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BROMINATION OF TRANS – STILBENE

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:

Experimental data was recorded in the following tables:

 

 

DISCUSSION:

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 240.2o to 241.7o 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 SN2 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 range from roughly 236 to 243 °C), while the racemic mixture melts far lower, near 113–115 °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. Under the GHS classification on PubChem, it is fatal if inhaled and causes severe skin burns and eye damage, so it must be handled only in a fume hood with gloves and eye 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, and some greener versions of this experiment use ethanol as the solvent.

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

REFERENCES:

Chemistry / Natural Sciences 5:37 pm , May 11, 2018 0
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