T H E RATE OF TRANSFORbfATIOY OF ACETYLCHLOROAhlIKOBEh'ZEh'E IKTO 0- AKD P-CHLOROAKILIDES AS A LIEASURE O F T H E CATALYTIC POWER O F T H E HYDROCHLORIC ACID. BY FREDERICK GEORGE SOPER
The study of the transformation of acetylchloroaminobenzene to 0- and r-chloroacetanilides, under the influence of hydrochloric acid as catalyst, is of importance in the theory of strong electrolytes and reaction yelocity and has been undertaken by Rivett', by Harned and SeltzZand by Akerlof.s The measurement of the rate at which the chloroamine was transformed was based on the fact that acidified potasssium iodide reacts quantitatively with a chloroamine with formation of iodine, whilst no iodine is liberated by the Ruclear chlorinated anilide. It will be shown that, owing to an overlooked factor, the results obtained for the rate of transformation when using the above method of analysis indicate a catalytic activity of the hydrochloric acid which is too high for the more concentrated solutions. The main essential features of the transformation were demonstrated by Orton and Jones'. It was shown that the production of free chlorine was an intermediate step in the reaction which was completed by extra-nuclear chlorination of the acetanilide, the successive stages being symbolisedA
+
C ~ H S N C ~ A CHCl+
C12
B + CsHsSHhC + CsH4 C1
+ HCl.
Since no indication of successive reactions can be observed from a study of the composition-time curve, it followed as a necessary corollary to the above mechanism that the velocity coefficients of the two stages had very different values, the rate being entirely governed by the slowness of either the first or the second of these successive reactions.5 Which reaction actually governed the speed in aqueous media was not definitely settled. It is now possible from a knowledge of the rate of formation of acetylchloroaminobenzene from acetanilide and chlorine, the determination of which is described below, combined with that of the equilibrium constant of the reaction: C1, C6H6KHXc CsH5SClhc HCl, to show that the velocity coefficient of the interaction of acetylchloroaminobenzene wlth hydrochloric acid agrees with that of the transformation. The first of the above consecutive reactions therefore governs the rate of the trmsformation and is
+
+
+
Z. physik. Chrm., 82, Z O I ; 85, 113 (1913). J. Am. Chem. Soc., 44, 1476 (1922). Medd. Sobel Inst., 6 , S o . 2 , I (1925). * B r i t . Ass. Reports, 1910, 85. 5 T h e formation of chlorine from a chloroamine and hydrochloric acid does not involve preliminary hydrolysis of the chloroamine to hypochlorous acid slnce phenol which reacts rapidly with hypochlorous acid does not react to a n appreciable extent with the chloroamine in aqueous solution a t 25' in an examined perioa of seven hours. The rate of hydrolysis of the chloroamine in aqueous solution is therefore relatively slow. 1
TRANSFORMATION OF ACETYLCHLOROAMINOBENZENE
1 I93
the slow stage. The addition of readily chlorinatable substances, e.g., acetanilide, should not therefore alter the rate of fall of titre of the system. Actually' an increase in the velocity was observed. This apparently conflicting observation is due to the fact that in the mixture of chloroamine and hydrochloric acid, the chlorine when formed undergoes two main simultaneous reactions, (a) with the acetanilide forming 0- and p-chloroanilides and (b) with the chloroanilides, which are present at a much higher concentration than acetanilide, forming chloroamines. These chloroamines of the chloroanilides are formed progressively and since they contribute to the iodine titre, the observed rate of fall of iodine titre is less than that of the formation of chlorine in reaction A. When excess of some readily Cchlorinatable substance is present competition for the chlorine reduces the N-chlorination of the chloroanilides to a negligible amount and the rate of fall of titre increases to a constant value equal to that of the formation of chlorine. The ratio in which the chlorine reacts to form Y- and C-chlorinated products varies with the strength of the hydrochloric acid, being respectively less in the concentrated acid solutions owing to attainment of the equilibria: ,/KHAc -c /NClAC HCl. Since the true "catalytic" acC h t 6 4\c1 C6H4,C1
+
+
tivity of the hydrochloric acid is measured by its rate of interaction with acetylchloroaminobenzene to form chlorine, the velocity determinations should be made in the presence of excess of substances which can remove the chlorine completely as formed. Suitable substances are acetanilide, phenol, and p-cresol. Results obtained at 25' using 0 . 2 , 0.4, and 0.8 molal solutions of hydrochloric acid are given in Table I . TABLE I Velocity coefficients of the transformation at 25.0' Concentration of HC1
a k = logl, t a - x k' in presence of excess acetanilide, phenol, or p-cresol Ratio kjk'
o.00172
0.4 0.00666
0.0282
0.00188
0.00711
0.0291
0.915
0.937
0,970
0.2
0.8M
s
These results show that the catalytic activity of the hydrochloric acid given by the rate of formation of chlorine is not proportional to the velocity coefficient, k, of the transformation as ordinarily measured. Such measurements contain a varying error indicating a catalytic activity which is, for example, 0.915of the true value in 0 . 2 M HCl and 0.970 of the true value in 0.8 31 HCl. Deductions based dn such measurements lead to a relative catalytic activity of the 0.8 h l HC1 which is some 6Yc too high in comparison with that of the 0 . 2 M HCl. Before attempting to correlate the thermodynamic activity of the hydrochloric acid with new measurements of its catalytic activity, other factors are being investigated. Orton and Jones: Brit. Am. Reports, 1910, 85.
