Kinetics and mechanism of the dehydrohalogenation of Ar2CHCX3 and Ar2CHCHX2 by strong N-bases

Article abstract of DOI:10.1016/S0022-2860(98)00546-8

The kinetics and mechanism of the elimination reactions of 1,1,1- trifluoro-, 1,1,1-trichloro-, 1,1,1-tribromo-, 1,1-dichloro- and 1,1-dibromo- 2,2-bis(4-nitrophenyl)ethanes and two strong N-bases: 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1

Full text of DOI:10.1016/S0022-2860(98)00546-8

Journal of Molecular Structure 476 (1999) 173–179
Kinetics and mechanism of the dehydrohalogenation of Ar2CHCX3
and Ar2CHCHX2 by strong N-bases
´
Grzegorz Schroeder, Bogusława Łe˛ska, Błaz˙ej Gierczyk, Bartosz Roz˙alski,
*
Bogumil Brzezinski
´
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780, Poznan, Poland
Received 10 June 1998; accepted 6 July 1998
Abstract
The kinetics and mechanism of the elimination reactions of 1,1,1-trifluoro-, 1,1,1-trichloro-, 1,1,1-tribromo-, 1,1-dichloro-
and 1,1-dibromo-2,2-bis(4-nitrophenyl)ethanes and two strong N-bases: 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-
methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) are discussed. ᭧ 1999 Elsevier Science B.V. All rights reserved.
Keywords: Kinetics; Strong N-bases; Elimination; Halogeno-compounds
1. Introduction
DBU) were also used for kinetics investigations
(isotope effect) of the dehydrohalogenation reactions
from various halogenocompounds [2, 14–19]. The
results obtained in the case of 1,1,1-trifluoro-2,2-
bis(4-nitrophenyl)ethane have demonstrated that the
multistep mechanism is realized. They indicate that
E1cB mechanism in acetonitrile as a solvent and an
E2H or mixed E1cB–E2H mechanism in less polar
solvents such as tetrahydrofuran and hexane takes
place (Scheme 1). The same results were reported
for 1,1,1-trichloro- and 1,1-dichloro-2,2-bis(4-nitro-
phenyl)ethane promoted by DBN in a series of aprotic
solvents [14, 19].
In this paper we studied the mechanism of the dehy-
drohalogenation reaction of five halogeno-2,2-bis(4-
nitrophenyl)ethanes (1,1,1-trifluoro-, 1,1,1-trichloro-,
1,1,1-tribromo-, 1,1-dichloro- and 1,1-dibromo-2,2-
bis(4-nitrophenyl)ethane) by N-bases with guani-
dine-like character. As N-bases we used two, TBD
and MTBD (Scheme 2).
In the last years very strong N-bases with small
nucleophilicity were synthesized and used for the
deprotonation reactions of C–H and N–H acids.
Amidines
(1,8-diazabicyclo[5.4.0]undec-7-ene,
DBU, and 1,5-diazabicyclo[4.3.0]non-5-ene, DBN
[1]), guanidines (tetramethylguanidine, TMG [1],
1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD [2], 7-
methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, MTBD
[3], and pentaisopropylguanidine [4]), proton sponges
(1,8-bis(dimethylamino)naphtalene [5–7] and 1,2-
bis(dimethylaminomethyl)benzene [8]), cryptands
(cryptand 111 [9]) and so called Schwesinger bases
(phosphazene base P1-tBu, phosphazene base P2-Et
and phosphazene base P4-tBu (P4-tBu)) belong to
such very strong N-bases [10–13].
In previous papers some strong N-bases (TMG and
* Corresponding author.
0022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00546-8

