Bromination Of Trans-Stilbene Lab Report
LAB REPORT!
EXPERIMENT#5,6! MANPREET KAUR KHAIRA! LAB PARTNER: VIAN RAIES!
The purpose of this lab exercise was to perform the bromination of (E)-1,2-diphenylethene (trans-stilbene) by addition reaction in which bromine was added across the double bond to yield a vicinal dibromide. The next step was to perform a double elimination reaction by product gained to synthesize an alkyne, that is, 1,2-diphenylacetylene. The two major techniques used in this lab were TLC analysis and UV-vis spectroscopy. TLC technique is non-destructive, that is, the molecules in the mixtures are separated physically without being chemically altered. Analytical TLC is used in drug analysis, consumer product monitoring, environmental pollution! studies, forensics and many other applications. Further, UV-vis method is based …show more content…
on the [rinciple that atoms containing π-electrons or non-holding electrons (n-electrons) can ingest the vitality as ultraviolet or unmistakable light to energize these electrons to higher hostile to holding sub- atomic orbitals.!
The major conclusions drawn from this lab include the melting points, percent yields, TLC and UV-vis analysis. TLC performed in this lab shows that trans-stilbene is the most polar and UV- vis depicts that maximum absorption occurs at the highest peaks of the absorbance vs. wavelength curve.!
INTRODUCTION:!
ABSTRACT:!
!
!
!
!
The bromination reaction of trans-stilbene is traditionally carried out by elemental bromine in a solvent that will dissolve the alkene but not participate in the reaction. Acetic acid was used as the solvent and pyridinium tribromide as the source of molecular bromine. The overall reaction followed is as follows:!
!
equation1:!
!
The mechanism accepted for the addition reaction to proceed is an ionic mechanism in which the electron-rich alkene trans-stilbene acts as the nucleophile and bromine as the electrophile. the bromine becomes more polarized when it approaches the alkene.The step-1 includes the transfer of the polarized Br atom to the alkene to yield a cyclic bromonium ion and a bromide anion as shown in the step 1 of equation 1.In the second step, one of the bromide anions attacks one of the carbon atoms of the cyclic bromonium ion to open the three-membered ring and yield the vicinal dibromide. The two bromine atoms can be attached to the double bond in two possible ways, either “syn" or “anti”. !
The stereochemistry of the reaction is controlled by the bromonium ion. The product 1,2- dibromo-1,2-diphenylethane has two chirality centres and can be represented by the following four possible stereoisomers:!
!
!
! ! ! Figure1: stereoisomers of 1,2-dibromo-1,2-diphenylethane.!
This lab experiment uses acetic acid as the source of molecular bromine, which is gradually released into the reaction medium through rapid equilibrium between pyridinium tribromide and pyridine hydrobromide + bromine. More the bromine is consumed in the reaction, more is the bromine produced by the pyridinium tribromide by the following mechanism:!
!
! equation 2:!
!
The action of potassium hydroxide in alcoholic solution on compounds containing halogen atoms on adjacent atoms results in the elimination of two molecules of hydrogen halide and the formation of two new pi-bonds to give a carbon-carbon triple bond. The reaction involving the elimination of two molecules of hydrogen halides is called the double elimination reaction. The second part of the lab includes formation of an alkyne by performing double elimination reaction on stilbene dibromide as following:!
<equation3!
! !
Various other tests had been performed in this lab which are discussed briefly. Chromatography is the process by which one separate compounds from one another by!
passing a mixture through a column that retains some compounds longer than others.
This separation technique is based on the principal that under the same conditions, the time between the injection of a component into the column and the elution of that component is constant. This characteristic is used to perform qualitative or quantitative analysis. Thin-layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures. Thin-layer chromatography is performed on a sheet of glass, plastic, or aluminium foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose. A small amount of the mixture to be analyzed is spotted near the bottom of this plate. The TLC plate is then placed in a shallow pool of a solvent in a developing chamber so that only the very bottom of the plate is in the liquid. This liquid, or the eluent, is the mobile phase, and it slowly rises up the TLC plate by capillary action. The underlined principle is that the components will differ in solubility and in the strength of their adsorption to the adsorbent and some components will be carried farther up the plate than
others.!
