NOTES
308 25
Temp., 'C. 45 55
35
65
75
Vol. 63
sively raised. This note is submitted with emphasis upon the method rather than upon the chemical reaction studied.6 Consider the reaction X
k + I' + ... -+-
Products rate = A exp( -E./RT) (X)m(Y)"
...
(1) (2)
Equation 2 can, in general, be solved completely even if T , the temperature, is not constant.6 The rate is a function of concentration, time and temperature. However, the most convenient method involves (i) an assumption as to the order (m n ...) of the reaction, (ii) a plot of the derived function for that order' and (iii) the determination of the slope of the derived curve a t a given temperature. The slope with small corrections is proportional to the rate constant k a t that temperature. The Arrhenius plot of log corrected slope versus 1/T is linear if the correct assumption concerning the reaction order has been made. The method requires a nearly continuous record of the optical density and temperature of the solution for construction of smooth curves of each with time. A continuous record of each is most desirable and provides an automatic method. The reaction chosen for study is that of octatomic sulfur and triphenylphosphine in benzene s s 4- 8(C&)sP +8(c&)zPs (3) forming the phosphine sulfide. Reaction 3 has been studied both by a titration method8 and by ultraviolet absorption analysisg using more conventional techniques. Evidence has been presented8.9 that the rate-determining step is a bimolecular reaction of the phosphine with Ss with ring opening forming a phosphonium octasulfide,
+ +
1.00
0
4000 8000 Seconds. Fig. 1.-Second-order plot of the reaction of sulfur and triphenylphosphine in benzene; upper abscissa, temperature in the cell as a function of temperature. Shown is the slope determined a t 55" which is then corrected +3%.
+
(CeH6),P-Ss-. Rapid displacement reactions with the phosphine produce the phosphine sulfide and lower phosphonium polysulfides. The activation energy of the reaction
+
1.5
&
fast
+2 (C6Hd3 ps (7) determined in separate kinetic experiments8J is 16.0 f 0.2 kcnl./mole (Table I). Figure 1 presents an experiment in which the temperature has been raised from 23 to 77'. The concentrations of the reactants are (S& = 2.00 X (C6Hs)a p -k (C6Hb)s $-&-
e
3
1.0
3.0
3.2 3.4 1O3/T. Fig. 2.-Arrhenius plot of log corrected d o e determined from the curve of Fig. 1 versus 1 K .
(0) It should be pointed out that the rate of change of any physical property proportional to the rate of reaction could be used to obtain the kinetic paraineters. (6) However, the reader should refer to ref. 3 for a discussion of 8ome of the mathematical difficulties involved for a solution in closed form. The rate of heating is required for solution. (7) First order, l o g concentration ueveus time. See A. A. Frost and R. G. Pearson, "Kinetics and Mechanism," John Wiley and Sons, Inc.. Hew 'I'ork, N. Y., 19.53, pp. 8-25, 147-188, for other reaction orders. (8) P. D. Bartlett and G. hIeguerlan, J . Am. Chem. SOC.,78, 3710 (1956).
(9) P. D. Bartlett, E. Cox and R. E. Davis, in preparation.
NOTES
Feb., 193J
M and (CeH&PO= 1.60 X A I ; therewhere OD is the optical fore, a plot of 1/OD-OD density, vemus time is the proper derived function for a second-order reaction with the reactants a t equal equivalent con~eiitrations.~ The upper abscissa has been added to show the t.emperature in the reaction cell as a function of time. The slope of Fig. 1 a t any given temperature is then corrected for the change in density of the solvent with temperature and for a small change in the molar extinction coefficient of sulfur with temperature. The resultant Arrhenius plot (Fig. 2) of log corrected slope versim 1/ T gives an activation energy of 16.5 f 0.4 kcal./mole, in agreement with 16.0 f 0.2 kcal./mole.
