In neutral or acidic solution trimethyl phosphate undergoes hydrolysis with carbon-xygen bond fission, and acid-catalysis is not observed. The second-order reaction with hydroxide ions, on the other hand, proceeds with phosphorus-oxygen bond fission. Similarly triphenyl phosphate is hydrolysed with phosphorus-oxygen bond fission in alkaline solution. The latter reaction, which has a low activation energy (ca. 10 kcal. mole-l), has been shown not to involve isotopic exchange a t the phosphoryl oxygen atom. SIMPLE trisubstituted phosphate esters are known to be relatively alkali-labile : lP3 with the exception of an investigation of trimethyl phosphate

The hydrolysis of monoaryl phosphates shows acid catalysis only if an electron-attracting substituent (e.g., NO,or CH,CO-) is present in the aryl group. There is then a rate maximum at a strong acid concentration in the range 3 -6 ~. A typical rate maximum a t ca. pH 4.0 due to hydrolysis via the monoanion species is observed in all cases. The reactions, both in aqueous strong acid and at pH 4, proceed by phosphorus-oxygen bond fission.Rate maxima in aqueous strong acid are also observed in the hydrolysis of diphenyl and triphenyl phosphates :the reactions atso proceed by phosphorus-oxygen bond fission. The maxima do not arise from complete protonation of the substrate but are apparently the result of a powerful negative electrolyte effect. EXPERIMENTAL Materials.-Disodium phenyl phosphate. A commercial sample (B.D.H.) was purified by recrystallisation from aqueous alcohol.

Rate data for the hydrolysis of 111ethy1, ethyl, n-propyl, n-butyl lnethanesulphonate have been determined over sufficient range of temperature to pcrlnit an evaluation of the corresponding enthalpy, entropy, and heat capacity terms for the activation process. Changes in these derived parameters are related to characteristic dilferences in solvation and mechanism as a consequence of changes in the struct~lre of the hydrolyzing ester.Our ltnowledge collcerning the details of hydrolysis of simple allcyl esters of strong acids in water have been extended recently by a study of isotope effects arising from changing the medium from Hz0 to D2O (1, 2, 3) and from various secondary deuterium isotope effects (4,5, 6). In principle, further information should be made available from a consideration of the changes in the derived parameters, AH*, AS*, ACT,* but the theoretical basis for the interpretation of changes in these parameters suffers from such uncertainty that to an important degree, relating changes in these paraineters with changes in structure amounts to no more than confirming changes anticipated from other evidence of mechanism. The greatest uncertainty would appear to lie in AH* and AC,,*, differences in As+ being somewhat easier to interpret. Our purpose in this paper, therefore, was to determine the rate data for hydrolysis of a series of alkyl metl~anesulphonates over sufficient range of teinperature that reliable values of the enthalpy, entl-opy, and heat capacity of activation could be determined, and to show how changes in these parameters are consistent with the solvation of sulphonates and with changes to be expected in mechanism from lengthening the carbon chain. EXPERIMENTAI, METI-IODS A N D i\/IATERl.AL,SThe methanesulpho~lic esters used in this study were synthesized by conve~ltional methods (7,8). The physical constants are given in Table I. T h e method used in determining rates of hydrolysis, in temperature measurement and control were identical in all respects with those used in determining ltinetic data for the be~lzenesulphonates (9)

No abstract

The application of kinetic methods to nucleophilic aliphatic substitution is a familiar topic in introductory courses on reaction mechanism. A suitable experiment was required to demonstrate to a group of thirty or more students the use of conductivity measurements as a method of following the rate of the reaction RX + R'3N -RfiR'3 + XĨ n the course in question (which is of 12-wk duration) a maximum of 5 hr practical work per week could be undertaken. Any reactions studied have to be fast enough to enable an infinity reading to be taken in one day; otherwise, if they are left until the following week uncertainties may arise from evaporation of solvent and reversibility. With the Menschutkin reaction, pseudo-first order kinetics could be secured by using an excess of amine and Guggenheim's method applied to the experimental results. The need for an infinity reading is thereby removed.The reaction chosen COCH.Br was studied in methanol (20-45°C) and fulfills the conditions imposed by available time.The advantages of studying this system are as follows:(1) It is a clean reaction to handle and can be considered irreversible in the time needed for measurements. (2) The kinetic behavior is reproducible.(3) It illustrates the application of the Guggenheim method for obtaining a rate constant. (4) It shows the difference between apparent kinetic order and mo-