Martin J. Nye University of Guelph Guelph, Ontario, Canada
Wooden Models of Asymmetric Structures
Wooden bloclcs were const,ructed by s a w ing a length of ivood as she\\-11 in Figure 1. The actunl dime~isiorisarc a matter of personal choice. A total of fifty bloclcs is sufficient for most. purposes. The resulting blocks are of tn-o types, each being the non-superimposable mirror image of the other (see Fig. 2). One type \vas given a light staili finish and the other a darker stain finish so that they were easily distinguishable. Applications
The TI-oodenblocks n-ere conceived to represent molecules of a pair of enantiomers, and w r e constructed so that they may be rcildily stacked toget,her to show crystal structure. Such crystal models can theu be used to illustrate discussions on the folloning topics: I4cmcrrtst.y aspcets of crystallogt.aphy, c.g., et.vstal lattices, unit rclls St~.urturcuf mrcmic motlilieatians, i.e., compounds, mirtures,snd solutions Diffcruncrs in nrulling point, solubilities, and densit.v1 hetaeen oor 1.- compounds and rnecn>ir modifications.
Just one example of the usc of the blocks is illust,rated in Figurcs 3 and 4. Figure 3 shows a possible crystal structure of a DL-racemic compound. However when one attempts to construct a similar crystal structure using just n- (or L-) bloclcs, it is fouud t,o be impossible. Thus one must coriclude that a DL-racemic compound must havc a different crystal struct,ure from the n- (or L-) s u b ~ t a u c eaod , ~ further, if these t,wo substances have diffcrent crvstal structures., thev., n-ill also have different physical properties such as melting point, solubility, and detisity. A possihlc crystal structure for the D- (or L-) substance is sho~r-11 in Figure 4 (note that the struc'In a discussion of tho physical prapcrties of optical isomers, a popular. tt:xtl~ook I l l a e ~ ~J.~ I)., s , AN, C A S E R I ~ 31. , C., "Basic P1.inaip1t.sof Chganic Chomist~~y," IV. A. Benjsmin, Inc., Ncw l m k , 1964, p. 5741 statcs that "right,- and lcft-handed ohjccts usually can be pnclwd in a box batter than all right- or all lefthanded objccts." If "better" means "more tightl,y" then this statcmmt implics that a. racumic compound should havc x hipher. dmsity than the u- (or L-) iorm, and this is thc hchavior which is actuitlly oha:rvcd for most compounds (Thcle is a small pel; wntage of cxecptions such ns glutamic arid). Ilowever, thc rrasouing hchirid tho concept is not substantiated using the woodm I h c k motluls, which in inct demonstmte the right- or Id-hsndrd ol,iccts c s n hc u d w d iust as tiehtlv ., ns a 50150 mixturc of hoth (SLY Figs. 3 and 4), and hence a mot.