Recrystallization, also known as fractional crystallization, is a procedure for purifying an impure compound in a solvent. The method of purification is based on the principle that the solubility of most solids increases with increased temperature. The choice of solvent depends on the quantity of impurities and temperature dependance. For better recrystallization, the quantity of impurities should be small and the solubility curve of the desired solute should rise rapidly with temperature, because as temperature increases, the amount of solute that can be dissolved in a solvent increases.!
The melting point determination is a method to determine the melting point of an unknown compound by the capillary tube method followed in this experiment. Melting point is one of those physical properties that has most often been used for identification and characterization of solids. This procedure is performed in order to identify a sample and establish it’s purity, because a pure, nonionic crystalline organic compound usually has a sharp and characteristic melting point which is detected by this method, hence proving this method really useful.!
The silver nitrate test has been performed during this lab exercise. Vicinal dihalides readily react with alcoholic silver nitrate to form a precipitate of the corresponding silver halide, which is used to characterize organic reaction products which contain bromine and chlorine atoms.!
UV-Vis spectroscopy is another method which had been performed at the end of this exercise with the final product, that is, 1,2-diphenylacetylene.!
The beer lambert law, that is the basis of UV-vis method, relates the attenuation of light to the properties of the material through which the light is traveling as follows:!
A=εlc!
where, A= absorbance of the solution, ε= molar absorption coefficient of the solution, l= length of the cuvette used for performing the experiment, c= concentration of the solution.!
!
MATERIALS AND METHODS:!
!
MATERIAL
STRUCTURE
MOLAR MASS
MELTING POINT
DENSITY
SOLUBILITY
(E)-1,2- diphenylethene (trans stilbene)
180.25 g/mol
122-125 C
0.9707 g/cm^3 practically insoluble in water.
Pyridinium tribromide
319.82 g/mol
127-133 C
-
soluble
1,2- dibromo-1,2dip henylethane (stilbene dibromide)
340.05 g/mol
241-242 C
1.613 g/cm^3
-
Diphenylacetyl ene
178.229 g/mol
59 C
1.1 g/cm^3
4.649g/ml at 25 C
!
The procedure is same as mentioned in the lab manual.!
!
RESULTS AND CALCULATIONS:!
!
The results have been divided into two parts depending on the experiment.!
A. BROMINATIONOFALKENE:! i) Limiting reagent calculations:!
! moles of trans-stilbene = 0.0055 mol!= 0.0055mol*180.25 g/mol= 1.988 g!
! moles of pyridinium tribromide = 0.0062 mol!=319.82 g/mol *0.0062 mol= 0.990 g!
! Therefore, trans-stilbene is the limiting reagent.! ! ii) Calculation of percent yield:!
! Actual Mass of stilbene dibromide = 1.006 g!
! Theoretical mass of stilbene dibromide = 1.988 g!
! percent yield = ( actual yield / theoretical yield )* 100 %!
! ! = ( 1.006/1.988 ) *100 = 50.60%! !! iii) Melting point = 249 C!
!
iv) Silver nitrate test observations: !
! When ethyl acetate was added, the mixture was clear, but when silver nitrate was !
! added, the solution became opaque, and the crystals didn't dissolve much.! !
!
v) TLC plate observations:! !!
!! Rf = distance travelled by component / distance travelled by solvent!
!! Rf for A = 4.6/5.9 = 0.779 ( product standard )!
!! Rf for B = 4.0/5.9 = 0.677 ( product )!
!! Rf for C = 4.7/5.9 = 0.796 ( starting material/reactant )!
B. ALKYNE SYNTHESIS:!
i) Limiting reagent calculations:!
! mass of stilbene dibromide = 1.006 g = 1.006 g/ 340.05 g/mol = 0.0029 mol!
! mass of KOH = 0.799 g = 0.799g/56.10g/mol = 0.014 mol = 2* 0.014 = 0.028 mol !
! (according to equation stoichiometry)!
! Therefore, stilbene dibromide is the limiting reagent.! ! ii) Calculation of percent yield: !
! Actual yield of 1,2-diphenylacetylene = 0.218 g!
