Solvent effect on reaction rate constants
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
95
nitrile. This scale of differences in k(free) is compared with
the inaccuracy of calculations caused by differences in
the activity coefficients of the components of equilibrium
of ArOH and Ar(OH)2 with the solvents and the reactions
of Ph2N• with ArOH and Ar(OH)2 in CCl4, toluene,
acetonitrile, and decane.31 Based on the obtained data,
we can conclude that the experimental rate constants of
the reactions of Ph2N• with ArOH and Ar(OH)2 in toluꢀ
ene and acetonitrile are lower than those in decane due to
the formation of Hꢀcomplexes of the solvents (toluene
and acetonitrile) with ArOH and Ar(OH)2 molecules. This
assumption has been advanced earlier5,6 and agrees with
the data on the effect of the solvent nature on the rate
constants of the reactions of phenols with other free radiꢀ
cals.25,31—37
The linear dependence (3) between logk and DOH in
acetonitrile was used to estimate DOH in 2,5ꢀdichloroꢀ
hydroquinone for which the respective data are lacking.
Substituting k = 1.45•106 L mol–1 s–1 into (3), we obtain
DOH(2,5ꢀCl2ꢀ4ꢀHOC6H2OH) = 346.6 kJ mol–1. This
value is by 5.4 kJ mol–1 lower than DOH in unsubstituted
hydroquinone, which is somewhat unexpected, because
the electronꢀwithdrawing substituents usually increase
DOH in the phenols and hydroquinones and the electronꢀ
releasing substituents decrease DOH (see Refs 16,18,
and 38).
References
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Thus, it follows from the obtained data that the
diphenylaminyl radicals are highly reactive in the reacꢀ
tion of H abstraction from phenols and hydroquinones;
the k rate constant depends on the nature of substituents
in phenols and hydroquinones and on the solvent nature.
The k values in decane at 294 K lie in a range of 106—108
L mol–1 s–1 and decrease by 6.5 and 50 times, on the
average, when decane is replaced by toluene and acetoꢀ
nitrile, respectively. The dependence of k on the solvent
nature is caused by the formation of Hꢀcomplexes of the
phenols and hydroquinones with the solvent. The logaꢀ
rithms of the rate constants logk for the reactions in deꢀ
cane, toluene, and acetonitrile depend linearly on the
dissociation of the O—H bonds in the phenols and
hydroquinones. Using this dependence and the rate conꢀ
stant of the reaction of Ph2N• with 2,5ꢀdichlorohydroꢀ
quinone in acetonitrile, we estimated the dissociation enꢀ
ergy of the O—H bond in 2,5ꢀdichlorohydroquinone
DOH = 346.6 kJ mol–1
.
The authors are grateful to E. T. Denisov for valuable
advice.
22. E. T. Denisov, A. L. Aleksandrov, and V. P. Shcheredin, Izv.
Akad. Nauk SSSR, Ser. Khim., 1964, 1583 [Bull. Acad. Sci.,
Div. Chem. Sci., 1964 (Engl. Transl.)].
23. N. M. Emanuel, G. E. Zaikov, and Z. K. Maizus, Rol´ sredy
v radikal´noꢀtsepnykh reaktsiyakh okisleniya organicheskikh
soedinenii [The Role of the Medium in Chain Radical Reacꢀ
tions of Oxidation of Organic Compounds], Nauka, Moscow,
1973, 279 pp. (in Russian).
This work was financially supported by the Russian
Academy of Sciences (Program of the Division of Chemꢀ
istry and Materials Science No. 1 "Theoretical and Exꢀ
perimental Investigation of the Chemical Bond Nature
and Mechanisms of the Most Important Chemical Reacꢀ
tions and Processes") and the Project "Integratsiya" (Grant
No. 2.2.1.1.7181).