In the Laboratory
Conformational Analysis in an Advanced Integrated Laboratory Course
W
David B. Ball* and Randy M. Miller Department of Chemistry, California State University, Chico, CA 95929-0210; *
[email protected] As the chemical industry and chemistry graduate programs integrate state of the art analytical equipment and techniques into their research efforts, undergraduate chemistry programs need to offer a curriculum that brings technology into both the classroom and the laboratory. Modern instruments for spectroscopic analysis have dramatically affected all areas of chemistry. Today chemical research is highly dependent on spectroscopic tools to elucidate structures, stereochemistry, conformational, and exchange phenomena in molecules. Undergraduate students of chemistry need to be exposed to these spectroscopic techniques to experience both the versatility and limitations encountered in their use. Our department has tried to partially meet these challenges by developing an advanced integrated laboratory sequence that incorporates the laboratory components of our physical, analytical, and inorganic chemistry courses.1 For this sequence we have developed several interdisciplinary experiments. One such experiment has been described in this Journal (1). Here we describe another project that incorporates synthesis, the use of 1H and 13C NMR spectroscopy, GC– MS, molecular modeling, and IR and UV spectroscopy. This article will highlight the spectral methods and analysis used and a future article will detail the molecular modeling aspects of this experiment. Conformational analysis of cyclohexane systems have been extensively studied using a variety of techniques that in-
1
O
HOAc
H
H
Br
O
O
Br
O
H
H
Br
Br
conformers
The format of this project is multifaceted. The six-week project (12 three-hour labs) include (i) synthesis, isolation, and purification, (ii) structure elucidation, (iii) determination of conformational preferences by spectral methods, and (iv) molecular modeling. Both oral and written reports by the students are required. The pedagogical goal of this project is to have students, in a research-like setting, collectively make initial predictions and then attempt to validate these predictions by generating the appropriate data utilizing the theoretical and analytical tools available to chemists. The Experiment Each student is required to complete each of the following parts of the experiment. But as we expect the class to work cooperatively in arriving at their final conclusions, we designate a specific individual as a “team leader” to coordinate the data collection and interpretation for each part.
NMR Spectroscopy 1H and 13C NMR spectra (in CDCl ) are obtained for 3 the starting ketone 1 and for the conformationally mobile bromide 2. COSY and DEPT spectra are acquired on bromide 2 to facilitate the analysis of the proton and carbon spectra. To show the nature of the dependence of the conformational equilibrium of 2 on solvent polarity, 1H NMR spectra are also run in DMSO-d6, CD3CN, CD3OD, and C6D12 (4).
enantiomers
O
Problem Outline
Synthesis Students start by brominating 3,3,5,5-tetramethylcyclohexanone, 1, giving the conformationally mobile racemic 2bromo product, 2 (Figure 1). GC–MS analysis of the reaction products is carried out.
PyrHⴙ Br3ⴚ
+
cludes IR and UV spectroscopy, optical rotatory dispersion, polarography, dipole moments, and most recently nuclear magnetic resonance spectroscopy (2). Cyclohexanone provides a fairly rigid backbone that, depending on other substituents present in the molecule, allows for the elucidation of static or dynamic conformational preferences. Spectroscopic analysis of the conformationally mobile 2-bromo-3,3,5,5-tetramethylcyclohexanone and comparison to model compounds leads to the quantitative determination of the variation of conformational preferences in solvents of differing polarities (3).
conformers 2
Figure 1. Bromination of the cyclohexanone showing the stereoproducts. For additional information see the Supplemental Material.W
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IR Spectroscopy IR spectra in CH3CN and CCl4 of bromoketone 2 and of ketone 1 are collected. UV Spectroscopy UV spectra of starting ketone 1 in C 6 H 12 and of bromoketone 2 in CH3CN and C6H12 are obtained.
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In the Laboratory
Molecular Modeling GAMESS version 16 Feb 2002 (R1) is used to obtain free energies, HOMO and LUMO energies, dipole moments, C⫽O IR frequencies, and C⫽O bond orders in DMSO and C6H12 (5). All computations are performed using the restricted Hartree–Fock method with a 3-21G basis set. The solvent is treated using the polarized continuum model. Hazards Most of the organic solvents and some of the reagents used in these experiments are flammable or corrosive. Specifically CD3CN and CD3OD are flammable and toxic. CCl4 and CDCl3 are extremely toxic and suspected cancer agents. Pyridinium tribromide is corrosive and a lachrymator. Column chromatography involves the use of fine mesh SiO2 and large volumes of volatile, flammable solvents. Adequate ventilation must be provided in making and using these columns. Appropriate disposal of waste products is required. Results The α-bromoketones required for this project are synthesized utilizing techniques that our students have become proficient at in their year organic sequence. Student yields of monobrominating ketone 1 are modest to good, ranging from 65 to 84% (Figure 1) after normal work up and chromatography. The student results of the IH NMR analyses of ketone 2 are shown in Table 1.2 All other NMR spectroscopy (13C, DEPT, and COSY) are consistent with the proposed structures. IR and UV absorbancies for ketones 1 and 2 are shown in Table 2. Computed carbonyl IR frequencies, bond orders, dipole moments and orbital energies of the HOMO, n, and LUMO, π*, for ketones 1 and 2 are shown in Table 3.
