Vol. 63 308 NOTES

Temp., 'C. 25 35 45 55 65 75

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%.

1.5 + 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 .

sively raised. This note is submitted with emphasis upon the method rather than upon the chemical re- action studied.6

Consider the reaction k

X + I' + ... -+- Products (1) rate = A exp( -E./RT) (X)m (Y)" . . . (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 de- rived function for that order' and (iii) the deter- mination of the slope of the derived curve a t a given temperature. The slope with small correc- tions 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

forming the phosphine sulfide. Reaction 3 has been studied both by a titration method8 and by ultraviolet absorption analysisg using more con- ventional techniques. Evidence has been pre- sented8.9 that the rate-determining step is a bi- molecular reaction of the phosphine with Ss with ring opening forming a phosphonium octasulfide, (CeH6),P-Ss-. Rapid displacement reactions with the phosphine produce the phosphine sulfide and lower phosphonium polysulfides. The activation energy of the reaction

(3)

+

fast (C6Hs)a p -k (C6Hb)s $-&- + 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 (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. (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.

The rate of heating is required for solution.

Feb., 193J NOTES 309

M and (CeH&PO = 1.60 X A I ; there- fore, a plot of 1/OD-OD where OD is the optical 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 tempera- ture. 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.

TABLE I RATE CONSTANTS IN BENZENE

T e m p . , 'C.O k2, 1. mole-' sec.-'b Ref. methode

7.35 7.50 x 10-4 0 Ultraviolet 25.00 4.40 x 10-3 8,9 Ultraviolet titr. 35.00 11.3 x 10-3 8,9 Ultraviolet titr.

a =t0.02. Rate = k?(Ss)((C6H&P). Ultraviolet an- alysis of sulfur, titr. iodometric analysis of the phosphine.

The main requirements for the use of this method are (i) the rate of reaction is moderately slow a t the lowest temperature; (ii) the tem- perature in the cell must be uniform; (iii) the acti- vation energy does not vary with temperature; (iv) the boiling point of the solvent cannot be ex- ceeded; and (v) Beer's law must be obeyed even though the molar extinction coefficient may change with temperature. Requirement (ii) restricts the size of the cell and the rate of heating. If the activation energy varies greatly with tempera- ture one would be unable to obtain the correct order of the reaction and the rate constants. The main disadvantage of this method is that only a small percentage of reaction is used to determine the rate at any one temperature. The advantage is obtaining the rate, the order of the reaction, the frequency factor and the activation energy in a single rapid experiment.

Experimental The purification of sulfur, triphenylphosphine and benzene

has been reported previously.* The use of the ultraviolet absorption spectrum of sulfur to study this reaction a t a constant temperature and with conventional techniques will be subject to a forthcoming publication.@ The thermo- stated cell compartment for a Beckman D U spectrophotom- eter has been discussed.1O The brass jacket was carefully made to ensure good thermal contact with a square Corex cell. To the top of the cell was sealed a 7 mm. Pyrex t8ube (15 cm. in length) through which a multi-junction thermo- couple was placed into the cell just above the light path The output of the thermocouple was applied to a Speedomax recorder. The reactants were mixed and placed in the cell (total volume 1 to 2.6 4). Water, circulated a t the rate of one gallon per minute through the compartment, was slowly heated. Optical density measurements were manually re- corded every 30 seconds at 345 mp. The heating rate does not enter into the graphic analysis of the data but averaged 0.5 degree per minute to cover the range of 23 to 77". Other heating rates can be used to cover only a 25' increase.

(10) P. D. Bnrtlett and R. E. Davis, J . Am. Chem. SOC., 80, 2513 (1958).

THE CHELATING TENDENCY OF RIBOFLAVIN'

BY T ~ 0 a r . k ~ R. HARKINS A N D HENRY FREISER~

Received August 4 , 1958

The structural similarity of riboflavin (I) and 8- hydroxyquinoline (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 un- expected. Of course, it is possible that the pres-

c~HiiO4

6 H I

I OH

I1

ence of a sterically hindering group in riboflavin (the fused benzene ring) would have a sufficiently greater effect on copper(I1) which is smaller thaii iron(I1). The sterically hindering group present in 2-methyl-8-hydroxyquinoline was shown to have a greater effect on the chelates of the smaller metal ions.4 Another interesting observation concerning metal-riboflavin complexes was made by Foye and Lange6 who prepared a series of such complexes whose composition corresponded to a two to one mole ratio of metal to riboflavin.

The stoichiometry and thermodynamics of the formation of metal-riboflavin reported in this paper was undertaken to evaluate these observa- tions.

Experimental Stock solutions of approximately 0.01 M metal ions were

prepared by dissolving their reagent grade perchlorates (G. Frederick Smith Co.) in water. The copper(I1) and cobalt( 11) solutions were standardized by electrodeposition. The nickel( 11) solution was standardized by precipitation with dimethylglyoxime. Zinc(I1) was standardized gravi- metrically as ZnNH4P04. The iron(I1) solution was pre- pared by dissolving high purity iron wire in perchloric acid under an inert atmosphere.

Riboflavin, obtained ofrom the Nutritional Biochemicals Corp., was dried a t 110 . Anal. Calcd. C, 54.25; H, 5.36. Found: C, 55.50; H, 4.99.

The titration apparatus and rocedure have been pre- viously described .e A slight mosification in preparing the solution for titration was undertaken to facilitate the dis- solution of riboflavin. Fifty-five milliliters of water was added to a weighed quantity of the reagent. A small meas- ured volume of standard base was added to bring about solution after which the perchloric acid and metal perchlor- ate were added and the titration proceeded in the customary

( I ) Abstracted from the thesis submitted by T. R. Harkins in partial fulfillment of the requirements for the Ph.D. degree at the University of Pittsburgh, June, 1956.

(2) Department of Chemistry, University of Arizona, Tucson. (3) A. Albert, Biochem. J . , 64, 646 (1953). (4) W. D. Johnston and H. Freiser, Anal. Chim. Acta, 11, 201

(1954). (5) W. 0. Foye and W. E. Lange, J . Am. Chem. SOC., 7 6 , 2199

(1954). (0) €1. Freiser, R. G. Charles and W. D. Johns ton , i t i d . , 7 4 , 1383

11952).