The rate of hydration of ethylene in dilute aqueous perchloric acid has been measured in the range 170-190 "C a t 100 bars and 100 to 3 000 bars a t 180 "C. The energy, entropy, and volume of activation are 33.3 =k -1 kcal mole-', -5.7 f -2.5 cal deg-1 mole-', and -15.5 f -1.5 cm3 mole-' respectively. The volume of activation shows that a t least one molecule of water is strongly bound in the transition state.The acid-catalyzed hydration of olefins has been much studied in recent years, particularly of relatively highly substituted olefins (see ref. 1 for references). Little has been reported on the hydration of the parent compound ethylene, probably largely because its hydration rate is fast enough for convenient study in dilute aqueous acids only a t temperatures above 150 OC. This is well above the boiling point of aqueous acids, and consequently the measurements can be done only under pressure.The hydration of ethylene is a reaction of considerable importance both theoretically, as the simplest olefin hydration, and technically, as a manufacturing method for both ethanol and (by the reverse reaction) ethylene. The hydration of ,propylene and isobutylene has recently been studied in this laboratory (I), and the same techniques have now been applied to ethylene. T h e volume of activation for propylene and isobutylene proved of considerable value in the determination of the mechanism, and consequently the effect of pressure as well as temperature has been studied. E X P E R I M E N T A L MaterialsEthylene was Phillips Petroleum Co. research grade, n-heptane was Anachemia Chemicals Ltd. pure grade, and other chemicals were reagent grade. Analysis of EthanolThe hydration of ethylene was followed b y estimating the alcohol produced. T h e method of analysis was reported e: . ;ier (1). Kinetic MeasurementsThe thermostat bath was a n aluminum block 14 in. long and 10 in. in diameter, with a hole of 4 in. in diameter and 12 in. long bored centrally to hold the reaction vessel. The outer surface of the block was covered with layers of asbestos paper and wound evenly with two coils of resistance wire, one to serve a s the main heater, and one to serve as the intermittent heater. The block was insulated on all sides b y transite or vermiculite. The temperature of the block was measured b y a platinum resistance thermometer inserted in a hole in the block. I t was controlled by a second platinum resistance thermometer connected to a Mueller 'N. R.C. No. 8476.

The effect of pressure LIP to 3 lcbar on the rate of the acicl-catalyzecl hydration of propylene and isobutylene has been measured. 'The volumes of activation are: for propylene a t 100' C -9.6 A -1.0 cm3 mole-', and for isobutylene a t 35" C -11.5 A -1.0 cm3 mole-'. The effect of temperature in the range 90-120" C 011 the rate of hydration of propylene a t 100 bar was measured. At 100" C the Arrhenius energy is 27

The effect of pressure on the rate of spontaneous solvolysis of t-butyl chloride in ethanol-water mixtures in the range 0 to 40% v/v ethanol at 0 -C has been measured up to a maximum pressure of 3 kbar. The results have been examined from several points of view and the conclusions are briefly described in the summary.

The extension of the concept of isokinetic conditions into the dimension of pressure is discussed. The use of the enthalpy and entropy of activation at constant pressure and of the energy and entropy of activation at constant volume to characterize activated processes is discussed, and it is concluded that neither set can be rigorously justified as easier to understand, but that the constantvolume parameters are probably to be preferred most of the time. The effect of pressure up to 3 kbar on the rate of the acid-catalyzed hydrolysis of methyl acetate and ethylene oxide in acetone+ water mixtures has been measured. The constant-volume parameters of activation for the acidcatalyzed hydrolysis of methyl acetate vary with solvent in a less complicated way than the constantpressure parameters, the minima in the constant-pressure parameters not being present in the constant-volume parameters. The existence of the minima in the constant-pressure parameters is ascribed mainly to the large change of thermal expansivity when an organic liquid is added to water.The effect of solvent on the rate of hydrolysis of methyl acetate is dependent on the pressure, and reverses sign at about 2 kbar. The implications of this observation in the determination of reaction mechanisms is discussed.

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