Identification of Methylcyclohexanones by NMR Jerome E. Gurst University of West Florida, 11000 University Parkway, Pensacola, FL 32514 I t is unlikely that the three isomeric methylcyclohexanones (see table) could be distinguished from each other a priori using ultraviolet spectroscopy or infrared spectroscopy. To use mass spectrometry, elaborate arguments involving deuterium labelling andlor fragmentation patterns might be invoked to help solve this problem, but NMR, and in particular, easy-to-interpret 2D-NMR experiments, make the analysis almost trivial. This example has been presented to students in beginning organic chemistry classes, both a t this institution and at area community colleges, as an introduction to modem NMH techniques. An excellent series of articles ( 1 4 1has appeared in this Journal presenting the theoretical and experimental bases for 2D NMR experiments. The 'H NMR spectra1 of the three ketones are rather complex, and not amenable to immediate analysis (although one might guess that the spectrum from the symmetrical 4-methylcyclohexanone might be the least complex of those shown). Deuterium labelling of the a-H's and good integration, in theory, should allow one to identify the 2-methylcyclohexanone from its two isomers. I3C NMP easily permits the identification of the 4-methylcyclohexanone from the other two due to symmetry. Only four upfield peaks are expected in the UC NMR spectrum of this compound as opposed to six npfield peaks for each of the others. The problem of cleanly distin&mishingthe 2-methyl com~ o u n dfrnm the 3-methvl ketone still exists. It is here that of two of the newer NMR techwe appreciate the niques. In an AFT (Attached Proton Test) spectrum, carbons with an odd number of attached protons (methyl and methine groups) appear in a negative direction while carbons with an even number of attached protons (methylene moups) have a positive sense. From the spectra, we see ;hat each cornpo;nd has two signals pointinidown, one for the CH1 and one for the CH. The other technique demonstrated -here the HKTCOR (HETeronuclear CORrela- ~ is - ~ tion) spectrum. Here one looks above the signal for a particular carbon atom in the I3C NMR spectrum. There will be a significant black "spot" or signal. Looking to the left from this spot, one can identify the signals in the 'H NMR spectrum for the hydrogens directly attached to the carbon in ouestion. In this wav. we can see that the hiehest field (or Lost shielded) carddn signal correlates witcthe highest field (or most shielded) moton sienal. which is a doublet andwhich integrates to a r e k i v e area of 3 H's. mu^. Clearlv. " . these sienals are from the methvl . . The lower field (more deshielded) negative signal in the APT spectrum must be for the methine carbon, which allows us to find the methine hvdnzen mine the HETCOR " spectrum. The methine hydrogen and metkne carbon are closer to the carbonyl group in the 2-methylcyclohexanone than in the 3-methylcyclohexanone. The electronegativity ~~
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Data for the Three Isomeric Methvlcvclohexanones
2-Methylcyclohexanone
3-Methylcyclohexanone
4-Methylcyciohexanone
no 1.4480,d 0.924
no 1.4450, d 0.914
nD
1.4460, d 0.914
and magnetic anisotropy of the carbonyl group is known (5, 6 ) to cause deshielding (downiield shifts) of carbons and
hydrogens close to it. For wnfirmation, we can determine that the methine carbon and hydrogen is in almost the same position in 3-methylcyclohexanone a s i t is in 4methylcyclohexanone. [The shielded, or upfield position (14.8 6 ) of the methyl signal in the 2-isomer relative to the 3- and 4-isomers suggests that this carbon atom lies in the
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'The 'H NMR spectra are shown on the left side of the figure. 2The 13CNMR spectra are shown at the bottom of each set, running from 160 ppm on the left to 0 pprn on the right. Signals for the carbonyl carbons appeared at 21&212 S units when the spectra were measured with a larger spectral window. Samples were dissolved in CDCI,, which accounts for the three signals centered at 77 ppm.
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Journal of Chemical Education
'H NMR spectra (300 Hz), 13cNMR spectra (75 MHz), APT, and HETCOR spectra of 2-methylcycI0hexanone (bottom); 3methylcyclchexanone (center);.and4-methylcyclohexanone(top).
shielding cone created by the magnetic anisotropy of the carbonyl group. This means that the methyl group is in the equatorial position as expected from conformational analysis and molecular modeling.] An Interesting O b s e ~ a t i o n Because each CH2group has an mial and an equatorial hydrogen attached to it, and because axial H's and equatorial H's generally have different chemical shift values, there are two signals, or "spots", above most of the CH2 signals. Axial H's generally have chemical shifts at higher fields than equatorial H's (5, 6). The spectra were measured using a General Electric QEPlus Spectrometer operating a t 300 MHz for proton and 75 MHz for carbon spectra. Samples (approximately 40 mg) were dissolved in CDC13(- 0.75 mL). A"macro" supplied by
GE was used tn program thc spccirometer Each complete set of s ~ e c t r atlH NMR. 13CM R . APT and HETCOR, was compleied in just under 10 minutes after autoshimming. Acknowledgment Purchase of the spectrometer was funded in part by a grant from the National Science Foundation's Instrumentation and Laboratory Improvement Program (Grant #USE-9050802). Literature Cited 1.King.R. W.:Williams. K R. J C h . Educ 1988,66,A213. 2.King.R.W;Wihsms.KR. J C h E d u c 1989,66,A243. 3. Wiltiams. K R.; King, R.W J. Chrm. Educ. 1990,67,A100. 4. William, K R.; f i g , R. W. J Chem Edue. 19W. 67.A125.
5. Kemp, W. 01gonic Spdrosmpy, 3rd ed.; Freeman: New Yo& 1991:Chaptu 3. 6. Retsch, E.; Clerc, T.; Seibl, J.; Simon, W. Toblea a f S p ~ f f d D o lfar a Struefumhter minorion o f O g ~ i Compounds, e 2nd d.(translated by Biemsnn, K.);SpILnge~ Verlag. Berlin, 1939.
Volume 69 Number 9 Se~tember1992
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