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Figure 2 shows kinetic curves for thermal decom-
position of AZT. The process follows the first-order
kinetics, as indicated by the linear plots in the coor-
dinates ln(m
m) , where m is the total weight
loss in the first decomposition stage (20% of the
initial weight), and m is the weight loss by a time
moment . The rate constants k1 105 at 140, 158,
169, 183, 192, and 201 C are, respectively, 0.35, 2.9,
1
6.8, 28, 81, and 147 s . Manometric study of the first
decomposition stage in the temperature range from
160 to 201 C also revealed the first-order kinetics.
The rate constants k1 105 at 160, 170, 181, 191, and
1
201 C, are respectively, 3.6, 9.2, 21, 64, and 146 s .
Fig. 2. Kinetics of thermal decomposition of AZT in air
at (1) 140 C, (2) 158 C, (3) 169 C, (4) 183 C, (5) 192 C,
and (6) 201 C; thermogravimetry, sample weight 40 mg.
The rate constants for thermal decomposition of AZT,
determined by the thermogravimetric (k1) and mano-
metric methods (k1), are very similar. The above listed
values fit a common Arrhenius equation (Fig. 3):
1
k = 1014.7 0.5 exp[ (38100 900)/RT] (s );
r = 0.997, s = 0.063, n = 11.
In thermogravimetric experiments decomposition
of AZT occurs in an open reaction system, whereas
manometric experiments are carried out in a vessel
with a limited volume. Therefore, the similarity
between the rate constants obtained in the two series
of experiments suggests that there is no appreciable
effect of AZT decomposition products on the kinetics
of the process. On the other hand, comparison of the
thermogravimetric (in air) and manometric kinetic
data (in a vacuum) indicates once more that the reac-
tion under study is not accompanied by thermo-
oxidative processes.
By cooling the gaseous AZT thermolysis products
in the Bourdon gage from 25 C to 196 C (liquid
nitrogen) we found that the volume fraction of nitro-
gen among these products is 95.2%. According to the
mass spectrometric data, nitrogen is the major gaseous
product formed in the first stage of thermal decom-
position of AZT (190 C, 10 h); also, carbon dioxide
(no more than 3 vol. %), methanol, and furan were
detected: m/z (Irel, %): 68 (1.0) [CH CHOCH CH]+;
44 (3.9), 42 (0.3) [CO2]+; 40 (0.3), 39 (1.4), 32 (0.4),
31 (0.7), 29 (1.3), 28 (100) [N2]+, 18 (3.2) [H2O]+,
17 (1.2), 16 (0.5), 15 (0.3), 14 (5.2), 12 (0.2).
Fig. 3. Arrhenius plot for the first-order rate constant of
thermal decomposition of AZT according to (1) thermo-
gravimetric and (2) manometric measurements.
The rate of thermolysis increases on further heating,
presumably as a result of thermooxidative processes.
It seems to be important that the kinetic curve for
wieght loss of AZT almost does not change up to
250 C on replacement of air by argon (1 atm). This
means that the first decomposition stage is a purely
thermal process which is not accompanied by oxida-
tion. Under conditions of a dynamic vacuum (con-
tinuous evacuation of the reaction vessel) above
170 C, the contribution of sample evaporation
becomes appreciable. Taking the above stated into
account, the subsequent termogravimetric study of
AZT decomposition under isothermal conditions was
performed in air at temperatures above the melting
point.
The solid residue obtained after complete decom-
position of AZT in the temperature range 140 201 C
has a dark color. Its weight fraction is about 80%.
The yield of volatile products approaches 20%, i.e.,
it is almost twice as large as the theoretical weight
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 5 2003