Kinetic vs. Thermodynamic Control of Organic Reactionss, Lecture notes of Thermodynamics

Download Kinetic vs. Thermodynamic Control of Organic Reactionss and more Lecture notes Thermodynamics in PDF only on Docsity! Experiment: Kinetic vs. Thermodynamic Control of Organic Reactions Kinetic vs. Thermodynamic Control During your study of reactions this year you have examined many mechanisms. You have used these mechanisms to predict which one of two possible organic products would be formed exclusively or as the major product. You have also used them to predict which product would form faster. Usually, the product formed in greater quantity is also the product formed faster. This is because, in the rate determining step, one of the two products is more stable than the other. The transition state for this step resembles the products, so the more stable product has a more stable transition state leading to it, which in turn lowers the energy of activation. The now familiar set of potential energy curves below shows this clearly. The more stable product is called the thermodynamically controlled product. The product formed by way of the lower energy of activation pathway forms faster and is referred to as the kinetically controlled product. In the case discussed above, the same case you have seen over and over this year, the kinetically controlled product (the one formed faster) and the thermodynamically controlled product (the more stable one) are the same. The more stable product is formed faster. But there are exceptions to this common scenario. One of them is the formation of 1, 2- and 1, 4-addition products from conjugated dienes. You have probably studied this reaction already, and it would be a good idea to review it before doing this experiment. Another is the formation of semicarbazones, reactions that you will be doing in this experiment. In these exceptions, the product that forms faster (the kinetically controlled product) is the less stable product, while the more stable product (the thermodynamically controlled product) has a higher energy of activation leading to it and is formed more slowly. In the reactions to be studied in this experiment, you will allow an aldehyde (2-furaldehyde) and a ketone (cyclohexanone) to react less stable product more stable product less stable TS with higher Ea Potential Energy Progress of Reaction with semicarbazide hydrochloride to form semicarbazone products called 2-furaldehyde semicarbazone (FS) and cyclohexanone semicarbazone (CS) respectively. O C H O NH2NH C O NH2 H+ O NH2NH C O NH2 H+ O NNH C O NH2 H NNH C O NH2 + 2-furaldehyde (F) semicarbazide (S) 2-furaldehyde semicarbazone (FS) semicarbazide (S) cyclohexanone C cyclohexanone semicarbazone (CS) + H2O + H2O One of these reactions is kinetically controlled. The product of this reaction is the one that forms faster. The other reaction is thermodynamically controlled. The product of this reaction is the more stable product. For this pair of reactions, the thermodynamically controlled product is not the same as the kinetically controlled product. One of the products, either FS or CS, will be the kinetically controlled product. The other product will be the thermodynamically controlled product. Let’s look at a generalized reaction in which A and B react with semicarbazide to form S and BS. Potential Energy Progress of Reaction AS (kinetic control) BS (thermodynamic control) A + S B + S A + S → AS B + S → BS As you can see from this melting point diagram, as more and more Y is added to X and the percentage of X decreases, the melting point of the mixture decreases. Similarly, as more and more X is added to Y, and the percentage of Y decreases, the melting point of the mixture decreases. The point where the two melting point lines meet is called the eutectic point. (Eutectic means “easy melting” in Greek). The mixture of X and Y that has the composition shown at this point is called a eutectic mixture. The eutectic mixture has the lowest melting point that a mixture of two substances, in this case X and Y, can have. Unlike all other mixtures of X and Y, the eutectic mixture has a constant melting point, much like that of a pure substance. It begins and finishes melting at the same temperature. In your pre-lab assignment, you will be making a melting point diagram like this from experimental melting point–percentage composition data for 2-furaldehyde semicarbazone and cyclohexanone semicarbazone. From this diagram and your own experimental data you will determine the composition of the mixtures you obtain from parts 3 and 4. It is important to note something here. On the diagram above, between the melting point of the eutectic mixture and 125ºC, each temperature corresponds to two different compositions of an X-Y mixture. You will have to take this into account when determining the compositions of your semicarbazone mixtures from parts 3 and 4 of the experiment. In some cases, it will not be possible to do more than narrow down to two possibilities the composition based on the melting point of your sample and the melting point diagram you will generate in the pre-lab work. Percentage of X (by mass) 125ºC Melting point of X 100% X (0% Y) 0% X (100% Y) Eutectic Point Temperature 175ºC Melting point of Y  Pre-lab Preparation 1. Read thoroughly the theory section of this experiment and the section in your lecture textbook dealing with kinetic and thermodynamic control. 