In the Laboratory
Determination of the Rotational Barrier for Kinetically Stable Conformational Isomers via NMR and 2D TLC
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An Introductory Organic Chemistry Experiment Gregory T. Rushton, William G. Burns, Judi M. Lavin, Yong S. Chong, Perry Pellechia, and Ken D. Shimizu* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208; *
[email protected] Conformational isomerism is of great importance in determining the three-dimensional structure of molecules and therefore can influence many structure-based molecular properties such as reactivity, melting point, and drug–receptor interactions. Undergraduate students encounter conformational isomerism early in first-semester organic chemistry when they are introduced to the gauche–anti–eclipsed forms of ethane and butane and the chair–boat–twisted-boat forms of cyclohexane (Scheme I) (1). Since the barriers of rotation for many of the commonly chosen textbook examples are low (e.g., 3 kcal兾mol for ethane), conformational isomers are difficult to study in the teaching laboratory. A laboratory experiment that studies the room temperature stable conformational isomers of diol 1 is presented. The high barrier of rotation about the two Caryl⫺Nimide single bonds in diol 1 display restricted rotation at room temperature owing to the steric interactions of the phenol oxygens and the imide carbonyls. The unusual kinetic stability of this system enables the separation and isolation of the respective conformational isomers and measurement of the barrier of rotation by following the kinetics of the anti to syn isomerization. Students are exposed to the concepts of organic synthesis, purification, two-dimensional thin-layer chromatography (2D TLC), conformational isomerism, kinetics, and 1H NMR (2). Fur-
thermore, this laboratory is designed for first-semester organic students as it requires only introductory-level synthetic and analytical techniques and was designed with the time and equipment constraints of a typical undergraduate laboratory. Although the experiment can be performed by first-semester students, the concepts introduced (e.g., imide formation, atropisomerism) can be emphasized over the fundamental aspects to make it more appropriate for an intermediate or advanced course as well. The synthesis was completed in a single step from the condensation reaction between a substituted aniline and 1,4,5,8-naphthalene tetracarboxylic dianhydride (Scheme II). The reaction produces a statistical mixture of two stereoisomers: the syn-form, in which both phenol groups are directed on the same side of the naphthalene surface, and the antiform, in which they lie on opposite sides of the plane. This system’s kinetic properties are fairly unique in that the isomers are stable at room temperature but can be interconverted upon heating (3). Initially, 2D TLC was used to investigate the stability of the isomers and estimate the approximate barrier of rotation of the system. A more accurate determination of the rotational barrier using an NMR time study was then carried out to define the kinetic parameters k (rate constant) and ∆G ‡ (energy barrier of rotation) for the system.
Scheme I. Interconversion barriers and half-lives at 298 K. In the case of butane (top) and cyclohexane (center), the barrier is too low to isolate the conformational isomers at room temperature. The system studied herein (bottom) has a sufficiently high barrier to allow for resolution of the isomeric and kinetic studies via TLC and NMR. Note that molecule 1 has two bonds with restricted rotation. Therefore, the rate of rotation around one of the Caryl–Nimide bonds is half the observed rate of isomerization.
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In the Laboratory
Scheme II. The preparation of the isomeric mixture is a one-step reaction from commercially available starting materials.
Experimental Procedure
Synthesis In a clean, dry 25 mL round-bottomed flask containing a magnetic stirrer, 250 mg of 1,4,5,8-naphthalene tetracarboxylic dianhydride was added to 8.0 mL of glacial acetic acid. While this mixture was stirring, 345 mg of 2amino-4-tert-amylphenol was added to the flask and then heated under refluxing conditions for 2.5 hours. The powdery light brown product (540 mg, 99%) was then filtered and dried in an oven at 110 ⬚C overnight. 2D TLC The product was analyzed to determine a rough estimate of the rotational barrier by 2D TLC using a 5% MeOH兾CH2Cl2 mixture on three identical silica gel plates (5 × 5 cm). After being run in the first direction, the plates were allowed to dry at room temperature for 7, 21, and 42 minutes, then run again in the orthogonal direction. The halflife (t1/2) was estimated by correlating the time to observe crossspots on the TLC plate to the rotational barriers given in a student handout (see the Supplemental MaterialW, Table 2).
