Experiment: Preparation of Adipic Acid by Oxidative Cleavage of Cyclohexene

Transcription

1 Experiment: Preparation of Adipic Acid by xidative Cleavage of Cyclohexene Under mild conditions, only the pi bond of the alkene is cleaved to form 1,2-diols or epoxides. Under more rigorous oxidation conditions, complete cleavage of both the sigma and the pi bonds may occur, leading to the formation of two carbonyl compounds. R 1 R 3 or [ox] R 1 R 3 [ox] R 1 R 3 + R 1 R 3 Figure 1: xidations of Alkenes R 3 The type of carbonyl products would then be dependent upon the substitution of the alkene and the type of oxidant used. If the cleavage product is a ketone, the oxidation process is generally complete but if the cleavage product is an aldehyde, it may be further oxidized to form a carboxylic acid functional group. Previous Methods of xidation: Potassium Permanganate: arsh oxidizer oxidizes many functional groups Difficult to work with doesn t dissolve readily Produces large quantities of an undesirable by-product, Mn 2 Nitric Acid: azardous (explosive, violent reaction) Produces large quantities of an undesirable by-product, N 2 Reaction Equation for Experiment 4: 2 2, Na 2 W 4 Eq. 1 Aliquat 336, KS 4 Today s xidation: Sodium Tungstate catalyst Avoids the need for strongly acidic/basic reaction solutions uses water as a solvent (water from 2 2 ) Generates water as the only by-product

2 xidizer used in catalytic amounts only a 1:1 mole ratio is not required as the catalyst reforms and recycles through the reaction over and over again. xidizing solution is actually able to be re-used for other oxidations (less waste disposal!) Also allows us to recycle an old product from another reaction, instead of having to pay to dispose of it! When cyclohexene and an aqueous solution of sodium tungstate with hydrogen peroxide are mixed, two layers will result and no reaction can occur. In order to allow the reaction to occur, in the presence of two phases, a phase transfer catalyst will be added to the solution. Phase Transfer Catalyst (PTC): Enhance the rate of reaction between two species in two different phases. Exists in aqueous solution because it s a polar salt (hydrophilic) Exists in an organic solution because of the large, non-polar alkyl groups attached (hydrophobic) CATALYST again, a species that will be reformed during the process and recycled over and over again Through ion pairing, the PTC can bring negatively charged anionic species into the less polar system, allowing the reaction to occur. Since the PTC can repeat this process again and again, it is generally used in catalytic amounts instead of a 1:1 ratio with the reacting species, hence the name phase transfer catalyst. Aliquat 336: N Cl 2

3 The Process: cyclohexene adipic acid 2 2 R 4 N + - S 4 (R (R 4 N) 2 [reduced W] 2 R 4 N N) 2 W 4 S organic aqueous Na 2 W 4 2 NaS 4 2 NaS 4 Na 2 [reduced W] 4 tungsten intermediate is reoxidized using 2 2 Figure 3: Phase Transfer Catalysis in the product of Adipic Acid Ion exchange will allow the PTC to take the oxidizing the tungstate oxidizer from the aqueous layer into the organic layer (Reaction 1 in Figure 3). This oxidizer is then able to oxidize the cyclohexene to adipic acid (Reaction 2), during which time the tungstate is reduced to a lower oxidation state. The PTC releases the reduced tungsten intermediate back into the aqueous layer (Reaction 3). ydrogen peroxide is present in the aqueous layer to re-oxidize the tungsten intermediate back to the tungstate oxidation state (Reaction 4) and the cycle begins again. Since both the Aliquat 336 and the sodium tungstate function catalytically, only the hydrogen peroxide is needed as a stoichiometric reagent, and we will be using it in large excess. This leaves the alkene as the limiting reagent for this reaction. Safety Concerns: Wear Gloves when handling 50% hydrogen peroxide solutions. Avoid getting hydrogen peroxide on you or your clothing. EXPERIMENTAL PRCEDURE: Reagents MUST be added in this order!!! D NT MVE REAGENTS FRM TEIR DESIGNATED BALANCES! 1. Place 125 mg of sodium tungstate dihydrate in a clean, dry 5-mL conical vial containing a magnetic spin vane. 3

