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Vol. 66
KOTES
FORMATION CONSTAST OF THE 1:1 PYRIDINE-IODINE COMPLEX BY ARTHURG. MAKIAND EARLEK. PLYLER National Bureau of Standards, Washington 16, D. C Recezved October 37, 1961
There has been considerable interest recently in the complexes of halogens with aromatic and amine-like compounds. Plyler and n'lulliken' have shown that solutions containing the pyridineiodine complex have an absorption band at 184 cm.-l. This absorption band now has been used to measure the formation constant for the pyridineiodine complex. ,4dditional measurements also were made on the pyridine absorption band a t 601 cm.-l. This band is shifted to 618 cm.-l in the complex and thus may be used to measure the concentration of uncompIexed pyridine. Experimental The far infrared measurements were made using a vacuum spectrometer recently constructed a t the Bureau of Standards. I n this instrument a 320 lines-per-inch grating was used. Black polyethylene and quartz windows were used as filters and tests showed that the stray radiation was quite negligible. Measurements in the 600 cm.-1 region were made with a double-beam infrared spectrometer with a KBr prism. The sample cells used were constructed of Teflon and had either KBr or quartn windows depending on the wave length studied. Cell lengths of 0.75, 1.0, and 2.0 cm. were used. All solvents used and the pyridine were spectro grade. The iodine used was reagent grade. The manufacturer states that the pyridine was 99 % pure and contained less than 0.1% moisture. Subsequent detailed studies of the infrared spectrum of this sample revealed no trace of impurity, thus indicating that the purity is considerably better than 99%. The spectrophotometric measurements were made on solutions freshly prepared by mixing two previously prepared solutions of pyridine and iodine. A11 measurements were completed within an hour after the solutions were mixed. In this way any aging process was eliminated. Considerably more time, of the order of 10 hr., was required to give any discernible changes in the solutions although this varied with the solution concentrations. No special effort was made to obtain accurate temperature regulation. Experimental error was sumcient to mask any small fluctuations in formation constant due to temperature changes. An average sample temperature of 26' is applicable to all measurements.
centration of the pyridine-iodine complex. To determine the concentration of the complex the absorption was graphically integrated over the entire range for which absorption was evident. This was done by using the integral
in which c = concentration in moles per liter of solution, b = path length in cm., and Y is the frequency in The value of E:!: was determined by using a large excess of pyridine and ai1 amount of iodine sufficient to yield a concentration of complex approximately equal to that found in the unknown solutions. That is to say, the solution used to determine E::;had an absorption band with about the same area as the absorption band in the unknown solutions. In determining E;:; a correction was made for the noli-infinite concentration of pyridine. For this purpose a tentative value of 100 for the formation constant was used. I n order to miiiiniize errors due to long term instrumental fluctuations the value of E;:; was remeasured for each determination of the complex concentration. TABLE I FORNATION COXSTAXT DATAFROM TION
Concn. in moles/liter
+
Results and Discussion The experimental determination of the formation constant was made by measurements on the bands a t 184 and 601 cm.-l. The measurements on the 601 cm.-l band mere used primarily as a check of the results obtained from the 184 an.-' band. For this check only the peak absorption coefficient was used. Duplicate runs were made on the same solution and five different mixtures were measured. I n order to minimize errors due to the non-linearity of a Beer's law plot, the extinction coefficient was measured for pyridine concentratioiis approximately equal to those measured for the pyridine-iodine solutions. The concentrations given in Table I and for all other measurements in the 200 cm.-l region were calculated from the known initial concentrations of iodine and pyridine and the value of the con(1) E. K. Plyler and R. s. hfulliken, d . Am. Chem. Soe., 81, 823 (1959).
THE 184 CM.-' AnsowBANDWITH CYCLOHEXANE AS SOLVENT
Pyridine
0.00592 ,00605 .00605 ,00522 ,00654 ,00715 ,00722 .00293 .00268 .00290 .00233
I2
Pyridine. Is complex
0.00586 ,00599 .00595 ,00578 ,00228 .00259 .00265 ,00767 .00742 ,00764 ,00707
0.00318 ,00305 .00305 .00388 .00256 ,00195 ,00188 .00183 ,00208 .00186 .00243
Ka/ (l./mole)
92 84 84 128 172 105 98 81 105 84 148
Av. 107 i 25
a
K
=
[PY~IzI/[P~I [Izl.
Since a stable ionic complex of pyridine with iodine e ~ i s t s ,care ~ , ~ vas exercised throughout this york to ensure that this stable complex, dipyridineiodine heptaiodide, was not present in appreciable quantities. The stable coniplex is ionic and consequently insoluble in most organic solvents. It has infrared absorption bands a t 636 and 437 em.-', neither of which were observed in the solutions used for this work. Upon standing for several days, however, these solutions were observed to yield a precipitate of this complex. The formation constants obtained independently from measurements on the two different infrared absorption bands and in the solvents CC14, aheptane, and cyclohexane all agree within the limits of experimental error, For the formation constant of the 1: 1 pyridine-iodine complex ( P y . 1 ~ )the best value obtained by this work is (2) A B. Presoott and P. F. Trowbridge, rbrd., 17,859 (1895). (3) 0. I-Iassel and 11. TTopo, A c f a Chem. Scnnd., 15, 407 (1961).
