EXPERIMENT 8. NMR STUDY OF A KETO-ENOL EQUILIBRIUM CONSTANT

Transcription

1 EXPERIMENT 8. NMR STUDY OF A KETO-ENOL EQUILIBRIUM CONSTANT The equilibrium constant (K) for the keto-enol tautomerization of 2,4- pentanedione will be studied using variable temperature (VT) Nuclear Magnetic Resonance (NMR) spectroscopy. The enthalpy and entropy of reaction will be calculated by examining the temperature dependence of K. Introduction Nuclear magnetic resonance (NMR) spectroscopy is a very powerful technique. It is an essential component of current research in chemistry and biochemistry. This technique also has important clinical applications in medicine. In this experiment you will gain experience in operating a modern NMR spectrometer, and will analyze the keto and enol form of 2,4-pentanedione in a quantitative manner. By measuring the relative concentrations of these two tautomers, you can calculate the equilibrium constant for the reaction. By measuring K as a function of T, you can determine Δ rhº and Δ rsº for this tautomerization. Figure 1. Keto-enol tautomerization of 2,4-pentanedione. The two enol structures exchange faster than the NMR timescale, so we can use the average enol structure (shown at the bottom) in our analysis. 1 Last Modified: 4/13/16

2 Theory The equilibrium constant for the tautomerization reaction is given by the expression: [ ] [ ] K = enol keto From thermodynamics, we know that the relationship between the standard Gibbs energy of reaction and the equilibrium constant is: (1) ΔG! = RT lnk (2) The standard Gibbs energy of reaction is related to the standard reaction enthalpy (Δ rhº) and the standard reaction entropy (Δ rsº) via: Combining equations (2) and (3) gives us: Δ r G! = Δ r H! TΔ r S! (3) ln K = Δ rh! R 1 T + Δ rs! R Equation 4 above suggests that a plot of ln K vs. 1/T should be linear, with a slope of -Δ rhº/r and an intercept of Δ rsº/r. In this experiment, you will measure the concentration of the keto an the enol forms of 2,4- pentanedione as a function of temperature in DMSO-d 6 (dimethyl sulfoxide 1 ): a polar aprotic solvent. From your temperature dependent data, you will be able to determine the standard reaction enthalpy and entropy change, and should be able to tell whether either the keto or the enol form of 2,4-pentanedione is stabilized. In an NMR spectrum, the area of the peak (the integral) is proportional to the concentration of this particular proton in solution. 2 (4) Figure 2. Proton integrals used to calculate the equilibrium constant. We can calculate the equilibrium constant, K, by dividing the integral from protons H b by onehalf of the integral from protons H a. 1 Residual DMSO signal will be observed at 2.50 ppm, and should be visible as a quintet with a J-coupling constant of 1.9 Hz. 2 Strictly speaking, this is only true if the pulse delay time in an FT-NMR experiment is much greater than the spin-lattice relaxation time (T 1) of the proton. A general rule of thumb is that the pulse delay time should be greater than or equal to 5T 1. 2 Last Modified: 4/13/16

3 [ ] [ ] = Integral H b (5) 1 2 Integral H a K = enol keto Procedure You will need to prepare a ~10 % mole fraction solution of 2,4-pentanedione in DMSO-d 6. The density of DMSO-d 6 is 1.19 g/ml at 20 ºC. DMSO is hygroscopic, so 1-mL ampules will be used. Calculate how much 2,4-pentanedione has to be added to 1-mL of DMSO-d 6 to prepare a 10 % mole fraction solution. Carefully transfer this amount of 2,4-pentanedione to a glass vial, and then add the entire contents of the ampule using a glass Pasteur pipette. Mix thoroughly with this pipette, and then transfer this solution into a clean, dry, NMR tube. Cap the NMR tube immediately after preparing the solution. Clean, dry, NMR tubes are kept in the glassware oven in the Organic Chemistry lab. Remove the screw-cap NMR tube from the oven using a hot glove. You might find it useful to use a 125- or 250-mL Erlenmeyer flask to transport the NMR tube and keep it upright. Make sure the tube has cooled down to room temperature before using it. The tube cap for this NMR tube should be kept in the beaker containing the DMSO ampules. Do not put this tube cap in the oven! Figure 3. Screw-cap NMR tube. You will need to measure the NMR spectrum of this sample at temperatures of 25 ºC, 30 ºC, 35 ºC, 40 ºC, and 45 ºC. Allow at least 10-minutes after the VT unit has come to temperature before collecting your spectrum. Use the method Keto-Enol 1H (16 Scans) for your experiments. To change the temperature using the VT unit, double left-click the Probe Temperature window. 3 Last Modified: 4/13/16

