WOOD : THE AFFINITY CONSTANTS OF XAX‘THINE. 1839 By JOHN KERFOOT WOOD. IN a previous paper (Trans., 1903, 83, 568) the author gave the results of the determinations of the affinities of a number of feeble bases, including, amongst others, several members of the xanthine series. Some of the values obtained for these xanthino derivatives were not in agreement with those expected on constitutional grounds. It was therefore decided to repeat some of the earlier experiments, and to extend the investigation so as not only to include other methyl derivatives of xanthine, but also to determine the acidic dissociation constants as well as the basic constants. Particulars Begarding the Substances Employed. Xunthiiie.-A portion of the same sample as was used in the earlier Heteroxantl,h~e.-This substance was prepared by Fischer’s method l’heobromine.-The sample used was Gbtained from Merck.experiments (Zoc. cit.) was employed. (Ber., 1897, 30, 2,400).1840 WOOD: THE AFFIKITY CONSTANTS OF Y'heophyZZine.-This substance was also obtained from Merck. The Paraxanthine.-This compound was prepared by Fischer's method Cafleine.-The sample used was supplied by Kahlbaum. It melted sample was crystallised from water and then dried at 140'. (ZOC. cit.). at 2349 Determinations of Basic Constunts. The methods used were those basod on : (1) The catalysis of methyl acetate (see Walker and Wood, Trans., (2) The solubility in water and in hydrochloric acid of known The majority of the experiments, as previously, were conducted at XccGmthine.-The basic constant of this substance, as in the earlier The following 1903, 83, 484).concentration. 40.1°, but a few determinations were also made at 25'. experiments, was determined by the solubility method. results were obtained at 4 0 ~ 1 ~ . 1000 C.C. water dissolved 0*1823 gram xanthine. 1000 C.C. N/lO hydrochloric acid dissolved 0.2183 gram xanthine. These figures give a value for the basic dissociation constant (kb) of 1.933 x K, where K is the dissociation constant of water. The value of K at 40*1° is 3.15 x 10-14 (Kohlrausch and Heydweiller, Zed. physibal. Chew., 1894, 14, 317), and therefore kb = 6.09 x This figure is of the same dimensions, but of rather greater magnitude, than that previously determined, viz., 4.6 x 10-14. Hetermanthim.-Determinations of the solubility at 40.1' gave the following results : 1000 C.C.water dissolved 0.7325 gram heteroxanthine. 1000 c,c. N/10 hydrochloric acid dissolved 1.003 gram hetero- Therefore kb/K= 3.754 and k b = 11-82 x 10-14. Theohornine.-Repeated determinations of the solubility of theo- bromine in water gave a result greater than that previously obtained ; the value, however, for the solubility in hydrochloric acid was in close agreement with that already given. The following are the mean values obtained : xanthine. 1000 C.C. water dissolved, at 40*1°, 0.8125 gram theobromine. 1000 C.C. N/10 hydrochloric acid dissolved, a t 40*1°, 0.939 gram From these figures the following values are given : theobromine. kb/K= 1.47 ; kb = 4.63 X wi4. The greater value obtained for the solubility in water on the presentXANTHINE AND ITS METHYL DERIVATIVES.1841 occasion has caused the value of kb to fall to about one-quarter of the figure previously determined. 2'heophyZZine.- With this substance it was found possible to employ both the previously mentioned methods. Velocitg Comtants at 25". N/20 solution of theophylline hydrochloride + methyl acetate . . . . . . , . . . . . 0*0001276 N/20-C1 solution (90 per cent. HCl, 10 per cent. KCl)+methylacetate ... 0.0001225 N/2O-Cl solution (95 ,, ,, 5 ,, ,, )+methyl acetate.. . 0*0001328 Calculating from these results, a N/20 solution of theophylline hydrochloride is hydrolysed to the extent of 92.52 per cent. at 25". Velocity Constants at 40.1 ". X/20 solution of tlieophylline hydrochloride +methyl acetate .. , . . .. . . . . 0.0005333 N/20-C1 solution (90 per cent. HC1, 10 per cent. KCl) +methyl acetate ... 0*0005140 N/20-C1 solution (95 ,, ,, 5 ,, ,,)+methyl acetate ... 0.0005460 These results show a N/20 solution of theophylline hydrochloride t o be hydrolysed to the extent of 92.57 per cent. at 40.1". Results of Solubilitp Det erminutiom Grams of theophylline 25" 6.607 7 -43 40'1 14.23 15.70 It is interesting to observe how much more soluble is theophylline than most of the other members of this series; caffeine, in fact, is the only other member possessed of a solubility comparable with that of theophylline. The values of the ratio kb/K, calculated from the foregoing data, are given in the following table : Grams of theophylline per 1000 C.C. of Temperature.per 1000 C.C. of water. N/10-hydrochloric acid. 25". 