April, 1962
767
SOTES
or less no effective competition occurs at conceiitratioiis of up to 5 mole%. The collision efficiency of the reaction between H atoms and benzene in the gas phase is 1.3(4) A. I. Popov and R. H. Rygg, J . Am. Chem. Soc., 79, 4622 7.9 X l O d 4 . I o Arguments have been presented (1957). to show that gas-phase data for these reactions are relevant to the liquid phase.li It consequently is THE MECHANISM OF T H E unlikely that benzene could compete with reaction “UNIMOLECULAR” YIELD OF HYDROGEN 2. The iiiconsistency between known collision efI N T H E RADIOLYSIS OF LIQUID ficiencies and those required for competition with CYCLOHEXANE a diffusion controlled reaction is seen again in this BYP. J. DYNE analogous argument. Our data, when plotted in the manner suggested by Roy, Hamill, and WilResearch Chemistry Branch, Atomic Eneruy of Canada Ltd., Chalk River, Ontario liams, and interpreted in terms of a diffusion-conReceined Noaember 1, 16Mi trolled reaction, show that the collision efficiency Some of the hydrogen produced in the radiolysis for the radical scavenger reaction for both benzene of cyclohexane is formed by a “unimolecular” or and iodine would have to be close to unity. Since “intramolecular” mechanism where both the hy- the collision efficiency of the reaction in the gas C6H12 4 Hz C6Hll is -1 X 10-7,11J2 drogen atoms come from the same hydrocarbon phase H molecule.1-3 Our experiments1 have det,ected it follows that benzene and iodine should compete this reaction by measuring the yield of D, produced with the homogeneous bimolecular reaction sein the radiolysis of c-C~H~~-C-C~DIZ mixtures which quence is first order m7ith respect to the concentration of C6H12 --+ C6Hil H C6D12. H CsHiz --+ H2 CeHii Ingalls4 reports similar studies on the radiolysis of deuterated toluene and suggest,s that the mech- a t coiicentrations mole fraction. Experianism of such a “unimolecular” yield may be written mentally, however, a concentration of benzene > (for cyclohexane) loe3 mole fraction is required to reduce G(H2) C-CSDI~ CsHii D to about one-half its original value. (1) Similar arguments can be applied to iodine where, D C6Dll --+ C D I O DZ (2) as previously has been argued by R. H. Schuler,13 where reaction 2 is a diff usion-controlled react’ioii between the atom and radical formed in reaction 1. its effect 011 G(H2) shows that the collision efThe kinetics of this type of reaction have been ficiency of the H atom-iodine reaction is much studied extensively by Hamill and Williams and less than unity. In studying the photolysis of HI in hydrotheir collaborator^.^-^ The yield of deuterium in this reaction sequence would be first order with carbon solvents, Nash, Williams, and Hamill14 observed no diff usion-controlled recombination of respect to the C6D112concentration. We recently have shown* t,hat the “unimolecu- H and I atoms at room temperature; this they lar” yield of D2 in these c-C&z--c-C6D12 mixtures attributed to “the small size of the H atom and is reduced on the addition of benzene and iodine. the relatively open structure of the solvents.” Since we assumed t’hat the “unimolecular” de- It consequently can be argued that there is a low composition did n o t involve the separation of free probability of reaction 2 occurring in a diffusioiideuterium atoms, we argued that the int,eraction controlled reaction. There seems to be no a priori reason why the must be by a quenching or energy-transfer type of process. A very reactive hydrogen atom scaven- “unimolecular” decomposition should not go by ger may, however, compete with reaction 2 and a true molecular splitting-Le., a reaction which our arguments consequently need re-examination.9 is mechanistically described by C6D12--+C6D10 Roy, Williams, and Hamills have shown that a Dz, since analogous reactions are well establishedl”16 scavenger can only compete with a diffusion-coii- in the vapor-phase radiolysis and photolysis where trolled radical reaction if the collision efficiency of no diffusion-controlled reactions can occur. the reaction between the scavenger and the radical We conclude that (1) we are justified in assuming is close to unity. If t,he collision efficiency is 0.1 that benzene and iodine reduce the“unimo1ecular” yield by a quenching or energy-transfer mechanism, (1) P. J. Dyne and W. M. Jenkinson, Can. J . Chem., 38,539 (1960). rejecting a scavenging interaction and ( 2 ) part, if (2) T. D. Nevitt and L. P. Remsberg, J . Phys. Chem., 64, 969 (1960). not all, of the “unimolecular” yield probably occurs ( 3 ) M. Burton and J. Chang, private communication. by a true molecular splitting.
107 i 25 l./mole in cyclohexane solution a t 26’. This compares quite favorably with the value of 101found by Popov and Rygg4in CC14.
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14) R. B. Ingalls, J . Phgs. Chem., 66, 1605 (1961). ( 5 ) J. C.Roy, R. R. Williams, Jr., and W. H. Hamill, J . Am. Chem.
76,3274 (1954). ( 6 ) J. C.Roy, W. H. Hasmill,and R. R. Williams, Jr., ihid., 77,2953 (1955). (7) J. C. Roy, J. R. Naah, R. R. Williams, Jr., and W. H. Hamill, ibid.. 78, 519 (1956). (8) P. J. Dyne and W. 14. Jenkinson, Can. J. Chem., 39, 2163 (1961). (9) The author is indebted to Dr. H. Sohwarz who first drew his atSoe.,
tention t o this noint.
(10) P. E. M. Allen, H. W. Melville, and J. C. Robb, Proc. Roy. SOC.(London), 8218,311 (1953). (11) T.J. Hardwiok, J . Phys. Chem., 66, 101 (1961). (12) H.E. Sohiff and E W.R. Bteaoie, Can. J . Chem., 29, 1 (1951). (13) R. H Sohuler, J. Phys. Chem., 61,1472 (1957). (14) J. R. Nash, R. R. Williams, Jr., and W-.H. Hamdl, J . Am. Chem. Soc., 82, 5974 (1960). (15) L. M.Dorfman. J. Phws. Chem.. 62,29 11958). (16) Myran C. Sailer, Jr., and L. M. Dorfmah, J . Chem. Phys., 35, 497 (1961).