174
G. Schroeder et al. / Journal of Molecular Structure 476 (1999) 173–179
Scheme 1.

G. Schroeder et al. / Journal of Molecular Structure 476 (1999) 173–179
175
Scheme 2.
2. Experimental
MTBD and TBD were purchased from Aldrich and
used without any purification.
1,1,1-Trifluoro-2,2-bis(4-nitrophenyl)ethane
(Ar2CHCF3),
1,1,1-trichloro-2,2-bis(4-nitropheny-
2.1. Kinetic measurements
l)ethane (Ar2CHCCl3), and 1,1-dichloro-2,2-bis(4-
nitrophenyl)ethane (Ar2CHCHCl2) were prepared
according to the previously described procedures
[19–21], using as substrates corresponding haloaceto-
phenones.
Generally, syntheses of 1,1,1-tribromo-2,2-bis(4-
nitrophenyl)ethane (Ar2CHCBr3) and 1,1-dibromo-
2,2-bis(4-nitrophenyl)ethane (Ar2CHCHBr2) were
carried out by mixing tri- or di-bromoacetic aldehydes
and benzene in sulfuric acid. A typical procedure was
described for the well-known pesticide DDT [22].
Then the obtained tri- and di-bromodiphenylethanes
analogues were nitrated by nitric acid at Ϫ 30ЊC
following the procedure given in ref. [23].
The kinetic runs were carried out using a stopped-
flow spectrophotometer (Applied Photophysics) with
the cell block thermostated to ^ 0.1ЊC. The kinetic
runs were completed under pseudo-first-order condi-
tions with large excess of the base concentration. The
observed rate constants were calculated from the
traces of absorbance versus time. The spectrophoto-
metric measurements were carried out at 320 nm. The
concentration of haloethanes was 5 × 10Ϫ5 M.
The observed rate constant, kobs, depends on the
base concentration and is given by the equation:
kobs ˆ k[B], where k is the rate constant for the elim-
ination reaction and [B] is the initial base

176
G. Schroeder et al. / Journal of Molecular Structure 476 (1999) 173–179
Table 1
negativity and quantity of halogen atoms at the Ca-
atom there is an increase in positive charge on the Ca-
atom and HϪb-atom, but simultaneously a decrease in
the positive charge on the Cb-atom is observed.
All the studied dehydrohalogenation reactions of
five halogeno-2,2-bis(4-nitrophenyl)ethanes with
TBD and MTBD lead to corresponding halogeno-
2,2-bis(4-nitrophenyl)ethenes (Scheme 3).
The charge distribution on Ca, Cb, Hb calculated by AM1 (Hyper
Chem 3.0) semi-empirical method
Compound
Charge on Ca
Charge on Cb
Charge on Hb
Ar2CHCF3
Ar2CHCCl3
Ar2CHCBr3
r2CHCHCl2
r2CHCHBr2
0.418
0.138
Ϫ 0.274
Ϫ 0.251
Ϫ 0.118
Ϫ 0.063
Ϫ 0.038
Ϫ 0.018
Ϫ 0.017
Ϫ 0.029
0.142
0.131
0.121
0.127
0.128
Kinetic parameters for all these reactions have been
calculated and are shown in Tables 2–5.
TBD, in comparison with MTBD, is the more
efficient elimination factor because of its basicity.
At 25ЊC the relationship of calculated rate constants
for studied reactions between TBD and the follow-
ing reagents, Ar2CHCF3:Ar2CHCCl3:Ar2CHCBr3:
Ar2CHCHBr2:Ar2CHCHCl2, equals 13:1:6:6:1, while
for the reaction with MTBD the relationship is
60:1:3:3:1. For the studied arrangements the large
differences of the rate constants values are connected
with the differences of the b-hydrogen atom acidity as
well as with the character of the leaving halogen
atoms. The compound Ar2CHCF3 has the highest
acidity and a poor leaving group (fluoride ion),
whereas Ar2CHCBr3 has a good leaving group
(bromide ion) and the lowest acidity in comparison
with the other studied trihalogeno-dinitropheny-
lethanes. A similar observation for corresponding
Ar2CHCHCl2 and Ar2CHCHBr2 dihalogeno-
compounds is found.
All the investigated reactions could be divided into
two groups considering their rate constants and reac-
tion parameter values. Reactions between Ar2CHCF3
and N-bases belong to the first group, and the other
described reactions to the second one. The values of
the rate constants for the first group are higher than for
the second one. The described arrangements are
characterized by difference in entropy–enthalpy
concentration. The rate constants for forward (k) reac-
tion were calculated by linear least-squares fit of the
variation of kobs versus base concentration. The acti-
vation parameters were calculated by linear least-
squares fit of lnk versus 1/T.
3. Results and discussion
TBD and MTBD molecules in acetonitrile are very
strong N-bases and their pKa values are 24.97 ^ 0.05
and 24.70 ^ 0.05, respectively [12].
The type and quantity of halogen atoms at the Ca-
atom determines the electronegativity of the –CX3
group and defines the acidity of Cb–H bonds. The
electronegativity values for CF3, CCl3, CBr3, CHCl2,
CHBr2 groups are 3.29, 2.79, 2.57, 2.63, 2.45, respec-
tively [24].
The values of steric effect described by the Taft
equation (Es) for the following substituents, CF3 ,
CCl3, CBr3, CHCl2, CHBr2, are Ϫ 1.16, Ϫ 2.06,
Ϫ 2.43, Ϫ 1.54, Ϫ 1.81, respectively [25].
The influence of halo-substituents on charge distri-
bution of the studied molecules was also calculated by
semi-empirical method AM1 (Hyper Chem 3.0), and
the obtained results are collected in Table 1. The data
in this table demonstrate that with increasing electro-
Scheme 3.