1
I94
FREDERICK GEORGE SOPER
S 2. Test of the mechanism of the transformation. Calculation of the speed of interaction of acetylchloroaminobenzene with hydrochloric acid. The velocity coefficient, kl, of the interaction of acetylchloroaminobenzene with hydrochloric acid can be expressed in terms of the equilibrium constant of the reaction, C6HjhTClAc HCl C6HjSHAc Cl2, and the velocity coefficient, kz, of the formation of acetylchloroaminobenzene from acetanilide and chlorine. The equilibrium constant of the reaction can be deduced from the hydrolysis constant, Kh, of the chloroamine’ and that of chlorine,2 Kj, Thus
+
+
e
The results of the measurements of the velocity coefficient, ICz, of the formation of the chloroamine of acetanilide and those of 0- and p-chloroacetanilides are given in Table 11.
TABLE I1 Velocity coefficients of N - and C- chlorination at 25.0’ Chloroamine formation, k2, Nuclear chlorination, k,,,
1.3 X
102
1.7 X
104
1.9X IO^ X IO*
2.1
6.1 X
102
45.
Substituting for k2 in ( I ) the value 130, for &, the value 7.3 X IO-’ and for K J 4.84 X IO^ one obtains 121 = 0.197. The observed value in 0.8 M acid where the activity is 0.40, is o.o291/log e, giving a value at unit acid activity of 0.17. These values are in satisfactory agreement. The calculated value of the velocity Coefficient of the transformation assuming that the second of the consecutive stages is the slower can be shown to be k = kliKh/KJ, where k,, is the velocity coefficient of nuclear chlorination of acetanilide, yielding a value some 130 times too great. The observed speed of the transformation is thus in satisfactory agreement with that calculated on the assumption that the,interaction of chloroamine and hydrochloric acid is the slow stage whilst no agreement is found with the value calculated on the alternative assumption that the interaction between acetanilide and chlorine is the slower.
S 3. Experimental. The measurement of the speeds of chloroamine formation depends on the fact demonstrated by Orton and Jones (loc. cit.) that chloroamine formation and nuclear chlorination are simultaneous side reactions, / chloroamine
‘lZ
-‘chloroanilide
Soper: J. Chern. SOC.,127, (1925).