! Theoretical yield of 1,2-diphenylacetylene = 0.0029 mol = 0.0029 mol*178.229 g/mol = !
! 0.516 g!
! Percent yield = ( 0.218/0.516 )* 100 = 42.24 %! ! iii) Melting point = 60.1 C!
!
iv) UV-vis spectroscopy results: !
!
!

!!
!
=> Standard diphenylacetylene = (0.103g / 25 mL) * (1mol / 178.23 g) * (1000 mL/L) = 2.3 * 10^-5 mol/L = c!
A=1.082!
A= εlc!
1.082= ε* 1 cm * 2.3*10^-5 mol/L!
Hence, ε=46807 L/cm/mol= 4.7*10^4 L/cm/mol ( ε_std )!
!
=> Experimental product = (0.102g / 25 mL) * (1/10) * (1/10) * (1/10) = 4.06 *10^-6 = c!
A= 0.989!
A= εlc!
0.989= ε*1 cm* 4.06*10^-6 mol/L!
Hence, ε=2.43*10^5 L/cm/mol ( ε_prod )!
!
Percent purity = [ ( ε_prod ) / ( ε_std ) ] * 100 = [2.43*10^5 L/cm/mol / 4.7*10^4 L/cm/mol ] *100 !
DISCUSSION: !
!
!
A. BROMINATIONOFALKENE:!
! The experiment had been a success till some extent, as the melting 249 C, is quite close to the melting point of pure stilbene dibromide. This would be possible due to the presence of impurities in the material. The percent yield came out to be almost half, that is 50.60 %. This could be due to spilling of material while performing the experiment, vacuum filtration might not have been done for a long time as was required. After the silver nitrate test was performed, precipitates were formed depicting the formation of the corresponding silver halide, that is stilbene dibromide. This means the experiment was performed correctly and the product had the organic reaction product desired. The TLC results show that the starting material travelled the most among the three materials depicting that it was the most non-polar among all.!
!
B. ALKYNE SYNTHESIS:!
! The experiment has not a been a success. The percent yield was out to be beyond the error limit, whereas the melting point has been found out correctly, that is 60.1 C. The possible sources of error include spilling of material and impurities present in it. The product is not pure as the percent purity is far beyond experimental error. The beer law explains that absorbance is directly proportional to the concentration of the solution and becomes linear with it when plotted against it. (A= εlc)!
CONCLUSION:!
!
!
The experiment performed consisted of two parts that depict trans-stilbene and stilbene dibromide as the limiting reagents. The melting points of stilbene dibromide and 1,2- diphenylacetylene are found out to be 249 C and 60.1 C, respectively. !
REFERENCES:!
!
!

1. "The Beer-Lambert Law." - Chemwiki. N.p., n.d. Web. 24 Nov. 2014.!
2. "Beer's Law - Theoretical Principles." Beer's Law - Theoretical Principles. N.p., n.d. Web. 23
Nov. 2014.!
3. "CAS # 5789-30-0, 1,2-Dibromo-1,2-diphenylethane, 1,1'-(1,2-Dibromo-1,2- ethanediyl)bisbenzene." CAS # 5789-30-0, 1,2-Dibromo-1,2-diphenylethane, 1,1'-(1,2-
Dibromo-1,2-ethanediyl)bisbenzene. N.p., n.d. Web. 25 Nov. 2014.!
4. "Diphenylacetylene." ChemSpider. N.p., n.d. Web. 25 Nov. 2014.!
5. "HPLC Basic Course 1.Principle and System Configuration of HPLC(1):Hitachi High-
Technologies Corporation." HPLC Basic Course 1.Principle and System Configuration of
HPLC(1):Hitachi High-Technologies Corporation. N.p., n.d. Web. 25 Nov. 2014.!
6. "Principles and Practice of Chromatography." Nature 152.3845 (1943): 35. Web.!
7. "Pyridinium Tribromide." ChemicalBook---Chemical Search Engine. N.p., n.d. Web. 25 Nov.
2014.!
8. "Recrystallization." - Chemwiki. N.p., n.d. Web. 25 Nov. 2014.!
9. "Thin Layer Chromatography (TLC)." Thin Layer Chromatography (TLC). N.p., n.d. Web. 22
Nov. 2014.
