chemica exemplified Auto Windows-Strong
but Self-Destructing
Illustrating principles of metastable equilibrium
Corning Glass Works The glass in auto windows is much more difficult to break than ordinary window glass, but when it does break it's in a way which minimizes hazards to persons in. the car. Car windows don't shatter like ordinary window glass, rather the whole window disintegrates into tiny fragments. One flaw or crack spreads by a chain reaction through the entire panel, and potentially lethal large pieces are converted to much less hazardous gravel-like material. It can be most puzzling to the car owner when, as occasionally happens, he does not know what caused the original crack and discovers that his car window has become an opaque mass. To make auto window glass shatterproof, the glass is chill-tempered. The surfaces have been instantaneously cooled from high temperature to "freeze in" the high temperature expanded equilihrium state of the molecular structure near the surfaces while the interior glass cools more slowly and shrinks more. The window then permanently retains stored energy, in the form of balanced me.chanical stresses-compressive a t the surface, tensile in the center. Prestressing imparts mechanical strength to the glass; it is a mechanical effect widely used in structural materials. The prestressed glass is in a state of metastable equilibrium. It loses its stored energy and attains stable equilihrium only by breaking into many small pieces if the surface becomes flawed, or by reheating t o relieve the stresses by viscous flow. The metastability produced by chill-tempering is not the only metastability involved in the chemistry of glass. Even the annealed stress-free glass in ordinary windows is in a metastahle equilihrium. Although it remains in the classy state for thousands of years. window alass at true equilibrium would he a mixed c&lomerac of silicate crvstals. The amorohous. alassv state retains more e n e m than the sum of the energies o i i t s crystalline components; hut it remains permanenth metastable because it is so rigid at ordinary temperatures that the molecules are restrained from rearraneinz to the stable crvstalline eauilibrium state.
Prince Rupert's Drops Suggested by Pmfe.wor Emil Slou,in.~ki, Macalester Colle~e After a career in military service, Prince Rupert (Count Palantine of the Rhine and Duke of Bavaria, 1619-82) devoted the last years of his life to art and science. Among
his discoveries was the fascinating glass novelty now named Prince Rupert's Drops. They are tempered, tear-shaped drops which are very strong, resisting even a hammer blow. Yet when the tip of the drop is broken off the entire drop explodes into harmless granules as the built-up stresses are relieved. Directions for making them are available in some books on glassblowing;' they provide an interesting demonstration or student project.
Stereospecificity in the Palm of Your Hand iiiusfrafing the reacfion between two optically active agents Contribution by Professor Richard S. Treptow, Chicago State University Although the two enantiomers of an optically active compound are aenerallv said to he indistineuishahle with regaid to chemical reactivity, there is a special class of reactions for which this is not the case. These exceptions to an otherwise simple rule are most commonly encountered in biochemistry and involve the reaction of one of the enantiomers with a second reagent which must also be optically active. This second reagent is frequently an enzyme. Because such reactions distinguish D and L isomers they are classified as stereospecific although the term "diastereomeric" would better emphasize their characteristic of involving two optically active reagents. Within everyday experience can he found many physical analogies to the stereospecific reaction. Consider, for example, a reaction possible only between two right hands
-
D-hand + D-hand
Handshake
While D-hand and L-hand are identical in many respects, they become clearly distinguishable when confronted with the stereospecific task of shaking another Dhand. There is also the example from the machine shop D-bolt + D-nut -Fastening device D-bolt and L-bolt are indistinguishable in most of their reactions, rusting in air or dissolving in acid, hut only one of them interacts with D-nut. In these physical analogies the course of the reaction often is used as the basis for designating the enantiomers. Thus, D-nut is by definition that enantiomer which reacts with D-bolt. From this it should not be concluded that a D isomer can never react with an L. Consider, for example, the stereospecific palms-together reaction possible only between D-hand and L-hand. In the chemical case we have enzymes, composed entirely of L-amino acids, acting as catalysts for D compounds. The phosphorylation 'Hammesfahr. J. E., and Stong, C. L., "Creative Glassblowing," W. H. Freeman and Co., San Francisco, 1968, p. 13. Volume 50. Number 2 , February 1973
/ 131
of D-ribose by rihokinase illustratks the point. Whether physical or chemical in nature, the course of a stereospecific reaction depends entirely upon the proper fitting together of the two optically active reactants in some sort of complex. The labels D and L (or R and S) are used to indicate molecular configuration; only if the transition complex is known in detail does this knowledge enable a prediction about the course of a stereospecific reaction.
132 /Journal of Chemical Education
As an aside, i t should he mentioned that L-handed people encounter stereospecific problems living in our predominantly D-handed world. It is awkward for them to use conventional D-sewing scissors or t o shift gears in our D-automobiles. Their problems, however, are much less severe than those which would he encountered by a hypothetical man having a D-amino acid enzyme system in his attempt to utilize our common D-glucose as a body fuel!