* Jakowkin: Z. physik. Chem., 29, 813 (1898)
+ HCl + HCl
TRANSFORMATION OF ACETYLCHLOROAMINOBENZEKE
I
I95
The ratio of chloroamine to chloroanilide formed gives the ratio of the respective velocity coefficiexits. The speeds of nuclear chlorination were themselves measured in the presence of sufficient H’ and C1’ ions to prevent appreciable formation of the chloroamine. The fall of iodine titre of a mixture of anilide and chlorine then gives a measure of the rate at which chlorine is disappearing from the system to form C-chlorinated derivatives. Measuremenl of the speeds of nuclear chlorinalion. The source of chlorine was a mixture of Chloramine-T and hydrochloric acid, C,HSO(ONa) : S C 1 + zHCl* C,H7S02NH2 NaCl C12. I n the presence of the excess of hydrochloric acid necessary to prevent N-chlorination of the anilide, the equilibrium between Chloramine-T and hydrochloric acid is displaced with production of 10057chlorine. This source of chlorine was used in preference to chlorine water owing to its greater stability and convenience in handling. The stability of chlorine in the presence of Chloramine-T and of p-toluene-sulphonamide was shown by examination of the titre of a mixture of Chloramine-T and ,If hydrochloric acid contained in a flask stoppered with a ground-in pipette and fitted with a side arm connected to a washbottle containing the same mixture. Removal of a quantity of the solution for analysis caused the entry of a gaseous mixture into the bottle which wm identical with that already present. If this precaution is omitted there is readjustment of the chlorine between the phases and consequent fall of the chlorine concentration in the liquid phase. Even under the conditions outlined a slight disappearance of chlorine was experienced but is negligible in comparison with that occurring in the presence of the anilides examined. I t was found, by aspirating air over the surface of the liquid mixture, that the rate of escape of chlorine when an anilide is present was also negligible except when the solution was agitated. Mixtures of Chloramine-T, hydrochloric acid and the anilide were made ~ Chloramine-T solution being added last to start the reaction. up at 2 5 . 0 ~the Portions of the mixture were withdrawn by a pipette at suitable time intervals, run into oxygen-free potassium iodide solution and the liberated iodine titrated against thiosulphate. The necessary concentration of hydrochloric acid required to cause the full production of chlorine from Chloramine-T was found to be from 3 to 4 molal, since further increase in concentration did not increase the speed of chlorination but actually caused a slight decrease, probably due to some slight removal of free chlorine as hydrogen tri-chloride. In the case of acetanilide portions were withdrawn every I O secs., introducing an unavoidably large timing error. Its effect on the velocity coefficient is estimated at about 10-157The ,. results obtained are given in Table 11. Measurement of the relative rates of chloroamine formation and o j nuclear chlorination.-The method adopted was to add the requisite amount of hypochlorous acid to a mixture of excess of the anilide and 0.I Mhydrochloric acid. The hydrochloric and hypochlorous acids form chlorine and since the chlorine is formed in situ it is surrounded by excess of the anilide, thus allowing the simultaneous side reactions to proceed unmasked by consecutive reactions which might be caused by local excess of chlorine. The iodine titre of the
+
+
1196
FREDERICK GEORGE SOPER
mixture was measured at suitable time intervals and was found to fall quickly to a practically constant value. The fall of titre represents the nuclear chlorination, whilst the residual titre represents the chloroamine formed. The concentration of hydrochloric acid which is necessarily present to prevent hydrolysis of the chlorine has no appreciable effect on the amount of chloroamine formed, since the rate of interaction of these chloroamines with M/IO hydrochloric acid is negligible compared with their rates of formation. The results obtained are given in Table I11 and lead to the velocity coefficients of chloroamine formation given in Table 11.
TABLE I11 Relative speeds of N - and C-chlorination of anilides at
25.0'
/SH.lc UHAc C s H J H A c C ~ H I \ ~ ~ ( C~ I); H I C ~ ( ~ )
Initial titre of I O c.cs. Final titre of I O c.cs. [chloroamine] / [chloroanilide] formed
IO0 0 0
I O 00
75
0.0076
10.00
c.cs.
4 . 7 5 c.cs.
9.37
0,905
13.2
The necessity of the presence of a large excess of the anilide is well exemplified in the case of acetanilide. When the chlorine and the anilide are equimolar, aconsiderable fraction of the chlorine reacts with the chloroanilides to form chloroamines which appear in the final titre. The ratio of chloroamine to chloroanilide formed becomes constant in the presence of an 8-fold excess of the anilide. The results are given in Table IF'. T.4BLE
[acetanilide]/ [chlorine] [chloroamine]/[chloroanilide]
I
0.064
Iv 2
0.010
4
8
0.008
0.00756
I2
0.00768
Of the chloroamine formed when chlorine and acetanilide interact in eqimolar proportions only I 2 % is acetylchloroaminobenzene, the remainder being composed of the chloroamines of the chloroanilides. Measurement of the rate of interaction of hydrochloric acid and acetylchloroaminobenzene.-The chloroamine was prepared by the method of Chattaway and Orton' and was dissolved in water redistilled from acid permanganate solution. The phenol and p-cresol used were redistilled Kahlbaum preparations, were quite colourless and when mixed with the chloroamine solution caused no detectable fall in the iodine titre in 7 hours at 2 5 ' . I n conclusion, I wish to thank Professor K. J. P. Orton, F. 11. S., for suggesting this work and for helpful criticism. University College of North Wales, Bangor. March 21, 1927
J. Chem. SOC., 73, 1046 (1899).