computed and scaled frequencies are 1744 cm᎑1 for axial 2 and 1753 cm᎑1 for equatorial 2. In 2 when the bromine is equatorial, the C⫺Br bond and the carbonyl are nearly parallel while when the bromine is axial they are nearly perpendicular resulting in a larger dipole moment for the equatorial isomer (8).4 The bond moments exhibited by the C⫺Br bond and the carbonyl and their relative position to each other should result in a higher carbonyl stretching frequency when the bromine is equatorial (the carbonyl being less polarized with more double bond character) and a lower carbonyl stretching frequency when axial (more carbonyl delocalization resulting in less double bond character). The IR data, judging from peak intensities, lead to the conclusion that in a nonpolar solvent the less polar conformation appears to be the greater contributor to the equilibrium while in a polar solvent the opposite is seen. The UV spectra of 2 in CH3CN and C6H12 as compared to the parent cyclohexanone 1 also confirm the solvent dependency of this equilibrium (Table 2). The shift to longer wavelength and the change in the molar absorptivity of the n–π* transition in nonpolar solvents is rationalized by the stabilization of the π* antibonding orbital of the carbonyl by the overlap of empty orbitals on the axial bromine. Only the axial bromine is in the correct orientation for this overlap resulting in the lower energy of this transition. A quantitative analysis of this conformational equilibrium is required of our students. This may be carried out using the Eliel method (eqs 1–3; ref 9). The analysis relies on being able to measure a property of the system in question that varies in a linear fashion as the equilibrium shifts and that can be directly correlated to a model system. In this case, the chemical shift of the α-H2 of 2 (R ) is that property used to arrive at the equilibrium determination,
Discussion After successfully synthesizing and isolating racemic αbromoketone 2, students are challenged by the characterization of this racemic mixture of conformers. The 1H NMR spectrum of 2, a chiral molecule, in CDCl3, lacking the symmetry of the parent ketone 1, has five different signals each integrating for a single hydrogen and four signals each integrating for three hydrogens. A DEPT spectrum indicates the compound has three quaternary carbons, one methine, two methylenes, and four methyl carbons. Analysis of a 1H–1H COSY spectrum of 2 gives evidence of geminal two-bond and W-type four-bond couplings.3 As α-bromide 2 undergoes rapid chair–chair conversion (being conformationally mobile; Figure 1) at a rate faster than the NMR timescale, a spectrum of the interconverting conformers gives a time-averaged absorbancy for the α-H. The students investigate this phenomenon further, noticing that the chemical shift of this α-proton is dependent on the polarity of the solvent (Table 1). Further information of this dynamic equilibrium being solvent dependent is gathered from the IR spectra of 2 in CCl4 and CH3CN. In each solvent there are two absorbancies for the carbonyl group; at 1717 and 1732 cm᎑1 in CCl4 with the latter being of greater intensity and at 1715 and 1728 cm᎑1 in CH3CN with the relative intensities of these peaks being reversed from that observed in CCl4 (7). Vibrational spectra computed with GAMESS aids in the assignment of these frequencies. The 122
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R = NaxRax + NeqReq
(1)
Nax + Neq = 1
(2)
Nax = (R − Req)兾(Rax − Req)
(3)
where N is mol fraction and Nax and Neq are the mol fractions of the axial and equatorial conformers of 2. The model system that allows for the desired equilibrium calculations in the solvents studied is exo- and endo-2-bromobicyclo[3.2.1]octan-3-one, 3a and 3e.5 Br
H
H
Br
O 3a
O 3e
Students arrive at the conclusion, because of the bicyclic structures, that 3a and 3e are diastereomers that are conformationally static and have different magnetic environments resulting in different chemical shifts for their α-hydrogens (Table 1).6 Assuming that the ethano bridge does not greatly perturb the cyclohexane ring as compared to nonbicyclic bromide 2, the chemical shifts of the axial and equatorial H2’s of 3e (Req) and 3a (Rax) will be the reference points.7 Using the NMR chemical shift data from Table 1 and eqs 2 and 3, students are able to calculate the equilibrium position of the conformationally mobile bromide 2 in various solvents (Table 4).