4 2. Add 0.15 ml Aliquat 336 to the 5 ml conical vial. Note: It is a very thick liquid. Pull back the plunger to about the 0.8 ml mark and let the Aliquat slowly move into the syringe barrel. Push the plunger in slowly. Air bubbles should generally be pushed back out in front of the liquid so do not panic if you see a lot of air bubbles in the syringe. 3. Add 2.0 ml of 50% hydrogen peroxide to the 5 ml conical vial. GLVES N FIRST! Treat all water spills as suspicious! Automatic pipettes are available for this step. Remember there are two possible points for depression one to fill the pipette to the desired amount and one to ensure all liquid is delivered. 4. At this time, add 92.5 mg ( g) potassium hydrogen sulfate and commence stirring without heat, in the Al block. 5. To transfer the cyclohexene into the reaction vial, obtain a clean dry syringe, with needle, from your kit. Draw up 0.62 ml of cyclohexene into your syringe and place the shield on the needle. Weigh to obtain the mass of the cyclohexene AND the syringe and record this weight in your notebook. Remove the shield, transfer the cyclohexene to the conical vial, replace the shield then QUICKLY return to your balance to reweigh the empty syringe, with its shield in place again. Record these weights (full syringe, empty syringe) in your notebook. Since you are working with a student sample in a large sample vial, you will not be able to flip the syringe upside down to remove the probably air bubble. Use the air bubble to determine your volume. Failure to reweigh quickly will mean that you won t actually know what was transferred (versus what evaporated away in the interim). This will lead to a falsely higher weight for the amount of starting material and your percent yield will appear lower! 6. Attach a water-cooled condenser to the top of the conical vial, attach thin-walled tubing to condenser, clamp the apparatus in the Aluminum block by clamping around the middle of the condenser. Turn on the water and begin stirring at maximum speed (10). Turn on the heat to a setting of 4.5. Maximum stirring is required for maximum interaction of the two phases, where the reaction must occur. Remember that the water tubing is hooked up so water goes IN at the bottom and UT at the top. To easily slide on the tubing, slightly wet the end of the tubing before pushing onto the glass projections. 4

5 7. Upon heating to boiling, allow to reflux for 15 minutes then, using a pipette, add approximately 0.5 ml of water through the top of the condenser, in order to wash down the inside of the condenser. Continue refluxing for another 15 minutes. A reflux is a process where a reaction is heated with a condenser attached so any vapors that form will re-condense and drip back into the reaction vial. No reagents or solvent is allowed to escape out the top. The reflux should occur so the reflux line is never more than halfway up the inside of the condenser. Inform instructor if it is refluxing too high inside your condenser. 8. After a total of 30 minutes at reflux, remove the conical vial from the hot plate and allow the solution to cool slightly (will still be really warm!) and separate into layers. While still really warm (when you can just barely tolerate touching the vial!), carefully remove only the aqueous layer, transferring it to a 30-mL beaker using a disposable pipette. Do NT remove any of the gooey, oily-looking catalyst with the aqueous layer. The catalyst will inhibit formation of the product crystals. Leave behind any aqueous layer that cannot be removed without the oily catalyst. Simply deflate the pipette bulb on your disposable pipette and place end of pipette in aqueous solution to be removed. Allow bulb to re-inflate, and it will pull the aqueous solution into the pipette. This avoids blowing bubbles into the solution after the pipette has been placed to remove what is desired. 9. Place the beaker containing the aqueous layer into an ice bath to form crystals. Gently stir with a glass rod to encourage crystal formation. Cold temperatures again why? Product is insoluble in water why? 10. Vacuum filter the resultant crystals. Wash the crystals in the irsch funnel twice using 1 ml of ice water each time. Allow the product to dry until the next class, at which time a weight and a melting point will be obtained. 5