2D TLC Students collected a 15 mg sample of the dried isomeric mixture, dissolved it in acetone, and prepared three identical TLC development plates using a solvent system that clearly separated the syn-isomers from the anti-isomers. The TLC plates were allowed to stand at room temperature for 7, 21, and 42 minutes, respectively, and then allowed to run orthogonally in the same solvent system (Figure 1). The plates were visually compared under UV light (254 nm) to determine the quantity of time necessary to begin observing the interconversion of the separated isomeric species. The conversion process is evidenced by the appearance of “cross-spots” on the plate. The plate run in the second direction after 42
NMR Time Study The precise determination of the rotational barrier between the two atropisomers was determined by following the reequilibration of an anti-enriched sample by 1H NMR as the sample equilibrated. An oil bath was heated to 60 ⬚C and in it were placed 15 NMR tubes containing 5 mg of the antienriched mixture in DMSO-d6. One NMR tube was removed from the oil bath every four minutes, allowed to cool to room temperature, and its isomeric ratio analyzed via 1H NMR. The values of k and ∆G ‡ were determined as described below. Hazards This procedure requires the proper handling of reagents that are irritants (1,4,5,8-naphthalene tetracarboxylic dianhydride, 2-amino-4-tert-amylphenol), toxic (methanol, dichloromethane), flammable (glacial acetic acid, methanol), and corrosive (glacial acetic acid). Proper safety attire should be worn at all times. Results Students used two methods to measure the conformational stability of the chosen atropisomeric system. First, 2D TLC demonstrated the two isomers were stable at room temperature but could be reequilibrated within minutes. Second, NMR was used to monitor the equilibration process so that the rotational barrier could be determined more precisely. 1500
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Figure 1. Silica plates showing the results of the 2D TLC experiment in 95%/5% (v/v) CH2Cl2/MeOH after 21 (top) and 42 minutes (bottom) of separating the isomers in the direction shown by the horizontal arrow. The appearance of significant cross-spots on the bottom plate indicates that interconversion has taken place.
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In the Laboratory
minutes of equilibration time showed significant quantities of each isomer, indicating that under these conditions the half-life is on the order of minutes. From this observation alone, the rotational barrier can be estimated to between 20 and 25 kcal兾mol. It is noteworthy that this procedure can only be completed at room temperature if the barrier lies in this narrow range, since at lower values the interconversion occurs too rapidly and the isomers could not be separated on the time scale of TLC. If the barrier were too high, the isomers would be separated when run in the first direction but would interconvert too slowly to observe in a reasonable time frame.
NMR Study The rotational barrier of this system was calculated more precisely using a simple NMR equilibration method. To observe the equilibration process, the students began with a sample that had been enriched in the anti-isomer. Fortunately, heating the neat mixture at 110 ⬚C between the first and second laboratory session (∼24 h) produced a material that was approximately 80 mole percent anti-1 (from the initial 1:1 mixture). Several samples, each containing a small quantity (∼5 mg) of this mixture dissolved in DMSO-d6 were prepared and observed by NMR at regular time intervals as the system
progressed towards equilibrium. In this solvent, the phenolic protons of the syn- and anti-isomers can be differentiated as distinct singlets appearing at 9.42 and 9.44 ppm, respectively (Figure 2). Heating the sample to 60 ⬚C allowed the students to collect enough samples in less than one hour to observe three half-lives and determine the rotational barrier. Integration of the peak areas was used to determine the mole ratios of the isomers at any time during the isomerization. Plotting the natural logarithm of the ratio [(R − Re )兾(R + 1)] versus time yielded a straight line (eq 1), indicative of the first-order kinetics (Figure 3) (4),
ln
R − Re R+ 1
= k isom t + C
(1)
where R represents the syn兾anti isomeric ratio at time t and Re is the equilibrium ratio. The least-squares regression equation gave the rate of isomerization (kisom) as the negative of the slope, which was 6.9 × 10᎑4 s᎑1. Since there are two bonds with restricted rotation in 1, the rate of rotation (krot) is half the observed rate of isomerization. Substitution of krot (3.5 × 10᎑4 s᎑1) into the Eyring equation (eq 2) yielded the rotational barrier, ∆G ‡, krot =
kBT − ∆G ‡/(RT ) e h
(2)
where kB is Boltzmann’s constant and h is Planck’s constant. The sample student data displayed in Figure 3 gave a ∆G ‡ of 25.2 kcal兾mol, consistent with the studies conducted on this system in our graduate research laboratory. This straightforward procedure and analytical method provides instructors an alternative to less intuitive techniques that may not be appropriate for students enrolled in an introductory organic chemistry course. WSupplemental
Figure 2. NMR spectra showing the change in isomeric ratio as the system equilibrates. From bottom to top, the NMR spectra show the relative increase in the syn to anti peak areas.
Figure 3. The plot of rate versus time indicates the pseudo-first order kinetics of the equilibration process. The slope of the best-fit line yields the rate constant, kisom.
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Material
Instructions for the students including pre- and postlab assessment and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Brown, W. H.; Foote, C. S. Organic Chemistry, 3rd ed.; Harcourt College Publishers: Orlando, FL, 2002; pp 65–83. 2. In contrast, previous submissions to JCE have studied systems with much lower barriers that require more advanced theory and techniques than is appropriate for an introductory organic course. Jarek, R. L.; Flesher, R. J.; Shin, S. K. J. Chem. Educ. 1997, 74, 978–982. Gasparro, F. P.; Kolodny, N. H. J. Chem. Educ. 1977, 54, 258–261. Morris, K. F.; Erikson, L. E. J. Chem. Educ. 1996, 73, 471–473. 3. Choi, D.-S.; Chong, Y. S.; Whitehead, D.; Shimizu, K. D. Org. Lett. 2001, 23, 3757–3760. Chong, Y. S.; Smith, M. D.; Shimizu, K. D. J. Am. Chem. Soc. 2001, 123, 7463–7464. Chen, Y.; Smith, M. D.; Shimizu, K. D. Tetrahedron Lett. 2001, 42, 7185–7187. 4. Stewart, W. E.; Siddall, T. H., III. Chem. Rev. 1970, 70 (5), 517–551.
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