40 '1 '. Methyl acetate catalysis uiethod ... 1.74 1.734 Solubility method.. . .. . . . . . . . . . . . . . . . . . . . 1'125 1 *305 The results, it will be observed, are all of the same dimensions. Probably most reliance can be placed on those arrived at by the methyl acetate method, in which errors are not as likely to occur as in the solubility method ; a slight error in the determination of one of the solubilities would cause a proportionately greater error in the value of kb/K. I n the case of the results obtained by the catalysis method, it will be noticed that the value of kb/K is unaffected by a change of temperature. Taking the value of kb/K found by the catalysis method, the basic dissociation constant of theophylline at 40*1° is calculated to be 5.46 x 10-l4.1842 WOOD: THE AFFINITY CONSTANTS OF Paraxanthine.-With this substance it was impossible to prepare a solution of sufficient concentration to permit of the methyl acetate method being employed.Determinations of the solubility at 40*1° gave the following results : 1000 C.C. water dissolved 1.06 grams paraxanthine. 1000 C.C. XI10 hydrochloric acid dissolved 1.17 grams paraxanthine. From these figures the following values are obtained : ka/K = 1.044; kb = 3.29 X Cuc$eine.-A determination by the methyl acetate catalysis method showed caffeine hydrochloride to be hydrolysed to the extent of 89 *7 per cent. in N/lO solution at 40.1". This figure is identical with that previously determined (Zoc.cit.). Therefore, as previously found, kb = 4.0 x 10 - 14. Determination of Acidic Constants. The method chiefly employed was that of the saponification of methyl acetate by the sodium salt of the substance (Shields, Zeit. physikal. Chem., 1893, 12, 167), the results being Calculated by means of the equation given in the preceding paper. The velocity of saponification of methyl acetate being much greater than that of the catalysis of the same substance, and the velocity increasing with the temperature, the determinations mere made at 25O, instead of at 40*1° as in the case of the determination of the basic constants. With one or two substances the acidic constant was calculated from the solu- bilities in water and in N/20 sodium hydroxide; these solubility experiments were conducted at 40.1'.Xanthine.-The solubility method was employed, and the following results obtained : 1000 C.C. water dissolved 0.1823 gram xanthine. 1000 C.C. N/20 sodium hydroxide dissolved 6.405 grams xanthine. From these data i t is found that ka/K=3767, and therefore at 2Zeteroxanthine.-With this substance also the solubility method 1000 C.C. water dissolved 0,7325 gram heteroxanthine. 1000 C.C. N/20 sodium hydroxide dissolved 7.781 grams hetero- Therefore ka/K=1276 and ka=4*019 x 2'heobrornine.-The determination of the velocity with which the sodium salt of this compound saponifies an equivalent quantity of 40.1' ka= 1.186 x lo-''. was employed. xant hine.XANTHINE AND ITS METHYL DERIVATIVES. 1843 methyl acetate is rendered more difficult by the slight solubility of the compound.After the action has proceeded to some extent, the theobromine, being no longer kept in solution as the sodium salt, begins to be precipitated. The conditions of equilibrium are thus changed, and it is impossible to obtain readings a t the most trust- worthy part of the change, namely, the middle portion. It is possible, however, to obtain a very good estimate of the velocity of saponification by comparing the readings during the initial period of the saponification of methyl acetate by solutions, of equal con- centration, of sodium theobromine and sodium phenoxide respectively. The figures for the two reactions are shown below; in both cases 25 C.C. of the solution of the alkali salt were mixed with an equal volume of the solution of methyl acetate.NjZ5 sodium theobromine + N/25 methyl acetate :- Time. 0 61 81 117 Titre. k. 7-92 - 5-72 0*00097 5 -39 0-00104 4'95 0'00111 N/25 sodium phenoxide + N/%5 methyl acetate : Time. 0 63 83 119 Titre. 