Quantitative estimation of the α-effect in the reactions yielding phosphorus-containing thiosemicarbazides

Article abstract of DOI:10.1134/S1070363206080111

The α-effect in the reactions of O,O-diaryl phosphorohydrazidothioates with phenyl isothiocyanate in benzene at 25°C was quantitatively evaluated from the kinetic data. The probable reaction mechanism was suggested, and the causes of the α-effect in these reactions are discussed. An approximate linear correlation is observed between the α-effect in the reactions of O,O-diaryl phosphorohydrazidothioates with phenyl isothiocyanate, basicity constants of the hydrazides, and σ^p constants of the substituents at the P atom. The value of the α-effect is largely determined by the structure of the transition state. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S1070363206080111

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 8, pp. 1240 1242.
Pleiades Publishing, Inc., 2006.
Original Russian Text N.I. Yanchuk, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 8, pp. 1291 1294.
Quantitative Estimation of the -Effect in the Reactions
Yielding Phosphorus-Containing Thiosemicarbazides
N. I. Yanchuk
Gnatyuk National Pedagogical University,
ul. M. Krivonosa 2, Ternopol, 46004 Ukraine
Received May 20, 2005
Abstract The -effect in the reactions of O,O-diaryl phosphorohydrazidothioates with phenyl isothio-
cyanate in benzene at 25 C was quantitatively evaluated from the kinetic data. The probable reaction mech-
anism was suggested, and the causes of the -effect in these reactions are discussed. An approximate linear
correlation is observed between the -effect in the reactions of O,O-diaryl phosphorohydrazidothioates with
p
phenyl isothiocyanate, basicity constants of the hydrazides, and
constants of the substituents at the P atom.
The value of the -effect is largely determined by the structure of the transition state.
DOI: 10.1134/S1070363206080111
It was found previously that phosphorus acid hy-
drazides exhibit increased reactivity toward phenyl
isocyanate, which was attributed to the -effect [1, 2].
In this study, aimed to check the above assump-
tion, we examined the kinetics of the reactions of
O,O-diaryl phosphorohydrazidothioates with phenyl
isothiocyanate. It is known that hydrazine derivatives
react with isothiocyanates in benzene relatively slowly
[4]. Therefore, we attempted to quantitatively evaluate
the -effect in these reactions if it takes place. We
found that O,O-diaryl phosphorohydrazidothioates
react with phenyl isothiocyanate in benzene quantita-
tively to form thiosemicarbazides:
It is also known that these reactions occur via
cyclic transition states; hence, the manifestation of the
-effect and its value are determined by the reaction
mechanism [1 3]. It was suggested that organophos-
phorus hydrazine derivatives would show increased
reactivity toward substrates of definite type.
S
S
RC H O
RC H O
6 4
6
4
P NHNH + S=C=N C H
P NHNHC(S)NHC H5
6
2
6
5
R C H O
R C H O
6 4
6
4
The kinetics of these reactions are described by
second-order equations. The rate constants calculated
by these equations do not noticeably change during
the process.
thiocyanates, as indicated by the rate constants rang-
ing from 0.949 10 to 0.970 10 l mol s .
4
4
1
1
Experimental data on the effect of substituents in
phenyl groups on the reactivity of O,O-diaryl phos-
phorohydrazidothioates toward phenyl isothiocyanate
show that the activity of the hydrazides is proportional
to their basicity constants (determined in 50% ethanol,
see table). The Brønsted equation for the reactions of
O,O-diaryl phosphorohydrazidothioates with phenyl
isothiocyanate is as follows:
To check whether the starting compounds or reac-
tion products exert a catalytic effect on the reactions
of the hydrazides with phenyl isothiocyanate, we
studied the kinetics of these reactions at different
concentrations of the reactants (hydrazides, 0.000625
0
.002 M; phenyl isothiocyanate, 0.000625 0.00625 M)
and with preliminary addition of the reaction products
0.000625 M) to the initial solutions. We found that
neither the starting reactants nor the reaction products
thiosemicarbazides) affect the rates of the reactions of
phosphorus-containing hydrazides with phenyl iso-
logk = ( 19.5 2.0) + (5.91 0.73)pKBH
n 7, r 0.961, s 0.116.
+
,
(1)
(
(
Thus, variation of the reactivity of the examined
hydrazides is related to variation of their basicity.
1240

