A Simple Dynamic Stereomodel for the Interconversion of Enantiomers via a HighEnergy Achiral Intermediate Introduction of molecular chirality to the elementary student is often effectively made by the comparison of familiar chiral and achiral objects. The concept thus implanted is then extended ta the molecular level. Frequent comparisons are that of the body as a whole with right and left hands, a tennis racket with a golf club (usually dextral because of the odd stereospecificity of nature as applied to homo sapiens), and the comparison of a knife (chiral) with fork and spoon (both achiral). Further comparison of simple familiar objects with more complex molecules can readily be made with springs and other helical species such as shaver cords. Interestingly, in these eases, the helix configuration appears to be quite arbitrary-cords on my desk leading to the telephone and to the dictating machine are wound with opposite chiralities. An important additional concept requiring implantation in early stages is that interconversion of molecules of opposite chiralities is an endo-energetic process and that this process frequently but not always occurs uio an achiral intermediate which reverts with equal probability to either enantiomer. An extremely simple model which illustrates this latter concept is the type of paper clip1 illustrated (Fig. 1).This elip, consisting of a long arm crossed by a short arm exists in right- (A) and left-handed (C) forms.' By careful twisting of the clip between thumb and forefinger, the two ends can be made to rest against one another in one plane. In this arrangement (B)the clip is echiral and the work (activation x) (Fig. 2) required to twist the clip into this planar configuration can clearly be felt through the fingers. If the elip in this metastable planar configuration is now dropped onto a table, the jolt is enough ta surmount the energy barrier (activation y ) , and the clip reverts ta one d the enantiameric arrangements.
Figure 1. Paper clip in c h i d and achiral forms.
L Figure 2. Energy profile for converiing paper clip from chiraltoachiral form.
This very simple model can be directly related to chemical reactions with precisely the same energy profiles. Typical examples are the solvolytic racemization of a chiral tertiary halide eqn. (1) and the base-catal~sedracemization of a ketone bearinga chiral a-carbon atom (eqn. (2)).
1 Spring washers or Fenske helices exhibit the same type of chirality but chiral forms are not so readily intereonverted and they cost more! 2 Assignment of chirality is arbitrary but a suitable convention can, no doubt, be agreed upon.
University College of North Wales Bangor, Caernarvonshire, United Kingdom
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Journal of Chemical Education
