KINETICS AND MECHANISM OF QUATERNIZATION
Here Ca is the concentration of acrylic acid, Cp is the
concentration of phosphine, kIII is the third order rate
constant, k' = kIIICa2 is the pseudofirst order rate
constant at Ca >> Cp.
825
log Ca
0.0
The validity of kinetic Eq. (1) for the aprotic
solvents is confirmed also by an example of
methyldiphenyl- and dimethylphenylphosphine and
also the other unsaturated carboxylic acid, the metha-
crylic one. Hence, the use of aprotic solvents permitted
proving directly that the quaternization process under
study has the general third order. For the alcohols and
carboxylic acids capable of playing the role of the
proton-donating agent such proof can be only indirect
because the order by solvent in contrast to the order by
substrate cannot be evaluated directly from the kinetic
experiment. The conclusion on the including the
concentration of the proton-donating solvent in the
kinetic equation of the reaction was made previously
on the basis of comparison of the kinetic data for the
reactions of triphenylphosphine with the unsaturated
carboxylic acids and their esters, of the study of the
kinetics of the reaction in the mixtures of acetic acid
with the aprotic solvent, acetonitrile, and also because
of the existence of the common isokinetic dependence
for different channels of proton transfer [1, 2]. For the
carboxylic acids (acetic, propionic) kinetic Eq. (2) is
valid.
x + log k'
Fig. 1. Evaluation of order with respect to acrylic acid in
the reaction with triphenylphosphine in the aprotic
colvents: (1) MeCN: log k' = 1.92 log Ca – 1.30, R = 1.000;
(2) sulfolane: 1 + log k' = 1.97log Ca – 0.41, R = 0.9999;
(3) (EtO)2CO: 2 + log k' = 1.94 log Ca + 0.38, R = 0.9994;
(4) BuOAc: log k' = 1 97 log Ca – 1,98, R = 0.9999; (5)
EtOAc: log k' – 1 = 2.02log Ca – 3.03, R = 1.000; (6) DMF:
log k' – 1 = 2.02log Ca – 4.88, R = 0.9995; (7) DMSO:
log k' = 2.01 log Ca – 4.09, R = 0.9995.
acid in the aprotic solvents, alcohols, and carboxylic
acids presented in Fig. 2. It can be complemented by
the points for methacrylic acid:
log kT+30 = 0.915 log kT + 0.481,
N 21, R 0.9995, s 1.23×10–3.
W = kIII,sCsCpCc.
(2)
For alcohols it has the form of superposition (3).
W = kIII,sCsCpCa+ kIIICpCa2.
Analogous isokinetic dependence was obtained
previously for the series of the reactions of triphenyl-
phosphine with unsaturated carboxylic acids in the
acetic acid medium [1]. It shows that all the reactions
studied in this work and in the previous ones belong to
one general array where the main features of the reac-
tion mechanism are preserved independent of the nature
of phosphine, of the unsaturated carboxylic acid, and
of the proton donor in the final act of the reaction.
(3)
Here Cs is the concentration of proton-donating
solvent, kIII,s is the third order rate constant with the
participation of the given solvent. Equations (2) and
(3) are partial cases of the Eq. (1).
Kinetic and activation parameters of the reactions
under study are listed in the table.
As seen from the table, the rate of the reaction
increases in the series triphenylphosphine–methyl-
diphenylphosphine–dimethylphenylphosphine with the
increase in the nucleophilicity of phosphine and the
decrease in sterical hindrances around the phosphorus
atom indicating the general nucleophilic character of
the reaction. The introduction of a methyl group
instead of one of the phenyl substituents increases the
reactivity of tertiary phosphine in this process
approximately ten-fold. It is known that the quater-
nization of phosphines with alkyl halides may be
described by the one-parametric Taft Eq. (5) [3]:
In this case the common isokinetic dependence
log kT+30–log kT is observed for the reaction of
triphenylphosphine with the acrylic acid in alcohols,
carboxylic acids, and the aprotic solvents. It includes
also data for the reactions of methyldiphenyl- and
dimethylphenylphosphine.
log kT+30 = 0.915log kT + 0.482,
N 19, R 0.9995, s 1.35×10–3.
(4)
Isokinetic dependence (4) including 19 points in the
range of more than four logarithmic units for the data
on the reactions of tertiary phosphines with the acrylic
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 5 2011