Jan., 1962
T HE IOKIZATION COR'STANT OF p-XITROPHESOL
Experimental Synthesis of Monomers.-The general methods of preparing the alkyl styrenes have been described.15 The purity of all the monomers was determined by vapor phase chromatography and infrared spectroscopy. Synthesis of Fluors-4-Bromo-p-terphenyl.-This was synthesized in 26% yield according to the procedure of France, Heilbron and Heylo by adding bromine to a solution of p-terphenyl in hot glacial acetic acid in the presence of a trace of iodine and heating until the brown color faded. The product had a melting point of 229-230'. The infrared spectrum Laken by means of a potassium bromide pellet showed abnorption for para substitution a t 12.25 p (strong) and a t 13.90 p (weak) and only a trace of absorption a t 12.92 p (a peak which is present in p-terphenyl). The ultraviolet spectrum of this compound showed a h2Fi8 a t 280 mp ( emax 38,500). 4-Iodo-p -terphexiyl.l~-To a mixture of p-terphenyl in 3400 ml. of glacial acetic acid a t 80' was added approximately 1 ml. of water. The reaction mixture was cooled to room temperature and then 31.6 g. (0.402 M ) of iodine, 15.2 g. of potassium iodate, 33.8 cc. of concd. sulfuric acid and 42.2 cc. of CC14 were added. The reaction mixture was heated to 80" and stirred for 48 hr. at, that temperature until the iodine color faded. The product precipitated on cooling and was recrystallized from benzene to yield 77.3 g. (50% yield) of 4-iodo-p-terphenyl, m.p. 245-247' (1it.I' 246-247'), X ~ 281 ~ n ~~ p (mnax " 40,200). 4-Methyl-p-terphenyl.-To a solution (0.426 M ) of 4bromo-p-terphenyl in ether was added a solution of n-butyllithium (0.554 M). When the reaction was complete methyl sulfate was added in sufficient quantity to react with all the p-terphenyllithium. The product was obtained in 38% yield; m.p. 206-208' (lit.18 m.p. 208'). The infrared spec-
-
(15) D. T.Mowry, M. Reno11 and W. F. Huber, J. A m . Chem. Soc., 68, 1105 (1946). (16) H.France, I. IM. Heilbron and D. H. Hey, J. Chem. Soc., 1364 (1938). (17) W. Kern, W. Gruber and H. 0. Wirth, Macromol. Chem., B37H3, 210 (1960). (18) H.Gilman and E. A. Weipert, J . Ow. Chem., '2'2, 446 (1957).
171
trum of this compound wae identical to that of an authentic sample supplied to us by Prof, Guido Daub of the University of New Mexico. The ultraviolet spectrum of this compound showed a 282 mp 29,700). 4-Phenylstilbene.-To an ethereal suspension of 20.9 g . of magnesium (0.88 M) was added 194 g. of 4-bromobi henyl (0.833 ill)in ether. When the reaction was completegeshly distilled 97.6 g. of phenylacetaldehyde (0,833 M ) was added dropwise to yield 45 g. of 2-phenyl-1-biphenyl-1-ethanol which then was dehydrated over alumina at 300" under vacuum to 4- henylstilbene, m.p. 221-222' m.p. 325 mp (emmax34,300). 221-222'; Sources of Other Fluors.-Naphthalene was recrystallized three times from methanol, m.p. 80-81. 1-Methoxynaphthalene was obtained from Eastman Distillation Products and redistilled, b.p. 122' (5 min.), 1.6222. p-Terphenyl-Dajac Laboratory-was scintlllation grade, m .p. 212-213'; 279 mp (emlLx 29,600). 4,4'-Diphenylstilbene was obtained from Pilot Chemical Co. and used without further purification; X2z:ia 340 mp (emmax 53,700). POPOP, scintillation grade, was obtained from Tracerlab and recrystallized from benzene. Preparation of Plastic Scintillators.-The plastic scintillators were prepared as cylindrical discs 1 / 2 in: high by l s / ~ o in. in diameter by polymerizing the substituted styrenes in glass vials of the right diameter (degassed under vacuum) and machining to the desired height. Determination of Pulse Height .-The method and apparatus for determining the relative pulse heights has been described! For p-irradiation, a Paz34(0.01 mc., 2,3-mev.) source was used. All pulse heights are relative to an anthracene crystal of the same dimensions.
