LITERATURE CITED i.

2.

3.

4.

5. 6o 7. So 9. i0. ii.

V. M. Berdnikov and G. A. Bogdanchikov, "Calculation of the probability of transition in reactions of outer-sphere electron transfer in dilute aqueous solutions," Zh. Fiz. Khim., 53, No. 2, 273-283 (1979). R. R. Dogonadze and A. M. Kuznetsov, "Kinetics of chemical reactions in polar solvents," Results of Science, Physical Chemistry Series. Kinetics [in Russian], Vol. 2, VINITI, Moscow (1973). G. M. Brown and N. Sutin, "A comparison of the rates of electron exchange reactions of ammine complexes of ruthenium(II) and (III) with the predictions of adiabatic outersphere electron transfer models," J. Am. Chemo Soc., i01, No. 4, 883-892 (1979). H. C. Stynes and J. A. Ibers, "Effect of metal ligand bond distances on rates of electron-transfer reactions. The crystal structures of hexaammineruthenium(II) iodide, [Ru(NH3)6]2, and hexaammineruthenium(IIl) tetrafluoroborate, [Ru(NH3)6][BF4]3," Inorg. Chem., iO, No. i0, 2304-2308 (1971). W. P. Griffith, "Infrared and Raman spectra of group VIII ammine complexes," J. Chem. Soc., A, No. 7, 899-901 (1966). M. B. Fairey and R. J. Irving, "The infrared spectra of some armnine nitrosyl compounds of ruthenium," Spectrochim. Acta, 22, No. 2, 359-369 (1966). Yu. I. Kharkats, "Calculation of the rate constant of electron transfer in a polar medium," Elektrokhimiya, 12, No. 4, 592-595 (1976). D. Waysbort and G. Navon, "Proton ~ and covalency parameters of ruthenium(III) hexaammine,"Jo Chem. Phys., 59, No. i0, 5585-5590 (1973). S. G. Entelis and R. P. Tiger, Kinetics of Reactions in the Liquid Phase. Quantitative Consideration of the Effect of the Medium [in Russian], Khimiya, Moscow (1975). T. J. Meyer and 11. Taube, "Electron-transfer reactions of ruthenium ammines," Inorg. Chem., 7, No. ii, 2369-2379 (1968). R. A. Ma--rcus, "On the theory of oxidation--reduction reaction involving electron transfer," J. Chem. Phys., 24, No. 5, 966-978 (1956)o

KINETICS AND MECHANISM OF THE REACTIONS OF DIAZONIUM SALTS WITII HYDROQUINONES IN ETHERS UDC 547.024.556.7

L. A. Rykova, L. A. Kiprianova, and I. P. Gragerov

In [i] we established that the reaction of benzene diazonium fluoroborate with hydroquinone (IIQ) in solution in aqueous dioxane in the absence of oxygen is a radical-chain process, which obeys the kinetic equation

w = k [PhN +]

[HQ

When the reaction was carried out in aqueous dioxane or aqueous tetrahydrofuran, chemically induced dynamic nuclear polarization (CIDNP) was observed in the hydrogen of the obtained benzene, but only in experiments with a high concentration of hydroquinone. This observation was explained by means of the reaction mechanism proposed in [i], in which the hydroquinone served as initiator: initiation: PhN~ + HO--C~H4-- OH - - ~ P h N 2 ' 9 HO§ -H+~N~

Ph--O--C6H4--OH 5

C6H40H 5

~--Ph + " o - % H 4 - O H ~

L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 17, No. 4, pp. 542-545, July-August, 1981. Original article submitted September 18, 1980.

0040-5760/81/1704- 0423507.50

9 1982 Plenum Publishing Corporation

423

chain propagation: /---X Ph" + Ok_jO ~

/---X" Ph 4- OM_/O ;

+

r-r --""Ph'+

0 O+N2; k--../

(l)

chain termination:

/--x 2 0 0 ~

inactive products.

The p o l a r i z e d benzene is produced from the phenyl radicals which appear in the initiation stage but not in the chain propagaticn stage, where there are no radical pairs. From the scheme it is seen that the rate of formation of polarized benzene and its fraction in the total amount of benzene is proportional to the concentration of hydroquinone, and the overall rate of accumulation of benzene is proportional to the square root of this concentration. The rate of accumulation of polarized benzene therefore increases with the concentration of hydroquinone more rapidly than the overall rate of formation of benzene, and at high concentrations of hydroquinone the polarized benzene is detected in the general mass of benzene.

