!I!HE SYSTEM CH3*COrnO*CH3+H,0 O H 3 * 0 H + ~ C O * O H . 2723 CCCLXXII1.-Equilibrium in the System : CH;CO0O*CH3 + H,O += CH;OH + CH;CO*OH. By GEORGE JOSEPH BURROWS. FROM the results of experiments on the rate of hydrolysis of methyl acetate by acids in the presence of various amounts of wafer and acetone it appeared that the equilibrium between the ester wafer, alcohol and acetic acid varied considerably. Jones and Lapworth (J. 1911 99 1427) have shown that the equilibrium constant of ethyl acetate varied between 6 and 9 for solutions containing hydro-chloric acid in which the ratio of molecules of water to hydrogen chloride varied from 6.2 to 4.6. In the case of methyl acefate, Berthelot and P6an de St. Gilles (Ann. Chim. Phys. 1863 68 225) found that if equivalent quantities of acetic acid and methyl alcohol were mixed 67.5% of each had combined at equilibrium, from which result K = 4-31.The value obtained from Mens-chutkin's results (Annden 1879 195 334) is K = 5.18 whilst Worley (Proc. Roy. Soc. 1912 A 87 582) deduced the value 6.6 by extrapolation for a solution containing no added catalyst. Results are now given for the value of K for solutions containing relatively low concentrations of water and the effect of diluting the system with various quantities of water methyl alcohol and acetone has been studied. The acetone was added with the intention of diluting the system with a substance not participating in the reaction. Special precautions were taken to dehydrate the alcohol eater and acetone. It has been found that the value of K is dependent no 2724 BUTCROWS EQUILIBRIUM IN THE SYSTEM : only on the ratio [H,O] [HCl] but also on the amount of methyl alcohol or acetone present in the solution.Only for solutions con-taining a large excess of methyl alcohol is K in the neighbourhood of 4 in all other cases it is considerably greater. For solutions containing approximately equal amounts of water and ester in which the ratio of molecules of water to hydrogen chloride is not greater than 7 to 1 K is greater than 12. As the amount of water relative to hydrogen chloride is increased the equilibrium constant decreases approaching 7 as the limiting value for a solution in which the ratio [H,O] [HCl] is about 1OOO. The value of K is also decreased by the addition of acetone to the solution the effect being smaller than that observed when the system is diluted with excess of water.A considerably greater effect is observed when the system contains a large excess of methyl alcohol. Thus for a solution in which the ratio [HCl] to [H,O] to [CH,*OH] was 1 to 7-8 to 2-2 R was 11.92, whereas the values 5-87 and 4-34 respectively were obtained for solutions in which the ratios were 1 to 9.6 to 32.6 and 1 to.625 to 2 639. This displacement of the equilibrium by hydrogen chloride indicates that the latter alters the activity of one or more of the reactants so that the total concentrations of water methyl acetate, acetic acid and methyl alcohol found at equilibrium in the usual way are in reality not the concentrations actually participating in the equilibrium.At present it is not possible to state what fraction of each of the substances is rendered inactive in this way but the results recorded here are capable of explanation by such a theory. It is now definitely established that in a solution of hydrogen chloride in water only a portion of the hydrogen chloride and water molecules are in an active condition. Thus from electromotive-force or vapour-pressure measurements of a series of such mixtures one can calculate the activity of water in the presence of different amounts of hydrogen chloride. The figures in the seventh column in Table I taken from the results of Dobson and Masson (J. 1924, 125 671) represent the fraction of the water molecules in each particular solution that are in the active condition.The figures in the eighth column are obtained by multiplying K by the ‘‘ water activity ” in each case and it will be seen that these numbers are constant and equal to about 7. This would indicate that the con-centration of water participating in the equilibrium is the same as the active concentration found from vapour-pressure measurements. Furthermore the constancy of this product points to the fact that, for the particular concentrations in this series the effect of the hydrogen chloride on the equilibrium constant is due almost entirel CH3*CO.O*CBj+H,O CH,*OH+CH3*CO*OH. 2725 to its effect on the activity