X:gia
Acknowledgment.-We are grateful t o Mr. Heinz Gunter Dickens for his help in carrying out some of the experiments and to Drs. S. Loshaek and B. D. Halpern for their encouragement and helpful suggestions. (19) F. Bergmann and J. Weizmann, ibid,, 9, 408 (1944). (20) R. Kuhn and T. Wagner-Jauregg, Helv. Chim. Acta, 13, 9 (1930).
THE IONIZATION CONSTANT OF p-NITROPHENOL FROM 0 TO 60' BY G. F. ALLEN,R. A. ROBINSON AND V. E. BOWER Nictional Bureau of Standards, Washington .26, D. C . Received September 6 , 1061
The spectrophotometric method has been used t o measure the ionization constant of p-nitrophenol in aqueous solution from 0 to 60'. The ionization constant is 6.98 x 10-8 a t 25". It is given as a function of the absolute temperature ( T ) by -log K = 2150.69/T - 3.8133 f- 0.01260T.
Some spectrophotometric measurements already have been made to determine ionization constants over a temperature range; thus, Sager and Siewersl determineld the ionization constant of 4-aminobenzophenone from 10 t o 40' and Sager and Byers2 that of 4-chloro-4'-aminodiphenylsulfonefrom 15 to 35 O. In order to determine changes in enthalpy and heat capacity occurring in the ionization of a weak acid, a knowledge of the ionization constant over a wide range of temperature is desirable. To see if the range could be extended, spectrophotometric measurements in the near-ultrasiolet now have been made wiih p-nitrophenol between 0 and 60'. The earlier measurements of Robinson and Biggs* (1) E. E. Sager and I. J. Siewers, J. Research Natl. Bur. Standards,
46, 489 (1950). ( 2 ) E E. Sager and F. C. Byers, ibid., 69, 245 (1957). (3) R. A. Robinson and A. I. Big& Tram. Faraday Soc., 61, 901 (1966).
a t 25' were repeated but the data were treated by a somewhat different method due to Bates and Sch~arzenbach.~ E.m.f. measurements5 of cells containing phosphate buffers yield unequivocal values of the quantity -log (VIH YH ycl). This was formerly called pwH but the symbolism is not consistent with that recommended recently.6 The results of spectrophotometric measurements give the ionization constant of p-nitrophenol as pK
=
--log
- log a/(1 - a) - log YNIYHN
(~HYH)
where a,the fraction of phenol ionized, is given by ( D - D ~ ) / ( D zT 01) ; D1,DZand D being the optical densities of acid, alkaline and buffer solutions con(4) R. G. Bates and G. Schwarzenbsch, Helw. Chim. Acta, 31, 1069 (1954). (5) R. G. Bates and 8. F. Acree. J . Research Natl. Bur. Standards, 34, 373 (1945); R. G. Bates, ibid., 39, 411 (1947). (6) €2. G . Bates and E. A. Gugssnbeim, Pure Appl. Chem., 1, 163 (1960).
