starch end-point \\ith 0.U5 N sodium thiosulfate. The sealed ampule technique \vas used and reactions werc quenched by placing ampules containing aliquots in an icebath. Aliquots were measured a t 25' with calibrated pipets and 5-ml. calibrated microburets were used for titrations. The titer changed about 1.5 ml. during the 3 5 4 0 % of completion to which the reactions were carefully followed and individual titrations were reproducible to 1 0 . 0 0 4 ml.The rate of reaction of cyclohexyl bromide with iodide ion was followed by potentiometric (Beckman model G potentiometer) titration of 5-ml. aliquots in 50 nil. of water with 0.15 N silver nitrate.Second-order elimination rate constants, k., were calculated from the equation where n is the initial concentration of iodide ion, b the initial dihalide concentration, t the elapied time, and $ the fraction of dihalide that has redcted (z.e., 4 = [ 1 3 -] / b ) a t time t . The values of k2 were corrected for solvent expansion by multiplying by the ratio dZ5,/dt,. The \ d u e s used were: d2j4 0.7870 (by interpolation), dSo4 0.7355, dloo4 0 714 20The values of k2 obtained in this way decreased as the reactions progressed. The rate coilstarits were linear with time to about 307; reaction and initial rate constants were determined by extrapolation to zero time by the method of least squares. Data for typical runs are presented in Tables VI and VII. The second-order constants for the s N 2 reaction of cyclohexyl bromide with iodide ion vxre determined from the relationshipwhere a is the initial concentration of iodidc ioii and of cy-( 2 0 ) Landolt-Btrnstein "Tabellen," J. Springer Verlag, Berlin, ~~ 1923, Vol. I, p. 278.

Experiments having outcomes unknown to the student heighten student interest and motivation and provide a semblance of real experimentation. We have designed five experiments that present puzzles in orientation, stereochemistry, and extent of reaction.The puzzles involve the generation of bromine from hydrobromic acid and potassium bromate in acetic acid as described by Schatz (1). The equation for the reaction isThis method produces controllable amounts of bromine and is convenient and safe.

The synthesis of 1, l-diphenyI-2-fluoroethylene is reported. This vinyl fluoride is converted to 1, l-diphenyl-2-ethoxyethylene by sodium ethoxide in ethanol. At 99.75°the kinetics are second-order, first-order with respect to each reactant, and the rate is 270 times faster than that of l,l-diphenyI-2-chloroethyIene. The results are consistent with an additionelimination mechanism.

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