7-92 5 '85 5.47 5 *lo k. 0*00080 0*00093 0-00094 - It will be observed that the values obtained for k are in both cases of gradually increasing magnitude. Such inconstancy is always noticed when results are calculated from observations made during the initial period of the reaction. The rate of increase of k: is almost the same in both series, the ratios between corresponding figures of the first and second series being respectively 1.21, 1.12, and 1.18. It is evident, therefore, that the saponification of the ester by the salt of theo- bromine is proceeding a t a rate which, on the average, is 1.17 times as great as that with which the saponification by sodium phenoxide proceeds.The velocity of saponification being inversely proportional to the acidic dissociation constant, and the dissociation constant of phenol at 25O being 1.3 x 10-1O (Walker and Cormack, Trans., 1900, 77, IS), it follows that the acidic dissociation constant of theobromine is approximately 1.1 1 x 10 - 10. Theodor Paul (Arch. Phurm., 1901, 239, 48j calculated the dis- sociation constant at 18' from the results of determinations of the solubility in water and in solutions of sodium hydroxide. The result obtained was 1.33 x but an error appears to have been made in1844 WOOD : THE AFFINITY CONSTANTS OF the calculation. A recalculation by the author from Paul's data gave a value for ka of 0.91 x 10-lo, which, allowing for the differ- ence in temperature, is in good agreement with the value given above.!Z'heophyllins.-No difticulty of the bind experienced with theo- bromine was met with in the case of theophylline, and it was there- fore possible to calculate the dissociation constant from the readings obtained during the middle period of the saponification. The results obtained were as follows : k = 0.0000845 ; ka = 1.62 x lo-'. Experiments on the solubility of theophylline in water and sodium hydroxide were made. It was found that the additional amount of theophylline which passed into solution because of the presence of the alkali was equivalent to the amount of the latter, thus showing that theophylline is a comparatively strong acid, the sodium salt of which is not appreciably hydrolysed in solution.Pumxanthine.-For this substance also, the saponification method was employed, giving as results : k = 0 ~ 0 0 0 0 6 1 6 and ka=2*22 x 10-9. Cafeine.-This compound is almost devoid of acidic properties. A solution of it in sodium hydroxide saponified methyl acetate with a velocity almost as great as that shown by the pure alkali. Moreover, the solubility in a solution of sodium hydroxide is only very slightly greater than that in pure water. The value of ka is evidently, there- fore, of dimensions less than 1 x Xurnrnary of Beszllts. NH*CO*C*NH, CO*NH-C-N~ Xanthine, I I ( CH ...... ... Heteroxanthine, 1 \ I \CH ... Theobromine, 1 11 \CH ... NH'CO'C'NMe CO-NH.C----N/ NH- CO-C -NMe CO-NM~*C----N~ NMe'CO'C'NH, Theophylline, I 11 'CH ...... CO*NMe*C-N/ NMe'CO 'C'NMe, Paraxauthine, 1 ( 1 \CH ...Caffeine, I 1 ) \CH ........ -NH-c---N/ NMe' CO 'C "Me CO'NMe'C---N/ kb x 1014. Temp 4.6 40'1" 11.82 40.1 4-63 40.1 5-46 40.1 3.29 40-1 4.0 40'1 ka x 11-86 4.019 11 -1 162.0 222.0 < 0 *OOl Temp. 40.1" 40 -1 25.0 25'0 25'0 25-0XANTHINE AND ITS METHYL DERIVATIVES. 1845 Before comparing in any way the results given in the above table, one or two points require to be specially mentioned. The methods used for the determination both of the basic and the acidic constants have not been the same for all the substances examined, and it might perhaps be urged that this difference i n method made any comparison of the results of uncertain value. This objection vanishes, however, when it is recalled that for each series of constants the results for one substance have been arrived a t by both methods. Thus, in the case of the basic constants, the constant of theophylline was determined both by the methyl acetate catalysis method and by the solubility method, the values obtained being of similar dimensions.I n the case of the acidic constants, the fact that, the dissociation constant of theo- bromine as found by the saponification method is in close agreement with that of the same substance calculated from Paul's solubility data shows that the results arrived at by the two methods are strictly comparable. The effect of temperature on the results must also be referred to, seeing that in the case of the acidic constants results are given at two temperatures.This difference in the working temperature was largely due to the desire to obtain results of the greatest possible accuracy. A comparatively low temperature was found to be the best in the