Bromination
Chemistry 212
Fall 2013
Experiment 5: Bromination – Evaluation Summary Last Name:
First Name:
Date:
Lab Day/TA/Group:
Labs reports must be typed and chemical structures must be drawn with ChemDraw. Report must not exceed five pages (including this page). Page limit does not include any attached spectra. ** Deductions for hand written report/structures and exceeding page limit ** Report Breakdown Intro/Results: _________________ /10 Discussion:
_________________ /10
Report Total:
_________________ /20
Other Lab Marks Performance:
_________________ /6
Prelab Quiz:
_________________ /3
TA Comments on Report/Performance
Report assessment TA (name):
TA (signature):
Performance assessment TA (name):
TA (signature):
1 of 5
Experiment 5: Bromination – Lab Report Guidelines Include the following in your report (in the following order, use provided table): 1. Title, Date, Your Name, Your Day/Time/TA 2. Objective: (1 mark) What is the purpose of this experiment? The purpose of this experiment is to compare the bromination products of alkene and aromatic compound and the mechanism by which they proceed. The active ingredient for both experiments is the same (elemental bromine) but the products are obtained through different mechanisms. 3. Introduction: (2 marks) Brief description of the concept/reaction studied – bromination reactions of alkenes and aromatic compounds – and why it is important Bromination of alkenes proceeds through an electrophilic addition reaction where a temporary dipole is induced on the bromine molecule as it approaches the double bond of the alkene. An intermediate is formed where one of the bromine molecule is partially bonded to both carbons on the double bond and the electron pair on the BrBr bond is heterolytically given to the other bromine atom, which forms a bromide ion. This intermediate means that there is no true carbocation. The bromide ion then attacks from the opposite side to which the initial bromine atom is bonded because of steric hindrance. Thus we get an anti product. This reaction is important because it produces a product that can be used as a starting material for various reactions, such as systhesis of alkynes. Bromination of aromatic compounds proceeds through electrophilic substitution reaction. As the substituent on bromine is an activating group no catalyst is required. The bromine molecule approaches the benzene ring and a dipole is induced. An electron pair is transferred to the del positive bromine atom and the BrBr bond breaks to give Br. The addition of bromine leaves the benzene ring positively charged. The Br then comes and removes a hydrogen from the carbon to which the bromine was bonded and this restores the electron cloud of benzene. This is an important reaction because, again, the product can be used as intermediate for organin synthesis as well as producing drugs such as aspirin etc. 4. Reaction Equation: (2 marks) Equation and data [molar mass, concentration, density, volume, mass, moles, etc – as appropriate – including theoretical yield] for reactants and products
2
Chemistry 212
Materials Estilbene Pyridinum Tribromide Ethanol Acetanilide Sodium Bromide Galacial Acetic Acid Sodium Hydroxide Stilbene Dibromide Bromoacetanilide Bleach Sodium Thiosulphate
Fall 2013
Molar Mass/g 180.25 319
Quantity
Density/ (g/mL)
Moles/ mmol 1.11 1.25
46 135 103 60
5ml .5g .9g 2.5mL
.789 1.05
85.8 3.70 8.73 43.7
40
6.5mL
2.13
26.6
.2g .4g
339 214 74.4 158
Theoretical yield/g
0.38 7mL .8mL
1.08 1.66
101.6 8.4
0.79
5. Procedure: (2 marks) Summary of lab procedure – what YOU did, include KEY observations – be concise! Experiment 1 Mixed the required amounts of Estilbene and Ethanol in erlenmeyer and gently heated with a stir bar. After Estilbene dissolved added 0.4 g pyridinum tribromide, using ethanol to rinse the 3 of 5
sides. Heated the flask again for 5 mins, cooled it, and placed it in ice bath to obtain the product, which was pale yellow in color at this stage. Carried out vaccum filtration to obtain white crystals of product. Dried it, weighed it and took melting point. Experiment 2 Added the required amounts of Acetanilide, sodium bromide, ethanol and acetic acid in an erlenmeyer in the fume hood. Placed a stir bar in flask and closed the opening with a cotton plug. Placed the flask in ice bath on the bench. Left it for 510 mins. The solution turned yellow. Removed plug to add 7mL bleach, and a pale yellow precipitate was formed. Removed flask from icebath can warmed to room temperature. Added 0.8mL sodium thiosulphate and 0.5 mL NaOH, a white precipitate was formed. Collected product by vaccum filtration, dried, weighed and took melting point of crude. Carried out recrystallizzation of crude with 5050 ethanol/water. Vaccum filtered to obtain product, dried, weighed and took melting point of purified product. 6. Results: (3 marks) a. Table 1 : Yield and Physical Properties Dibromostilbene
Bromoacetanilide (crude/pure)
Yield (g)
.31g
Crude=.62g, Pure= .58g
Yield (%)
82
Crude yield= 78, pure yield=73
White crystals
White crystals, both
238240
Crude=157159; Pure= 163165
Appearance Melting Point Recovery (%)
94
7. Discussion: (10 marks) a. Discussion of Yields – crude, purified, % recovery – why less than 100%? Crude: The reaction might not have reached completion Purified: Recovery: All of the product might not have recrystallized. Also, the mass of the crude was pure product + impurities so after the impurities was removed the mass of pure product had to be less than the crude. 100% recovery would mean no impurities was present. b. Discussion of Physical Properties – evidence for correct product i. Analysis of Melting Point – pure/impure? The melting point of the product of the first experiment was 238240°C and the melting point of stilbene bromide is 238240 °C. As the two melting temperatures coincide the product obtained pure The melting point of the initial product of reaction 2 was 157159°C where was the melting point of Bromo acetanilide is 165169°C. This shows that the product obtained was impure. After recrystallization the melting point of the product was 163165°C, this is a range closer to the actual melting point of bromoacetanilide, so this was a more pure product. 4
Chemistry 212
c.
