Pre-lab write-up (Sections 1-Vl) Answers to Assigned Problems (Section X) Product (yield, purity, appearance)

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1 Chemistry Name Preparative Experiments TA Name: Experiment # 15 Lab Day: Pre-lab exercises Pre-lab write-up (Sections 1-Vl) Completed report (Sections VII-IX) Answers to Assigned Problems (Section X) Product (yield, purity, appearance) TOTAL SCORE (13 points) (60 points) (26 points) (16 paints) (16 points) (133 points) (percent) This is your report cover. Please fill this form out and attach it to your prelab.

2 Experiment 15 The Wittig Reaction: The Preparation and Purification of Cis- and Trans-Stilbenes Featuring Chromatography General Safety Considerations 1. Benzyltriphenylphosphonium chloride, benzaldehyde, ethanol, ethyl acetate and methylene chloride should all be viewed as toxic, flammable, irritants. Gloves and goggles must be worn continuously when working with these chemicals. It is best if all work is done in the hood. As usual, no flames are allowed in the Lab. When heating any of these chemicals, do so conservatively. If chemical exposure to any of these chemicals occurs, flush the exposed area for at least fifteen minutes with cold water.benzyltriphenylphosphonium chloride is especially toxic and you should use extra caution when handling it. Double glove, use a clean spatula and let someone know if you spill it. Do not sample the odor of any compound. Keep your hands away from your face. 2. The 10M NaOH solution is extremely corrosive. Wear gloves and goggles when handling it. If exposure to this chemical occurs, flush the exposed area with cold water for at least fifteen minutes.

3 Experiment 15 The Wittig Reaction The Preparation and Purification of Trans and Cis-Stilbenes Featuring Column Chromatography In this experiment, a mixture of trans- and cis-stilbenes will be prepared from benzyltriphenylphosphonium chloride and benzaldehyde using the Wittig reaction. trans-stilbene cis-stilbene Reaction: O P() 3 + Cl - 10 M NaOH (aqueous) dichloromethane + cis-stilbene + PO 3 An interesting aspect of this reaction is the fact that it occurs in two phases and that a phase transfer catalyst is at work. A phase transfer catalyst is a species that can travel between the phases allowing physical contact between the reagents and thus a more efficient reaction. In this reaction, the benzyltriphenylphosphonium chloride acts as the phase transfer catalyst. Upon addition of the aqueous sodium hydroxide (and establishment of the two phase system), this ionic species travels into the base solution, where it is deprotonated, forming the net neutral ylid. The ylid travels back into the methylene chloride layer where the Wittig reaction with benzaldehyde occurs. A more classic way to explain the phenomenon is to say that the

4 benyltriphenylphosphonium chloride passes into the aqueous layer. In the aqueous layer, it exchanges anions and passes back into the organic layer with hydroxide instead of chloride. Somewhere en route or in the organic layer deprotonation occurs. Study the scheme below. What is unusual about this particular phase transfer reaction is that the phase transfer catalyst is the reagent as well as the catalyst. More typically, a big lipophilic salt which has solubility in both the organic and aqueous phases will serve simply to move ions from one phase to another. It will not be involved in the chemistry. What is unusual about this Wittig reaction is more often, n-butyl lithium is used as the base. Sometimes we jokingly call it n-brutal lithium because it is a very strong base. In this case, we can get away with hydroxide as the base because of the acidity of the proton (why is it particularly acidic?) and the equilibrium involved in the phase transfer. Why is benzyltriphenylphosphonium chloride capable of dissolving in an organic layer even though it is a salt? Wittig Reaction with ase Transfer Catalysis P() 3 + OH - NaOH P() 3 + Cl - aqueous P() 3 + OH - P() 3 + Cl - organic, dichloromethane O ylide + H 2 O P() benzaldehyde stilbenes, side product In this scheme, stands for phenyl. The benzyltriphenylphosphonium chloride reagent exchanges anions for hydroxide in the aqueous layer. Because of its ability to dissolve in both layers it makes its way into the organic layer. Somewhere in route, it forms the ylide. This is shown as happening in the organic layer. The ylide is neutral and should stay preferentially in the organic layer where it will react with benzaldehyde to form stilbenes. This final step is the Wittig.

5 In addition to the minor stilbene (cis) being viewed as a side product, it should be noted that benzaldehyde is air sensitive and can be oxidized to benzoic acid. The longer benzaldehyde is exposed to air, the more benzoic acid there will be in the benzaldehyde reagent bottle. Generally, you should not be using benzaldehyde that has any crystalline solid on the bottom. This crystal is benzoic acid. Benzoic acid will not undergo a Wittig reaction and the extent to which it is present will reduce your yield. Also, the yield of the reaction is generally poor, due to the two phase nature of the reaction and the formation of cis-stilbene. You might want to note that the more vigorously you stir, the better your yield will be. Finally, the product is difficult to purify because of the organic nature of the biproduct, i.e., triphenylphosphonium oxide is similar in solubility to the crystalline trans-stilbene. cis-stilbene is a liquid which facilitates its removal in the recrystalllization process. The Wittig reaction has been covered thoroughly in lecture and in your text, but you should review your lecture notes and your text on this very important, Nobel prize winning reaction before doing this reaction. Why is this reaction important? Under what synthetic circumstances is it useful? Pre-Lab Qu es tion s 1. Write the standard starting materials for benzyltriphenylphosphonium chloride. In other words what is the reagent made from (think substitution reaction). 2. Why do you suppose vigorous stirring is necessary in this particular Wittig reaction? Pro ced u re This procedure may be scaled up or down depending on its function in a total synthesis. Remember, when scaling one has to increase or decrease all reagents and solvents proportionately and increase or decrease glassware proportionately (as much as one can given limitations of equipment and glassware). One does not change reaction times. Think of baking a half batch of cookies - it still takes the same amount of time to cook, concentrations are the same. Solids should be measured as grams and solutions and pure liquids as milliliters. Please note modifications on your flow chart or in your procedure before you begin. Better still, do so in your laboratory notebook. 1. Into a 50 ml round bottom flask, add 5 mmole of benzaldehyde, 5 mmole of benzyltriphenylphosphonium chloride and 5 ml of methylene chloride

