H 2 SO 4 heat + H 3 O +

Transcription

1 Dehydration of Cyclohexanol DISCUSSION OF TE EXPERIMENT In this experiment, cyclohexanol is dehydrated by aqueous sulfuric acid to produce cyclohexene as the sole product, and carbocation rearrangement will not be an issue since we are forming the most stable carbocation initially. O 2 SO 4 heat + 2 O The equation above indicates this reaction is reversible, and Le Chatlier s principle states that the equilibrium will shift to relieve whatever stress there is on the system. In this reaction, the position of equilibrium is shifted to the right by continuously removing cyclohexene as it is formed using a packed fractional distillation column. As the product is removed, the reaction equilibrium shifts to form more products. Keep in mind that the addition of heat/catalysts is not considered to be a driving force because they do not ensure 100% product formation. The presence of heat/catalyst will help establish the equilibrium quicker but not force the formation of product once the equilibrium is established. The mechanism for this reaction depends on the class of alcohol being dehydrated, and the one shown is for 2 and 3 alcohols. O + O 2 O + 3 O + Compelling evidence for the intervention of a carbocation intermediate comes from the observation that secondary carbocations derived from certain 2 alcohols may undergo molecular rearrangement to a more stable carbocation. Fractional distillation also ensures that the product, cyclohexene (bp 83 C) is not contaminated with the starting material cyclohexanol (bp 161 C). The distillate does not, however boil at 83 C because cyclohexene and water form a minimum-boiling azeotrope (a mixture that has a single boiling point, below that of either single boiling

2 points) whose boiling point is 70 C; it is this cyclohexene-water mixture that is collected as the distillate. This is the first reaction this semester that the product is not simply obtained by crystallization. The process will be a little more involved to isolate the desire product, which is a liquid. Consider the steps involved in the running of a reaction : 1. Purification of starting materials, when necessary 2. Finding a procedure on the same or similar type material. 3. Setting up the reaction. 4. Monitoring the reaction (TLC, GC) until it is completed. 5. Isolating the product from by-products. 6. Purification (distillation, recrystallization) if desired 7. Analysis of Product In our organic lab, Steps 1 and 2 are already done for you. Step 3 for this reaction requires you to set up a fractional distillation. Step 4 is not necessary, as you will know the reaction is complete when your distillation stops distilling anything. Step 6 is not necessary today. The product is reasonably pure after doing the required work-up process. Step 7 will require you to obtain a weight and an IR on your compound. Step 5 is new. The removal of other compounds from your desired product can be done through a couple of different methods, depending on what these are that need to be removed. Removal of by-products There are two terms we use when removing other compounds from organic products: 1. To remove inorganic compounds, we perform a wash. We add aqueous solutions to our organic compounds so they wash away the impurities. To remove acids, we add bases. To remove bases, we add acids. To remove salts, we just sometimes wash with water. 2. To remove organic compounds from aqueous solutions, we sometimes perform an extraction. We add an organic solvent to dissolve the organic compound. The organic solvent has a different density than the aqueous solution and we separate the two layers, keeping the desired organic component.

3 Both washes and extractions are done in a separatory funnel, when working on a larger scale. The separatory funnel has a stopcock on the bottom to help drain out the bottom layer. It has a stopper to plug up the top, when you want to shake and mix the layers. To properly use a separatory funnel, one uses an iron ring to hold the separatory funnel. First add the compound to be washed, and then add the appropriate aqueous solution. The stopper is placed on top. The separatory funnel is inverted and shaken vigorously, then vented (the stopcock is opened while the funnel is still upside-down). This venting prevents any vapors from building up inside the funnel. The separatory funnel is then righted and set into the iron ring while the two layers separate. Once the layers are separated, the stopper is removed and the bottom layer may be drained from the bottom. Generally, the top layer is just poured out the top. Why is the stopper removed? To prevent formation of a vacuum inside the separatory funnel (keeps the liquids from draining out the bottom) Drying the Product : Water must commonly be removed from liquid organic compounds. Not only did your product distill over with water as an azeotrope but you washed your product with an aqueous solution. The bulk of water is usually separated away during washing and then trace amounts must be removed using what is called a drying agent. Drying agents are anhydrous compounds that form hydrates, meaning they adhere to water molecules to form complexes. The job of the drying agent is to enter a liquid (either a solution where something is dissolved in an organic solvent, or a neat liquid compound) and search out water molecules and complex with them.

