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HARIFI‐MOOD AND SADRZADEH
TABLE 2 The second‐order rate coefficients of the reaction (at
2.0 × 10−4 M of piperidine) and solvatochromic parameters in
glyceline‐DMSO mixtures at 25°C
decreases as HBD acidity of the media increases. Because
of the base‐catalyzed mechanism of the reaction, the exis-
tence of free piperidine molecules is a necessity in the
reaction media, and therefore HBD interactions reduce
the available piperidine molecules as catalyst.
xglyceline
kA (M−1 s−1
)
ETN
π*
α
β
0.0
425
0.44
1.01
0.13
0.98
The activated complex leading to the zwitterionic
intermediate (Scheme 1) is expected to have the zwitter-
ionic character and to be favored by the increase in the
normalized polarity parameter, because zwitterionic mol-
ecules were more stabilized in high‐polar media than in
the media with lower polarity. In spite of our expectation,
it is interesting that a reverse result was observed. This is
in contrast to previous researches in ionic liquids
media.[10] In spite of that reline and glyceline as 2 com-
mon DESs have ionic characterization like ionic liquids,
they don't show similar effects on this SNAr reaction as
a solvent. It might be due to weak solute‐solvent interac-
tion in zwitterionic intermediate on one hand and the
strong interaction of the solvent with reactants and also
preferentially solvating by DES on the other hand.
Among dual‐parameter correlations, it seems that all
of them except 1 case show similar acceptable results. A
meticulous investigation in data, especially in all mix-
tures, confirms that the best correlation with reasonable
statistical factor consisting of regression coefficient, stan-
dard deviation, Fischer factor, and the significant values
of the parameters is between logkA with π* and α. It is
exactly compatible with single parameter correlations that
showed the best correlated data of logkA with ENT and α,
individually and separately. Because the normalized
polarity parameter is a blend of dipolarity‐polarizability
and the HBD acidity of the media,[1] we can claim that
the correlation of logkA with π* and α is the pure model
based on LFER method. Figures 3 and 4 show the ability
of mentioned dual‐parameter correlation in the reproduc-
tion of the reaction‐rate coefficients.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.1
1.0
43.9
19.5
5.90
5.85
3.67
2.83
2.80
2.82
2.97
3.03
0.61
0.65
0.69
0.72
0.75
0.77
0.79
0.80
0.83
0.85
1.07
1.09
1.10
1.10
1.10
1.10
1.10
1.10
1.10
1.09
0.43
0.50
0.59
0.66
0.71
0.75
0.80
0.82
0.88
0.94
0.94
0.94
0.88
0.86
0.81
0.77
0.73
0.67
0.65
0.59
multi‐parameter correlations between the logkA of the
reaction and the solvatochromic parameters of the media.
Multi‐parameter regression can be usually used when
combination of some of the solvent properties can affect
the rate coefficient. The solvatochromic parameters for
solutions of DMSO with ethaline and glyceline at various
compositions have been determined in our laboratory
(Tables 1 and 2).[29]
The hydrogen bond acceptor (HBA) ability, β, values
of the solvents decrease slowly with increasing of the
mole fraction of DES; thus, it can be expected that the
reaction rate coefficient depends on β. Despite this agree-
ment, the comparative statistical data of Table 3 demon-
strate a weak correlation between logkA and β in all
mixtures. While a different trend is observed in the π*
parameter, the correlations of logkA with π* show a little
bit better results than those with β but not reasonable.
On the other hand, solvatochromic parameter data in
Tables 1, 2 demonstrate an increasing trend, but sharply,
in ENT and α values. In contrast to this variation, the rate
coefficients of the reaction show a sharp decreasing to
its corresponding value in DES in all mixtures. These var-
iations accompany with a smooth positive and negative
deviation in ENT and α values and rate coefficients data,
respectively. Statistical data show the better correlation
of logkA with these parameters than the others, and there-
fore it can be demonstrated that the empirical polarity
and HBD of the media play an important role in DMSO‐
DES mixtures. As can be seen, the second‐order rate coef-
ficient of the reaction decreases with the increase in the
normalized polarity parameter and HBD of the media.
In the presence of piperidine, DESs are known to act
as HBD, and therefore, piperidine is stabilized via hydro-
gen‐bonding interactions with HBD and the reaction rate
On the other hand, standardized coefficients of the
solvent properties in dual‐parameter correlation show
that the HBD ability of the media as a specific interaction
of the solvent is a preferable and effective parameter,
compared with nonspecific interactions such as
dipolarity‐polarizability of the media. This different effect
is considerable in ethaline‐DMSO mixtures.
3 | EXPERIMENTAL
3.1 | Materials
Choline chloride (>99%), ethylene glycol (99.8%, anhy-
drous), glycerol (>99.5% spectrophotometric grade), and
spectroscopic grade of DMSO were purchased from Merck
(all materials in high purity grade). FDNB was available