Mechanism of dehydrochlorination of 2,2-diaryl-1,1,1-trichloroethanes with nitrite and halide anions

Article abstract of DOI:10.1134/S1070363217030033

The reactivity of 2,2-diphenyl-1,1,1-trichloroethane toward halide ions in dipolar aprotic solvents has been studied, and the mechanisms of its reactions with nitrite and halide ions have been compared. The results of kinetic and DFT quantum chemical stud

Full text of DOI:10.1134/S1070363217030033

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 3, pp. 381–385. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.N. Kazin, M.B. Kuzhin, S.G. Sibrikov, A.V. Sirik, E.A. Guzov, V.V. Plakhtinskii, 2017, published in Zhurnal Obshchei Khimii,
2
017, Vol. 87, No. 3, pp. 371–375.
Mechanism of Dehydrochlorination
of 2,2-Diaryl-1,1,1-trichloroethanes
with Nitrite and Halide Anions
V. N. Kazin*, M. B. Kuzhin, S. G. Sibrikov, A. V. Sirik, E. A. Guzov, and V. V. Plakhtinskii
Demidov Yaroslavl State University, ul. Sovetskaya 14, Yaroslavl, 150003 Russia
*
e-mail: kaz@bio.uniyar.ac.ru
Received July 14, 2016
Abstract—The reactivity of 2,2-diphenyl-1,1,1-trichloroethane toward halide ions in dipolar aprotic solvents
has been studied, and the mechanisms of its reactions with nitrite and halide ions have been compared. The
results of kinetic and DFT quantum chemical studies suggest a common bimolecular elimination mechanism
for both dehydrochlorination reactions.
Keywords: 2,2-diaryl-1,1,1-trichloroethanes, nitrite and halide ions, dehydrochlorination, kinetics, DFT
calculations
DOI: 10.1134/S1070363217030033
Diaryl-1,1,1-trichloroethanes and their derivatives
(4-chlorophenyl)ethane (DMF, 363 K, substrate-to-reagent
ratio 1 : 3). Alkali metal bromides and iodides are less
active, and chlorides occupy an intermediate poisition.
are intermediate products in the manufacture of various
dyes and pigments, dietary supplements, polyfunc-
tional organic reagents, and monomers. Polymers
containing trichloroethane fragments possess such
important properties as incombustibility and self-
extinction, but their thermal stability is not high. En-
hanced heat resistance is favored by the transformation
of trichloroethane group to dichloroethene or carbonyl.
Reagent
Time, h
KNO
0.5–1 0.1–0.3 2.5–3.0 3–3.5 4–4.5
96–98 94–96 93–95 92–94
2
KF
KCl
KBr
KI
Yield, % 95–97
The kinetics of dehydrochlorination of 2,2-diaryl-
,1,1-trichloroethanes with nitrite ions were studied in
1
The set of available dehydrochlorinating agents is
fairly large; it includes anhydrous metal alkoxides,
solid alkalies and their solutions in water or organic sol-
vents, and organic bases such as pyridine and aromatic
and aliphatic amines. We previously reviewed [1]
mechanisms of dehydrochlorination of 2,2-diaryl-1,1,1-
trichloroethanes and the effects of substrate structure
and reagent and solvent nature on this process. Despite
vast chemical information on functionalization and
functional transformations of 2,2-diaryl-1,1,1-trichloro-
ethanes, only a few data are available on the reactivity
of these compounds toward alkali halides and nitrites.
In particular, dehydrochlorination of 2,2-diaryl-1,1,1-
trichloroethanes by the action of alkalies and inorganic
salts in various solvents was studied, and optimal
reaction conditions were found [2]. It was shown that
alkali metal fluorides and nitrites are the most efficient
dehydrochlorinating agents for 1,1,1-trichloro-2,2-bis-
[
3], and an E2 mechanism was proposed on the basis
of the experimental data and results of quantum
chemical simulation. However, there are no published
data on the kinetics and mechanism of dehydrochlorina-
tion of 2,2-diaryl-1,1,1-trichloroethanes with halide
ions in dipolar aprotic solvents.
The goal of the present work was to analyze how
the reagent nature affects the reactivity and mechanism
of dehydrochlorination of 2,2-diaryl-1,1,1-trichloro-
ethanes. The kinetic studies were performed using
1,1,1-trichloro-2,2-diphenylethane as model substrate
and potassium chloride as dehydrochlorinating agent.
The effect of the substrate concentration [S] on the
reaction rate was studied in DMF at 363 K with excess
reagent, i.e., under pseudofirst-order reaction condi-
tions. Table 1 contains the rate constants for the reac-
tions of 1,1,1-trichloro-2,2-diphenylethane with potas-
3
81