309
THE CHELATING TENDENCY OF RIBOFLAVIN' BY T ~ 0 a r . kR. ~ HARKINS A N D HENRY FREISER~ Received August 4 , 1958
The structural similarity of riboflavin (I) and 8hydroxyquinoline (11) has been noted previously by Albert3 in accounting for the ability of the riboflavin to complex various metal ions. His results indicated an unusual metal stability order in that iron(I1) (log KI = 7.1) formed a more stable complex thaii did copper(I1) (log KI = 6.5). Inasmuch as 8-hydroxyquinoline chelates follow the usual stability order, this result was unexpected. Of course, it is possible that the presc~HiiO4
TABLE I RATECONSTANTS I N BENZENE T e m p . , 'C.O
k2, 1. mole-'
sec.-'b
Ref. methode
7.50 x 10-4 0 Ultraviolet 4.40 x 10-3 8,9 Ultraviolet titr. 11.3 x 10-3 8,9 Ultraviolet titr. a =t0.02. Rate = k?(Ss)((C6H&P). Ultraviolet analysis of sulfur, titr. iodometric analysis of the phosphine. 7.35 25.00 35.00
6H
I
OH I1
I ence of a sterically hindering group in riboflavin (the fused benzene ring) would have a sufficiently The main requirements for the use of this greater effect on copper(I1) which is smaller thaii method are (i) the rate of reaction is moderately iron(I1). The sterically hindering group present slow a t the lowest temperature; (ii) the tem- in 2-methyl-8-hydroxyquinoline was shown to have perature in the cell must be uniform; (iii) the acti- a greater effect on the chelates of the smaller metal vation energy does not vary with temperature; ions.4 Another interesting observation concerning (iv) the boiling point of the solvent cannot be ex- metal-riboflavin complexes was made by Foye and ceeded; and (v) Beer's law must be obeyed even Lange6 who prepared a series of such complexes though the molar extinction coefficient may change whose composition corresponded to a two to one with temperature. Requirement (ii) restricts the mole ratio of metal to riboflavin. The stoichiometry and thermodynamics of the size of the cell and the rate of heating. If the activation energy varies greatly with tempera- formation of metal-riboflavin reported in this ture one would be unable to obtain the correct paper was undertaken to evaluate these observaorder of the reaction and the rate constants. tions. The main disadvantage of this method is that only a Experimental small percentage of reaction is used to determine Stock solutions of approximately 0.01 M metal ions were the rate at any one temperature. The advantage prepared by dissolving their reagent grade perchlorates is obtaining the rate, the order of the reaction, the (G. Frederick Smith Co.) in water. The copper(I1) and frequency factor and the activation energy in a cobalt( 11)solutions were standardized by electrodeposition. The nickel( 11) solution was standardized by precipitation single rapid experiment. with dimethylglyoxime. Zinc(I1) was standardized gravi-
metrically as ZnNH4P04. The iron(I1) solution was prepared by dissolving high purity iron wire in perchloric acid The purification of sulfur, triphenylphosphine and benzene under an inert atmosphere. Riboflavin, obtained ofrom the Nutritional Biochemicals has been reported previously.* The use of the ultraviolet Anal. Calcd. C, 54.25; H, 5.36. absorption spectrum of sulfur to study this reaction a t a Corp., was dried a t 110 constant temperature and with conventional techniques Found: C, 55.50; H, 4.99. The titration apparatus and rocedure have been prewill be subject to a forthcoming publication.@ The thermostated cell compartment for a Beckman D U spectrophotom- viously described .e A slight mosification in preparing the solution for titration was undertaken to facilitate the diseter has been discussed.1O The brass jacket was carefully made to ensure good thermal contact with a square Corex solution of riboflavin. Fifty-five milliliters of water was cell. To the top of the cell was sealed a 7 mm. Pyrex t8ube added to a weighed quantity of the reagent. A small meas(15 cm. in length) through which a multi-junction thermo- ured volume of standard base was added to bring about solution after which the perchloric acid and metal perchlorcouple was placed into the cell just above the light path The output of the thermocouple was applied to a Speedomax ate were added and the titration proceeded in the customary recorder. The reactants were mixed and placed in the cell ( I ) Abstracted from the thesis submitted by T. R. Harkins in (total volume 1 to 2.6 4). Water, circulated a t the rate of partial fulfillment of the requirements for the Ph.D. degree at the one gallon per minute through the compartment, was slowly University of Pittsburgh, June, 1956. heated. Optical density measurements were manually re(2) Department of Chemistry, University of Arizona, Tucson. corded every 30 seconds at 345 mp. The heating rate does (3) A. Albert, Biochem. J . , 64, 646 (1953). not enter into the graphic analysis of the data but averaged (4) W. D. Johnston and H. Freiser, Anal. Chim. Acta, 11, 201 0.5 degree per minute to cover the range of 23 to 77". Other (1954). heating rates can be used to cover only a 25' increase. (5) W. 0. Foye and W. E. Lange, J . A m . Chem. SOC.,7 6 , 2199
Experimental
.
(10) P. D. Bnrtlett and R. E. Davis,
(1958).
J . A m . Chem. SOC.,80, 2513
(1954). (0) €1. Freiser, R. 11952).
G. Charles and W. D. J o h n s t o n , i t i d . , 7 4 , 1383
