Experiment 18: Bromination of trans-Cinnamic AcidReading Assignment: Mohrig 3rd ed, p. 38-42, p. 111, Figure 10.7b

Your Organic I textbook: review stereoisomers, electrophilic addition of bromine to alkenes and nomenclature

MSDS for bromineOne of the most important aspects of organic chemistry is the study and application of organic reactions. Due to the large number of different reactants that can be used in any particular reaction, it is virtually impossible to memorize the products that would form from all possible starting materials. Fortunately for the organic chemist, specific functional groups undergo very predictable chemical reactions. These chemical reactions generally occur by very specific reaction mechanisms. So instead of trying to memorize all possible products that could be formed, the organic chemist learns the mechanism for a particular type of reaction and then applies that mechanism to any starting material with the appropriate functional group.

A reaction mechanism is a step-by-step pathway by which starting materials are converted to products. The mechanism shows the structures of all reactants, intermediates, and products and uses curved arrows to show the movement of electrons as bonds are broken and formed during the reaction.

A variety of terms are often used when describing organic reactions and their mechanisms. Two terms, regioselective and stereospecific, are especially important. A regioselective reaction is one in which two or more constitutional (structural) isomers can form, but one of the isomers is formed in larger amounts than the others. A good example of a regioselective reaction is the Markovnikov addition of HCl to an alkene such as 1-hexene. In this reaction the major product isolated is 2-chlorohexane. Little or no 1-chlorohexane, the other possible constitutional isomer, is formed.

Stereospecific reactions are those in which a specific stereoisomeric reactant gives specific stereoisomeric products. The SN2 reaction of sodium methoxide with (R)-2-bromobutane to form (S)-2-methoxybutane is one example of a stereospecific reaction. The R enantiomer used in the reaction forms a very specific stereoisomeric product, the one formed by inversion of configuration.

The hydroboration-oxidation of 1-methylcyclohexene is another example of a stereospecific (as well as regiospecific) reaction. This reaction occurs by syn addition and gives a racemic mixture of the anti-Markovnikov products.

In this experiment, you will carry out the bromination of trans-cinnamic acid and determine the stereochemistry of the products as a means of supporting or disproving the accepted mechanism for this reaction.

Since 2,3-dibromo-3-phenylpropanoic acid has two asymmetric carbons, as many as 22 or 4 stereoisomers can form as a result of this reaction. Fischer projections for these four stereoisomers are shown below:

The first two structures are commonly referred to as the erythro isomers due to their structural similarity to the sugar, erythrose. The two erythro isomers are enantiomers of each other and consequently have the same melting point (literature MP = 204oC). The last two structures are commonly called the threo isomers due to their structural similarity to the sugar, threose. The threo isomers are also enantiomers of each other (literature MP = 95oC). A variety of mechanisms can be suggested for the addition of bromine to an alkene. If the bromination of trans-cinnamic acid is stereospecific, the addition of bromine to the C = C may occur by either syn addition or anti addition, giving a mixture of two specific stereoisomers in each case. If the reaction is not stereospecific a mixture of both syn addition and anti addition products will form.As part of your pre-lab assignment, you will first draw three dimensional structures for the stereoisomers that would form by syn addition, by anti addition, and by a mixture of both syn and anti addition. You will also use your Organic I text book to draw the accepted mechanism for the addition of bromine to an alkene. After determining the melting point of your product, you will decide whether or not the product(s) obtained in this experiment is/are consistent with the accepted mechanism.

Pre-Lab Assignment:

1. Read the appropriate pages in your Organic I textbook to review stereochemistry and the properties of stereoisomers. Read the following experimental procedure.

2. Complete the pre-lab questions. Transfer the appropriate information to your lab data sheet.3. Remember, your pre-lab assignment must be submitted no later than the start of the pre-lab period. Late pre-lab assignments will not be accepted for credit. If you are completing this lab as a make-up, you must submit the pre-lab assignment but will not receive any credit.

Experimental Procedure:

SHAPE \* MERGEFORMAT

1. In this experiment, you will be using 3 pieces of very expensive glassware (a 3-necked round bottom flask, a reflux condenser, and an addition funnel). Each one costs $100 - $150. Please be very careful with them.2. Obtain a large ring stand, lab jack, and stirrer plate. Place the lab jack on the base of the ring stand and carefully balance the stirrer plate on top of it.