2. Carefully go over the procedure for this experiment, especially parts 3 and 4, to be sure that you understand the purpose of each procedural step. Since you will be working with a partner for this experiment, determine before you come to lab, which parts of the experiment each of you will do. A logical breakdown of responsibilities would be for one of you to do part 1 and either part 3 or 4 with the other of you doing the remaining parts. 3. Answer the following questions about the AS and BS reactions discussed in the background portion of this experiment. a. When a competitive reaction occurs between A and B and a limited amount of S, which product, AS or BS, would you expect to form in largest yield at 0ºC? Briefly explain. b. Which product would you expect to form in largest yield at high temperatures? Briefly explain. c. If BS and A were mixed together with the appropriate solvents used throughout the experiment and heated to the same high temperature as in “b” above, what would you expect to happen? Briefly explain. 4. Using the following melting point-percentage composition data, draw a melting point diagram like the one drawn for X and Y in the background section of this experiment. Use the average melting point data for your graph, not the melting point range. Graph to show the best fit line. Do not just “connect the dots”. To draw the best fit lines, do not attempt to begin and end at data points. Instead, draw a line interpolating between all points. To get a good graph, the lowest temperature on your y-axis should be 100ºC and the graph should be sufficiently large to allow you to use it to answer post-lab questions. % by mass of CS Melting point range (ºC) Average melting point (ºC) 100 165.2 - 166.8 166.0 90 144.9 - 160.0 152.5 80 140.0 - 144.0 142.0 70 143.7 - 148.4 146.1 50 143.0 -160.0 151.0 20 171.0 - 180.0 175.5 0 201.6 - 202.4 202.0 Experimental Procedure ! Safety Considerations ! Semicarbazide hydrochloride is carcinogenic. Use gloves when working with this compound. If you get any on your skin, wash it thoroughly with soap and hot water. 1. Preparing Cyclohexanone Semicarbazone In a 50-mL Erlenmeyer flask, dissolve 1.0 g of semicarbazide hydrochloride and 2.0 g of dibasic potassium phosphate (K2HPO4) in 25 mL of water. Using a 10-mL graduated cylinder, measure out 1.0 mL of cyclohexanone and pour it into a test tube containing 5 mL of 95% ethanol. Pour this ethanol solution into the solution containing the semicarbazide and swirl the mixture thoroughly to mix the reactants. Let the mixture sit for 5-10 minutes to allow crystals of cyclohexanone semicarbazone to form. Note: The reactions of aldehydes and ketones with semicarbazide hydrochloride require that the pH of the solution be acidic but not too much so. The function of the dibasic potassium phosphate is to assure the optimum pH for the reaction. Note: Throughout this experiment, you will be allowing solutions to sit, waiting for crystals of the semicarbazones to form. Sometimes these crystals form readily, and sometimes they do not. Crystallization is a tricky thing in organic lab. When crystals are slow in forming, cooling in an ice bath can help. So can scratching the sides of the container with a glass stirring rod. The scratch mark made by the stirring rod can provide a spot just right for molecules to collect and start developing a crystal. Once crystals start to form, other crystals will form more rapidly. Filter the crystals with suction using a Hirsch funnel. (A Hirsch funnel is a small, conical funnel designed for suction filtration. It works exactly like a Buchner funnel except it is used for filtering small amounts of solid. Use small pieces of filter paper that just cover the holes at the base of the funnel.) Wash the crystals with 2-3 mL of cold water. Remove the crystals, place them between two pieces of filter paper folded on the sides to keep the crystals from falling out, and label them. Note the color of the crystals. Set this solid aside for use in part 4. The melting point of pure cyclohexanone semicarbazone is 166oC.