G. Schroeder et al. / Journal of Molecular Structure 476 (1999) 173–179
177
Table 2
Kinetics parameters ( ^ standard deviation) for reaction between Ar2CHCX3 and TBD in acetonitrile
k
obs (sϪ1
)
Temp. (ЊC)
0.001 M
0.002 M
0.003 M
0.004 M
0.005 M
k (MϪ1 sϪ1
)
Ar2CHCF3
5
15
25
35
0.09
0.72
1.64
4.03
7.52
0.30
1.25
3.01
8.35
15.99
0.50
1.74
4.63
12.51
23.77
0.79
2.47
6.01
16.51
30.49
1.03
3.08
7.82
20.34
38.49
237 ^ 9
594 ^ 24
1536 ^ 42
4078 ^ 56
7640 ^ 161
45
Ar2CHCCl3
5
15
25
35
0.03
0.06
0.27
0.31
0.39
0.07
0.13
0.38
0.75
1.05
0.10
0.26
0.46
1.02
1.83
0.14
0.34
0.61
1.53
2.51
0.18
0.43
0.75
1.83
3.34
37 ^ 1
95 ^ 5
120 ^ 5
382 ^ 20
736 ^ 16
45
Ar2CHCBr3
5
15
25
35
0.47
0.61
0.83
1.05
1.20
0.91
1.21
1.64
1.85
2.45
1.32
1.82
2.35
2.89
3.45
1.75
2.34
3.21
3.85
4.51
2.35
3.01
3.97
4.75
5.56
460 ^ 20
593 ^ 11
785 ^ 11
940 ^ 20
1080 ^ 23
45
Ar2CHCHBr2
5
15
25
35
0.39
0.54
0.72
0.89
1.01
0.78
1.19
1.43
1.65
1.98
1.18
1.79
2.10
2.39
2.82
1.58
2.21
2.85
3.16
3.82
1.97
2.85
3.54
4.01
4.75
396 ^ 18
564 ^ 13
706 ^ 15
775 ^ 18
932 ^ 17
45
Ar2CHCHCl2
5
0.01
0.03
0.17
0.21
0.35
0.04
0.10
0.29
0.47
1.05
0.08
0.19
0.42
0.74
1.65
0.11
0.27
0.52
1.0
0.14
0.35
0.64
1.25
2.87
33 ^ 2
81 ^ 3
117 ^ 5
261 ^ 13
612 ^ 19
15
25
35
45
2.13
distribution. The difference in DG± values for reac-
tions between Ar2CHCF3 and MTBD or TBD is
5 kJ molϪ1, while for the other reactions this differ-
ence equals 9 kJ molϪ1. The difference in DG± values
for these two groups of reactions demonstrates the
different structures of the transition states. The reac-
tions between Ar2CHCF3 and N-bases are led by E1cB
mechanism, while the other reactions by E2H
mechanism. The course of reaction and the structures
of the transition states are presented in Scheme 1.
The fluoride anion abstraction from carbanion
formated by alkane in the fast reversible step is the
Table 3
Activation parameters ( ^ standard deviation) for reaction between Ar2CHCX3 and TBD in acetonitrile
Compound
DH± (kJ molϪ1
)
DS± (J molϪ1 degϪ1
)
DG± (kJ molϪ1
)
Ar2CHCF3
62.9 ^ 1.9
51.7 ^ 5.6
13.6 ^ 1.0
12.6 ^ 1.7
49.0 ^ 2.0
27.1 ^ 6.2
Ϫ 29.0 ^ 18.8
Ϫ 144.6 ^ 3.2
Ϫ 148.9 ^ 5.7
Ϫ 39.3 ^ 13.6
54.8 ^ 1.9
60.3 ^ 5.6
56.7 ^ 1.0
57.0 ^ 1.7
60.7 ^ 4.0
Ar2CHCCl3
Ar2CHCBr3
Ar2CHCHBr2
Ar2CHCHCl2