3. Examine the amount of sand in the sand bath at your lab station. Remove or add sand until it is about one third to one half full of sand. Place the sand bath on the center of the stirrer plate.4. Obtain a 3-necked 50-mL round bottom (RB) flask, add a magnetic stirring bar to it, and then clamp the flask to the ring stand high enough so that the sand bath can be removed easily later in the experiment. DO NOT PRE-HEAT YOUR SAND BATH.5. Raise the sand bath until the flask is resting in the sand and verify that the stirring bar moves easily when the stirrer is turned on. If it does not, you may need to remove a little more sand from your sand bath before proceeding. Turn off the stirrer plate.6. Weigh out about 0.5 g of trans-cinnamic acid (0.49 0.51 g) and record its mass to 3 decimal places on your data sheet. Transfer the solid to the 3-necked 50-mL RB flask. 7. Add 9-10 mL of dichloromethane to the flask. Immediately place a reflux condenser into the center neck of the flask and seal the remaining two necks with stoppers (or corks). Be sure to grease the joints before assembling (no grease for joints using corks) and carefully support the reflux condenser with an appropriate clamp. Attach the water tubing for the condenser ensuring that water goes in the bottom and out the top. VERY CAREFULLY (SLOWLY) turn on the water to the condenser so that you have a slow but steady stream of water passing through the condenser.8. Turn on the magnetic stirrer. Stir your mixture fast enough so that a small vortex forms in the liquid but not so fast that the mixture splashes. Allow the solid to dissolve while continuing with the next step.

9. Obtain a 10-mL addition funnel, another stopper (or cork), a 25-mL RB flask, and a 400-mL beaker. Grease the stopper (no grease if using a cork) and place it in the top of your addition funnel. Grease the bottom joint of the addition funnel and place it in the 25-mL RB flask. Carefully place this whole apparatus into the 400-ml beaker (this is nothing more than a support and a way to keep it upright). Remember that the pressure equalizing arm of the addition funnel is very fragile!

10. Use gloves for the next two steps. In the hood, carefully add about 2.0 mL of 2.0 M Br2/CH2Cl2 into the addition funnel. Be sure that the stopcock on the addition funnel is closed before adding the bromine. Use the marks on the addition funnel. Return to your lab bench.11. Remove one of the stoppers (corks) from the neck of your 3-necked RB flask. Quickly (but carefully) remove the addition funnel and 25-mL flask from the beaker, pull off the 25-ml flask, and place the addition funnel into the neck of your 3-necked flask. Re-stopper the 25-mL flask and return it to the 400-mL beaker to use later if you need additional bromine.

12. Turn on your sand bath (~30 on the variac) and heat the solution to a gentle reflux.

13. Once the solution reaches reflux, add the bromine solution from the addition funnel about 10 15 drops at a time. Allow the color of the bromine to disappear before adding additional bromine. Continue this process until the bromine color no longer fades. (If you have added all of your bromine and the mixture is colorless, you may need more bromine. Consult your professor for instructions).

14. If you still have bromine left in your addition funnel, quickly remove the addition funnel from the 3-necked flask and place the addition funnel into the 25-mL flask (after removing its stopper) that should still be in the 400-mL beaker. Replace the stopper in the 3-necked flask as quickly as possible. Put the addition funnel/flask bacl in the 400 mL beaker.15. Continue heating the mixture at reflux for at least 10 minutes. While waiting, take your addition funnel to the hood and destroy any excess bromine by carefully adding a minimum amount of cyclohexene. Be sure to destroy the bromine trapped in the stopcock as well. Dispose of this solution in the halogenated organic waste container.

16. After refluxing for 10 minutes, remove the sand bath and cool to room temperature while stirring. You will probably need to turn the magnetic stirrer off and then on again to get good agitation. Once the mixture is at room temperature, carefully add 1-2 mL of dichloromethane from a disposable pipet through the top of the condenser in order to wash out any residual bromine. Be careful that you do not break the disposable pipet off in the condenser!

17. To destroy the excess bromine present in your solution, add cyclohexene dropwise with stirring through one of the necks of the flask until the mixture is colorless or pale yellow. (Warning: Cyclohexene stinks! Do this quickly and keep the bottle of cyclohexene closed!)

18. Cool the flask in an ice bath for 5 10 minutes. You should also cool ~ 5 mL of dichloromethane in a separate container in the ice bath at the same time.

19. Assemble a vacuum filtration apparatus using a Buchner funnel and 250-mL filter flask. Wet the filter paper with a small amount of cold CH2Cl2 and filter the solid. Before filtering, be sure to clean all grease out of the neck of the RB flask that you pour your sample through! Use small amounts of the cold dichloromethane to help remove any residual product from the flask.20. Allow air to pass through your solid for at least 5 10 minutes to give your product time to dry. 21. Collect your product. Spread it out on filter paper or weighing paper and allow it to air dry for 5 10 minutes. Weigh your product and determine its melting point.22. Calculate the yield and the % yield of your product.23. Save your sample until your professor has approved your data.

