Kinetic and quantum chemical studies of the mechanism of dehydrochlorination of 2,2-diaryl-1,1,1-trichloroethanes with nitrite ions

Article abstract of DOI:10.1134/S1070428016090049

The E2 mechanism has been proposed for the dehydrochlorination of 2,2-diaryl-1,1,1-trichloroethanes with nitrite ion, leading to 2,2-diaryl-1,1-dichloroethenes, on the basis of experimental kinetic study and quantum chemical simulation.

Full text of DOI:10.1134/S1070428016090049

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 9, pp. 1277–1281. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © V.N. Kazin, M.B. Kuzhin, A.V. Sirik, E.A. Guzov, 2016, published in Zhurnal Organicheskoi Khimii, 2016, Vol. 52, No. 9,
pp. 1290–1294.
Kinetic and Quantum Chemical Studies of the Mechanism
of Dehydrochlorination of 2,2-Diaryl-1,1,1-trichloroethanes
with Nitrite Ions
V. N. Kazin,* M. B. Kuzhin, A. V. Sirik, and E. A. Guzov
Demidov Yaroslavl State University, ul. Sovetskaya 14, Yaroslavl, 150000 Russia
*
e-mail: kaz@bio.uniyar.ac.ru
Received December 1, 2015
Abstract—The E2 mechanism has been proposed for the dehydrochlorination of 2,2-diaryl-1,1,1-trichloro-
ethanes with nitrite ion, leading to 2,2-diaryl-1,1-dichloroethenes, on the basis of experimental kinetic study
and quantum chemical simulation.
DOI: 10.1134/S1070428016090049
We previously [1–3] studied the reactivity of
,2-diaryl-1,1,1-trichloroethanes toward alkali metal
nitrites in dipolar aprotic solvents, N,N-dimethylform-
amide (DMF), N,N-dimethylacetamide (DMA), and
dimethyl sulfoxide (DMSO) (Scheme 1). The reactions
Several elimination mechanisms have been consid-
ered in the literature, in particular E1, E1cB, and E2,
which differ from each other by the order of C–H and
C–Cl bond cleavage. Therefore, it is fairly difficult to
distinguish elimination mechanisms as applied to
particular reactions [7, 8].
2
were carried out at a substrate–reagent (KNO or
2
NaNO )–solvent ratio of 1 :(3–15):(20–40) at 80–
2
The present work was aimed at studying the
mechanism of dehydrochlorination of 2,2-diaryl-1,1,1-
trichloroethanes with alkali metal nitrites in dipolar
aprotic solvents. As model substrate we selected
1
30°C. Under these conditions, compounds 1a–1d
reacted with KNO or NaNO to give dehydrochlorina-
2
2
tion products, 2,2-diaryl-1,1-dichloroethenes 2a–2d in
5–98% yield. However, compound 1e containing
electron-withdrawing nitro groups in 2–3 h at 100–
10°C was unexpectedly converted to 4,4′-dinitro-
9
1
,1,1-trichloro-2,2-diphenylethane (1c). The reaction
of 1c with potassium nitrite gives 1,1-dichloro-2,2-di-
phenylethene (2c) in 96–98% yield.
1
benzophenone (3) in 94–97% yield.
The effect of substrate concentration on the reaction
rate was studied at 363 K using excess reagent, i.e.,
under pseudofirst-order conditions. The effective rate
constants were calculated from the changes of the con-
centrations of 1c and 2c. Taking into account similar
values of the rate constants calculated from the sub-
strate consumption and product accumulation and the
data of material balance experiments, neither interme-
diate nor by-products accumulated during the process.
The results of the kinetic study of the transforma-
tion of 1,1-dichloro-2,2-bis(4-nitrophenyl)ethene (2e)
into ketone 3 by the action of nitrite ion were reported
in [4–6]; quantum chemical simulation of a probable
reaction mechanism was also given. The mechanism
of dehydrochlorination of 2,2-diaryl-1,1,1-trichloro-
ethanes with alkali metal nitrites in dipolar aprotic
solvents was not studied.
Scheme 1.
R
R
R
R
O N
NO2
2
NaNO2
or
CCl3
CCl2
2a–2e
O
1
a–1e
3
2
R = MeO (a), Me (b), H (c), Cl (d), O N (e).
1
277