In the present work we investigated the effect of substituents in the molecules of the hydroquinone and diazonium salt on the kinetics and mechanism of the reaction and on the accompanying polarization phenomena. By the N ~ method we studied the reduction of PhN2+BF~ - by aqueous tetrahydrofuran (THF:H20 = 4:1), which had been purified from oxygen by blowing with argon, in the presence of additions of hydroquinone, durohydroquinone (DHQ), and chlorohydroanil (CHA). We found that catalytic amounts of the hydroquinones lead to quantitative reduction of the diazonium salt to benzene. Chemically induced dynamic nuclear polarization in the benzene (emission) is only observed with high concentrations of the hydroquinones. These data show that all the investigated reactions are radical-chain processes. The introduction of both electron-donating and electron-accepting substituents into the ring of the hydroquinone increases the overall rate of the transformation. At 40~ and with the diazonium salt at concentrations of 0.25 M the presence of 0.25 M of durohydroquinone increases the rate of the chain process by eight times, and the presence of 0.25 M of chlorohydroanil increases it by 2.5 times compared with the rate of the reaction in the presence of hydroquinone. If it is assumed that a mechanism of the same type as in scheme (i), in which the hydroquinones only take part in the initiation stage, is retained in the reactions with substituted hydroquinones and account is taken of the fact that the overall rate of the process is proportional to the square root of the initiation rate Wo, the ratio of the rate constants for initiation by the investigated hydroquinones varies in the following way: DHQ CHA HQ DHQ CHA H Q Wo :Wo :Wo = k b :ko :ko - - ~ 6 4 : 6 , 2 5 : 1 .

(2)

Increase in the initiation rate with the introduction of substituents into the hydroquinone ring shows up in the decrease of the concentration of hydroquinone at which CIDNP is detected in the benzene. In the case of the reaction of 0.25 M of a solution of PhN=+BF~- at 40~ with hydroquinone CIDNP appeared at [HQ] > 1.5 M, and in the case of the reaction with durohydroquinone it appeared at [DHQ] > 0.3 M. With the introduction of an electron-donating substituent p-CH30 into the ring of the benzenediazonium fluoroborate the character of the reaction changes. Proportionality is observed between the degree of transformation of p-CH3OC6H4N2+BF4 - and the amount of initial log W l,o

ao / ~._,-1

06

...... I

lo;~[Arlq.2j -,~0

E

log W I,o

F,2

~ , I~_1

- ,O~

o,~

o,2 I.

0

log

I

I

~

F"

- Z2 2,2 / / , za

[

I

- ,I~

1

-,zO

"02 l

w "

424

-o,6

Fig. i. Dependence of the initial rate Wo of the reaction between p-CH3OC~II~N2+BF~ and durohydroquinone (DHQ) in aqueous tetrahydrofuran at 20~ a) on the concentration of the salt; [DHQ] = 0.005 M; b) on the concentration of DHQ; [CH3OC6H~N2 +BF~-] = 0.i M.

DHQ. The reaction becomes insensitive to the action of inhibitors of radical-chain processes. (Ionol and benzoquinone were tested as inhibitors)o The given data indicate a nonchain mechanism for the transformation. The change in the reaction mechanism is probably due to a decrease in the electron-withdrawing capacity of the methoxy-substituted diazonium salt compared with the unsubstituted salt and to the resultant sharp retardation or complete cessation of chain propagation, during which electron transfer occurs from the radical of the ether to the diazonium salt [see scheme (i)]. The concentration dependence of the rate of the reaction of p-CH~OC6H4N2+BF4 - with durohydroquinone in a 4:1 mixture of tetrahydrofuran and water at 20~ was investigated by a volumetric method from the nitrogen evolution rate. The obtained reaction order was first in each of the components, i.e., the diazonium salt and durohydroquinone (Fig. i). These data confirm the scheme given above for the initiation of the chain reaction. The effect of the nature of the hydroquinone on the initiation rate of the chain transformation shows that the slow stage of initiation cannot be electron transfer, since the rate of transfer would be determined by the ionization potentials of the investigated hydroquinones and would decrease with increase in the ionization potentials IDH Q < IHQ < ICHA [2], and this is not consistent with the obtained sequence of rates. Initiation also cannot be limited by the formation of the complex x

x

ArN~. H O ' ~ O H

X

(X=CI,H, CH:~),

X

since the rate of this reaction must also change in parallel with the changes in the ionization potentials of the hydroquinones or their basicity and decrease with the introduction of bulky substituents into the hydroquinone ring, and this contradicts the experimental data. We suppose that the slow state of initiation in the investigated chain transformations and the slow stage in the nonchain reaction of methoxy-substituted salt with DHQ is dissociation of an intermediate compound of the diazonium salt with the hydroquinone. The process can be represented by the following scheme: s •

x

AFN~+ H0

x

OH k ~

x

ArN2:

OH ~ 3 slow

k2 x

x

x

x

•

. ArN2.. ~l

OH

H

x

x

x

3 X

X

_Nz ,'-At9- 0

X

X

OH ----,'--Ar" + "0-x

x

OH~

x

w h i c h c o r r e s p o n d s to a k i n e t i c e q u a t i o n o f second o r d e r i f scheme e x p l a i n s the e f f e c t o f the s u b s t i t u e n t s i n the r i n g

x

the k l / k 2 v a l u e i s s m a l l . This o f the h y d r o q u i n o n e , s i n c e the

dissociation of the intermediate product must occur more readily, the more stable the semiquinone radical-cations which form. It is known [3, 4] that the radical-cations I and II are fairly stable and are more stable than the radical-cation III: CH3

CH3

Cl

CI

OH~ CH,,3

i

CH3

el.