case of the saponification method, whilst in the case of the solubility method it was considered that, the solubilities of some of the sub- stances being very small, more accurate results could be obtained by working at a higher temperature. For purposes of comparison, it will be sufficient to state that at 40.1' k, will have a value rather greater than twice the value which it has at 25'; this result was arrived at by experiments made with theophylline by the saponification method. Discussion of Results.It has been pointed out by Walker (Trans., 1903, 83, 182 ; Proc. Roy. Soc., 78, A , 140) that the value kb is a composite expression con- taining not only the real ionisation constant of the base, but also a hydration constant. If the hydration remained constant, it might be expected that the values of kb would gradually increase as the number of methyl groups in the xanthine nucleus increased. It seems reasonable to suppose that with substances of similar constitution such as those examined the degree of hydration will not vary t o any great extent, and on this assumption it will be evident that there can likewise be no great difference in the values of the true ionisation constants, since for all the substances kb has nearly the same magnitude.1846 WOOD : THE AFFINITY CONSTANTS OF XANTHINE. The matter becomes simpler when acids are under consideration, for in such cases the process of hydration does not as a rule form part of the operation of solution. The general effect of the substitution of a hydrogen atom by a methyl group might be expected to be a very slight diminution in the value of the acidic constant, and it will be observed that in the transition from xanthine to heteroxanthine such an effect on the value of k, is produced.But when we proceed further and replace a second hydrogen atom by a methyl group, it is observed that k,, instead of undergoing a further slight diminution, assumes in all cases a much greater value, the increase being most noticeable in the cascs of paraxanthine and theophylline.The imino-groups which are now strongly acidic were present in the parent substance xanthine, and yet that substance has an acidic dissociation constant which is only a small fraction of those possessed by the dirnethylxanthines. This great augmentation of the value of k, can only be explained by-tho assumption of some stereochemical change taking place on the intro- duction of the second methyl group, the change produced being least when substitution takes place at position 3 of the xanthine nucleus. The almost complete freedom from acid characteristics shown by caffeine is in accordance with what might be expected on constitutional grounds, since it contains no imino-group. A coaparison of the values of k, given by the isomeric dimethyl- xanthines is also of interest when taken in conjunction with their respective constitutions.From our general knowledge as to the negative character of the carbonyl group, it might have been reasonably expected that theobromine, in which the imino-group is attached to two carbonyl groups, would possess a value for k, greater than those given by paraxanthine and theophylline, in both of which substances the imino-group is only connected with one carbonyl group. This view is supported by some of the results described in the previous paper ; for example, those obtained with the dimethyluracils. The results obtained, however, with the dimethylxanthines show that the positions are entirely reversed, paraxanthine having the highest and theobromine the lowest value for k,. Were it not for the fact that the constitu- tion of these substances appear to have been fixed with certainty by Fischer and others, it might almost be believed that some confusion between the two had taken place, and that theobromine should really have the formula ascribed to paraxanthine, and vice versd. In the circumstances, however, this view cannot be entertained, and we must therefore conclude that in the case of the dimethylxanthines the ordinary influences of the carbonyl group are not seen because of the stereochemictil influence of the methyl groups being more powerful andRUHEMANN : XANTHOXALANIL AND ITS ANALOGUES. 1847 varying in magnitude according t o the positions in the xuiithirie nucleus occupied by the methyl groups. I n view of the interesting results obtained, the investigation is being extended to other purine derivatives. UNIVERSITY COLLEGE, DUNDEE.