1048
TIMCHENKO et al.
0.9
4
3
3
2
0.6
0.3
2
1
1
0
20 40 60 80 100 120
, min
1
4000
3000
2000
1000 , cm
Fig. 4. Kinetic curves of thermal decomposition of
thiourea in a nitrogen flow: (1) 181, (2) 190, (3) 200,
and (4) 210 C.
Fig. 3. IR spectra of products of isothermal heating of an
SC(NH ) melt at 190 C for (1) 1, (2) 3, and (3) 8 h.
2 2
1
bands at 1470 and 1090 cm are characterstic bands
thiocyanate. Ammonium thiocyanate, isomerizing into
Thio, supports this process. Cessation of the weight
loss corresponds to complete decomposition of both
Thio and NH4SCN and formation of a homogeneous
melt of guanidinium thiocyanate, stable at the given
temperature.
1
of Thio; and the bands at 1635 and 500 cm are due
to vibrations of the guanidinium ion [7].
The melt heated for 1 h was a mixture of ammoni-
um thiocyanate, guanidinium thiocyanate, and a minor
amount of Thio. A number of strong absorption bands
1
in the range 3000 3400 cm can be assigned to the
Figure 4 shows the kinetic curves of thermal de-
composition of the melt in the coordinates degree of
stretching vibrations of the amino groups of all the
three compounds.
conversion
time of isothermal heating. The cal-
culated kinetic parameters of thermal decomposition
of thiourea under isothermal and nonisothermal condi-
tions are listed in Table 2. Additionally we calculated
the activation energy from the linear dependence of
In the IR spectrum of the cooled melt that had been
heated for 3 h (after the melt boiling fully ceased), the
1
intensity of the absorption bands of Thio (1470 cm )
and NH+4 (3130, 1400 cm ) considerably decreased,
1
log(1/ 0.5) on 1/T (
is the conversion half-time);
0.5
whereas that of the guanidinium thiocyanate band
noticeably increased.
1
we obtained the value of 117.2 kJ mol .
The results of the nonisothermal experiments are
affected by the endoeffect of the Thio melting. There-
fore, E was determined with a larger error than from
the isothermal experiments. However, the correlation
between the values of E obtained by different methods
(under isothermal conditions, gravimetrically; under
nonisothermal conditions, from DTA curves) shows
that this parameter, on the whole, has been determined
with a reasonable accuracy and that it characterizes
formation of guanidinium thiocyanate.
After heating for 8 h, the melt ceased to lose
weight, and its major component was guanidinium
thiocyanate. This is confirmed by the absence of the
polymorphous transition of NH4SCN (95 C) in the
heating curves of the cooled melt and by the fact that
the IR absorption bands of Thio and ammonium thio-
1
cyanate at 1470, 1400, and 1090 cm were on the
background level (Fig. 3). At the same time, as judged
from the intensity of the bands at 2060, 1635, and
1
500 cm , the amount of guanidinium thiocyanate
remained virtually unchanged compared to the previ-
ous sample.
Since reaction (2) is the limiting step, formation of
activated complexes presumably involves hydrogen
bonding between the sulfur atom of one Thio mole-
cule and amino group protons of another molecule.
Then the most signficant contribution to the activation
energy will be made by cleavage of the C S bond
Hence, reaction (2) does not fully stop after the end
of boiling, since isomerization (1) starts to proceed in
the reverse direction, slowly compensating the loss of
Thio from the melt.
1
whose energy in the Thio molecule is 100.5 kJ mol
[8].
Thus, under isothermal conditions at 190 C, reac-
tions (2) and (3) occur successively, and the reactive
cyanamide formed by reaction (2), apparently, instan-
taneously transforms by reaction (3) into guanidinium
On reaching 60 70% degree of decomposition
under isothermal conditions, opalescence appeared
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 7 2004