A Simple Demonstration of Conformational Equilibrium The existence of stable (staggered bonds) and unstable (eclipsed bonds) conformations of, for example, the ethane molecule, is well documented in most advanced organic chemistry texts, often being described with the aid of a diagram plotting potential energy against torsional angle. A consideration of the butane molecule reveals stable conformations of differing potential energy depending on whether the methyl groups on Cp and Cg are syn-clinal or anti-periplanar. The evidence commonly cited for the existence of stable conformations includes dipole moment measurements, the result of infrared, Raman, microwave, and nuclear magnetic resonance spectroscopy and X-ray and electron diffractions studies.' Of particular interest is the effect of intramolecular hydrogen-bonding on the infrared spectrum of ethane-1.2-did, but the practical application of this is hindered by the low solubility of the d i d in the most suitable solvent, tetrachlorowith its greater carbon content is more soluble and can conveniently methane. However, pinacol(2,3-dimethylbutane-2,3-diol) be used to illustrate eonformational equilibrium. The infrared spectra of solutions of pinacol in tetrachloromethane with molarities of 0.02 M o t 0.2 M a r e scanned in the region 3800-3200 cm-'. A variable path-length cell should be used to ensure that the same number of absorbing molecules is present each time, e.g., 1mm path-length for the 0.2 M solution increasing to 10 mm for the 0.02 M solution. The results are shown in Figure 1.
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Figwe 2. Stable mnformationai isamrs of pinacol.
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The broad band, maximum wavenumber 5450 cm-1, illustrates the presence of intermolecular hydrogen-bonding, being due to the stretching vibrations of the weakened O-H bonds. This hand is clearly concentration denendent. Mare interestine, however. is the -"-o+ +2M **a occukence of two sharp peaks a t 3610 em-' and 3 5 6 5 a confrming ~ ~ Figure Infrared Of solutionsOf in letra- the presence of the different stable confornational isomers of pinacol, (0.20 Mto 0.02 4. chlw-thane I and lIlllI resoectivelv.. deoicted in Fimre "~ 2. Weak intramulecular hydrogenbonding in the equally-abundant n n ~ l m w t i u n a lenantwmers I1 and 111 results m n slight weakening of the 0 H bunduand hence asmall lwvrring in w a v r n u d w fur the stretch~ngvibration compartd wrth the iree 0 H in I. The use o l such wwenumbrr diffrrrnces ss o n w s u r r c , i rhc strength of the hydrugen uund has t w n inve~tigated.~,3 Z
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Eliel, Ernest L., "Stereochemistry of Carhon Compounds," MeCraw-Hill Book Co., 1962, pp. 124-137. Reference (I),pp. 254-5. Kuhn, L. P., J. Amer. Chem. Soe., 76,4323 (1954); 80,5950 (1958). D. A. Partridge Cambridgeshire College of Arts and Technology Cambridge, England
508 1 Journal of Chemical Education
