1
016
B. Ambrozini et al.
which 99.80 % of the initial sample mass is lost corre-
sponding to decomposition of the drug. In the same tem-
perature range, there is an endothermic peak at 312.0 °C in
the DTA curve.
In agreement with the TG curve, one can observe an
intense gas release in the 27.5 to 40.0 min interval, cor-
responding to 290–420 °C. According to the database
records, in this temperature range there are the character-
istic peaks in the FTIR spectra of 1-naphthol at 3647, 1525,
Figure 3 presents propranolol heat–cool–heat DSC
curves. In the first heating, an endothermic peak at
-
1
1412, 1286, 1240, 1205, 1046, 1008 cm ; isopropylamine
-1
-
1
1
66.7 °C (Tonset = 163.9 °C, DHfus = 16.31 kJ mol
)
at 3055, 2967, 2914, 2872 cm
and HCl in the
-
1
relative to the melting process was observed in agreement
with previously reported results [12]. In the first cooling,
crystallization was not observed, instead of that a glass
transition, typical of amorphous compounds, was detected
c.a. 30 °C.
2840–2660 cm range. A comparison of the experimental
and data base peaks for the three compounds suggested
here is presented in Fig. 5. A tentative attribution of such
peaks is presented in Table 1.
Thus, one can conclude that propranolol decomposed in
the 199.6–369.9 °C range releasing 1-naphthol, isopropy-
lamine and HCl. It should be expected the release of iso-
propanol once it is also part of the molecule. However, a
secure detection of this alcohol could not be performed
once the isopropanol IR spectra peaks are similar to those
of the other constituents, but we can suggest the evolution
of such alcohol during decomposition as well, but not
confirmed. A tentative mechanism for such decomposition
is presented in Fig. 6.
In the second heating, one can observe the reversion on
the glass transition, represented by baseline deviation c.a.
3
0 °C, an exothermic event centered at 125.3 °C
-1
(
DHcryst = 13.10 kJ mol ), probably associated with a
cold crystallization event, followed by melting
endothermic process with peak at 165.7 °C (T
a
=
onset
-
1
1
61.7 °C, DHfus = 15.42 kJ mol ) [12]. During the sec-
ond cooling, the same characteristics of the first cooling
were observed.
The investigation of gases evolved during the propra-
nolol thermal decomposition revealed peaks of gas evolu-
tion at 7, 31 and 67 min (respectively at 90, 330 and
It is interesting to note that in other b-blockers atenolol,
nadolol [15] and carvedilol [16] already investigated it was
found that the molecules start decomposition from the
aliphatic branch including the isopropylamine. However, in
the present case the structure of the molecule seems to
provide conditions for the entire decomposition followed
by volatilization of the 1-naphthol, a volatile compound not
present in the other b-blockers.
6
90 °C), as presented in Fig. 4. The identification of the
species was made according to Nicolet TGA Vapor Phase
and EPA Vapor Phase database library contained in the
Omnic 8.0 software (Thermo Scientific) [13].
In the first peak, CO and traces of H O were detected,
2
2
and in the last one, only CO appeared although this gas
2
could be detected all along the experiment. The first
detection could be related to carbon dioxide from the
residues on the line/furnace and at higher temperature the
gas was released from the decomposition of the residues of
the sample.
CH3
N
H
CH3
O
(s)
(l)
OH
HCl
Table 1 Tentative FTIR peak attribution for the evolved gases from
propranolol decomposition
CH3
CH3
-
1
a
Compound
-Naphthol
Wave number/cm
Attribution
N
O
H
OH
1
3647
O–H stretch
C=C ring stretch
O–H bend
HCl
1
1
1
1
525, 1412
286
205
C–O stretch
C–H
008
Isopropylamine
3055
N–H stretch
C–H stretch
C–N stretch
H–Cl stretch
OH
?
CH
HCl
H C
CH3
2
1
914, 2872
092
3
3
+
+
+
HN
CH3
OH
Hydrochloric acid
2840–2660
a
According to Silverstein et al. [14]
Fig. 6 Tentative mechanism for the thermal behavior of propranolol
1
23
Ambrozini, Beatriz