EXPERIMENT#5,6! MANPREET KAUR KHAIRA! LAB PARTNER: VIAN RAIES!
The purpose of this lab exercise was to perform the bromination of (E)-1,2-diphenylethene (trans-stilbene) by addition reaction in which bromine was added across the double bond to yield a vicinal dibromide. The next step was to perform a double elimination reaction by product gained to synthesize an alkyne, that is, 1,2-diphenylacetylene. The two major techniques used in this lab were TLC analysis and UV-vis spectroscopy. TLC technique is non-destructive, that is, the molecules in the mixtures are separated physically without being chemically altered. Analytical TLC is used in drug analysis, consumer product monitoring, environmental pollution! studies, forensics and many other applications. Further, UV-vis method is based …show more content…
on the [rinciple that atoms containing π-electrons or non-holding electrons (n-electrons) can ingest the vitality as ultraviolet or unmistakable light to energize these electrons to higher hostile to holding sub- atomic orbitals.!
The major conclusions drawn from this lab include the melting points, percent yields, TLC and UV-vis analysis. TLC performed in this lab shows that trans-stilbene is the most polar and UV- vis depicts that maximum absorption occurs at the highest peaks of the absorbance vs. wavelength curve.!
INTRODUCTION:!
ABSTRACT:!
!
!
!
!
The bromination reaction of trans-stilbene is traditionally carried out by elemental bromine in a solvent that will dissolve the alkene but not participate in the reaction. Acetic acid was used as the solvent and pyridinium tribromide as the source of molecular bromine. The overall reaction followed is as follows:!
!
equation1:!
!
The mechanism accepted for the addition reaction to proceed is an ionic mechanism in which the electron-rich alkene trans-stilbene acts as the nucleophile and bromine as the electrophile. the bromine becomes more polarized when it approaches the alkene.The step-1 includes the transfer of the polarized Br atom to the alkene to yield a cyclic bromonium ion and a bromide anion as shown in the step 1 of equation 1.In the second step, one of the bromide anions attacks one of the carbon atoms of the cyclic bromonium ion to open the three-membered ring and yield the vicinal dibromide. The two bromine atoms can be attached to the double bond in two possible ways, either “syn" or “anti”. !
The stereochemistry of the reaction is controlled by the bromonium ion. The product 1,2- dibromo-1,2-diphenylethane has two chirality centres and can be represented by the following four possible stereoisomers:!
!
!
! ! ! Figure1: stereoisomers of 1,2-dibromo-1,2-diphenylethane.!
This lab experiment uses acetic acid as the source of molecular bromine, which is gradually released into the reaction medium through rapid equilibrium between pyridinium tribromide and pyridine hydrobromide + bromine. More the bromine is consumed in the reaction, more is the bromine produced by the pyridinium tribromide by the following mechanism:!
!
! equation 2:!
!
The action of potassium hydroxide in alcoholic solution on compounds containing halogen atoms on adjacent atoms results in the elimination of two molecules of hydrogen halide and the formation of two new pi-bonds to give a carbon-carbon triple bond. The reaction involving the elimination of two molecules of hydrogen halides is called the double elimination reaction. The second part of the lab includes formation of an alkyne by performing double elimination reaction on stilbene dibromide as following:!
<equation3!
! !
Various other tests had been performed in this lab which are discussed briefly. Chromatography is the process by which one separate compounds from one another by!
passing a mixture through a column that retains some compounds longer than others.
This separation technique is based on the principal that under the same conditions, the time between the injection of a component into the column and the elution of that component is constant. This characteristic is used to perform qualitative or quantitative analysis. Thin-layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures. Thin-layer chromatography is performed on a sheet of glass, plastic, or aluminium foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose. A small amount of the mixture to be analyzed is spotted near the bottom of this plate. The TLC plate is then placed in a shallow pool of a solvent in a developing chamber so that only the very bottom of the plate is in the liquid. This liquid, or the eluent, is the mobile phase, and it slowly rises up the TLC plate by capillary action. The underlined principle is that the components will differ in solubility and in the strength of their adsorption to the adsorbent and some components will be carried farther up the plate than
others.!