of the water. There can be no doubt that the catalyst affects to some extent the activity of the other reactants but it would appear from these results that this effect is negligible in comparison with the effect on the water or else the individual effects on the ester alcohol and acetic acid neutralke one another or have a constant value in these cases.Figures are not available for the effect of hydrogen chloride on the activity of these three substances. McBain and Kam (J. 1919, 115,1332) have recorded results for the vapour pressures of mixtures of water and acetic acid a t the boiling point with and without the addition of neutral salts. These authors concluded from their results that “many salts enhance the partial vapour pressure of acetic acid in aqueous solution by very appreciable amounts. The undissociated acid must be regarded as exhibiting enhanced chemical potential in the presence of such salts.’y The increase in the partial pressure is proportional to the concentration.The results given in Tables VI and VII can be explained at least qualitatively on the assumption that the addition of hydrogen chloride to an aqueous solution of acetic acid has an effect on the activity of the molecules of the latter similar to that resulting from the addition of a neutral salt. The addition of a large excess of methyl alcohol to the system under discussion results in a marked decrease in the value of K . But an increase in the concentration of methyl alcohol relative to hydrogen chloride conesponds to a decrease in the concentration of acetic acid relative to hydrogen chloride, i e . to an increase in hydrogen chloride relative to acetic acid.The observed decrease in K with increasing alcohol concentration can thus be explained on the assumption that it results from an increase in the activity of the acetic acid under these conditions. At present it is not possible to treat the subject quantitatively but experiments are now in progress from which it is hoped to determine the actual effect of hydrogen chloride on each of the reactants in a system such as this. E x P E R I M E N T A L. Freshly distilled methyl acetate mixed with the desired quantity of water and hydrochloric acid was kept for 2 or 3 days in a stoppered flask until the mixture had become homogeneous. The weight of hydrogen chloride in a given weight of the acid used was previously determined. Small quantities of this stock solution were then mixed with various amounts of water acetone methyl alcohol or methyl acetate and sealed in hard glass tubes which had previously been steamed and dried.All quantities of the different liquids were weighed. The liquids used were p d e d and dehydrated by suitable means and their purity was ascertained by boiling poin 2726 BURROWS EQUILIBRXUM IN THE SYSTEM : and density determinations. The acetone and methyl alcohol were dehydrated with metallic calcium. The methyl acetate was freed from acid by means of sodium carbonate and dehydrated with calcium chloride. It was then distilled the middle portion of the distillate being used in these experiments. A sample treated in this way was hydrolysed with barium hydroxide solution and the amount of methyl acetate found was 99.7% of the theoretical.The tubes containing the Merent solutions were kept in a thermostat a t 25.0" for Merent intervals of time varying from 2 to 10 weeks according to the amount of hydrochloric acid present. The tubes were then opened under neutral sodium acetate solution, and the amount of acetic acid was determined by titration with barium hydroxide solution. Blank experiments were performed which showed that the solutions had no determinable.effect on the glass tubes. The figures given in the following tables represent the number of gram-molecules of the merent substances present a t equilibrium. The values of the equilibrium constant were calculated from the equation K = [AcOMe][H,O]/[AcOH]~eOH]. The effect of hydrogen chloride on the equilibrium constant is shown by the results in Table I.For the first four experiments in which the amount of water was comparatively small K is much larger than is the case for the solutions for which the ratio [H,O] [HCI] was high. As stated above the products of these high values of K and the '' water activity " are approximately constant. TABLE I. [HClI x 103. 5-57 4.245 4.49 4-59 4.53 5-01 0.446 0.444 0.0576 [AcOMe] 84.86 44.57 34-10 34.00 24.8 1 2-76 32-44 25-14 19-65 x 103. LH#I x 103. 