3
82
KAZIN et al.
Table 1. Rate constants of the dehydrochlorination of 1,1,1-trichloro-2,2-diphenylethane with potassium chloride and nitrite
(
363 K, DMF, [reagent] = 0.6 M)
Initial substrate
KCl
KNO
2
5
–1
5
–1
4
–1
4
–1
concentration
S] , M
k
ef × 10 , s (with respect
k
ef × 10 , s (with
k
ef × 10 , s (with re-
kef × 10 , s (with
[
0
to the substrate)
respect to the product)
spect to the substrate)
respect to the product)
0
0
0
0
.03
.04
.05
.06
7.50±0.33
7.13±0.31
7.39±0.35
7.19±0.34
7.12±0.35
7.26±0.33
7.27±0.34
7.04±0.35
6.35±0.34
6.26±0.35
6.27±0.33
6.11±0.32
6.55±0.31
6.27±0.29
6.33±0.34
6.14±0.34
sium chloride and nitrite. Similarity of the rate con-
stants calculated from the substrate consumption and
product formation, as well as material balance data,
indicated no accumulation of intermediate or by-
products in the reactions with both potassium chloride
and potassium nitrite. It was also found that the rate
constant for the reaction with potassium chloride is
lower by an order of magnitude than the rate constant
for the reaction with potassium nitrite.
in a binary mixture of aprotic and protic solvents,
DMF–EtOH, at volume ratios of 80 : 20 and 60 : 40.
As a result, a correlation was found between log kef
and Dimroth solvatochromic parameter ET:
log kef = (4.78 ± 1.47) – (0.048 ± 0.01)E
T
;
r = 0.963, s = 0.096, N = 5.
As the E value increases, the reaction slows down,
T
and addition of protic solvent sharply reduces the rate
constant. The negative coefficient at the Dimroth
parameter in the above correlation indicates that the
transition state is less polar than the initial system and
that no charge appears or disappears in the transition
state [4]. Analogous trend was also observed for the
reaction with nitrite ion [3].
The thermodynamic parameters of the dehydro-
chlorination reactions were derived from the tempera-
ture dependences of the rate constants (Table 2). The
high negative entropies of activation for the reactions
with both chloride and nitrite ions suggest higher
ordering of the transition state compared to the initial
system, which is typical of bimolecular processes.
The effect of halide ion nature on the kinetics of
dehydrochlorination of 1,1,1-trichloro-2,2-diphenyl-
We also examined the effect of solvent nature (DMF,
N,N-dimethylacetamide, DMSO, ethanol) on the
reaction kinetics. No dehydrochlorination was observed
in ethanol. It was interesting to carry out the reaction
ethane in DMF at 363 K ([S] = 0.03 M, [Hlg] = 0.3 M).
0
0
We failed to measure the rate constant for the reaction
with KF at 363 K because of its very high rate;
therefore, it was determined by extrapolation. The
4
reactivity of halide ions dicreases in the series (k × 10 ,
Table 2. Rate constants and thermodynamic parameters for
ef
–
1
–
–
–
the reactions of 1,1,1-trichloro-2,2-diphenylethane with potas-
s ): F (251.42 ± 12.84) > NO (6.35 ± 0.34) > Cl
2
a
–
–
sium chloride and nitrite (DMF, [reagent] = 0.3 M)
(0.75 ± 0.03) > Br (0.33 ± 0.02) > I (0.25 ± 0.02).
KCl
KNO
2
The mechanism of dehydrochlorination of 2,2-
diaryl-1,1,1-trichloroethanes with halide ions was
simulated by quantum chemical calculations of the struc-
tures of the initial reactants, prereaction complexes,
transition states, and final products. The probability of
E1cb and E2 mechanisms was estimated by
preliminary DFT B3LYP/6-31G++(d,p) calculations in
the ENERGY mode with inclusion of solvent effects
4
–1
4
–1
T, K
kef × 10 , s
T, K
333
343
353
363
373
kef × 10 , s
3
3
3
4
4
53
63
83
03
23
0.32 ± 0.01
0.75 ± 0.03
2.85 ± 0.14
12.59 ± 1.43
33.98 ± 2.62
0.49 ± 0.01
1.52 ± 0.07
4.28 ± 0.24
6.35 ± 0.34
11.62 ± 0.78
E
a
83.34±1.04 kJ/mol
ΔH = 80.18±1.14 kJ/mol
log A = 18.09±1.45
a
E 80.60±0.62 kJ/mol
ΔH = 77.68±0.73 kJ/mol
log A = 19.39±1.49
(
(
DMF) in terms of the polarizable continuum model
PCM). These mechanisms imply initial attack of the
≠
≠
anionic reagent on the Ar CH hydrogen atom. The
2
≠
–1 –1
≠
–1 –1
ΔS = –104.66±4.25 J mol K ∆S = –93.20±4.53 J mol K
distance between the reagent and CH carbon atom was
fixed at 3 Å. Successive extension of the C–H distance
a
The entropy of activation was calculated for 363 K.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 3 2017

