A. Liberlesl, A. Greenberg, and J. E. Eilers Foirleigh Dickinson University Teaneck, New Jersey 07666 Frostburq- State Colleqe Frostburg, Maryland The Mathematical Institute University of Oxford Oxford, England
I
Discussions of steric effects in organic chemistry usually emphasize the repulsive terms arising from interactions between nonbonded atoms or groups. Thus, groups of greater "bulk" or "size" tend to be equatorial in cyclohexanes, anti in 1,2-disubstituted ethanes, or trans in alkenes with respect to other bulky groups (I). It then comes as somewhat of a surprise that axial preference can be found (2), some 1,2-disubstituted alkenes are more stable in the cis form (3, 4), and that 1-bromo- and 1-chloropropane are largely in the gauche conformation (5). Other examples exist (ti), and since in each of these cases bulky groups prefer to be in closer proximity, there must be inadequacies in the bulk approach that is frequently employed to explain steric interactions. Cis- and Trans-2-Butene
The inadequacy of the bulk approach can be shown most dramatically by referring to the calculated energies of cis- and tmns-2-butene in different methyl-rotated geometries. The ground states of trans- and cis-2-butene were calculated by Radom and Pople (7) to be I and 11, respectively. However, each methyl group can be rotated about its local Ca-axis by an angle 9. Rotation of both methyl groups in each isomer by 60" leads to geometries I n and IV, and in these geometries, cis-2-butene (IV) is actually 0.54 kcal/mole more stable than trans-2-butene In. We indicate in Tahle 1 the cis-trans energy difference as the two methyl groups in each isomer are rotated. H
H'
H ' I
n
H' 111
IV
In geometry IV, we still have both methyl groups on the same side of the molecule, yet this cis-geometry is actually more stable than 111. Clearly, the bulk of the methyl groups is not a satisfactory explanation here. Simply rotating these groups leads to a favorable interaction in the cis isomer, an attractive steric effect, and this causes the methyl rotational barrier to be much smaller in this isomer (8, 9). Energy-Partitioning One may well wonder how to explain the cis-trans energy difference, and a partition of the total energy employed extensively by Allen affords a reasonable explanation of the facts (10-12). 'Joint contribution from the Departments of Chemistry of Fairleigh Dickinson University, Teaneek, N. J., Frostburg State College, Frostburg, Maryland, and the Mathematical Institute, The University of Oxford, Oxford, England. 676
/ Journal of Chemical Education
Attractive Steric Effects
Table 1. The Cis-Trans Energy Dilference in the 2-Butenes AEc,"~,~".
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Table 2. The Change in the Energy Components Upon Passing From Trans- to Cis-2-Butene AT A"..
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