An Integrated NMR and Photochemical
George Glaros and Norman H. CrornweW University of Nebraska
Organic Chemistry Experiment
Lincoln, 68508
In a previous paper,%we suggested a synthetic organic chemistry scheme which we felt gave products having interesting nuclear magnetic resonance spectra illustrative of many basic principles. In this paper, we describe a photochemical experiment involving one of the compounds previously prepared. The Claisen-Schmidt condensation of 4,4-dimethyl1-tetralone with henzaldehyde gives the thermodynamically more stable trans-2-benzal-4,4-dimethyl-l-tetralone.= s
.;.
Figure 2. Nmr spectrum of 100 mg tranl-2-benzal-4,4-dimethyl-1 -tetralone in 0.3 ml of CDCC irradiated for 4 hr.
It has been shown that the ring proton P to the carbonyl group is deshielded by the anisotropy of the carbonyl group and appears as a multiplet downfield of the rest of the ring proton^.^ The vinyl proton of trans-2-benzal4,4-dimethyl-l-tetralone is in a similar position relative to the carbonyl group as is the ring proton P to the carbonyl. Thus, a downfield shift of the vinyl proton is also to be expected.= This is indeed the case as can be seen in Figure 1where the triplet a t 463Hz is assigned Figure
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Nmr spectrum of tmns-2-benrol-4,4-dimethyl-l-tetralone.
to the vinyl proton. The doublet a t 175Hz (5 = 2-3 Hz) is assigned to the methylene protons. The pure trans isomer can he isomerized to a mixture of cis and trans-2-benzal-4,4-dimethyl-l-tetraloneby irradiation using a B-100A Blakray s ~ u r c e . ~
204 / Journal of Chemicol Educofion
as in Figure 2 after
24 hr of irradiation.
flask or on a smaller scale in a test tube or nmr sample tube. The latter method allows several students to perform the experiment a t the same time with one source. Figure 2 shows the nmr spectrum of a 100-mg sample of trans-2-benzal4,4-din1ethyl-l-tetralone in 0.3 ml of CDCla, held 1in. from the source after irradiation for 4 hr.6 There are new signals observed for the geminal methyl groups, the methylene protons, and the vinyl proton. The methyl proton signals are shifted approximately 8 Hz downfield, the methylene signals are shifted approximately 10 Hz upfield, and the vinyl proton resonance is shifted 65 Hz upfield to 398 Hz."igure 3
' Author to whom correspondence regarding this ~ r t i d eshould
be addressed.
N . H., J. CHI:M.EDUC.,46, 854 Z G ~ ~ ~G.,o AND s , CROMWELL, (1969). a HASSNICR, A., AND CROMWELL, N. H., J . A w m . Chent. Soe., 80. 893 ll95Xl. , ~-~ 'IMBACH, J. L., POHLAND, A. E., WEILER,E. D., AND CROMWISLL, N . H., Telmhedrm, 23, 3931 (1967). KEYILL, 1). N., WICILI,:R,E. U., A N D CROMWI:LL, N. H,. J . 074. Chem., 29, 1276 (1964). The s a m ~ l eis heated when close to the source and some solvent is lost
.
This can be performed on a preparative scale in a Pyrex
3. Spectrum of some sample
'
~~~
shows that the same sample after 24 hr irradiation contains only 15% of the tram isomer. The strong deshielding effect of the carbonyl group is very evident in this experiment. Although the magnitude of the coupling constant is used to determine the stereochemistry of vicinal protons as well as cis and trans relationships in olefins, allylic systems have many exceptions to the general rule7 that JCCIOM > JtlanSOzd.In this case, the very poor resolution of the vinyl triplet and methylene doublet of the cis isomer reflect the smaller coupling constant for the cisoid form. Therefore, the best way to assign stereochemistry about the double bond is by the chemical shift of the vinjl proton. Indeed, although infrared and ultraviolet spectroscopy are the usual methods to determine cis and trans relationships, they have been found to be of little value in this s y ~ t e m . ~ The two isomers can be separated by column chromatography6 to obtain the pure cis isomer, hut little would be gained by the studpnt in performing this additional experiment.
This experiment, which requires very little sample and can be performed by several students a t the same time, illustrates the application of nmr to a stereochemical problem for which other methods fail. The anisotropy of the carbonyl group is again clearly demonstrated, and the integration of nmr theory into an undergraduate synthetic organic experiment has again been accomplished. Acknowledgment
This work was supported in part by a Special Departmental Science Development Award to the department of chemistry from t,he National Science Foundation. Grant No. GU-2054: One of us (G. G.) held an Avery Fellowship from the University of Nebraska and wishes to acknowledge this award.
7 JACKMAN, L. M., AND STERNHILL, S., "Application of Nuclear Magnetic Resonance Spectroscopy in Organic Chemist~y" (2nd ed.), Pergamon Press, New York, 1969, p. 316.
Volume 48, Number 3, March 1971
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