Photochemical Catalysis of an Aliphatic Substitution Reaction
( A then Ed,
Suhmitirrl 1 1 1 ~ : I-Iarold Hart, Michigan State University, East Lansing Chmkrrl b?l: D. A. Driesbach, Hiram College, Hiram, Ohio PREPARATION
wrap an.8-in. test tubc completely with friction t,ape, being certain that there arc no gaps for light to leak in. Mount two 8-in. test tuhes, onc clear and one wrapped, side by side on a ring stand. Above each, mount a small (30 ml) long-stemmed addit,ion funnel. On a separate ring stand, mount an ordinary sun lamp (Westinghouse Type RS, 275 watt), so that the middle of the lamp is at a level about one inch above the bottom of the test tubes. Prepare a 5yo (by volume) solution of Bra in CC14. Moist blue litmus and/or a bottle of concentratrd ammonia should be available. DEMONSTRATION
Place 25 ml of hexane in each test tube, and in each additional funnel place 5 ml of Brz solution. Bring the sun lamp within 1 4 inches of the tube, turn the lamp
A Demonstration Answer to a Stereochemical Puzzler
on, and add Brz rapidly by drops (total time 2-3 min) simultaneously to each of the hexanr solnt~ions. The red color will disappear as rapidly as the Brpis added to the unwrapped t,uhe. Evolution of HBr can be demonstrated by breathing out, across the mouth of the tube, by t.he moist blue litmus, and by hringing NH8 near. After each addition is complete, pour thc contcnt,~ of the wrapped tube into a clear vessel; the solution will be deep red-brown, and will not show HBr evolution by the above test,s. REMARKS
The sun lamp should not bc too close to the hexane solutions, or the latter will get hot; but ultraviolet radiat,ion is dissipated rapidly with distancc and is absorbed by the Pyrex glass. I t is therefore necessary to strike a balance ( 1 4 inches) in positioning the lamp.
( A then
m- 1
Submitted by: Harold Hart, Michigan State University, East Lansing, Michigan. Thc assistance of Mr. Roger Wheaton, who drew the figure, is appreciated. PREPARATION
A drop of oil is placed on the threads of a clamp holder
/="
(see figure; CENCO 12264 or one of similar design will do) to ensure that the thumb screws turn freely. The ends of the clamp holder or the heads of the thumb screws may be painted different colors for identification. One thumb screw should be turned all the way in; the other extended nearly all the way out. INTRODUCTION
St,udents sometimes ask, when studying a molecule with an asymmetric carbon atom, why the different orientations of the molecule in solution do not result in cancelling the rotation of plane-polarized light. Why does not molecule B, located further down the ~olarimeter tube and oriented 180' to molecule A, reverse any rotation induced by A? This demonstration is designed to provide an answer using a mechanical analogv. DEMONSTRATION
The student is shown a clamp holder, the ends of which consist of right-handed threads oriented a t
180' to one another (corresponding to two asymmetric molecules A and B, similarly oriented), ~h~ thumb screws correspond to plane-polarized light which must pass through the molecules (from one end of the clamp holder to the other; see arrows in the figure), ~t is obvious from the figure that in order to pass through must be turned clockmolecules A and B the thumb wise for both molecules. Therefore, despite the different molecular orientations, the light (thumb screws) will be rotated in the same direction. Journal of Chernicol Eduealion
. April l96l
