James F. McConnell State Universitv of New York College at Cortland Cortland, New York 13045
I
Dehydration of 3-H~X~IIOI
There has been a trend in undergraduate organic chemistry away from traditional "cookbook" experiments toward project-oriented laboratory experiences (1, 2). Experiments (3-5) have been published dealing with the dehydrations of certain alcohols. These experiments impart a good general understanding of the fundamentals of carbonium ion chemistry as well as familiarizing the student with routine techniques of organic chemistry and newer methods of analysis. Recently our students in undergraduate organic chemistry have been carrying out similar studies on a series of alcohols which includes 3-p'entanol, tertpentyl alcohol, 2-methyl-2-hexanol, and 3-hexanol. We have incorporated a unique kink in our experiments, however, which utilizes nuclear magnetic resonance as an analytical tool in conjunction with preparation gas chromatography. The Experiment Each member of the class selects one of the alcohols and is asked to dehydrate the alcohol to obtain a crude olefin mixture. A procedure for the preparation of cyclobexene is supplied as a guide. The student is then asked to analyze his olefin mixture by gas chromatography, determining the number of components in the mixture and the percent of each component. I n addition, he is asked to isolate enough of the major component in the mixture to identify it by nuclear magnetic resonance spectral analysis. The most interesting results are obtained with 3-hexanol since this alcohol presents a slightly more complex analytical problem. There are four major olefin products formed when 3-hexanol is dehydrated
552 / Journal of Chemical Education
experiment for undergraduates
These are cis3-hexene (I), trans-3-hexene (II), cis-2hexene (111), and trans-2-hexene (IV). We have been unable to find a gas chromatography column which will give four well-resolved peaks with this mixture. The olefin mixtures are, in general, most effectively separated on a column in which the liquid phase is a saturated solution of silver nitrate in henzyl cyanide. This column separates olefins by virtue of their varying complex stability with the silver ions in the liquid phase. This quality is highly dependent on steric hindrance factors in the complex. Consequently, cis isomers are efficiently separated from trans isomers. The cis isomers form the more stable complex and therefore are retained longer on the column. However, it is not possible to effect a practical separation of the 2-hexenes from the 3-hexenes with this column. At this point it was discovered that a delightfully simple application of nmr spectroscopy could be used to determine the relative quantities of these materials. The cis-isomers are collected together, as are the trans-isomers, and an integrated nmr spectrum is obtained for each of these mixtures. The analysis is made simple by the fact that the [-CH=CH-CHI] methyl protons adjacent to the double bond in the 2-olefins gives a resonance in a unique location (between
Table 1.
Treatment of Raw Data
Olefins
Ratios
Trans-2 and 3-hexenes Cis-2 and 3-hexenes
Table 2.
Figure 1. The fin1 peok from the right Ipeok I ) includes the tron~-2- and 3-hexener. The second psok from the right (peok 21 includes the cis-2m d 3-hexener Note that partial resolution i s obtained for the cis oleflnr. Cutting out and weighing the individual peaks ir the suggested method of integration for the closr.
Comments
.59.8% Obtained from integration of 40.2% peaks in gas chromatograph illustrated in Fig. 1. 57.2% Obtained from integrstion of 48.27, nmr spectrum shown in Fig. 2 at bottom. 74.1% Obtained from integration of 25.9% nmr spectrum shown in Fig. 2 at top.
Final Results
Olefin
Percent
Trans-Zhexene Trans-3-hexene
34.2 25.7
Acknowledgment
1.65 and 1.85~)in the spectrum. Therefore, simple algebraic manipulation of the integration data, enables one to arrive at tvans-2-hexene to trans-3-hexene ratios and eis-2-hexene to cis-3-hexene ratios. Procedure
A routine procedure is used for the dehydration of 3-hexanol. The analysis is done directly on the crude olefin product. The separation is accomplished using a 3/&in. column packed with saturated silver nitrate in benzyl cyanide as the liquid phase and 60-80 mesh chromosorb W as the support phase. With this column it is found that enough material for nmr analysis can be collected after two or three injections with a 50-wl syringe. The collections are made in a Dry Ice cooled trap attached to the elution port of the instrument. The collected sample is washed directly from the collection vessel into an nmr tube with a small amount of carbon tetrachloride.
The author is indebted to Mrs. Patricia Quackenbush for her photography work on the spectra reproduced here. Literature Cited (1) FIFE.W. K., J. CXEM.Eoac.. 45, 416 (1988). . 44, 112 (1967). (2) Ross, J.. J. C x e ~ Eouc.. (3) T n ~ s n R. . L., &r*nC ~ ~ u P r o n , C.. J. C x e ~ Eonc., . 44, 020 (1867). (4) NIENHOOBE, E. J . , J . CXEM.EDUC.,46, 765 (1969). ( 5 ) T ~ s e n R. , L., Gnommnst. G. D.. AND CXAMPION. C., J. CHEU. Ennc., 46, 849 (1969). A. A,,J . A m ? . Cham. Soo.. 66, 1649 (6) HENNE.A. L..A N Y MATUSZAK. (1944).
W.
W.
Results of a Typical Experiment
Tables 1 and 2 are the results of a single typical experiment using 3-hexanol. The accompanying gas chromatograph (Fig. 1) and nmr spectra (Fig. 2) are the raw data on which these results are based. Conclusions
These results are well explained and are predictable from currently popular theories concerning dehydration reactions, but are quite different from the only published results on the dehydration of 3-hexanol. Henne and Matuszak report the formation of 2-hexene to 3-hexene in the ratio of 24.7:75.3 (6). This vari* tion is not surprising when one also considers the limited methods of analysis available t o Henne and Matuszak a t the time of their research. Their analysis consisted of first ozonation, then oxidation to the acids. After separation of the acids by fractional distillation of the ethyl ester or crystallization of the anilide, neutralization equivalents were determined.
Figure 2. Top-NMR spectrum of mixture of cis-2- ond 3-haxener. This mixture war collected a t the elution port of the go. shromatogroph a%peok 2 (Fig. 11 war being recorded. Bottom-NMR spectrum of mixture of trans-2- and 3-hexener This mixture war collected a t the elution port of the gos chromatograph ar peok 1 (Fig. 1 ) war being recorded.
Volume 48, Number 8, August 1971
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