Recrystallization, also known as fractional crystallization, is a procedure for purifying an impure compound in a solvent. The method of purification is based on the principle that the solubility of most solids increases with increased temperature. The choice of solvent depends on the quantity of impurities and temperature dependance. For better recrystallization, the quantity of impurities should be small and the solubility curve of the desired solute should rise rapidly with temperature, because as temperature increases, the amount of solute that can be dissolved in a solvent increases.!
The melting point determination is a method to determine the melting point of an unknown compound by the capillary tube method followed in this experiment. Melting point is one of those physical properties that has most often been used for identification and characterization of solids. This procedure is performed in order to identify a sample and establish it’s purity, because a pure, nonionic crystalline organic compound usually has a sharp and characteristic melting point which is detected by this method, hence proving this method really useful.!
The silver nitrate test has been performed during this lab exercise. Vicinal dihalides readily react with alcoholic silver nitrate to form a precipitate of the corresponding silver halide, which is used to characterize organic reaction products which contain bromine and chlorine atoms.!
UV-Vis spectroscopy is another method which had been performed at the end of this exercise with the final product, that is, 1,2-diphenylacetylene.!
The beer lambert law, that is the basis of UV-vis method, relates the attenuation of light to the properties of the material through which the light is traveling as follows:!
A=εlc!
where, A= absorbance of the solution, ε= molar absorption coefficient of the solution, l= length of the cuvette used for performing the experiment, c= concentration of the solution.!
!
MATERIALS AND METHODS:!
!
MATERIAL
STRUCTURE
MOLAR MASS
MELTING POINT
DENSITY
SOLUBILITY
(E)-1,2- diphenylethene (trans stilbene)
180.25 g/mol
122-125 C
0.9707 g/cm^3 practically insoluble in water.
Pyridinium tribromide
319.82 g/mol
127-133 C
-
soluble
1,2- dibromo-1,2dip henylethane (stilbene dibromide)
340.05 g/mol
241-242 C
1.613 g/cm^3
-
Diphenylacetyl ene
178.229 g/mol
59 C
1.1 g/cm^3
4.649g/ml at 25 C
!
The procedure is same as mentioned in the lab manual.!
!
RESULTS AND CALCULATIONS:!
!
The results have been divided into two parts depending on the experiment.!
A. BROMINATIONOFALKENE:! i) Limiting reagent calculations:!
! moles of trans-stilbene = 0.0055 mol!= 0.0055mol*180.25 g/mol= 1.988 g!
! moles of pyridinium tribromide = 0.0062 mol!=319.82 g/mol *0.0062 mol= 0.990 g!
! Therefore, trans-stilbene is the limiting reagent.! ! ii) Calculation of percent yield:!
! Actual Mass of stilbene dibromide = 1.006 g!
! Theoretical mass of stilbene dibromide = 1.988 g!
! percent yield = ( actual yield / theoretical yield )* 100 %!
! ! = ( 1.006/1.988 ) *100 = 50.60%! !! iii) Melting point = 249 C!
!
iv) Silver nitrate test observations: !
! When ethyl acetate was added, the mixture was clear, but when silver nitrate was !
! added, the solution became opaque, and the crystals didn't dissolve much.! !
!
v) TLC plate observations:! !!
!! Rf = distance travelled by component / distance travelled by solvent!
!! Rf for A = 4.6/5.9 = 0.779 ( product standard )!
!! Rf for B = 4.0/5.9 = 0.677 ( product )!
!! Rf for C = 4.7/5.9 = 0.796 ( starting material/reactant )!
B. ALKYNE SYNTHESIS:!
i) Limiting reagent calculations:!
! mass of stilbene dibromide = 1.006 g = 1.006 g/ 340.05 g/mol = 0.0029 mol!
! mass of KOH = 0.799 g = 0.799g/56.10g/mol = 0.014 mol = 2* 0.014 = 0.028 mol !
! (according to equation stoichiometry)!
! Therefore, stilbene dibromide is the limiting reagent.! ! ii) Calculation of percent yield: !
! Actual yield of 1,2-diphenylacetylene = 0.218 g!
! Theoretical yield of 1,2-diphenylacetylene = 0.0029 mol = 0.0029 mol*178.229 g/mol = !