MECHANISM OF DEHYDROCHLORINATION OF 2,2-DIARYL-1,1,1-TRICHLOROETHANES
383
(
b)
ΔH
a
= 37.50 kJ/mol
(c)
(
a)
ΔH = –73.86 kJ/mol
Reaction coordinate
Fig. 1. Energy profile for the dehydrochlorination of 1,1,1-trichloro-2,2-diphenylethane with fluoride ion (interatomic distances are
given in Å): (a) prereaction complex, (b) transition state, (c) products.
with a step of 0.05 Å (without subsequent optimiza-
tion) was accompanied by increase of the total energy
to a certain maximum value, and then the energy de-
creased. The structure corresponding to that maximum
was taken as starting point in the search for transition
states via gradient optimization. Taking into account
the results obtained, in the further treatment we
2,2-diphenylethene, hydrogen fluoride, and chloride
ion). The transition state is characterized by consider-
2
1
able elongation of the C –Cl bond (to 2.13 Å against
2
1.83 Å for all three C –Cl bonds in the prereaction
1
2
1
complex). As the H moves apart, the C –Cl distance
3
2
synchronously increases to 3.28 Å due to sp –sp -
rehybridization of the corresponding carbon atom with
formation of 1,1-dichloro-2,2-diphenylethene (Fig. 1c).
considered only attack of reagents on the Ar CH
2
hydrogen atom of the substrate in keeping with the E2
or E1cb mechanism.
We previously showed that 2,2-diaryl-1,1,1-trichloro-
ethanes exist in antiperiplanar conformation; in the
transition state formed by 2,2-diaryl-1,1,1-trichloro-
ethanes with nitrite ion all five O···H···C–C–Cl atoms
appear in one plane [3]. Analogous stereochemical
structure was found for the transition states in the
reactions of 1,1,1-trichloro-2,2-diphenylethane with
halide ions. The torsion angle in the transition state for
all structures and reagents varies within 3°.
As follows from the kinetic data, fluoride ion is the
most reactive among halide ions. As an example, Fig.
1
shows the energy profile of the dehydrochlorination
of 1,1,1-trichloro-2,2-diphenylethane with fluoride ion.
1
Movement of H toward the fluoride ion (Fig. 1a) leads
to the transition state (Fig. 1b) whose Hessian matrix
contains only one imaginary frequency corresponding
1
1
1
to the linear oscillation of H between C and F .
Descent along the intrinsic reaction coordinate (IRC)
leads to prereaction complex or products (1,1-dichloro-
Figure 2 shows the energy profile for the dehydro-
chlorination of 1,1,1-trichloro-2,2-diphenylethane with
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 3 2017

