EvereW J. Nienhouse
Deportment of Physical Sciences Ferris State College Big Rapids, Michigan 49307
A Unique Laboratory-Lecture in Organic Chemistry
Currently, our students in organic chemistry meet four times a week for lectures but have a single 3-hr laboratory session. Such a system necessitates efficient usage of time in order to accomplish the main objective of the organic chemistry laboratory, namely: to provide interesting and challenging experiences that (1) teach fundamental laboratory techniques, and (2) give substance to the theoretical concepts treated in the lecture sessions. We feel that the laboratory-lecture is an excellent means of instruction available to accomplish this latter objective. During the course of our discussion on the preparation and reactions of alkenes, we formulated what we feel is a unique laboratory-lecture which we wish to share with others. The lecture centers about the products obtained from the dehydration of &methyl-2-pentanol.' The first portion of the lecture deals with a prediction of all the possible products that might be obtained from the dehydration of this alcohol. Carbonium ion theory leads to the introduction of the concept of 1,2-shifts as Presented in part a t the 154th National Meeting of the American Chemical Society, Chicago, Ill., September, 1967. 1 ADAM, JOHNSON, AND WILCOX, "Laboratory Experiments in Organic Chemistry" (5th Ed.), i\?scmillan Co., New York, N.Y., p. 171. ROBERTS,GILBERT, RODEWALD, AND WINGROVE, "Modern Experimental Organic Chemistry," Ilolt, Rinehart, Winston, Ine., New Yark, 1969, p. 103. The prediction of quantities of the bottom three compounds in the table may seem curious in view of the fact that there would appear to be little driving force for the first formed earbonium ion to rearrange to other secondary carbonium ions of approximate stability. The occurrence of large amounts of 2-met,hyl-2pentene and trace quantities of 2-methyl-1-pentene and 3-methyl2-pentene paints to t,he often neglected fact that carboniurn ions can and do rearrange not only to more stable carbonium ions, but to d h e r ions of similar stzbility. Presumably the activation energies between such carbonium ions are relatively law. a Authentic alkenes may be purchased from eit,her the Aldrich Chemical Co. or Phillips Petroleum.
applied to hydrogen and alkyl groups. Utilization of this concept and alkene stability considerations brings one not only to a prediction of tuhich products would presumably be obtained in the dehydration reaction, but also the relative amounts of the alkene that can be expected. Thus, the carbonium ion below, a secondary carbonium ion formed by loss of a water molecule from the protonated alcohol CHs cHsbcH3-~~-cma
I
H
may undergo a number of interesting things eventu-
Chromatogr~mof the product, of the dehydration of 4.methyl-2-pentanol. This analysis wor made with a 10-ft. X %-in. column pocked with 1 Oyo OV-l on 6 0 8 0 Chromosorb W at 3S°C.
Volume 46, Number I I, November 1969
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765