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In the Laboratory Table 1. 1H NMR Data: Dependence of Chemical Shift on Solvent Chemical Shift of α-H, δ/ppm
Compound
O
H
Br
H
O Br
2 H Br O 3e Br H O 3a
Solvent
Multiplicity of α-H, J/Hz
4.95
DMSO-d6
d; 1.1
4.63
CD3CN
t; 0.7
4.62
CD3OD
d; 1.5
4.28
CDCl3
d; 1.3
3.94
C6D12
t; 1.1
DMSO-d6 CD3OD
m
4.98
CD3CN
m
4.77
CDCl3
td; 1.8, 3.5
4.52
C6D12
m ddd; 1.5, 1.8, 3.2
4.29
DMSO-d6
4.17
CD3OD
ddd; 1.5, 1.8, 3.5
4.17
CD6CN
ddd; 1.5, 1.8, 3.5
4.13
CDCl3
ddd; 1.5, 1.8, 3.2
3.94
C6D12
m
These results, in general, agree with prior studies of this system based on IR, UV, and dipole moments (8a, 8b) and 1 H NMR spectroscopy (4, 12). But the results are in contrast to what students would predict from the knowledge they obtained in the year organic course that groups experience less gauche interactions in an equatorial position as compared to an axial position in cyclohexane systems.8 The solvent polarity dependence of the conformational preference of bromine in 2 leads to the conclusion that the bond moments of the C⫺Br bond and of the carbonyl must play a significant role in the equilibrium position shown in Table 4. The axial preference for α-bromo substituents in cyclohexanones has been attributed to the equatorial isomer having a higher energy than the axial isomer because of the dipole–dipole repulsion and the bromine–oxygen steric repulsion present when the bromine is in an equatorial position. It has also been suggested that the energy of the axial isomer is lowered by the overlapping of the C⫺Br σ* and the π bond of the C⫽O (the anomeric effect; eq 5; ref 13, 14 ): O H Br
Oⴚ H
ddd; 1.5, 1.8, 3.2
5.23 5.04
equatorial isomer increases in comparison to the axial isomer. This equilibrium shift versus solvent polarity is seen in all three of the spectral techniques used in this experiment. Directly and indirectly, the IR, UV, and NMR data indicate that the axial conformer is, by far, the predominant conformation in nonpolar solvents while, as the solvent polarity increases, the equatorial bromide becomes a greater contributor to the equilibrium. The computational results confirm the latter observation. However, the equatorial conformer is also computed to be favored in nonpolar solvents. So the computational results are consistent in terms of trends, but not in terms of specific results. This is not completely surprising given the relatively low level of calculation employed. Table 2. IR and UV Data: Solvent Dependency C⫽O Stretching Frequency/ cm᎑1 (solvent)
Compound
O
=
(5)
Brⴙ Br
The polarity of the solvent affects the conformational equilibrium by changing the relative solution stability of each conformation of 2. As the polarity of the solvent increases, the Coulombic interaction (repulsion) of the nearly parallel dipoles in the equatorial isomer is lowered. Another significant contributing factor to the equilibrium position is that there is a greater interaction of the more polar equatorial conformer with the polar solvent (15). As the polarity of the solvent is decreased the solution energy of the more polar www.JCE.DivCHED.org
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λmax/nm (molar absorptivity, solvent)
1715 (CCl4)
298 (ε = 16, C6H12)
1728 (CH3CN)
303 (ε = 36, CH3CN)
O H O
Br
1732 (CCl4)
O 1715 (CH3CN)
H
1717 (CCl4)
Br
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In the Laboratory
Table 3. Molecular Modeling Results Compound
C⫽O Stretching Frequencya/cm᎑1
Dipole Moment
C⫽O Bond Order
HOMO, n Orbital Energy/hartrees
LUMO, π* Orbital Energy/hartrees
1729
3.41
1.826
᎑0.3791
0.1644
1753
4.81
1.864
᎑0.3789
0.1476
1744
4.19
1.845
᎑0.3788
0.1117
O H
Br
O
O H Br a
The computed frequencies have been scaled by 0.9.
