Relative Carbonium Ion Stabilization Energies: An NMR Experiment for the Undergraduate Organic Laboratory A meaningful nmr experiment Lasting three laboratory periods and requiring only two or three different nmr speetra can he based on the method of Young e t al. for the quantitative determination of relative carbonium ion stabilization energies.' This method requires that equimalar amounts of a stable carbonium ion salt (R+Y-) and an appropriate carhonium ion precursor (&X) he mixed in a suitable solvent to yield an equilibrated mixture (as shown in eqn. (1))in which the relative amounts of R+ and &+are a direct measure of these two carbonium ions' free energy difference.
R++&XtRX+&+
(1)
The relative amounts d R + and &+ are determined fmm the change in ehemical shit3 of a methyl group in Ro. Our students used the mmmereially available trityl hexachlomantimonate as RfY- and mono-p-methyltrityl ehloride as RoX (Table I in the Young article lists alternatives). The students synthesized the mono-p-methyltrityl chloride following the same procedure that many laboratory texts give for the preparation of trityl chloride, merely substituting an equimolar amount of p-tolyl chloride for phenyl bromide. The chemical shifts of the methyl protons were determined from the spectrum of the equilibrium mixture in methylene chloride and the spectrum of mono-pmethyltrityl chloride. The chemical shift of the mono-p-methyltrityl hexachlomantimonate was supplied from the Young article, although this compound could have easily been gvnthesized.2 From these data, our students calculated an equilibrium constant for eqn. (1) and, of course, the free energy difference between the trityl and mono-p-methyltrityl carbonium ions.
E., Sandel, V. R., and Freedman, H. H., J. Amer Chem. Soe., 88.4522 (1966). L. F., and Fieser, M., "Reagents forOrganicSynthesis,"John Wiley& Sons, Inc., New York, 1967, p. 42.
1 Young, A.
2 Fieser,
Michigan Teehnalogkal University Haughton, Michigan 49931
David H. Kcnnv Vernon K. Sandel
Rruec R. O ~ t e r b y
Volume 51, Number 4, April 1974
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