4 Drying agents in a dried solution commonly remind you of those tourist toys, where the ball has the snowflakes you shake up. If a drying agent is still dry and there s no water to hydrate to, the drying agent stays dry and freely floats around in the solution. If a drying agent finds water and complexes to form the hydrate, the drying agent becomes sticky and heavy. Typically it falls to the bottom of the flask or vial and sticks there, not moving around. The object of the drying of a compound is to still have some amount of drying agent freely floating in the solution. This tells you there are not any water molecules left floating in the liquid. There are several drying agents to choose from, including magnesium sulfate (MgSO 4 ), potassium carbonate (K 2 CO 3 ) and calcium chloride (CaCl 2 ). In this experiment, we will use calcium chloride, which is a compound formulated as little round balls, not a powder. They have a limited surface area and need a bit more time to completely absorb all the water present. Once the reaction is complete, the distillate is transferred to a separatory funnel and aqueous NaO solution is added to the funnel. The NaO removes any traces of acid that may have co-distilled with the product. The bottom aqueous layer is carefully removed, leaving the product in the funnel. ow do you know which layer is which? Density! ow do you know the density of a NaO solution? All aqueous solutions, acid or base, have a density about the same as water (1 g/ml). After washing with the NaO solution, the wet product is then transferred to a large sample vial and treated with calcium chloride, CaCl 2, to remove traces of moisture that are contained in the cyclohexene. The preparation concludes by filtering the dry product from the solid drying agent, CaCl 2. FOR YOUR SAFETY 1. Wear gloves at all times when handling the aqueous sulfuric acid at the beginning of the reaction and when disposing of the acid remaining in the reaction flask after the distillation of the crude cyclohexene is complete. 2. Make certain the reaction flask is COOL before disassembling the distillation apparatus and cleaning the flask.

5 EXPERIMENTAL PROCEDURE 1. Into a clean 100-mL round-bottom flask (it does not have to be dry why?) place 10 ml of cyclohexanol (density = g/ml) and then add 5 ml of 9 M sulfuric acid (wear gloves when handling the acid.) Mix the contents of the flask by swirling it carefully. Clamp it, and place a heating mantle under it. 2. Place 2-3 boiling chips in the reaction flask, and assemble a fractional distillation apparatus using a packed column. (This apparatus is the same as that used for fractional distillation.) ave your instructor check your apparatus to be sure it is correct. 3. Turn on the power to the VARIAC, set it to 90-95, and heat the reaction mixture. Collect all the distillate in a clean (why not dry?) 25-mL round bottom flask that is Keck-clipped to the vacuum adapter. Discontinue heating when no distillate comes off, at which time the vial should be 1/3 to 1/2 full, or when the head temperature rises above 70 C and no oily layer is visible in the reaction flask. Most likely, you will see the temperature begin to drop when nothing else is distilling. You may see smoking occurring inside your round-bottom flask it s a sign that you can quit heating the reaction! 4. Transfer the distillate to a clean separatory funnel (why not dry?), making sure the stopcock is closed. Add about 10 ml of aqueous 3 M NaO solution to the funnel and shake well with periodic venting. Drain the lower aqueous layer from the funnel (remove the stopper from the funnel before trying this) into a clean 25-mL Erlenmeyer flask, and leave the upper organic layer in the funnel. Do NOT discard the aqueous layer until the end of the experiment. 5. Pour the organic layer into a clean, dry large sample vial. Add enough anhydrous CaCl 2 so that the bottom of the vial is covered. Cap the vial (make sure the cap has a plastic insert in it or it may leak), and swirl it for about 5 min. If the organic product is not clear or if the CaCl 2 clumps and sticks to the bottom, the drying agent is completely hydrated. Continue to add CaCl 2 in small amounts (each time with swirling) until the solution is clear and the CaCl 2 is freely moving. 6. Take a clean, dry short-stemmed glass funnel, place a piece of fluted filter paper and clamp the funnel by the stem with a finger clamp (or place the funnel in the iron ring, if its cool again). Place a dry, pre-weighed large sample vial under the stem of the funnel (be sure the funnel is positioned so the stem extends into the vial) and pour the dried cyclohexene into the filter paper/funnel. This procedure is useful for removing solids, such as drying agents, from an organic liquid. You are going to lose some product, as it is stuck to the surface of the CaCl 2 when you filter! And note how much organic liquid is sucked up by the filter paper! 7. When the filtration is complete, reweigh the vial to determine the weight of product obtained. This is accomplished by subtracting the weight of the empty vial from the weight of the vial containing the product. 8. Run an IR spectrum of your product, and hand-in the properly and completely labeled product.

6 WASTE DISPOSAL AND CLEAN-UP 1. Place the aqueous basic aqueous layers from the extraction (Step 4) in the appropriate labeled container. 2. Add a little water to the reaction flask and swirl. Place the acid solution from the reaction flask (Step 3) in the appropriate labeled container for acidic waste. Be sure the flask is cool before handling it. 3. Wash the round bottom reaction flask (Step 3) with water and clean the flask with a brush as well as possible. Then wash the flask with acetone. Acetone washes should go in acetone waste bottle, for later recycling. 4. Place the packing from the fractional distillation column in the used packing container 5. Clean all glassware that was used in this experiment, and place it in the proper locker your personal drawer or the common locker. If you wash any glassware with acetone, wash it again with water before putting it away. Do not put any glassware containing acetone in the glassware kit in the common locker. The foam liners dissolve when acetone comes in contact with them.