Fall 2013
Mechanisms – detailed mechanism for each bromination reaction Experiment 1
Experiment 2
d.
Discussion Questions – i. Comment on the stereoselectivity of the bromination of Estilbene (assign stereochemistry of dibromo product, why selective?, is this molecule chiral?) Bromine is added to Estilbene to form an anti product. This happens because no true carbocation is formed in the reaction, instead we have an intermediate where one of the bromine atom is partially bonded to both carbon atoms of the double bond. This prevent and bond rotation around the CC bond. The steric hindrance prevents the Br
5 of 5
from being added to the same side as the first bromine atom. So the bromination process is stereoselective. The product formed is chiral. ii. Comment on the regioselectivity of the bromination of acetanilide (assign regiochemistry of bromo product ortho/meta/para, why selective?) The regiochemistry of the product is para. The substituent present on the benzene ring has a nitrogen right next to the carbon of the benzene. The lone pain of the nitrogen can delocalize over the benzene ring and activate it. An activating group is ortho/para directing because the carbocation formed by this arrangement gives the most stabilized resonance structures. Here we get majority of the para product because there is steric hindrance in the ortho position as the substituent is a large group. iii. Why do we use alternative brominating agents in this experiment? Aqueous bromine is highly toxic and very volatile so alternative source has to be used for safety reasons.
6
Fall 2013
Experiment 5: Bromination – Evaluation Summary Last Name:
First Name:
Date:
Lab Day/TA/Group:
Labs reports must be typed and chemical structures must be drawn with ChemDraw. Report must not exceed five pages (including this page). Page limit does not include any attached spectra. ** Deductions for hand written report/structures and exceeding page limit ** Report Breakdown Intro/Results: _________________ /10 Discussion:
_________________ /10
Report Total:
_________________ /20
Other Lab Marks Performance:
_________________ /6
Prelab Quiz:
_________________ /3
TA Comments on Report/Performance
Report assessment TA (name):
TA (signature):
Performance assessment TA (name):
TA (signature):
1 of 5
Experiment 5: Bromination – Lab Report Guidelines Include the following in your report (in the following order, use provided table): 1. Title, Date, Your Name, Your Day/Time/TA 2. Objective: (1 mark) What is the purpose of this experiment? The purpose of this experiment is to compare the bromination products of alkene and aromatic compound and the mechanism by which they proceed. The active ingredient for both experiments is the same (elemental bromine) but the products are obtained through different mechanisms. 3. Introduction: (2 marks) Brief description of the concept/reaction studied – bromination reactions of alkenes and aromatic compounds – and why it is important Bromination of alkenes proceeds through an electrophilic addition reaction where a temporary dipole is induced on the bromine molecule as it approaches the double bond of the alkene. An intermediate is formed where one of the bromine molecule is partially bonded to both carbons on the double bond and the electron pair on the BrBr bond is heterolytically given to the other bromine atom, which forms a bromide ion. This intermediate means that there is no true carbocation. The bromide ion then attacks from the opposite side to which the initial bromine atom is bonded because of steric hindrance. Thus we get an anti product. This reaction is important because it produces a product that can be used as a starting material for various reactions, such as systhesis of alkynes. Bromination of aromatic compounds proceeds through electrophilic substitution reaction. As the substituent on bromine is an activating group no catalyst is required. The bromine molecule approaches the benzene ring and a dipole is induced. An electron pair is transferred to the del positive bromine atom and the BrBr bond breaks to give Br. The addition of bromine leaves the benzene ring positively charged. The Br then comes and removes a hydrogen from the carbon to which the bromine was bonded and this restores the electron cloud of benzene. This is an important reaction because, again, the product can be used as intermediate for organin synthesis as well as producing drugs such as aspirin etc. 4. Reaction Equation: (2 marks) Equation and data [molar mass, concentration, density, volume, mass, moles, etc – as appropriate – including theoretical yield] for reactants and products
2
Chemistry 212
