Date Lab Submitted: Group A, B, or C: Comments for Grading TA: (Please indicate if you performed the lab on a day other than your regularly scheduled day and/or with a TA other than your regular TA).
Page Limit: report must not exceed FIVE pages (including this page) LIMIT DOES NOT INCLUDE ANY GRAPHS, SPECTRA, OR REFERENCES (Please see General Lab Report Guidelines for detailed descriptions of all other requirements) DEDUCTIONS FOR HANDWRITTEN REPORT/STRUCTURES/ GRAPHS AND EXCEEDING PAGE LIMIT LATE PENALTY IS 2 MARKS PER DAY LATE (NOT ACCEPTED AFTER 7 DAYS)
Solo Experiment 3: Bromination – Lab Report Guidelines Include the following in your report (in the following order, use provided table): Objective: (3 mark)* What is the purpose of this experiment? The purpose of this experiment is to compare brominated products of alkene and an aromatic compound. Both experiments have the same active ingredient, but the products are obtained through different mechanisms. Introduction: (3 marks)* Brief description of the concept/reaction studied – bromination reactions of alkenes and aromatic compounds – and why it is important The bromination of alkene proceeds via an electrophilic addition reaction where a temporary dipole is induced on the bromine molecule while it approaches the double bond of alkene.
An intermediate is formed where one bromine molecule is bonded to both carbons on the double bond; the electron pair on Br-Br bond is given to the other bromine atom, which forms a bromide ion. This intermediate indicates that there is no carbocation. The bromide ion then attacks from the backside to which the initial bromine atom is bonded. Thus, an anti product is expected. This reaction is important because it produces a product that can be used as a starting material for various reactions. The bromination of aromatic compounds proceeds via an electrophilic substitution reaction. No catalyst is required since the substituent on bromine is an activating group. 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 Br-Br bond breaks in order to give Bromine.
The addition of bromine leaves the benzene ring being positively charged. The Bromine then comes and removes hydrogen from the carbon to which the bromine was bonded. This restores the electron cloud of benzene. The reaction is important because the product can be used as intermediate for organic synthesis as well as producing drugs. Reaction Scheme: (3 marks) (Balanced chemical equation and data [structures, molar mass, concentration, density, volume, mass, moles, etc – as appropriate – including theoretical yield] for reactants and products) Materials Molar Mass (g/mol) Quantity Density (g/ml) Melting Point (°C) Moles Yields E-stilbene 180.25 0.2 g 0.97 122-125 1.11 Pyridinum Tribromide 319.82 0.48 127-133 1.25 Ethanol 46.07 5 ml 0.789 -114.0 85.8 Acetanilide 135.17 0.5 g 1.121 114.3 3.70 Sodium Bromide 102.89 0.9 g 3.21 747 8.73 C2H402 60.05 2.5 ml1.05 16 43.7 Sodium Hydroxide 39.99 6.5 m/2.13 318 26.6 Stilbene Dibromide 340.05 241 0.38 Bromoacetanilide 214.06 164-167 0.79 Bleach 74.44 7 ml 1.08 18 101.6 Sodium Thiosulphate 158.11 0.8 ml1.66 48.3 8.4 Procedure: (3 marks)* Summary of lab procedure – what YOU did, include KEY observations – but be concise! Someone should be able to repeat your experiment from this summary alone. Synthesis of Stilbene Dibromide: mix correct amounts of Estilbene and Ethanol in an Erlenmeyer and heat gently while stirring.
After Add 0.4 g pyridinum tribromide and use ethanol to rinse sides. Heat for five more minutes; cool down to room temperature, then ice bath. Collect product by vacuum filtration. Weight dry product and take melting point. Synthesis of Bromoactanile: add correct amounts of acetanilide, sodium bromide, ethanol and acetic acid in an Erlenmeyer. Add a stir bar and plug the top with cotton. Place the flask in ice bath to cool. Add 7ml bleach and continue to stir for a while. Remove flask from ice bath and warm to room temperature. Add 0.8ml sodium thiosulphate and 0.5ml NaOH. Collect product by vacuum filtration. Weight dry crude and take melting point. Use 50/50 ethanol/water solution to purify and recrystallize bromoacetanilide. Collect product by vacuum filtration.
Weight dry product and take melting point. Results: (3 marks) Table 1: Yield and Physical Properties Dibromostilbene Bromoacetanilide (crude) Bromoacetanilide (purified) Yield (g) 0.20 0.64 0.58 Yield (%) 53% 81% 73% Appearance White crystals White crystals White crystals Melting Point 238-240 °C 157-159 °C 163-165 °C Recovery (%) 91% Discussion: (15 marks)* a. Discussion of Yields – crude, purified, % recovery – chemical reasons The yield of crude showed that the reaction did not reach completion. The reason that the recovery was less than 100% might be not all of the products have recrystallized. Since the weight of the crude was pure product plus the impurities, after the impurities were removed, the weight of pure product had to be less than the crude. If it were 100% recovery, it would indicate that no impurities were present at all.
b. Discussion of Physical Properties – evidence for correct product i. Analysis of Melting Point – pure/impure? The melting point of product obtained from the first experiment was 238-240°C. It seemed very close to the theoretical melting point of stilbene dibromide which was 241°С. Since the two temperatures were relatively coincide, the product was concluded to be pure. The melting point of the initial product obtained from the second experiment was 157-159°C, where
the theoretical melting point of bromoacetanilide was 164-167°C. This showed that the product was impure. After recrystallization process was completed, the new melting point of the product was 163-165°C. This temperature range was much closer to the theoretical melting point of bromoacetanilide, and showed the product to be much more pure. C. Mechanisms – detailed mechanism for each bromination reaction 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 was added to Estilbene to form an anti product. This happened because no true carbocation was formed in reaction. Instead, there was an intermediate where one of the bromine atoms was partially bonded to both carbon atoms of the double bond.
This prevented bond rotation around the carbon-carbon bond. The steric hindrance prevented the bromine from being added to the same side as the first bromine atom. Thus, the process of bromination was stereoselective and the product formed was 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 was para.
The substituent presented on the benzene ring had nitrogen right next to the carbon of the benzene. The lone pair of nitrogen could delocalize over the benzene ring and activate it. An activating group was ortho or para directing because the carbocation formed by this arrangement gave the most stabilized resonance structures. The majority was the para product because there was steric hindrance in the ortho position as the substituent was a large group.