QUANTITATIVE ESTIMATION OF THE -EFFECT
1241
Therefore, it could be expected that aniline would be
more reactive than O,O-diaryl phosphorohydrazido-
thioates toward phenyl isothiocyanate. However, actu-
ally less basic phosphorus acid hydrazides are more
reactive (see table). Hence, abnormally high reactivity
of O,O-diaryl phosphorohydrazidothioates cannot be
attributed solely to their basicity, and other factors
should be take into account.
stronger the influence of substituents on the value of
the -effect. Thus, it is possible to determine from the
Brønsted
constants how introduction of electron-
donor and electron-acceptor substituents will influence
the value of the -effect.
Analysis showed that the value of the -effect
varies in the same direction with the basicity constants
p
of the hydrazides and
constants of substituents at
As shown previously, the effect of polarizability on
reactions of this type is insignificant. Apparently, the
increased reactivity of O,O-diaryl phosphorohydrazi-
dothioates is due to the effect of the nitrogen atom
bearing a lone electron pair in the position adjacent to
the nucleophilic center, i.e., to the so-called -effect.
the P atom in the hydrazides [7]. The correlations are
described by Eqs. (3) and (4), respectively.
-
Effect = ( 7059 1879) + (2735 699)pK , (3)
BH+
n 7, r 0.868, s 0.110,
(930 225) p,
-
Effect = (242 42)
n 7, r 0.879, s 106.
(4)
In this study we characterized this -effect quanti-
tatively. The -effect is manifested as a positive devi-
ation of the point for the -nucleophile from the
Brønsted straight line for compounds with normal
reactivity [6]. The -effect is evaluated quantitatively
as the ratio of the rate constant of the reaction with
a given hydrazide to that of the reaction with an aryl-
amine having the same basicity [3]. The values of the
The -effect is attributed at present to several fac-
tors, but none of them, considered separately, fully
explains the nature of this phenomenons [1, 2, 8].
Assuming that the -effect is largely determined by
the structure of the transition state [9, 10], we can
explain the increased reactivity of phosphorus acid
hydrazides by a specific structure of the transition
state. Hydrazine derivatives, being bifunctional re-
agents, can form cyclic complexes in the transition
state [1].
-effect in the reactions of O,O-diaryl phosphorohy-
drazidothioates with phenyl isothiocyanate, estimated
using in some cases the rate constants calculated from
the Brønsted equation for the reactions of aromatic
amines with phenyl isothiocyanate in benzene {the
basicity constants were determined in 50% ethanol;
Eq. (2) [3]}, are given in the table.
It was shown that reactions of hydrazine deriva-
tives with isocyanates involve a nucleophilic attack of
the isocyanate carbon atom by the hydrazide. The bi-
molecular addition of the hydrazide to the electrophil-
ic carbon atom of the isocyanate occurs in one step
via a cyclic transition state [11]. The similar structure
of the transition state can be realized in the reactions
of O,O-diaryl phosphorohydrazidothioates with phen-
yl isothiocyanate, taking into account our results and
the relevant published data [12 16]. Thus, the general
mechanism of the noncatalytic reaction of O,O-diaryl
phosphorohydrazidothioates with phenyl isothiocya-
nate in benzene is, presumably, as follows:
logk = 9.44 + 1.17pKBH
r 0.997, s 0.088.
+
,
(2)
It is seen that the substituents in the aromatic rings
of the hydrazides influence the value of the -effect:
electron-donor substituents enhance, and electron-
acceptor substituents, on the contrary, suppress it.
The larger the difference between the constants in
the Brønsted equations for hydrazides and amines, the
S
S
S
RC H O
6
4
k
RC H O
RC H O
6 4
6
4
P NHNH + S=C=N C H
2
6
5
P N
P NHNHC(S)NHC H
6 5
R C H O
6
4
R C H O
R C H O
6 4
6
4
H HN
H
/
.
.
<
+
/
S===C == =N C H5
6
^
A
According to the mechanism suggested, the reac-
tion occurs in one step via a cyclic transition state A,
with the hydrogen bonding of the S and N atoms of
phenyl isothiocyanate with the H atoms of the imino
and amino groups of the hydrazide and formation of
a new bond between between the nucleophilic N atom
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 8 2006