taining p-nitrophenol and YN and YHN the activit y strength 0.1, measurements of optical dcnsity were coefficients of the nitrophenolate ion and the nitro- made a t thrce wave lcngths, 306, 406 and 416 mp; phenol molecule, respectively. Thus, from a large scale plot of optical density 11s. tcmperature, values of optical density were read a t PK = --log ( ~ B Y H Y C I )- log 4 1 0)- log (YNIYCIYHV) values of the tcmpcrature and the degrec of The last term should be negligiblc a t low ionic round ionization calculated. lj'or brevity, only the averstrength. age of the three a-valucs at each temperature is Some measurements of optical density a t a wave length of 406 mp were made in phosphate buffers recorded. The following results were obtained 0 5 10 15 20 at 25' and values of a / ( l - a) calculated; thcse, OC. combined with -log (V~H YH YCI) data,' and with -log ( ~ ~ Y R Y c ~ ) 7.091 7.057 7.029 7.006 6.988 the last term in the above equation neglected, gave a 0.280 0.300 0.323 0.347 0.371 the following p K values PK 7.501 7.425 7.350 7.281 7.216
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Solution -log ( ~ H Y B Y C J a
PK
1 2 3 4 5 6.912 6.974 6.992 7.040 7.080 0.354 0.397 0.407 0.436 0.454 7.173 7.156 7.155 7.152 7.160
Solutions 1 to S contained equimolal KIlzPQaand Na2HP04 a t total ionic strengths of 0.2, 0.1, 0.08, 0.04 and 0.02 mole/kg., respectively. They were prepared from Standard Samples of the Sational Bureau of Standards. p-Nitrophenol was Fisher's Reagent material, recrystallized from water. It is evident that four solutions give concordant values of pK and it is justifiable to equate the YN/(YHNYCI) term to unity. One solution gave a high pK value but, as it had a total ionic strength of 0.2, it is not surprising that the assumption is no longer valid. The mean of the four concordant results is 7.156; other determinations have given 7.14,' 7.149,' 7.1!ju4 Measurements were made over a temperature range 0 to 60' using a Beckman Model DU instrument. The cell compartment was fitted with jackets through which water could be circulated from a thermostat. A thermometer, inserted into the cell compartment and allowed to come to temperature equilibrium, showed a variation of less than 0.05' over periods of time much longer than those needed to make measilrenients of optical density. The maximum in the absorption spectrum of alkaline solutions of p-nitrophenol shows some change with temperature of both the wave length at which the maximum is found and the extinction coefficicnt at that wave length ( 2 O , 397 mp, 17,700; 25O, 402 mp, 17,900; SOo, 404 mp, 18,200). Using the equimolar phosphate buffer of total ionic (7) Recalculated recently from the original data by R . G . Bates and R. Gary, J . Reaeorch Natl. Bur. Standard%,668, 495 (1961). (8) C. M. Judson and M. Kilpatrick. J . Am. Chem. SOC.,11, 3110 (1949).
"C. -log
(~HYHYCJ
Ly
PK
OC. -log a
(~~YBYCI)
25 30 35 40 45 6.974 6.964 6.956 6 951 6.949 0.397 0.422 0.449 0.471 0.495 7.156 7.101 7.046 7.001 6 958 60 50 55 6.948 6.950 6.954 0.520 0.543 0.566 6.914 6.875 6.839
PK The pK values can be represented by the equation p K = 2150.69/T
- 3.8133 + 0.012602'
The corresponding change of entropy on ionization is calculated to be 71 j. deg.-l mole-'. The change of enthalpy is 16,730 j. mole-' at 2 5 O , compared with 19,660 j. mole-' (4,700 cal. mole-') from the calorimetric measurements of Fcrnandez and Ilep1 ~ r .We ~ hope to improve the tempcrature control of the cell; at present, it is difEcult to assess the accuracy of our results but the agreement with this direct measurement of the enthalpy change is encouraging. For Dhenol itself Laidler. el d . 1 " found ARo = 23,4305. mole-' (5,600 cal: mole-') and AI?" = 22,430 j. mole-' can be calculated from the data of Binns." Similarly, 22,260 j. mole-' can be calculated for o-cresol.12 For p-phcnolsulfonic acidla in its second stage, whcre thc ionization is of a different charge type, A H 0 = 16,883j. mole-'. Thus all of these phenolic acid?, in contrast to the aliphatic acids, have large MIOvalues a t 25' and their ionization constants are, therefore, markedly temperature-sensitive. (9) L. P. Fernandez and L. G.FIegler, itid. 91, 1783 (19.59). (10) H. hl. PapBa, W. J. Canady, T. W. Zawidski and K. J. Laidlrr, Trans. Faraday floc., 86, 1734 (1959). (11) E. H . Binns. tbid., 66, 1900 (1959). (12) K. D. Louise, t b i d . , 66, 1633 (1960). (13) R. G.Bates, G. 13. Siege1 and S. F.Acree, J. Rexearch Not2 RUT Standards, 31, 205 (1945).