Conclusion As the introduction for our three-semester integrated laboratory sequence, this experiment has been extremely successful from the perspectives of both the students and the faculty. Students make a relatively easy transition from their chemistry-major organic laboratory class, making good use of and extending their prior synthetic and analytical skills. They are also challenged by having to go beyond the common one-dimensional approach normally seen in experiments that are based on specific subdisciplines of chemistry. This experiment incorporates organic synthesis, spectral analyses for structural elucidation and equilibrium determination (qualitatively and quantitatively), and molecular modeling studies used to corroborate student predictions and conclusions. Acknowledgments We thank California State University, Chico for the continued financial and administrative support of the curricular and pedagogical changes that the chemistry department has implemented in the integrated laboratory sequence. We also thank the students and our colleagues who have actively participated in the integrated laboratory sequence. The funding of a CCLI grant proposal by NSF is gratefully acknowledged. We thank Will Chrisman of UCSB for valuable spectra and discussions that contributed to this work. Acknowledgment is made to Phil Crews of UCSC who introduced DBB to the viability of this experiment some 20 years ago. W
Supplemental Material
Support materials are available in this issue of JCE Online, including: general procedures (equipment and instrumentation), detailed experimental procedures for the preparation of 124
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2, 3a, 3e, and cis- and trans-2-bromo-4-butylcyclohexanone, CAS registry numbers of chemicals, a student lab handout, IR spectra of bromoketone 2 in CCl4 and CH3CN, and selected 1H spectra of bromoketones 2, 3a, and 3e. Notes 1. The integrated laboratory sequence was the basis of a funded National Science Foundation Course, Curriculum and Laboratory Improvement Program (grant #99-50413) that provided for the purchase of the Varian Mercury VX300 BB NMR spectrometer used in this study. 2. The recorded chemical shifts of 2, 3a, and 3e in Table 1 are in good agreement with prior studies (4, 11). 3. Though not expected of our students, an in depth analysis of the COSY spectra of 2 in C6D12, CDCl3, CD3CN, and DMSOd6 show that the axial isomer of 2 is the predominant species in C6D12 and CDCl3 and the equatorial isomer dominates in both CD3CN and DMSO-d6. Long range W-type four-bond couplings in the two former solvents are seen between Ha, Hb, and Hd, while these couplings are missing in the latter solvent but now Ha and Hb are coupled in a non-W-type four-bond coupling (6). H Hd
H
H
O
Br Hb Ha
Hb Ha
O
Br
2
4. Although the dipole moments of these interconverting diastereomeric bromides have not been experimentally determined, they have been calculated to be 3.00 D and 4.34 D for the axial and equatorial conformers respectively and 4.19 D and 4.81 D in the computational component of this lab (8a, 8b). It has also been
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In the Laboratory Table 4. Conformational Preference of α-Bromide 2 and Related Thermodynamic Values Solvent
% Axial Preference
∆G⬚298 / (kcal/mol)
K = [ax]/[eq]
DMSO-d6
30
0.42
0.50
CD3CN
43
0.76
0.16
CD3OD
48
0.93
0.041
CDCl3
77
3.3
᎑0.70
C6D12
~100
> 3.3
< ᎑0.70
calculated that the dipole moment of axial bromide 3a (3.62 D, experimentally 3.71 D) is less than that of the equatorial bromide 3e (4.23 D) (8c). The dipole moments computed using GAMESS are 3.73 D for 3a and 4.78 D for 3e. 5. The stored FIDs of the proton spectra of racemic 3a and 3e in the various solvents are accessed by the students on the Sun computer of the Varian NMR system. These are used to generate the proton chemical shift data for 3a and 3e. As an extension of this experiment, 3a and 3e may be part of the synthesis required (see Supplemental MaterialW ). 6. It has been pointed out that NMR chemical shifts can change as a function of solvent alone and not necessarily totally the result of conformational changes. In fact, the 1H NMR spectrum of bromoketone 2 in C6D6 shows a change of relative chemical shifts for the axial and equatorial hydrogens at C4 from those chemical shifts seen in CDCl3. However, the observed proton chemical shift dependency on solvent polarity for the α-H’s of 2 versus 3a and 3e are entirely compatible with the analysis given herein. 