! 0.516 g!
! Percent yield = ( 0.218/0.516 )* 100 = 42.24 %! ! iii) Melting point = 60.1 C!
!
iv) UV-vis spectroscopy results: !
!
!

!!
!
=> Standard diphenylacetylene = (0.103g / 25 mL) * (1mol / 178.23 g) * (1000 mL/L) = 2.3 * 10^-5 mol/L = c!
A=1.082!
A= εlc!
1.082= ε* 1 cm * 2.3*10^-5 mol/L!
Hence, ε=46807 L/cm/mol= 4.7*10^4 L/cm/mol ( ε_std )!
!
=> Experimental product = (0.102g / 25 mL) * (1/10) * (1/10) * (1/10) = 4.06 *10^-6 = c!
A= 0.989!
A= εlc!
0.989= ε*1 cm* 4.06*10^-6 mol/L!
Hence, ε=2.43*10^5 L/cm/mol ( ε_prod )!
!
Percent purity = [ ( ε_prod ) / ( ε_std ) ] * 100 = [2.43*10^5 L/cm/mol / 4.7*10^4 L/cm/mol ] *100 !
DISCUSSION: !
!
!
A. BROMINATIONOFALKENE:!
! The experiment had been a success till some extent, as the melting 249 C, is quite close to the melting point of pure stilbene dibromide. This would be possible due to the presence of impurities in the material. The percent yield came out to be almost half, that is 50.60 %. This could be due to spilling of material while performing the experiment, vacuum filtration might not have been done for a long time as was required. After the silver nitrate test was performed, precipitates were formed depicting the formation of the corresponding silver halide, that is stilbene dibromide. This means the experiment was performed correctly and the product had the organic reaction product desired. The TLC results show that the starting material travelled the most among the three materials depicting that it was the most non-polar among all.!
!
B. ALKYNE SYNTHESIS:!
! The experiment has not a been a success. The percent yield was out to be beyond the error limit, whereas the melting point has been found out correctly, that is 60.1 C. The possible sources of error include spilling of material and impurities present in it. The product is not pure as the percent purity is far beyond experimental error. The beer law explains that absorbance is directly proportional to the concentration of the solution and becomes linear with it when plotted against it. (A= εlc)!
CONCLUSION:!
!
!
The experiment performed consisted of two parts that depict trans-stilbene and stilbene dibromide as the limiting reagents. The melting points of stilbene dibromide and 1,2- diphenylacetylene are found out to be 249 C and 60.1 C, respectively. !
REFERENCES:!
!
!

1. "The Beer-Lambert Law." - Chemwiki. N.p., n.d. Web. 24 Nov. 2014.!
2. "Beer's Law - Theoretical Principles." Beer's Law - Theoretical Principles. N.p., n.d. Web. 23
Nov. 2014.!
3. "CAS # 5789-30-0, 1,2-Dibromo-1,2-diphenylethane, 1,1'-(1,2-Dibromo-1,2- ethanediyl)bisbenzene." CAS # 5789-30-0, 1,2-Dibromo-1,2-diphenylethane, 1,1'-(1,2-
Dibromo-1,2-ethanediyl)bisbenzene. N.p., n.d. Web. 25 Nov. 2014.!
4. "Diphenylacetylene." ChemSpider. N.p., n.d. Web. 25 Nov. 2014.!
5. "HPLC Basic Course 1.Principle and System Configuration of HPLC(1):Hitachi High-
Technologies Corporation." HPLC Basic Course 1.Principle and System Configuration of
HPLC(1):Hitachi High-Technologies Corporation. N.p., n.d. Web. 25 Nov. 2014.!
6. "Principles and Practice of Chromatography." Nature 152.3845 (1943): 35. Web.!
7. "Pyridinium Tribromide." ChemicalBook---Chemical Search Engine. N.p., n.d. Web. 25 Nov.
2014.!
8. "Recrystallization." - Chemwiki. N.p., n.d. Web. 25 Nov. 2014.!
9. "Thin Layer Chromatography (TLC)." Thin Layer Chromatography (TLC). N.p., n.d. Web. 22
Nov. 2014.