C5 by a bond up from the carbon chain and to CI by a hond pointing below the chain as illustrated in V; a t best this is a misleading representation. The solution to the oroblem. develo~edin 1950 bv one of us (TSW) following ~ h e l a n d , bis to L v e the C ~ - O Hin I1 ~ o i n t i n ninto the olane before cvclization. This can be done by usinithe permitation convenkon, which Whelande defines in the following way. Any operation that is equivalent to an add number of single interchanges of atoms or groups attached to the same c h i d atom is considered to invert the configuration of that atom, but any operation that is equivalent to an even number of such interchanges at the same atom is considered to leave the configuration unchanged. Interchanging the hydroxyl and hydrogen on Cs in 11, gives VI, a diastereoisomer of I1 in which the hydroxyl group is now pointing into the plane of the paper. Then exchanging the hydrogen and methylhydroxyl groups on C5 in VI gives VII. Structure VII (the result of 2 interchanges on Cs in 11) is identical in mnfiguration with Il hut C s has been rotated to the right about the(?,--C~ . .hond through 120°.'l'he hsdroxvl groupon Cs in VII is now pointing into'the plane of the pap& and is in the correct orientation for cvclization to VIIIa and VJllh (as can be confirmed by examining models). The relation between 11, VII, thecyclicstructures VIlIaand VIIIb7nnd the Haworth forms Ia and Ib can easily be seen? groups to the left of the carbon chain in VII and VIII are up in the Haworth structure; those to the right are down. In drawing VIII as well as the corresponding structures XI and XIV we place the ethereal oxygen (which is behind the carbon chain) to the left. The conventional usage (as shown in IV) would place the oxygen to the right as in IX. Structures VIII and IX represent the same molecule; we prefer VIII because simply turning the diagram to the right though 90° shows a t once its relation to I. The furanose forms of glucose (and other aldohexoses) can be similarly derived by making the interchanges of groups on C4. Using this approach it is easy to deduce the Haworth structure corres~ondingto anv oDen chain monosaccharide. ( ~ the ) pyranose form, For example to convert ~ - f m c t b s ~into Cn and Cz must be connected through oxygen. As CRis achiral, no interchanges of groups are needed i d XIa and XIb may he written directly. The corresponding Haworth forms are XIIa and XIIb. T o derive the fructofuranose structures, two interchanges on Cs in X are made to give XIII, and hence the corresponding cyclic forms XIVa and XIVb. These lead to the Haworth forms XVa and XVb. The ooerations described above ~ r o v i d ea simnle wav of converting an open chain sugar structure to the correct cyclic structure. We have not found anv undereraduate textbook that uses this method (cf, ref. (2), 6.24) alihough a few books show a ~ersnectivediaeram of the onen chain structure and point o i t that the rotacon of Cs aboit the C4-Cs bond must precede cvclization. We9 have used the a ~ o r o a c hin lectures for 30 ye& and find that students gr& it without much trouble, and have little difficulty in converting open chain into Haworth structures.
Reference in footnote 5, Sec. 7.7.
'The doned lines in sb'ucubes, VIII, IX, XI, and XIV indicate that the
-
oxvaen is behind the chain of carbons. , Whelandk book (footnote 5 ) , ftrst published in 1949, pointed out IMequivalence of I and Vlll but derived Vlll from Uw inmrrect structue
.IV..
9TSW 1950-1962; DMSW since 1954; MMW since 1972,
970
Journal of Chemical Education
Some Misunderstandings about Millikan's Oil Drop Experiment Mary E. Castelllon 340 Briar Bras Rd. Slamlord, CT 06903
John C. Bailar Jr. University of Illinois Urbana, IL 61801
Nearly all texthwks of general chemistw contain descriotions of the famous experihent by which ~ i l l i k a nmeasured the charge on the electron. However, there seems to be widespread misunderstanding of the experiment. Most of the books that describe the experiment in any detail state that the ionization of the air in the apparatus caused the liberation of electrons and that these electrons were picked up by the oil dronlets. This was not the case: in Millikan's discussion of the experiment, it is stated clearlithat ions were attached to the droolets ( I ) . Each oil droplet could hold several ions (anywhere from one to a hundred and fifty, hut usually from six to seventeen). Knowing the density of the oil and the rate of fall of a given drop when there was no charge on the olates. Millikan was able t o Ealcu~atethe weight of the drop: ~ h e the h plates were charged (3000-8000 V, but usuallv close to 8000 V). the drodet eithir rose or fell, depending onihe chargeon the iuns which were attached tu it. The charge on the plates and the rate uf motion, up or down, could he used ta calculate the charge, and hence the number of ions, on the drop. .Most of the attached ions were negative, but some were positive. Several textbooks state that the air in the apparatus was ionized by X-rays or radium. 'Millikan showed this oossibilitv. most of . but, in ~ ---~ - - his --experiments, he relied on the slight, natural ionization of air. Using radiation be learned, by practice, that he could force either positive or negative ions onto a droplet (by its proximity to one plate or the other). On a t least one occasion, a single droplet was observed as i t rose and fell for four and a half hours. In his earliest experiments, Millikan used droplew of water or alcohol, but found them to he unsatisfactorv hecause of their volatility; in a few experiments,, he used droplets of metallic mercury, hut these fell so rapidly that it was difficult to time them precisely. In other experiments, he used watch oil, castor oil, and glycerine, but the material which was employed in the famous series of experiments was gas engine oil. It is sometimes stated that Millikan's apparatus was exceedingly simple. While it might he thought so in comparison with some of today's equipment, it was, in reality, quite sophisticated. Care had to be taken to maintain a constant temperature, to keep theair in thechamber stagnant so the droplet under observation would not drift sideways, to keep the charge on the d a t e s constant. and to time the rate of rise and fall very precisely. Millikan's maior DaDer on this subiect . (1) . . should be studied by everyone who d&ts to improve his writing of scientific reports. Not only does i t describe elegant ex~erimentalwork. h;t dso it is wriiten in beautiful, precise ~ n i l i s h~. r e l i m i n a j reports were published earlier t2.31. In these Daoers. Millikan not only reported measurrments of the charg&un the electron hut also currected Stokes' Law and discussed obsewatims of the kinetic energy of a molecule which provided proof of the kinetic molecular theory. ~~
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Literature CRed
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