1242
YANCHUK
Quantitative evaluation of the -effect in the reactions of
O,O-diaryl phosphorohydrazidothioates [(RC H O)
R C H O)PSNHNH ] with phenyl isothiocyanate in
REFERENCES
6
4
(
1. Grekov, A.P. and Veselov, V.Ya., Fizicheskaya khi-
miya gidrazina (Physical Chemistry of Hydrazine),
Kiev: Naukova Dumka, 1979.
6
4
2
benzene at 25 C
4
pKBH
5]
+
k 10 ,
R
R
-Effect
2. Grekov, A.P. and Veselov, V.Ya., Usp. Khim., 1978,
[
l mol s 1
1
vol. 47, no. 7, p. 1200.
3
4
. Shandruk, M.I., Yanchuk, M.I., and Grekov, A.P.,
Dopov. Akad. Nauk Ukr. RSR, Ser. B, 1974, no. 4,
p. 349.
p-CH3
p-CH3
m-CH3 m-CH3
H
H
p-CH3
H
2.76
2.74
2.73
2.70
2.68
2.63
2.58
3.90 0.24
2.39 0.17
2.48 0.19
0.958 0.029
0.920 0.025
0.465 0.014
0.372 0.011
634
410
437
183
185
107
98.2
. Yanchuk, N.I. and Balukh, V.M., Zh. Obshch. Khim.,
H
1
984, vol. 54, no. 12, p. 2663.
p-Cl
p-Cl
m-Cl
p-Cl
m-Cl
5. Shandruk, M.I., Yanchuk, N.I., and Grekov, A.P., Zh.
Org. Khim., 1974, vol. 10, no. 11, p. 2357.
Aniline
4.19 0.0301 0.0016
6
7
8
9
. Dixon, J.E. and Bruice, T.C., J. Am. Chem. Soc.,
972, vol. 94, no. 6, p. 2052.
. Mastryukova, T.A. and Kabachnik, M.I., Usp. Khim.,
969, vol. 38, no. 10, p. 1751.
. Fina, N.J. and Edwards, J.O., Int. J. Chem. Kinet.,
973, vol. 5, no. 1, p. 1.
. Gregory, M.J. and Bruice, T.C., J. Am. Chem. Soc.,
967, vol. 89, no. 17, p. 4400.
1
of the hydrazide and the electrophilic C atom of the
isothiocyanate. This is accompanied by cleavage of
the C=N bond in the isothiocyanate and N H bond in
the hydrazide, with the proton transfer from the amino
group of the hydrazide to the isocyanate group (for-
mation of a new N H bond) and formation of the re-
action product, thiosemicarbazide derivative.
1
1
1
10. Hudson, R.F., Structure and Mechanism in Organo-
phosphorus Chemistry, London: Academic, 1965.
The occurrence of reactions via cyclic transition
states is energetically favorable, and therefore such
reactions should be relatively fast.
1
1. Grekov, A.P., Shandruk, M.I., and Yanchuk, N.I.,
Dokl. Akad. Nauk SSSR, 1974, vol. 214, no. 5,
p. 1077.
1
1
2. Yanchuk, N.I., Zh. Obshch. Khim., 1990, vol. 60,
EXPERIMENTAL
no. 5, p. 1033.
3. Yanchuk, N.I., Zh. Obshch. Khim., 1993, vol. 63,
Benzene for kinetic studies was purified according
to [17]. O,O-Diaryl phosphorohydrazidothioates were
prepared from the corresponding acid chlorides and
hydrazine hydrate and were purified as described in
no. 4, p. 832.
14. Yanchuk, N.I., Kinet. Katal., 1994, vol. 35, no. 4,
p. 568.
[
18, 19]. Phenyl isothiocyanate was vacuum-distilled
1
1
5. Yanchuk, N.I., Grod, I.N., and Ivanets, L.N., Zh.
Obshch. Khim., 2002, vol. 72, no. 11, p. 1889.
just before use.
6. Yanchuk, N.I., Ivanets, L.N., and Yanchuk, S.N., Zh.
Obshch. Khim., 2003, vol. 73, no. 2, p. 211.
The reaction progress was monitored by the content
of unchanged phenyl isothiocyanate [4, 20]. The ki-
netic experiments were performed in benzene at 25
17. Litvinenko, L.M. and Grekov, A.P., Ukr. Khim. Zh.,
0
.05 C. The initial concentrations (M) were as follows:
O,O-diaryl phosphorohydrazidothioates 0.000625
.002 and phenyl isothiocyanate 0.000625 0.00625.
1954, vol. 20, no. 2, p. 194.
18. Shandruk, M.I., Yanchuk, N.I., and Grekov, A.P., Zh.
Obshch. Khim., 1973, vol. 43, no. 10, p. 2194.
0
Three replicate experiments were performed in each
case; the experiments involved 6 8 measurements and
were performed to no less than 80% conversion.
1
9. Klement, R. and Knollmuller, K.O., Chem. Ber.,
960, vol. 93, p. 1088.
1
2
0. Shandruk, M.I., Yanchuk, N.I., and Grekov, A.P., Zh.
Obshch. Khim., 1974, vol. 44, no. 11, p. 2424.
The accuracy of the results was evaluated by meth-
ods of mathematical statistics [21] at a confidence
level of 0.95.
21. Doerffel, K., Statistik in der analytischen Chemie,
Leipzig: Inst. fur Analytische Chemie, 1966.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 8 2006