96
HARATI AND GHOLAMI
log k2 = −3.783( 0.067) − 0.690( 0.076)α
+ 0.783( 0.029)Sp
(7)
(n = 21, r = 0.993, p = 0.0000,
S = 0.020, F-ratio = 396.30)
Since the p-value in statistical models for aqueous so-
lutions of methanol as well as ethanol is less than 0.01,
a statistically significant relationship between the vari-
ables at the 99% confidence level is presented. The
correlation coefficient indicates that the model as fitted
explains 99.3% of the variability in log k2.
Figure 2 Plot of experimental values of log k2 vs the pre-
dicted values from Eq. (8) in the all-aqueous solutions at
25◦C.
It can be seen that the solvophobicity and HBD acid-
ity of the media have opposite effects on the reaction
rate. In the presence of aniline, alcohols are known to
act as HBD, and there is plentiful evidence of strong
hydrogen-bonding interactions between aniline and al-
cohols [19]. Therefore, aniline will be stabilized via
hydrogen-bonding interactions with hydrogen-bond
donors. Hence, the reaction rate decreases with the in-
creasing HBD acidity of the media. According to dual-
parameter regression, both Sp and α coefficients are
significant, but Sp plays a more important role in deter-
mining the reaction rates. This dual-parameter model
represents a significant improvement with regard to the
previous single-parameter models. Because of the op-
posite effects of HBD acidity and solvophobicity of the
media, a peak appears in the plots of second-order rate
constants of the reaction vs mole fraction of water.
Furthermore, a dual-parameter correlation of log k2
vs Sp and α was obtained in the all-aqueous solutions
of the alcohols (Eq. (8)).
best dual-parameter correlation is for log k2 vs α and
π*, because piperidine is completely soluble in wa-
ter. But when the primary amine (aniline) instead of
the secondary one (piperidine) is used, the best-fitted
statistical model is the relation of log k2 vs α and Sp,
because aniline is more hydrophobic than piperidine.
The authors thank Professor Ikenna Onyido (University of
Ibadan, Ibadan, Nigeria) for helpful advice on base-catalyzed
reactions and Mr. M. Izadyar (Sharif University of Technol-
ogy) for helpful comments.
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log k2 = −3.532( 0.108) − 1.004( 0.115)α
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(8)
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In this equation, the difference effects of Sp and α on
the reaction rate constant are high, and, because Sp and
α have the opposite effect, a maximum appears in the
plot of rate constant vs Xw.
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parametercorrelations, experimentalvaluesoflog k2 vs
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there is good agreement between the experimental and
the calculated values of log k2 in all-aqueous solutions
(r = 0.958).
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In our previous work [12], we studied the solvent
effect on reaction rate constant between 1-choro-2,4-
dinitrobenzene and piperidine. We figured out that the