JUNE, 1947
A SIMPLE APP,ARATUS TO DEMONSTRATE WALDEN INVERSION C. R. NOLLER Stanford University, California
THE
explanation to students of the way in which simple displacement reactions take place and of the mechanisd of Walden inversion is simplified greatly by the use of an adequate model. A simple device for this purpose that can be made by anyone and closely simulates what actually happens in a displacement reaction is illustrated in Figure 1. It consists of three wooden balls of different colors mounted as a rigid triangle and a fourth light-weight ball of tinned iron sheet attached by threads to the three wooden balls in such a way that when the threads are extended, it forms the center of a tetrahedron. It is held in this extended position by an Alnico magnet which forms the fourth corner of the tetrahedron. The magnet is attached by means of a saddle to a slotted bar running in a guide. At the opposite end of the slotted bar is an identical magnet mounted in the same.way. A center stop in the slot limits the movement of the slotted bar in such a way that as the magnets are moved r a p i u from one extreme position to the other, the metal ball swings from one magnet to the other, thus inverting the tetrahedron. If desired, the magnetsmay be made t o simulate spheres by filling in the center and rounding out the sides with Plaster of Paris. It is convenient to have tbree magnets, two of like color and the third of a different color, so that one may demonstrate racemization as well as retention of activity. Although suitable dimensions can he arrived a t readily, i t may he of value to describe the construction of a satisfactory model in some detail. Three wooden balls, approximately 2.5 cm. in diameter and differing in color (from a molecular model set), were made the apexes of an equilateral triangle by boring holes in them a t the proper angle and joining with 18-cm. lengths of 3/a2-inchwelding rod. A fourth ball was fashioned from the wall of a tinned food can by cutting from a flattened section an eight-petalled daisy, having petals 4 cm. long and 1 cm. wide, and bending the petals to form a sphere. The points of the petals were fastened
together by a drop of solder. Equidistant about the circumference of the sphere were soldered three eyelets made as small as possible from 22-gage copper wire. Black silk threads were tied to the eyelets and the ends of the threads plugged into holes in the three wooden balls, adjusting the lengths of the threads so that $hey held the metal ball in the desired position when they were extended. The rigid triangle was fixed in a vertical position by screwing two of the balls to a hoard 25 cm. wide and 35 em. lpng. The magnets were mounted on the slotted brass bar by saddle posts, the center of the magnets being 6 cm. above the base and the distance between the posts 29.5 dm. The slot was 16 cm. long and 0.5 cm. wide and the center stop 1 cm. long. It is well to have the slot long enough so that the magnets come almost in contact with the extended metal ball. Then they can be kept the desired distance away from the ball by mak'mg the cent& stop of the proper length. The slotted bar was limited to a linear motion by brass guides. All parts except the spheres and magnets were painted a flat black.
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Model to Dernonstrmt. Wdd.n I n . e .i o n
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