1
278
KAZIN et al.
4
The effective rate constants (k ×10 ) at [1c] =
The rate constants for the dehydrochlorination of
substituted 1,1,1-trichloro-2,2-diphenylethanes with
potassium nitrite at 363 K in DMF are given in table.
The following correlation with the Hammett constants
σ was observed:
eff
0
0
3
6
.03 M and [KNO ] = 0.3 M in DMF at 333, 343, 353,
63, and 373 K are 0.42±0.01, 1.38±0.07, 4.7±0.2,
.4±0.3, and 15.6±0.8 s , respectively. The thermo-
dynamic parameters of the reaction at 363 K are as
follows: E = 81.7 ± 4.0, ∆H = 78.7 ± 3.9 kJ/mol,
logA = 19.9+1.5 s , ∆S = –90.5+4.5 J mol K .
The large negative value of ΔS indicates higher
2
–
1
≠
a
–1
≠
–1
–1
logkeff = (0.28±0.01) + (3.76±0.24)σ;
r = 0.994, s = 0.205, N = 5.
≠
ordering of the transition state (TS) compared to the
initial reactant system. It may be presumed that the
dehydrochlorination follows a bimolecular elimination
mechanism.
The positive ρ value and the fact that the correlation
was obtained with the Hammett constants σ (rather
than with nucleophilic constants σ ) suggest formation
of neither substituent-stabilized nor destabilized
charge-transfer complex in the transition state. Our
results led us to presume that the dehydrochlorination
of 2,2-diaryl-1,1,1-trichloroethanes does not follow
E1cB mechanism since carbanion intermediate cannot
be stabilized by electron-withdrawing nitro group.
–
The effect of solvent nature (DMF, DMA, DMSO,
aliphatic alcohols) on the kinetic parameters was
studied. No dehydrochlorination occurred in a protic
solvent such as ethanol. It was interesting to carry out
the reaction in a binary mixture of aprotic and protic
solvents. The kinetic studies in DMF–EtOH at volume
ratios of 80:20 and 60:40 revealed the following
correlation between logkeff and the Dimroth parameter
We also estimated the possibility of dehydrochlo-
rination according to single-electron transfer from
nitrite ion to the trichloroethane fragment with inter-
mediate formation of radical anion. For this purpose,
the reaction mixture was analyzed by ESR spectros-
copy. The ESR spectrum of the reaction mixture con-
tained no signals assignable to radical species; how-
ever, the lifetime of radical anion may be insufficient
for its accumulation in a concentration necessary to
detect it by ESR. Therefore, we cannot strictly rule out
the possibility of the SET mechanism; probably, this
reaction path will be studied in the future. Thus, the
kinetic data indicate E2 (bimolecular elimination)
mechanism of the reaction under study.
(
ET) of the solvent:
logkeff = (8.64±1.34) – (0.064±0.01)E
T
;
r = 0.982, s = 0.09, N = 5.
As the E parameter increases, the reaction slows
T
down, and addition of the protic solvent sharply
reduces the rate constant. No substrate consumption
was observed in pure ethanol. The negative coefficient
at the Dimroth parameter indicates that the transition
state is less polar than the initial reactant system
and that no charges appear or disappear in the
transition state.
The dehydrochlorination mechanism was refined by
quantum chemical calculations of the structure of the
initial reactants, prereaction complexes, transition
states, and reaction products in antiperiplanar con-
formation. The calculations were performed for the E1,
E1cB, and E2 mechanisms at the DFT B3LYP/
In order to elucidate the reaction mechanism, the
effect of the substituent in the benzene rings on the
kinetic parameters was studied. Apart from dehydro-
chlorination, the reaction of nitrite ion with compound
6
-31G++(d,p) level of theory in the ENERGY mode
1
e containing an electron-withdrawing nitro group in
with solvent (DMF) effects included in terms of the
polarizable continuum model (PCM).
the benzene rings involved further transformation of
the dichloroethylidene group into carbonyl. We have
found that the rate of consumption of 1e at 363 K
is so high that the dehydrochlorination rate constant
could not be determined at that temperature. Therefore,
we initially determined the activation parameters
for the dehydrochlorination of 1e to 2e in the tem-
perature range from 283 to 308 K, and the rate
constant at 363 K was then calculated. Compound 2e
was not converted into 4,4′-dinitrobenzophenone (3)
at 283–308 K.
The E1 mechanism implies abstraction of chlorine
atom by nitrite ion with subsequent formation of
carbocation. Movement of the chlorine atom to nitro-
gen is accompanied by exponential growth of the total
energy of the system. Therefore, stabilization of transi-
tion state is impossible, and hence E1 mechanism may
be excluded. The E1cB and E2 mechanisms involve
initial attack by the oxygen atom of nitrite ion on the
hydrogen atom of the bridging CH group. Simulation
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 9 2016

