provocative opinion A Walk Through Snow Charles E. Dills California Polytechnic State University, San Luis Obispo, California,93407 I can hear them say 'Yeah, I know, you had to walk three miles through snow to go to school!" I am Emeritus and fromNorth Dakota, so they are right. But having seen a lot of time does not mean one is "old-fashioned or out of touch. The teaching of chemistry has changed enormously, and many of the changes were good ones. I t would be Panglossian to believe they were all for the best. One change I am observing does not seem to be for the best. Budgets, large class sizes, and potential liabilities have contributed to a change that is creating a potential hazard. The logic being, I suppose, that hazard is all right as long as it isn't on our turf. I'm an organic chemist. There is a trend today toward micropreps. I t saves money and time and minimizes the waste-disposal problem. But what happens when our students go out into the big cruel world and are expected to make a mole of something? Without experience, they are walking time bombs. They can follow a preparation from Organic Syntheses and probably be all right. But if they find a prep in the literature for a half gram and scale it up to 100 grams, they may be in serious trouble. The problem of scale is well-appreciated by industry, hence the existence of pilot plants. When one scales up a reaction and attempts to choose appropriately sized equipment, a n unfortunate situation exists. The mass of material increases by the cube of the linear scaleup factor. Because the heat produced is proportional to the mass, this also increases by the cube. But the area through which that heat is to he radiated only in-
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creases by the square. There comes a point at which the heat cannot be radiated fast enough. The temperature will rise. and a n ex~losioncan ensue. I learned thi's the hard way in graduate school by runniw a reaction that was on the brink. I had done it m a w t&s on the 1- or 2-g scale, &d then tried 50 g. In spite df my heroic attempts to control it, the content erupted all over the hood-only! Fortunately, there was no explosion. I was just plain lucky On the other hand if you take a large-scale reaction and cut it down, you're likely to fmd that, instead of having to cool it, you may have to heat it to achieve the proper temperature. This problem of scale must be taught, and it should be experienced.Students should be allowed a t least one largescale preparation, along with an intensive discussion of the problem of scale. Perhaps the same reaction could be done both micro and macro. The problem of scale is well-known and is totally pervasive in life. J. B. S. Haldane wrote an essay, "On Being the Right Size", in 1927. It can be found in many compendia in any library. In this essay Haldane gives a reason for the difference in the geometries of an elephant and chicken bones. He also explains why a mouse can fall down a mineshaft unharmed, while a horse will splash. He explains why a butterfly, which has nothing to fear from gravity, finds surface tension to be lethal. It should be required reading for chemists and engineers.
