J. K. M. S a n d e r s Universitv Chemical Laboratory
1
Prochirality and the English Beer ( 3 a ~
It is a relatively simple matter to convince students that chirality is important in chemical reactions. They can see that interaction of a right-handed glove with a right hand is different from that with a left hand. Generalization to diastereoisomerism, nmr non-equivalence or enzyme stereospecificity is then straightforward. Prochiralityl presents more problems. I t is not easy to see how the apparently identical methylene protons, HAand HB of ethanol (Fig. la) can he different and distinguishable. Even if it is pointed out that the apparent chirality of CI viewed from HA (Fig. l h ) is the opposite of that seen from Hg (Fig. lc), the chemical implications are not obvious. I suggest that the English beer glass2 provides an ideal model for illustrating the concepts of prochirality and asymmetric induction. The glass is clearly prochiral (Fig. 2) with a plane of symmetry through the handle apparently making both halves of the rim equivalent. However, when a chiral "molecule"-the hand-picks up the glass two complexes can I Far a comprehensive discussion see Bently, R. "Molecular Asymmetry in Biology," Academic Press, N.Y. and London, 1969, Vol. 1, p. 148; for a history see Bentley, R., Nature, 276,673 (19781. 2Coffee mugs also make good models, but in a beer-oriented student society they provoke less enthusiasm. Goering, H. L., Eikenberry, J. N., Koermer, G. S., and Latimer, C. J., J.Amw Chem. Sac., 96,1493 (1974);McCreary, M. D.,Lewis,D. W., Wernick, D. L., and Whitesides, G. M., J.Amer. Chem. S n c , 96, 1038 (19741. Pirkle, W.H., Muntz, R. L., and Paul, I. C., J. Amer. Chem. Sac., 93, 2817 (1971). "~Nicol, D. D., and Rycroft, D. S., Tetrahedron Letters, 2173 (1977).
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Figure 1. (a) The ethanol molecule. (b) The view of C1 from HA.(c)The view fmm HB.
he formed: the stable one (Fig. 3) and the unstable one (Fig. 4). In the stable complex the thumb always touches the same side of the glass. I t can touch the other side only hy forming the unstable complex. We have therefore efficiently distinguished the equivalent sides of a symmetrical "molecule" by binding it to a chiral site. Drinking from the stable complex is always from the same side of the rim and therefore the chemical reaction is also stereos~ecific! N,gt(. thilt h i d we uied our arther hand to pick up the glass, rhe srahle com~lexwould have the ormositc chin~lits, .. and we would drink frbm the other side of &e glass. Once these ideas are understood, the principles of asymmetric induction in synthesis are obvious. Two simple examples in other areas of chemistry are (1) Isoprapanal is proehlral and its two methyl groups are therefore chemically equivalent in an nmr sense. However, they can be rendered non-equivalent in principle by interaction with c h i d shift reagent,3 chiral solvent,' or ~yclodextrin.~ (2) In oxidizing alcohols to aldehydes, dehydrogenases stereospecifically remove one proton of a pair. In principle, it should be nossible to observe enzvme-indueed asymmetry in the nmr spectrum of a bound prochiral substrate, but to my knowledge this has yet to be achieved
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Figure 2. glass.
A
prochiral
beer
Acknowledgment I am grateful toPatricia and John Stewart for the loan of a elass Figure 3. me stable complex between Figure 4. The unstable complex between a hand and Louise Sanders for the loan of a hand and a glass. and a glass. a hand.
594 1 Journal of Chemical Education