KINETIC AND QUANTUM CHEMICAL STUDIES
1279
1
(a)
(b)
(c)
1
O
1
O
O
1
H
1
H
1
H
1
C
1
C
1
C
2
2
2
C
C
C
1
Cl
1
Cl
1
Cl
Structures of the (a) prereaction complex, (b) transition state, and (c) products of dehydrochlorination of 1,1,1-trichloro-2,2-di-
phenylethane with nitrite ion (interatomic distances are given in Å).
of this attack showed increase of the total energy of the
system, which attained some maximum value and then
decreased.
molecular elimination mechanism of the dehydro-
chlorination of 1,1,1-trichloro-2,2-diphenylethane with
nitrite ion.
Taking these results into account, further calcula-
tions were performed only for the attack by nitrite ion
The results of analogous simulations performed for
substituted 1,1,1-trichloro-2,2-diphenylethanes are
given in table. The torsion angles in the transition
states derived from substituted 1,1,1-trichloro-2,2-di-
phenylethanes correspond to planar arrangement of the
O···H···C–C–Cl fragment and its antiperiplanar
conformation. The torsion angles are almost indepen-
dent of the substituent nature. The steric structure of
the transition states indicates E2 mechanism of the
dehydrochlorination of all compounds under study.
on the Cl CCH hydrogen atom according to the E2 and
3
E1cB mechanism. As an example, figure shows the
structures of the prereaction complex, transition state,
and dehydrochlorination products of 1,1,1-trichloro-
1
2
,2-diphenylethane. Movement of the H atom toward
the nitrite ion results in the formation of transition state
see figure; a, b). The corresponding Hessian matrix
contains only one imaginary frequency due to linear
(
1
1
1
oscillation of the H atom between C and O . Descent
along the intrinsic reaction coordinate and potential
energy profile leads to prereaction state and reaction
products (1,1-dichloro-2,2-diphenylethene, nitrous
acid, and chloride ion).
We have revealed a correlation between log k
values and the enthalpy of activation ΔH , which also
eff
≠
Effective rate constants, torsion angles in the transition state
structures, and enthalpies of activation of the dehydro-
chlorination of 2,2-diaryl-1,1,1-trichloroethanes (363 K,
The stabilized transition state is characterized by
2
1
1
DMF, c = 0.03 M, cKNO2 = 0.3 M)
considerably extended C –Cl bond (2.45 Å against
2
1
.81 Å for the three C –Cl bonds in the prereaction
1
Comp.
no.
4
–1
Torsion angle
O···C–C–Cl, deg
≠
complex). Further movement of the H atom is accom-
panied by synchronous increase of the C –Cl distance
to 3.49 Å, which is related to sp →sp rehybridization
of C with the formation of 1,1-dichloro-2,2-diphenyl-
k
eff ×10 , s
ΔH , kJ/mol
2
1
3
2
1
1
1
1
1
a
b
c
1.25±0.04
3.46±0.04
176.69
176.18
177.75
175.98
175.57
86.6
86.1
85.7
83.3
76.5
2
ethene (see figure, c). The O···H···C–C···Cl five-
atom fragment of the transition state is almost planar.
This is a necessary condition for the E2 mechanism,
taking into account stereospecificity of E2 reactions.
The given data indicated concerted elimination of the
chlorine and hydrogen atoms, which confirmed bi-
6.35±0.29
d
e
134.67±5.2700
10724±22500
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 9 2016

