CALORIMETRIC INVESTIGATION OF COMPLEXATION OF PYRIDINE

WITH ORGANIC PEROXIDES

M. A. Kovbuz, I. I. Artym, A. B. Khotkevich, and Yu. A. Serguchev

UDC 541.49

The literature data on complexation of peroxides with amines are few and contradictory [1-3]. Interest in the study of these complexes, decomposing with the formation of radi- cals, is due both to their use as initiators of polymerization reactions [4] and to the fact that the complexation process affects the synthesis of polyfunctional peroxides carried out in pyridine [5]. In the present paper, we investigated the thermodynamics of the com- plexation process of pyridine with a series of organic peroxides of different classes by microcalorimetry, making it possible to determine the thermodynamic parameters of the processes directly and with greater accuracy than would be possible by spectrophotometric and other indirect measurements.

The investigations were carried out with a DAK 1-1-model differential microcalorimeter equipped with a feed device making it possible to feed a liquid reagent with a total volume of up to 1 ml into calorimetric cells in several portions. In most cases, the measuring cell contained a 0.03-0.1 M solution of the peroxide with a volume of 0.6-1.0 mi, the reference cell was an equal volume of the solvent, and the containers of the feed device contained a 1-3 M solution of pyridine. Such an order of mixing made it possible to eliminate the thermal effect appearing during change of the pyridine concentration in the cell. Chloro- benzene was used as the solvent. The choice of the solvent was dictated mainly by the technical possibilities of the instrument.

The total amount of heat Q evolved in the equilibrium reaction of compounds D and A in a !:I ratio with equilibrium constant K in volume V is

Q = ( - - AH) dN = ( - - AH) V [DAI.

For [D] >> [A], t h e f o l l o w i n g e q u a l i t y becomes v a l i d :

[DA] = K[D] [A]/(1 + KID]);

t h e r e f o r e , Q/V(--AH)=K[D][A]/(I+K[D]). C a r r y i n g o u t s i m p l e t r a n s f o r m a t i o n s , we o b t a i n

[A] V 1 -k K [D] 1 Q = K [DI ( - - AN) = ( - AH)-I K [DI ( - - AH)

I f we p l o t a g r a p h in c o o r d i n a t e s X = [ D ] -1, Y=[A]V/Q, t h e r e c t i l i n e a r i t y of t h e r e l a t i o n w i l l i n d i c a t e r e a c t i o n of t he r e a g e n t s i n a 1 :1 r a t i o , t h e o r d i n a t e a t t h e z e r o p o i n t w i l l g i v e the value of (--AH) -i, and the tangent of the angle of inclination of the relation will be I/K(--AH). Thus, it seems possible to determine --AH and K of the equilibrium compound by treating the relation of the total thermal effect to the amount of the added reagent.

The mentioned procedure was used to investigate the reaction of pyridine with the following peroxides: tert-butyl hydroperoxide (TBHP), tert-butyl peroxide (TBP), benzene peroxide (BP), lauryl peroxide (LP), methyl tert-butylmonoperoxysuccinate (TBMPS), and the polyfunctional peroxide (O,O'-dicarbo-tert-butylperoxy)succinyl peroxide (DTBPS). For the monofunctional peroxides, the Y(X) relation is practically linear (r~0.99), and for DTBPS r = 0.975.

From the calculation results given in Table i, it is evident that the greatest value of AH of formation of the complex corresponds to the reaction of tert-butyl hydroperoxide with pyridine. Apparently, this is because the starting molecule contains a hydroxyl group tending to form a hydrogen bond. For complexation of the diacyl peroxides (BP and LP),

Institute of Organic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Trans- lated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 20, No. 5, pp. 631-633, September-October, 1984. Original article submitted July 18, 1983.

0040-5760/84/2005-0595508o50 �9 1985 Plenum Publishing Corporation 595

TABLE i. Thermodynamic Constants of the Reaction of Pyridine with Organic Peroxides in Chlorobenzene at 303.2~

Peroxide

TBHP BP LP TBMPS DTBPS TBP

--AH. K, k J/mole ~ters/mole

24,4q-1,0 1, O1 =t::0,10 3,5::t=0,3 3 ,1~0 ,2 3 ,4 • 9,3=t:1,0 4,9-4-0,3 0 ,51• 7,1-t-O,6 I 1,0-+-0,4

no react ion observed

AS, J /mole �9 K

--55 +17 +26

- -2 ,8 - -2 ,9

a positive value of AS of the process is characteristic, which may be a consequence of ligand exchange in the chlorobenzene--peroxide--pyridine ternary system. For the per ester (TBMPS), such ligand exchange is manifested to a lesser degree, as indicated by the negative value of AS. Complexation of pyridine with the peroxide containing one diacyl group and two per ester groups (DTBPS) apparently occurs predominantly at the per ester fragments, as indicated by the equality of the values of AS of formation of the complexes of DTBPS and TBMPS.

On the whole, the values of the enthalpies of formation and the equilibrium constants of the investigatedcomplexes do not confirm the previously stated [i] conclusions concerning their high stability. In particular, for the alkyl peroxide (TBP), no heat evolution at all was observed in the reaction with pyridine. The obtained data attest to the very low com- plexing ahility of peroxide compounds with respect to medium-basicity amines.

The anomalously high values of the equilibrium constants of the reaction of benzene peroxide with pyridine given in [i] may be partially due to Vartapetyan et al.'s choice of dimethylformamide as the solvent. The article does not describe the procedure for preparation of the solvent, but dimethylformamide purified by the usual methods may contain impurities of dimethylamine and other peroxide-reactive substances in concentrations comparable with the concentrations of the solutions used by Vartapetyan et al. (~i0 -3 mole/liter). The specificity of such a system is also indicated by the results of [3], where for the benzoyl peroxide-piperidine pair in dimethylformamide an anomalously high value of AH of reaction (--117 kJ/mole) was determined, and in more inert solvents no complexation was observed at all.

The values of the thermodynamic constants of the systems that we investigated indicate that the complexation effects are comparable in magnitude with the solvation effects of the starting molecules; therefore, it is more correct to consider the thermodynamics not of a binary system, but of a ternary system. The discrepancy of the literature data on the com- plexation of peroxides with amines is most probably a consequence of the limited applicability of indirect measurement methods in investigation of such systems.

LITERATURE CITED

I. O.A. Vartapetyan, O. A. Chaltykyan, and S. A. Khachatryan, "Complexation between benzoyl peroxide and pyridine in dimethylformamide," Arm. Khim., Zh., 25, No. 9, 735- 739 (1972).

2. G.L. Bitman, Yu. Z. Karasev, and M. E. Elyashberg, "Investigation of molecular associ- ation of tert-butyl hydroperoxide with benzene derivatives by IR spectroscopy," Zh. Obshch. Khim., 44, No. 6, 1361-1365 (1974).

3. O.A. Vartapetya-~, O. A. Chaltykyan, and S. A. Khachatryan, "Complexation between benzoyl peroxide and piperidine in dimethylformamide," Zh. Fiz. Khim., 47, No. 6, 1371- 1373 (1973).

4. K. O'Driscoll and E. Richezza, !'Polymerization with redox initiators. 2. Complex for- mation between benzoyl peroxide and dimethylaniline," J. Polym. Sci,, 46, No. 147, 211- 216 (1960).

5. I.I. Artym, S. S. Ivanchev, M. M. Soltys, and M. A. Kovbuz, "Optimization of the syn- thesis of tert-butyl monopersuccinate by mathematical experiment design," Zh. Prikl. Khim., 49, No. 9, 2056-2059 (1976).

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