4 Set the Target Gas Flow to 535 lph, and then set the Target Temperature to 298 K (or whatever temperature you need). Click the On button next to VTU state, and wait for the VT unit to come to the desired temperature. You can close the Temperature Control Suite window once you are done setting the temperature. The color of the temperature in the Probe Temperature window at the bottom of the screen will change from Blue to Green once the temperature is within 0.5 ºC of the desired temperature. Be sure to wait at least 10-minutes after the temperature reaches its set-point before collecting a spectrum. You may need to increase the Maximum Heater Power to 15 % (in the Configuration tab of the VTU window) if the temperature does not reach the target temperature. Turn off the VT unit at the end of your experiment, and change the Target Gas Flow back to 270 lph. Be sure to remove your sample from the NMR, and place the 10 % ethyl benzene/cdcl 3 back in. You should re-lock on CDCl 3 before leaving. 4 Last Modified: 4/13/16

5 Clean-up While wearing gloves, clean the NMR tube using acetone from a wash bottle. Rinse it several times before placing the NMR tube back in the oven. The NMR screw-cap should be returned to the beaker containing the vials of DMSO. Calculations Your lab report should include a table of the peak integrals, K, and Δ rgº for each temperature. A plot of ln K vs 1/T can be used to determine Δ rhº and Δ rsº. Using the errors in the slope and intercept from regression analysis, carry out a propagation of errors to determine the uncertainty in Δ rhº, Δ rsº. Use the uncertainty in the y-intercept from your curve-fit as the uncertainty in ln K. Use this uncertainty to determine the error in each experimentally determined value of K and Δ rgº value. Clearly show how you carried out the propagation of uncertainty in the results section of your lab report. Be sure to compare your experimental values with literature values. 3 Questions Write a discussion section for this lab report in place of the normal answers-to-questions section. This section should be 3 4 pages long, and should discuss such things as: The physical interpretation of the signs (positive or negative) of the standard reaction enthalpy and entropy change. The polarity of DMSO, and its ability to stabilize/de-stabilize the keto and enol tautomeric forms. Literature values of the standard reaction enthalpy and entropy change for 2,4- pentanedione in other solvents. An explanation of why it is impossible to distinguish between the two enol forms (enol- 1 and enol-2) using NMR. A comparison of your experimental results in DMSO with the literature, and reasons for any observed deviations and possible ways to improve your measurements. Pre-Lab Questions 1. Qualitatively predict the NMR spectrum of the keto tautomer of 2,4-pentanedione. Refer to the NMR chapter in your Organic Chemistry textbook for details. Be sure to predict and explain: how many signals should be present, their splitting pattern, and where they will appear relative to each other. 3 Allen, G; Dwek, A., J. Chem. Soc., B, 1966, Last Modified: 4/13/16

6 2. Qualitatively predict the spectrum of the enol tautomer. Assume that during the timescale of the NMR experiment, the average enol form represents the structure that the NMR sees. Be sure to predict and explain: how many signals should be present, their splitting pattern, and where they will appear relative to each other. 3. Calculate how many grams of 2,4-pentanedione need to be added to 1-mL of DMSOd6 to prepare a 10 % mole fraction solution. The density of DMSO-d 6 is 1.19 g/ml at 20 ºC. 6 Last Modified: 4/13/16