Materials Estilbene Pyridinum Tribromide Ethanol Acetanilide Sodium Bromide Galacial Acetic Acid Sodium Hydroxide Stilbene Dibromide Bromoacetanilide Bleach Sodium Thiosulphate
Fall 2013
Molar Mass/g 180.25 319
Quantity
Density/ (g/mL)
Moles/ mmol 1.11 1.25
46 135 103 60
5ml .5g .9g 2.5mL
.789 1.05
85.8 3.70 8.73 43.7
40
6.5mL
2.13
26.6
.2g .4g
339 214 74.4 158
Theoretical yield/g
0.38 7mL .8mL
1.08 1.66
101.6 8.4
0.79
5. Procedure: (2 marks) Summary of lab procedure – what YOU did, include KEY observations – be concise! Experiment 1 Mixed the required amounts of Estilbene and Ethanol in erlenmeyer and gently heated with a stir bar. After Estilbene dissolved added 0.4 g pyridinum tribromide, using ethanol to rinse the 3 of 5
sides. Heated the flask again for 5 mins, cooled it, and placed it in ice bath to obtain the product, which was pale yellow in color at this stage. Carried out vaccum filtration to obtain white crystals of product. Dried it, weighed it and took melting point. Experiment 2 Added the required amounts of Acetanilide, sodium bromide, ethanol and acetic acid in an erlenmeyer in the fume hood. Placed a stir bar in flask and closed the opening with a cotton plug. Placed the flask in ice bath on the bench. Left it for 510 mins. The solution turned yellow. Removed plug to add 7mL bleach, and a pale yellow precipitate was formed. Removed flask from icebath can warmed to room temperature. Added 0.8mL sodium thiosulphate and 0.5 mL NaOH, a white precipitate was formed. Collected product by vaccum filtration, dried, weighed and took melting point of crude. Carried out recrystallizzation of crude with 5050 ethanol/water. Vaccum filtered to obtain product, dried, weighed and took melting point of purified product. 6. Results: (3 marks) a. Table 1 : Yield and Physical Properties Dibromostilbene
Bromoacetanilide (crude/pure)
Yield (g)
.31g
Crude=.62g, Pure= .58g
Yield (%)
82
Crude yield= 78, pure yield=73
White crystals
White crystals, both
238240
Crude=157159; Pure= 163165
Appearance Melting Point Recovery (%)
94
7. Discussion: (10 marks) a. Discussion of Yields – crude, purified, % recovery – why less than 100%? Crude: The reaction might not have reached completion Purified: Recovery: All of the product might not have recrystallized. Also, the mass of the crude was pure product + impurities so after the impurities was removed the mass of pure product had to be less than the crude. 100% recovery would mean no impurities was present. b. Discussion of Physical Properties – evidence for correct product i. Analysis of Melting Point – pure/impure? The melting point of the product of the first experiment was 238240°C and the melting point of stilbene bromide is 238240 °C. As the two melting temperatures coincide the product obtained pure The melting point of the initial product of reaction 2 was 157159°C where was the melting point of Bromo acetanilide is 165169°C. This shows that the product obtained was impure. After recrystallization the melting point of the product was 163165°C, this is a range closer to the actual melting point of bromoacetanilide, so this was a more pure product. 4
Chemistry 212
c.
Fall 2013
Mechanisms – detailed mechanism for each bromination reaction Experiment 1
Experiment 2
d.
Discussion Questions – i. Comment on the stereoselectivity of the bromination of Estilbene (assign stereochemistry of dibromo product, why selective?, is this molecule chiral?) Bromine is added to Estilbene to form an anti product. This happens because no true carbocation is formed in the reaction, instead we have an intermediate where one of the bromine atom is partially bonded to both carbon atoms of the double bond. This prevent and bond rotation around the CC bond. The steric hindrance prevents the Br
5 of 5
from being added to the same side as the first bromine atom. So the bromination process is stereoselective. The product formed is chiral. ii. Comment on the regioselectivity of the bromination of acetanilide (assign regiochemistry of bromo product ortho/meta/para, why selective?) The regiochemistry of the product is para. The substituent present on the benzene ring has a nitrogen right next to the carbon of the benzene. The lone pain of the nitrogen can delocalize over the benzene ring and activate it. An activating group is ortho/para directing because the carbocation formed by this arrangement gives the most stabilized resonance structures. Here we get majority of the para product because there is steric hindrance in the ortho position as the substituent is a large group. iii. Why do we use alternative brominating agents in this experiment? Aqueous bromine is highly toxic and very volatile so alternative source has to be used for safety reasons.
6