1
280
KAZIN et al.
Scheme 2.
≠
N
O
O
R
R
R
R
R
R
H
H
NO2
–
–
Cl , –HONO
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
suggests a common E2 mechanism for all the sub-
strates examined.
a Clarus 680 chromatograph equipped with a flame-
ionization detector and a 30 m×0.32 mm capillary
column (stationary phase 5% of diphenyl- and 95% of
dimethylpolysiloxane, film thickness 1 μm); injector
temperature 270°C, oven temperature 250°C, detector
temperature 280°C; carrier gas nitrogen, flow rate
2.1 mL/min; hydrogen flow rate 45 mL/min, air flow
rate 450 mL/min.
≠
logkeff = (28.96±2.2) – (0.38±0.02)ΔH ;
r = 0.982, s = 0.882, N = 5.
The results of the kinetic study and quantum chem-
ical simulation of the dehydrochlorination of 2,2-di-
aryl-1,1,1-trichloroethanes with nitrite ion showed the
following: (1) high negative value of the entropy of
activation indicated that the transition state is more
ordered than the initial reactant system, which is
typical of bimolecular elimination; (2) the positive
ρ value and correlation with the Hammett constants
σ ruled out formation of substituent-stabilized or desta-
bilized charge-transfer complex in the transition state;
The concentrations of 1e and 2e in the reaction of
1
,1,1-trichloro-2,2-bis(4-nitrophenyl)ethane with
potassium nitrite were determined by HPLC on
a Perkin Elmer instrument equipped with a UV
detector (λ 254 nm) and a 15 cm × 4.6-mm column
packed with C18 (grain size 3.5 μm, pore diameter
1
1
00 Å); eluent water–acetonitrile (20:80), flow rate
.0 mL/min; column temperature 30°C; 1,1,1-tri-
(
3) elimination of the chlorine and hydrogen atoms
from the α- and β-carbon atoms is a concerted process;
4) the O···H···C–C–Cl five-atom fragment of the
chloro-2,2-bis(4-chloro-3-nitrophenyl)ethane was used
as internal standard.
(
Geometry optimizations, search for transition
states, and calculations of the thermodynamic param-
eters were performed at the DFT B3LYP/6-31G++(d,p)
level of theory with solvent effects (DMF) included
according to the polarizable continuum model (PCM)
using Firefly program. Transition states were sought
for by the open-shell unrestricted Hartree–Fock (UHF)
method.
transition state is almost planar, which is a necessary
stereochemical condition for E2 reaction; (5) the pres-
ence of only one maximum on the potential energy
profile (one-step process) is typical of E2 mechanism.
Thus, we can conclude that the dehydrochlorination
of 2,2-diaryl-1,1,1-trichloroethanes with nitrite ion in
dipolar aprotic solvents follows the E2 bimolecular
elimination mechanism (Scheme 2).
Compounds 1a–2e and 2a–2e were synthesized
according to the procedures described in [9–11].
EXPERIMENTAL
REFERENCES
The kinetic studies were performed in a glass reac-
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with an accuracy of ±0.5°C, and a required amount of
potassium nitrite was added with stirring. Samples
were withdrawn during the reaction, cooled, and
analyzed by GLC and HPLC.
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