7. Initially the proposed model system was a mixture of cisand trans-2-bromo-4-t-butylcyclohexanone (see Supplemental MaterialW ). Because of the large butyl group, the cis and trans isomers are conformationally static or biased and have different magnetic environments resulting in different chemical shifts for the α-hydrogens bearing the bromine (at δ 4.38 for the trans isomer and 4.69 for the cis isomer in CDCl3). It has been stated that the tbutyl group and the opposing bond moments of the C⫺Br bond and the carbonyl provide a conformational anchor to the molecule making the trans isomer a reliable model compound (4). However, inspection of the chemical shifts in Table 1 recorded for 2 in CDCl3 and C6D12 (δ 4.28 and 3.94, respectively) do not compare favorably to those of trans-2-bromo-4-t-butylcyclohexanone in the same solvents (δ 4.37 and 4.19, respectively) indicating that it is not an appropriate model compound for this study. It has been argued that steric repulsion between the syn–axial methyls in 2 cause the axial groups at C2 and C4 to come closer to each other than normally seen in cyclohexanones (this has been called the Reflex Effect; ref 10a). This change in relative position for the equatorial α-H in 2 must result in a different chemical shift for this hydrogen as compared to the α-hydrogen in trans-2-bromo-4-t-butylcyclohexanone. The chosen model 2-bromobicyclo[3.2.1]octan-3-one (3a and 3e) does provide the chemical shift range in the solvents studied needed to calculate the equilibrium position of 2. Yet it has been argued that the ethano bridge while locking the molecule to inversion “pinches” in the ring pulling the axial substituents on the other side of the ring away from each other lowering their steric repulsions (this has been called the Antireflex Effect; ref 10b). Thus a criticism might
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be raised that the observed chemical shifts of 3a and 3e are not a true indication of the solvent dependence of the equilibrium position of mobile bromide 2. The molecular modeling results obtained however validates our use of 3a and 3e as model compounds. 8. It may also be instructive to point out that relative proton shifts for axial and equatorial hydrogens in cyclohexanones are reversed as compared to cyclohexane; that is, axial hydrogens are more deshielded than are equatorial hydrogens (11).
Literature Cited 1. Ball, D. B.; Wilson, R. J. Chem. Educ. 2002, 79, 112–115. 2. (a) For a review on six-membered ring conformational analysis, see Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley and Sons, Inc.: New York, 1994; pp 686–754. (b) Midland, M. M.; Beck, J. J.; Peters, J. L.; Rennels, R. A.; Asirwatham, G. J. Chem. Educ. 1994, 71, 897–898. (c) Fitzgerald, J. P. J. Chem. Educ. 1993, 70, 988–990. (d) Eliel, E. L. J. Chem. Educ. 1975, 53, 762–767. (e) Kaloustain, M. K. J. Chem. Educ. 1974, 51, 777–780. (f ) Eliel, E. L. J. Chem. Educ. 1960, 37, 126–133. 3. For a qualitative evaluation of the conformational preferences of 2, see Jefford, C. W.; McCreadie, R.; Muller, D.; Pfyffer, J. J. Chem. Educ. 1973, 50, 181–185. 4. Baretta, A.; Zahra, J. P.; Waegall, B.; Jefford, C. W. Tetrahedron 1970, 26, 15–26. 5. Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347–1363. 6. Jackman, L. M.; Sternhell, S. Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry, 2nd ed.; Pergamon Press: London, 1969; pp 334–341. 7. Allinger, J.; Allinger, N. L. Tetrahedron 1958, 2, 64–74. 8. (a) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley and Sons, Inc.: New York, 1994; p 711. (b) Cantacuzene, J.; Jantzen, R.; Ricard, D. Tetrahedron 1972, 28, 717–734. (c) Jefford, C. W.; Waegell, W. Tetrahedron Lett. 1963, 28, 1981–1986. 9. Basso, E. A.; Kaiser, C.; Rittner, R.; Lambert, J. B. J. Org. Chem. 1993, 58, 7865–7869. 10. (a) Waegell, B.; Ourisson, G. Bull. Soc. Chim. Fr. 1963, 496– 503. (b) Fournier. J.; Waegell. B. Tetrahedron 1972, 28, 3407– 3437. 11. Wellman, K. M.; Bordwell, F. G. Tetrahedron Lett. 1963, 25, 1703–1708. 12. Jefford, C. W.; Waegell, B. Bull. Soc. Chim. Belges 1970, 78, 431–436. (b) Artenius, M.; Schamp, N.; De Pooter, H. B. Bull. Soc. Chim. Belges 1967, 76, 541–551. (c) Pouzard, G.; Hall, L. D.; Zahra, J. P.; Waegell, B. Org. Mag. Reson. 1977, 9, 627– 630. 13. (a) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley and Sons, Inc.: New York, 1994; p 733. (b) Corey, E. J.; Burke, H. J. J. Am. Chem. Soc. 1955, 77, 5418– 5420. 14. (a) Kirby, A. J. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer: New York, 1983. (b) Deslongchamps P. Stereoelectronic Effects in Organic Chemistry; Pergamon: New York, 1983. 15. Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley and Sons, Inc.: New York, 1994; p 608.
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