3
84
KAZIN et al.
(
a)
(b)
(
c)
ΔH
a
= 85.67 kJ/mol
ΔH = –32.81 kJ/mol
Reaction coordinate
Fig. 2. Energy profile for the dehydrochlorination of 1,1,1-trichloro-2,2-diphenylethane with nitrite ion (interatomic distances are
given in Å): (a) prereaction complex, (b) transition state, (c) products.
nitrite ion. Analysis of the transition states with
fluoride and nitrite ions revealed changes of the distance
almost planar arrangement of the Hlg···H···C–C–Cl
pentad in the transition state (Fig. 1b) (this is a stereo-
chemical condition for E2 mechanism), and single-step
process typical of E2 mechanism (one maximum on
the potential energy profile) (Scheme 1).
1
2
between the departing Cl atom and C (2.13 and
1
1
2
1
.45 Å), as well as of the H –C distance (1.25 and
.42 Å). The shorter distance between the reaction
centers and more compact structure of the transition
state with fluoride ion corresponds to the lower energy
barrier in comparison to nitrite ion. The calculated
enthalpy of activation (37.50 kJ/mol) confirms higher
reactivity of fluoride ion relative to nitrite ion
Thus, the results of our kinetic and theoretical
studies in combination with our previous data [3] led
us to conclude that the dehydrochlorination of 2,2-
diaryl-1,1,1-trichloroethanes with alkali metal nitrites
and halides in dipolar aprotic solvents follows E2
bimolecular elimination mechanism.
(
85.67 kJ/mol). This may be rationalized by the
reagent structure since fluoride ion is characterized by
the smallest atomic radius among halide ions, so that it
is capable of approaching the substrate most closely.
EXPERIMENTAL
2
,2-Diaryl-1,1,1-trichloroethanes and 2,2-diaryl-
1
,1,1-trichloroethens were synthesized according to the
The results of quantum chemical simulation of the
dehydrochlorination of 1,1,1-trichloro-2,2-diphenyl-
ethane according to the E2 and E1cb mechanisms
indicated bimolecular character of the process,
specifically synchronous elimination of chlorine and
hydrogen atoms from the α- and β-carbon atoms,
procedures described in [5, 6].
The kinetic measurements were performed using a
glass reactor equipped with a stirrer, reflux condenser,
thermometer, and gas-inlet capillary for nitrogen. The
reactor was charged with a solvent and substrate, the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 3 2017

MECHANISM OF DEHYDROCHLORINATION OF 2,2-DIARYL-1,1,1-TRICHLOROETHANES
385
Scheme 1.
X
_
=
/
R
R
R
R
R
R
−
X
H
H
_
HX
Cl
_
−
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2 2
R = H, Cl, NO , OEt, Me, OCMe; X = F, Cl, Br, I, NO .
mixture was adjusted to a required temperature with an
accuracy of ± 0.5°C, and a required amount of the
reagent was added with stirring. Samples of the
reaction mixture were withdrawn during the reaction,
cooled, and analyzed by GLC (Clarus 680 chromato-
graph equipped with a flame ionization detector;
capillary column, 30 m × 0.32 mm; stationary phase
REFERENCES
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2. Kazin, V.N., Sibrikov, S.G., Kuzhin, M.B., and Savinskii, N.G.,
Bashk. Khim. Zh., 2011, vol. 18, no. 3, p. 156.
5
% of diphenyl- and 95% of dimethylpolysiloxane;
3
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 3 2017