceiling and 9.43 g is the right denominator. The 5.67 g isolated contains 0.0167 mol of product and therefore 2.66 g of bromine.

Experimental data was recorded in the following tables:

DISCUSSION:

The direction of the argument matters, because this reaction is stereospecific rather than merely stereoselective. trans-Stilbene carries a phenyl group at each end of the double bond, so the two alkene carbons are equivalent and the bromonium ion they form is itself symmetrical, with a two-fold axis that makes its two ring carbons identical. Bromide can open the ring at either carbon, but both openings lead to the same molecule. Anti addition to the trans geometry therefore has exactly one stereochemical outcome available to it, the meso (1R,2S) dibromide, and the expected share of the racemic pair is not a small fraction but zero. Any racemate in the sample would therefore mean that some addition had not been cleanly anti, not that a second expected product had formed alongside the first.

The melting point carries the argument because it is sensitive to exactly that difference. The meso isomer is a single species with an internal mirror plane, and identical molecules of one configuration pack into one lattice that melts at a single temperature, at about 241 °C with decomposition, reported between 241 and 243 °C. The racemic material melts near 114–115 °C, and the gap of more than 120 °C is what makes the measurement decisive rather than suggestive. The observed span of 1.5 °C is narrow for a melting range, which is the signature of a pure crystalline solid. A contaminated sample would not simply read slightly low: melting in a mixture begins at the eutectic, below the lower of the two pure melting points, so surviving trans-stilbene (mp 124 °C, Merck Index via PubChem) would drive the first drop well below that figure. An onset at 240.2 °C rules out any significant amount of it.

The 60.1% yield should not be folded into that argument. Mass balance cannot distinguish 60% of pure meso dibromide from 60% of the racemate, since the two weigh the same and the calculation contains no stereochemical information. What the yield measures is the fraction of stilbene that survived the work-up as isolable solid, and the missing 39.9%, about 3.76 g expressed as product, was not measured. The report’s own sources of error name stilbene left on the weigh boat, a short time in the ice bath and crystals left in the Buchner funnel, and some product is also lost to the mother liquor and the cold wash. That last route is not incidental. The dibromide was crystallized from dichloromethane to begin with, so it has real solubility there, and washing with 15 cm³ of cold solvent removes the impurities that would depress a melting point while carrying some product through the filter. A moderate yield with a sharp melting point is the expected pair.

Optical rotation would not settle the question either. A meso compound is achiral and shows no rotation, but a racemic mixture shows none either, so a reading of zero fits both the product this reaction should give and the one it should not. A mixed melting point against authentic meso dibromide would separate them. Neither appears in the report, so the melting point of the isolated solid remains the evidence for the assignment, supported by the literature value it matches.

SOURCES OF ERRORS:

• When transferring 5g of trans-stilbene to the conical flask, there was still some trans-stilbene left on the weigh boat. (Systematic Error)
• More crystals would have been formed and a better percentage yield could have been obtained if the flask was left in the ice bath for a bit longer. (Systematic Error)
• All the crystals were not transferred from the Buchner funnel to the petri dish. (Systematic Error)

CONCLUSION:

Trans-stilbene was brominated and 5.67g of 1,2-dibromo-1,2-diphenylethane were obtained. This is a percentage yield of 60.07%. The melting point range of 1,2-dibromo-1,2-diphenylethane was found to be 240.2^o to 241.7^o C.

Why Anti Addition Gives the Meso Product

The two bromine atoms add to opposite faces of the double bond (anti addition). In the first step, the alkene’s π electrons attack Br₂ and form a three-membered bromonium ion, releasing Br⁻. The bridging bromine blocks one face of the molecule, so in the second step the bromide ion attacks one of the ring carbons from the opposite face, much like an S_N2 reaction. The ring opens with the two bromines anti to each other.

The starting geometry then fixes the stereochemistry. In trans-stilbene the two phenyl groups start on opposite sides of the double bond. Anti addition to this symmetric trans alkene gives the (1R,2S) isomer, meso-1,2-dibromo-1,2-diphenylethane, which has an internal mirror plane and is optically inactive. Because the bromonium ion is symmetric, attack at either carbon gives the same meso product. By the same reasoning, anti addition to cis-stilbene would give the racemic mixture of the (1R,2R) and (1S,2S) enantiomers.

The melting point is a quick way to tell these products apart. The meso compound melts at about 241 °C (with decomposition; reported values from 241 to 243 °C), while the racemic mixture melts far lower, near 114–115 °C. The observed range of 240.2–241.7 °C therefore shows that the product was the meso isomer, as anti addition predicts.

Notes on the Procedure and Reagent Safety

The cyclohexene added at the end reacts with leftover bromine and removes the excess, which is why the bromine color disappears. Elemental bromine is the most hazardous reagent here. Fisher Scientific’s safety data sheet for bromine (revision 7, 19 December 2025) gives the signal word “Danger” with the hazard statements “Fatal if inhaled” and “Causes severe skin burns and eye damage”, notes that it is “Very toxic to aquatic life”, and directs “Use only under a chemical fume hood” and “Wear protective gloves/protective clothing/eye protection/face protection”. Many teaching labs instead use pyridinium tribromide, a stable solid that releases Br₂ in solution and is easier to measure and handle than liquid bromine or its solutions. Dichloromethane is also a health hazard: Fisher’s sheet for methylene chloride (revision 12, 18 December 2025) lists “Causes skin irritation”, “Causes serious eye irritation”, “May cause drowsiness or dizziness”, “May cause cancer” and “May cause damage to organs through prolonged or repeated exposure”. Some greener versions of this experiment use ethanol as the solvent.

For a related experiment, see Stereochemistry of Bromine Addition to an Alkene.

Further reading: Organic Chemistry (2nd ed.) by Jonathan Clayden, Nick Greeves and Stuart Warren — explains how bromine adds to an alkene through a bromonium ion and why that addition is stereospecific (anti), the chemistry behind brominating trans-stilbene to 1,2-dibromo-1,2-diphenylethane.
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REFERENCES:

• McMaster University, Chem 2O06 laboratory manual, Experiment 5: “Preparation of meso-Stilbene Dibromide and its Conversion to Diphenylacetylene” — meso-1,2-dibromo-1,2-diphenylethane, m.p. 241 °C (dec). chemistry.mcmaster.ca
• Clemson University CH 2270 handout, “Addition of a Halogen to an Alkene – Synthesis of Stilbene Dibromide” — meso isomer 241–243 °C, racemic mixture 114–115 °C. PDF
• OpenStax Organic Chemistry, 8.2: Halogenation of Alkenes (bromonium ion, anti addition)
• PubChem: Bromine (hazard classification)
• PubChem: meso-1,2-dibromo-1,2-diphenylethane