31-14 31.84 34-74 35-96 135.7 261-6 36-8 113-6 27.80 [AcOH] = [MeOH] x 103. 14.36 10.80 10.13 19-65 12.33 19-40 9.935 9.565 8-667 [H*Ol. [HCll 6.82 7-50 7-74 7-83 30.0 52.2 82.5 255.9 482-7 Water activity, K .a. K a. 12-82 0-525 6-8 12-17 0-56 6-8 12.01 0.57 6.8 11.92 0.58 6.9 7.59 7-27 * Accurate figures for the value of the " water activity " in dilute solutions are not available but by interpolation from the other values the activity in these five cases is found to vary from about 0.9 to 1. The product K x " water activity " for these dilute solutions is therefore approximately 7. In the next series of experiments the effect of diluting the system with acetone was studied. The results are in Tables 11 111, and IV 2727 [Hal x 103. 4-49 4-53 4-48 4.48 4.49 1-34 [AcOMe] 34.10 34.34 33-78 33.64 33.61 9-85 x 103. TABLE g . [H,O] [AcOH] =[MeOH] [Me,CO] x 103. x 103. x 103.K. 34.74 9-936 - 12.01 35-00 10.17 8.00 11-63 34.44 10.14 21.1 11.32 34.30 10-31 63.7 10.85 34-27 10.43 79.7 10.60 10.05 3.147 106.0 9-99 The ratio [H,O] [HCl] is nearly constant throughout the above series and is equal to 7.7. TABLE m. [Ha] [AcOMe] [H,O] [AcOH]=[MeOH] [Me,CO] x 103. x 103. x 103. x 103. x 103. K . 4.59 34-00 35.96 10.13 - 11-92 4.59 33-86 35.83 10.32 14.3 11.4 4-54 33-25 35-15 10-43 30-0 10.75 4.49 32-81 34.74 10.38 53.4 10.58 Although the results in these two tables are very similar they represent two distinct sets of experiments the solutions being prepared from entirely different samples. TABLE IV. [Hal x 103. 0-446 0.444 0-391 0446 0.446 0.444 0.445 [AcOMe] x 103. 32.44 32-21 28.28 32-12 32.02 31-82 31.82 [HsO] [AcOH] =[MeOH] [Me,CO] x 109.x 103. x 103. 36-80 12-33 -36-57 12.33 8-26 32.12 10.91 2.52 36-47 12.64 16-0 3640 12.75 29.2 36-15 12.79 38-8 36- 19 12-83 47.2 K. 7.85 7.75 7-63 7.33 7-17 7.04 7-00 The ratio [H,O] [HCl] in this case is 81. The results in these three tables show that the presence'of acetone decreases the value of the equilibrium constant but when the ratio [H,O] [HCl] is low the effect is less than that caused by the addition of the same number of molecules of water to the system (Tables I1 and III) whereas it is apparently greater in the case of a solution in which there is a great excess of water over hydrogen chloride (Table IV). In the former case a solution containing 7.7 molecules of water and 79 of acetone to 1 of hydrogen chloride gives a value for R equal to 10 whereas a solution containing 86 molecules of water to 1 molecule of the acid would have a value of 7.8.In the latter case however the value for a solution containing 80 molecules of water and 106 of acetone to 1 of hydrogen chloride is less than that for a solution containing 186 molecules of water 2728 THE SYSTEM CH~.CO.O.CH~+H~O e CH~.OH+CH~.CO.OH. The effect of increasing the concentration of methyl acetate is shown by the results in Table V ; K increases slightly with increasing concentration of ester. TABLE V. WCI] [AcOMe] [H,:] [AcOH]=[MeOH] x 103. x 103. x 10. x 103. I<. 4.69 34-00 35-96 10.13 11-92 4-25 44-57 31.84 10.80 12-17 4-64 64.61 33.42 13-22 12-36 4-68 79-88 31-61 14.08 12.74 The effect of adding methyl alcohol to the system is shown by the results in Tables VI and VII.TABLE VI. [HCI] [AcOMe] [H,O] [MeOH] [AcOH] x 103. x 103. x 103. x lo8. x 103. (MeOH]:[HCI]. K . 4.59 34.00 35-96 10.13 19.13 2.2 11.92 4.57 38-45 40-37 33.57 5.51 7-3 8-39 4-59 40-34 42-30 63-16 3.794 13.8 7-12 4.60 42-13 44-08 149.4 2.118 32.6 5-87 In this series the ratio [H20] [HCl] increases from 7.7 to 9.6. TABLE VII. [HCI] [AcOMe] [H,O] [MeOH] [AcOH] x 103. x 103. x 103. x lo3. x lo3. [MeOH] [HCI]. K. 0-0576 19.65 27.80 8.667 8.667 150-5 7-27 0-0576 26.03 34-22 79-20 2.418 1375 4.65 0.0576 27-63 36.02 152.0 1-51 2639 4.34 In this series the ratio [H20] [HCl] increases from 483 to 625. The usually accepted value for the equilibrium constant was found only in the last two experiments of this particular series. It is concluded from the results given in the last column of Table I, that the true equilibrium constant corrected for the effect of hydrogen chloride is 7 and that the low results given in Table VII are due to the effect of the hydrogen chloride on the acetic acid just as the high values of K in Table I are attributable to its effect on the activity of the water. The author is indebted to the McCaughey Research Fund Com-mittee for a grant to defray the expense of this investigation. THE UNIVERSITY SYDNEY. [Received September 16th 1925.