CI

L

HO'+---~OH.

m

Evidently, the stability of I is higher than that of II on account of the electron-donating nature of the CHa groups and of their participation in ~,~ conjugation. Therefore, the stability of the radical-cations probably varies in the order I > II > I I I , which corresponds to the order of decrease in the rate of initiation of the reduction of benzenediazonium fluoroborate by ethers with the addition of the corresponding hydroquinones.

425

LITERATURE CITED i.

o

3. 4.

L. A. Rykova, L. A. Kiprianova, A. F. Levit, and I. P. Gragerov, "Investigation of the mechanism of the chain reaction of benzenediazonium fluoride borate with ethers in the presence of hydroquinone," Teoro Eksp. Khim., 14, No. 2, 256-260 (1978). V. N. Kondrat'ev (editor), Dissociation Energies of Chemical Bonds, Ionization Potentials, and Electron Affinity [in Russian], Nauka, Moscow (1974). J. R. Bolton and A. Carrington, "Line-width alternation in the electron spin resonance spectrum of the durosemiquinone cation," Molec. Phys., ~, No. 2, 161-167 (1962). A. F. Levit, L. A. Kiprianova, T. G. Sterleva, and I. P. Gragerov, "Effects of chemical polarization of nuclei during the oxidation of phenylhydrazine by 1,4-benzoquinone or tetrachloro-l,4-benzoquinone," Teor. Eksp. Khim., 12, No. 3, 406-409 (1976).

KINETICS OF THE ELEmeNTARY STAGES IN THE OXIDATION OF PHENYLHYDRAZINE BY QUINONES L. A. Rykova, L. A. Kiprianova, and I. P. Gragerov

to of of in

UDC 547.024.556.8

During the investigation of the oxidation of phenylhydrazines by quinones, which leads the formation of ArH compounds, hydroquinones, and nitrogen, it was suggested on the basis the kinetic data [i, 2], the kinetic isotope effect of deuterium [2, 3], and the effect the nature of the solvent and quinone on the process [4] that transfer of a hydrogen atom a reversibly formed complex takes place in the controlling stage: 5

I The aim of the present work was to confirm scheme (i), to obtain the numerical values of the rate constants kl, k2, and k3 of its elementary stages on the basis of kinetic data, and to establish the relations between these constants with variation in the nature of the quinone. The reaction kinetics were investigated by a volumetric method from the nitrogen evolution rate. We studied the temperature dependence of the kinetics for the oxidation of phenylhydrazine by duroquinone, 2,6-di-tert-butylquinone, and p-benzoquinone and established that at certain temperatures the curves for the accumulation of nitrogen acquire an S-shaped character. In Fig. 1 such curves for the reaction of phenylhydrazine with duroquinone are given as an example. With increase in temperature the inflection point is shifted toward the origin along the time axis. At 60~ the S-shaped character disappears, and the form of the kinetic curve corresponds to a first-order reaction. At the given temperature the time for the attainment of the inflection point (tinf) depends on the nature of the quinones; for the oxidation of phenylhydrazine (0.25 M) by duroquinone, 2,6-di-tert-butylquinone, and pbenzoquinone (0.05 M) at 20~ tin f amounts to 1600, 500, and 5-10 sec, respectively. During oxidation of 2,3-dicyano-5,6-dichloroquinone in acetonitrile the inflection on the kinetic curve is absent even at 0~C. It is known [5] that the S-shaped character in kinetic curves can be observed either in radical-chain or in consecutive transformations. A chain mechanism was ruled out, since we found that the rate of release of nitrogen does not depend on the presence of inhibitors of radical-chain processes. On the other hand, the scheme of consecutive reactions (i), as will be shown below, not only explained the S-shaped character of the kinetic curves and the dependence of tin f on the nature of the quinone and experimental conditions qualitatively but also made it possible to obtain the numerical values of kl, k2, and k3 for the reaction of phenylhydrazine with duroquinone.

L. V. Pisarzhevskii Institute of Physical.Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 17, No. 4, pp. 545-549, July-August, 1981. Original articlesubmitted September 18, 1980o

426

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9 1982 Plenum Publishing Corporation