24. Clean your glassware according to the following directions. Keep your clean glassware at your bench until they have been inspected by your professor. Once approved, they will be placed on the drying rack as described below.a. Remove all grease from the ground glass joints of your flasks, stoppers, condenser, and addition funnel using rolled up paper towels and/or KimWipes. You may need to wipe it out several times. If you do not remove all grease from the joints, you will contaminate the test tube brushes and the inside of your glassware with grease, which is very difficult to remove. b. Wash both RB flasks as usual using a test tube brush, soap, and water. Rinse with DI water. Once approved by your professor, hang them on a peg on the drying rack.c. After removing the grease from the joints of your addition funnel, remove the stopcock and wash with soap and water. Rinse with DI water and then replace the stopcock. Be sure to put the o-ring between the white Teflon washer and the blue nut. Once approved by your professor, hang it on a peg on the drying rack.

d. After removing the grease from the joint of your reflux condenser, rinse the inside of the condenser with copious amounts of tap water. Rinse with DI water. Flush the cold finger with acetone to remove the water and prevent build-up of hard water deposits. Once approved by your professor, place the condenser on the drying rack. DO NOT hang it on a peg.25. Clean up the other glassware, equipment, and your lab bench as usual.Clean Up:1. After your sample and data have been approved by your professor, dispose of your product in the solid waste container.2. Return all equipment and chemicals to their designated areas.3. If it is your turn for general lab clean-up, you are responsible for organizing the equipment/chemical cart or areas, cleaning the balance area, washing any glassware left in the sinks, cleaning around the sinks, the common lab benches, and in the hoods (if used for the experiment), and turning off the hood lights.Experiment 18 Pre-Lab Assignment Name

Bromination of trans-Cinnamic AcidYou may want to keep a copy of this pre-lab assignment to use while writing your conclusion.1. Draw three dimensional structures of the two stereoisomers that would form if bromination of trans-cinnamic acid occurs via stereospecific syn addition. Do not use Fischer projections. Give the IUPAC name for each of the possible products. Transfer the names of these products to Part B of your data sheet. (2 pts)2. Does stereospecific syn addition of bromine to trans-cinnamic acid give the erythro stereoisomers or the threo stereoisomers? (1 pt)

3. Draw three dimensional structures of the two stereoisomers that would form if bromination of trans-cinnamic acid occurs via stereospecific anti addition. Do not use Fischer projections. Give the IUPAC name for each of the possible products. Transfer the names of these products to Part B of your data sheet. (2 pts)4. Does stereospecific anti addition of bromine to trans-cinnamic acid give the erythro stereoisomers or the threo stereoisomers? (1 pt)

5. Complete the table in Part B of your data sheet with the names and expected melting point of the products formed by non-stereospecific addition of bromine to trans-cinnamic acid.

6. Your organic chemistry textbook provides the accepted mechanism for the bromination of an alkene. Draw the accepted mechanism for the bromination of trans-cinnamic acid. (Note: Your text does not show one of the arrows, but you must show how all bonds form and break!) Include all appropriate stereochemistry in the reactants, intermediates, and products. Use the back if needed. Note: You will need this mechanism when you write the conclusion paragraph for this experiment. (4 pts)Experiment 18 Data Sheet Name______________________________

Bromination of trans-Cinnamic AcidApproved ______________________________Part A: Data

Mass of trans-cinnamic acid________________________

Mass of product isolated________________________

Theoretical yield (in grams) of product ________________________

(Show calculation in the space below.)

Percent yield________________________

Experimental melting point of product________________________

Describe the physical appearance of the product that you recovered:

Part B: Products from the Possible Mechanisms

Complete the following table by providing the IUPAC name of all possible stereoisomers formed by the following possible reactions. Estimate the melting point you would expect to obtain from each type of reaction.

Type of Addition ReactionIUPAC Names of ProductsEstimated MP(oC)

Syn addition

Anti addition

Both syn and

anti addition

Part C: Results and Conclusions (5 pts)

Write a results and conclusion section for this experiment. In the first (results) paragraph, identify what you were trying to achieve and how you accomplished it. Remember, your conclusion should not include procedure. Present all relevant data pertaining to your product. In the second (conclusion) paragraph, you should draw the accepted mechanism for the bromination of trans-cinnamic acid and discuss whether or not your data supports this mechanism. Be sure to clearly explain the meaning/implications of your data.

Part D: Questions

1. What would the observed rotation be for the product you obtained by the addition of bromine to trans-cinnamic acid? (1 pt)

2. Based on the accepted mechanism for bromination of an alkene, draw three-dimensional structures of all distinct stereoisomers that would be expected to form during the bromination of trans-1,2-diphenylethene. (2 pts)3. What two experimental techniques studied in lab so far this semester could you use to distinguish between a sample of trans-cinnamic acid and the brominated product you made in this experiment? (2 pts)Caution: Bromine is very toxic and corrosive. It can cause severe chemical burns on skin contact. Bromine vapor is also very harmful. Measure the bromine in the hood. Replace the lid on the bromine bottle immediately. Notify your professor of any bromine spills immediately.

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