178
G. Schroeder et al. / Journal of Molecular Structure 476 (1999) 173–179
Table 4
Kinetics parameters ( ^ standard deviation) for reaction between Ar2CHCX3 and MTBD in acetonitrile
Concentration
Temp. (ЊC)
0.001 M
0.002 M
0.003 M
0.004 M
0.005 M
k (MϪ1 sϪ1
)
Ar2CHCF3
5
15
25
35
0.007
0.025
0.3
2.1
3.64
0.013
0.049
0.51
3.68
7.64
0.019
0.078
0.72
5.55
11.42
0.026
0.110
1.0
7.29
15.41
0.032
0.120
1.25
9.31
19.22
6.3 ^ 0.1
25.1 ^ 2.0
239.0 ^ 8.0
1803.0 ^ 41
3893.0 ^ 20
45
Ar2CHCCl3
5
15
25
35
0.001
0.003
0.008
0.010
0.021
0.002
0.005
0.012
0.018
0.038
0.004
0.009
0.017
0.030
0.050
0.005
0.013
0.020
0.039
0.062
0.007
0.016
0.024
0.049
0.076
1.5 ^ 0.1
3.4 ^ 0.2
4.6 ^ 0.5
9.9 ^ 0.3
13.4 ^ 0.5
45
Ar2CHCBr3
5
15
25
35
0.005
0.009
0.012
0.029
0.034
0.008
0.015
0.026
0.038
0.067
0.012
0.025
0.035
0.075
0.096
0.017
0.032
0.048
0.099
0.134
0.025
0.038
0.064
0.113
0.160
4.9 ^ 0.6
7.5 ^ 0.4
12.6 ^ 0.6
22.9 ^ 2.4
31.9 ^ 0.9
45
Ar2CHCHBr2
5
15
25
35
0.003
0.006
0.008
0.020
0.028
0.007
0.013
0.019
0.040
0.055
0.012
0.019
0.031
0.060
0.085
0.017
0.025
0.042
0.086
0.112
0.021
0.031
0.053
0.097
0.139
4.6 ^ 0.7
6.2 ^ 0.5
11.3 ^ 0.9
20.0 ^ 1.3
27.9 ^ 1.5
45
Ar2CHCHCl2
5
0.001
0.002
0.004
0.006
0.012
0.002
0.004
0.009
0.014
0.025
0.004
0.006
0.012
0.025
0.037
0.005
0.008
0.017
0.032
0.050
0.006
0.011
0.020
0.039
0.062
1.3 ^ 0.1
2.2 ^ 0.1
4.0 ^ 0.2
8.4 ^ 0.4
12.5 ^ 0.2
15
25
35
45
rate determining step. The transition state for this slow
step is product alkene-like. The HCl or HBr elimina-
tion for other arrangements is realized by concerted
E2H mechanism. This transition state is strongly
polarized.
enthalpy DH± is determined by the enthalpy–entropy
coupling and charge distribution in the transition state.
In conclusion, bases such as MTBD and TBD,
demonstrating good basicity and small nucleophili-
city, may be applicable to elimination processes and
are more efficacious than other weaker N-bases.
For the studied reactions the value of activation
Table 5
Activation parameters ( ^ standard deviation) for reaction between Ar2CHCX3 and MTBD in acetonitrile
Compound
DH± (kJ molϪ1
)
DS± (J molϪ1 degϪ1
)
DG± (kJ molϪ1
)
Ar2CHCF3
123.9 ^ 9.4
37.8 ^ 3.3
33.3 ^ 1.7
32.7 ^ 2.5
40.7 ^ 2.1
215.0 ^ 31.7
Ϫ 104.8 ^ 11.0
Ϫ 111.9 ^ 5.7
Ϫ 115.2 ^ 8.0
Ϫ 96.4 ^ 7.0
59.7 ^ 9.4
69.0 ^ 3.3
66.7 ^ 1.7
67.0 ^ 2.5
69.5 ^ 2.1
Ar2CHCCl3
Ar2CHCBr3
Ar2CHCHBr2
Ar2CHCHCl2