6 (dichloromethane). Clamp this flask to the monkey bars and attach a reflux condenser in the standard fashion. Equip the flask for magnetic stirring. Use one of our larger magnetic stir bars. Magnetic stirrers work best if they are just above the center of the stir plate. The stir plate should be clamped to the monkey bars. Supporting it with an iron ring interferes with the stirring. The best way to fix stirring problems is to turn the stirring off completely, let the stir bar settle down and then turn it up slowly. Usually the problem is the placement of the stir plate or the stirrer being up too fast. 2. While stirring vigorously, add 3.3 ml of 10 M sodium hydroxide solution through the neck of the flask. If the reaction is working, the flask should become warm and the contents should turn yellow. Note any other color changes that occur over the course of the reaction. Add a heating mantle and establish a gentle reflex. Set the step control (power mite) somewhere in the range of volts, Allow the reaction to reflux with vigorous stirring for thirty minutes. Note: it is very difficult to stir through a metal heating mantle. You must patiently establish stirring as described above. Do not start timing the reaction until it is both stirring and refluxing. If you can't get it stirring, ask for help from your instructor or teaching assistant. Do not give up, it is almost always possible to get it stirring. Why is vigorous stirring so important? Look at scheme above. 4. Remove the heat source and cool the reaction flask in an ice bath. 5. When the reaction mixture is cool, add it to a 125 ml separatory funnel. Then add approximately 3 ml of water and about 10 ml of dichloromethane to help distinguish the layers and to enhance the organic layer. When the layers appear distinct, separate them and determine which is the organic layer. Remember, if you do not discard layers, you can never get in trouble in a layer separation. 6. Dry the organic layer over anhydrous sodium sulfate. Remember to use the drying agent sparingly. Decant the organic layer off the sodium sulfate into a ml tared round bottom flask (the size will depend on the scale of your reaction) and rinse the spent drying salts with two three ml portions of fresh dichloromethane. These rinsings should be combined with the organic material in the round bottom flask. This procedure will retrieve any product adhering to the drying salts. 7. Remove as much of the methylene chloride as possible using gentle rotary evaporation. Set the hot water bath at approximately 30 or 35. If you are lucky, you will solids crystallize in the flask. 8. Purify the remaining material using recrystallization techniques.

7 a. First, dissolve the material in approximately ml of absolute ethanol with warming. Use the solvent sparingly. You should be working in your smallest Ehrlenmeyer, using gentle heating on the hot plate. This volume is just a recommended amount. You might use less or more depending on your yield. Remember the goal is to have your crude product dissolve in the minimum amount of hot crystallization solvent. It is very easy to boil off all the solvent and "fry" the precious product. It is also very easy to add too much solvent. Take your time and do it carefully, it truly pays off. Allow the resulting solution to cool to room temperature and note any crystallization. If sufficient crystallization does not occur, cool the room temperature solution in an ice bath whereupon crystals should form. Isolate any crystals by vacuum filtration. This process primarily removes cis-stilbene which as mentioned previously is a liquid. Why do you think cis-stilbene is a liquid and trans-stilbene is a solid at room temperature. b. Further purify the trans-stilbene by adding approximately 3 ml of absolute ethanol to the crystals. Bring the mixture to a gentle boil and add 2-3 drops of water, Allow the solution to cool slowly to room temperature. isolate the crystals (pure trans-stilbene) by vacuuurn filtration. Again, this is just a recommended amount and you need to follow the advice given in the first recrystallization. Once again you should be working in your smallest Ehrlenmeyer and working very carefully. This procedure primarily removes the very problematic triphenylphosphine oxide which is an organic biproduct formed in equimolar quantities to the desired products. Write-up In addition to the normal components of a preparative lab report, you should answer the following post-lab questions. Note: Often this lab is done as part of a total synthesis so, you may not have to do the following. Please follow instructions given in class. Often in a total synthesis some or all of the following questions are assigned. 1. Using reactions studied in this course, outline a synthesis (reactions only- no procedures) of trans-4-octene from bromobutane, triphenylphosphene and any required inorganic reagents. 2. Write the product(s) from the reaction of cis- a n d tr an s- stilb en e with bromine. Include the stereochemistry of the products in your answer. 3. Look up the proton NMR spectrum of either cis- or trans-stilbene using our web site and the spectra data bases found therein.

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