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.

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The pyrolysis of cis-and trans-l,2-dimethylcyclohexyl acetates leads to the formation of large amounts of 2-methylmethylenecyclohexane. However, the cis isomer gives 5 1 0 % 1,2-dimethylcyclohexene which is specifically absent from the products from the trans-acetate. In comparison experiments it was found that I-methylcyclohexyl acetate gives about 86% methylenecyclohexan e and 14% 1-methylcyclohexene. The isomeric 1,2-dimethylcyclohexyl bromides undergo second-order elimination with sodium hydroxide in 98y0 ethanol at rates which differ by a factor of twelve. The trans structure has been assigned to the fast isomer. The trans-bromide is stoichiometrically converted to 1,2-dimethylcyclohexene under conditions favorable for second-order elimination, but the cis isomer gives largely 2-methylmethylenecyclohexane.

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