G. Schroeder et al. / Journal of Molecular Structure 476 (1999) 173–179
179
[13] M. Fletschinger, B. Zipperer, H. Fritz, H. Prinzbach, Tetrahe-
dron Lett. 28 (1987) 2517.
References
[14] G. Schroeder, A. Jarczewski, J. Prakt. Chemie 332 (1990) 269.
[15] G. Schroeder, B. Leska, A. Jarczewski, B. Nowak-Wydra, B.
Brzezinski, J. Mol. Struct. 344 (1995) 77.
[16] A. Jarczewski, G. Schroeder, M. Dworniczak, J. Chem. Soc.
Perkin Trans. II (1986) 55.
[17] A. Jarczewski, G. Schroeder, M. Waligorski, M. Dworniczak,
J. Chem. Soc. Perkin Trans. II (1991) 665.
[18] A. Jarczewski, M. Waligorska, K.T. Leffek, Can. J. Chem. 63
(1985) 1194.
[19] G. Schroeder, A. Jarczewski, J. Prakt. Chemie 331 (1989) 964.
[20] K.T. Leffek, G. Schroeder, Canad. J. Chem, 60 (1982) 1696.
[21] A. Jarczewski, G. Schroeder, W. Galezowski, K.T. Leffek, U.
Maciejewska, Canad. J. Chem 63 (1985) 576.
[22] A.I. Vogel, A Textbook of Practical Organic Chemistry,
Longman, London, 1978.
[23] E.J. Skerrett, D. Woodcook, J. Chem. Soc. (1952) 2807.
[24] J. March, Advanced Organic Chemistry, Reaction,
Mechanism and Structure, John Wiley, New York, 1992.
[25] K. Schwetlick, Kinetische, Methoden zur Untesuchung von
Reaktionsmechanismen, VEB, Berlin, 1971.
[1] K.T. Leffek, P. Pruszynski, K. Thanapaalasingham, Canad. J.
Chem. 67 (1989) 590.
[2] G. Schroeder, B. Leska, Wiadomosci Chem. 50 (1996) 369.
[3] R. Schwesinger, J. Willaredt, H. Schlemper, M. Keller, D.
Schmitt, H. Fritz, Chem. Ber. 127 (1994) 2435.
[4] G. Schroeder, J. Przybyl, B. Leska, Polish J. Chem. 67 (1993)
273.
[5] R.W. Adler, P.S. Bowman, W.R.S. Steele, D.R. Winterman, J.
Chem. Soc., Chem. Comm. (1968) 7232.
[6] R.L. Benoit, D. Lefebvre, M. Frchette, Can. J. Chem. 65
(1987) 996.
[7] L.A. Kurasov, A.F. Pozharskii, V.V. Kuzmenko, Zh. Org.
Khim. 19 (1983) 859.
[8] G. Schroeder, B. Brzezinski, A. Jarczewski, E. Grech, J.
Nowicka-Scheibe, J. Mol. Struct. 344 (1995) 89.
[9] J. Cheney, J.-M. Lehn, J. Chem. Soc., Chem. Comm. (1972)
487.
[10] R. Schwesinger, Nachr. Chem. Tech. Lab. 38 (1990) 1214.
[11] T. Pietzonka, D. Seebach, Chem. Ber. 124 (1990) 1837.
[12] H.-J. Gais, J. Vollhardt, C. Krger, Angew. Chem. 100 (1988)
108.