Thermal analysis of salts from 4-nitrophenol and aliphatic amines

Article abstract of DOI:10.1007/s10973-019-08205-9

Solid samples obtained from 4-nitrophenol and aliphatic amines, in excess of amine, were submitted to thermal analysis. The TGA/DTA showed two endothermic events, with exception of tert-butylamine, which showed three endothermic peaks. The first event was the melting of the ionic salt, which temperature does not follow a pattern, and is maxim for ethylammonium minimal for butylammonium salt. For methylammonium and ethylammonium, the fusion is followed by mass loss corresponding to one amine by 4-nitrophenol, while for the others ammonium salts, this event marks the beginning of the mass variation, that is maintained until the decomposition of the 4-nitrophenol at 485?K. The temperature of the event depends of the length of the chain of the amine, while the second occurs at the same temperature for all amines.

Full text of DOI:10.1007/s10973-019-08205-9

Journal of Thermal Analysis and Calorimetry
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Thermal analysis of salts from 4-nitrophenol and aliphatic amines
1
•
•
•
´
Marcio Lazzarotto
Francine Furtado Nachtigall2 Geisa Liandra de Andrade de Siqueira3
Simone Rosa da Silveira Lazzarotto4 Marcelo Lazzarotto4
•
Received: 6 December 2017 / Accepted: 22 March 2019
´
´
Ó Akademiai Kiado, Budapest, Hungary 2019
Abstract
Solid samples obtained from 4-nitrophenol and aliphatic amines, in excess of amine, were submitted to thermal analysis.
The TGA/DTA showed two endothermic events, with exception of tert-butylamine, which showed three endothermic
peaks. The first event was the melting of the ionic salt, which temperature does not follow a pattern, and is maxim for
ethylammonium minimal for butylammonium salt. For methylammonium and ethylammonium, the fusion is followed by
mass loss corresponding to one amine by 4-nitrophenol, while for the others ammonium salts, this event marks the
beginning of the mass variation, that is maintained until the decomposition of the 4-nitrophenol at 485 K. The temperature
of the event depends of the length of the chain of the amine, while the second occurs at the same temperature for all amines.
Keywords Ammonium salts Á 4-Nitrophenol Á Proton transfer Á Thermal analysis
Introduction
[2], whereas ammonium cations are weaker acids (pKa
around 10.5 in water and 18 in acetonitrile) [3]. The dif-
ference of pKa values from ammonium cations is slightly
above the DpKa threshold for ionic compounds, albeit the
chemical equilibrium between hydrogen-bonded and ionic
species is dependent from the dielectric constant of the
solvent [4, 5], and the absorption in the UV/Vis region of
aggregates formed by 4-nitrophenol with diisopropy-
lamine, butylamine and imidazole in aprotic solvents is
dependent of the temperature [6]. Complexes from amines
and nitrophenols have been studied by UV/IR/Raman
spectrophotometry [7] and conductivimetry, and some
structures have been determined by X-ray.
The interaction of amines and phenols has been understood
by means of hydrogen bonding and electrostatic interac-
tion. Proton transfer yields ammonium cation and pheno-
late ion, but this chemical process occurs when the
difference between pKa of ammonium cation is 3 pKa units
above the phenol [1]. Below this threshold, the structure is
not ionic and the aggregation is kept by hydrogen bond
interaction. In both cases, the hydrogen bonds direct the
relative position of the ammonium and the phenol units in
order to increase the number of interactions and the optimal
angle.
The 4-nitrophenol is most acidic than the majority of the
phenols, with pKa = 7.15 in water and 21.3 in acetonitrile
We used thermal analysis to achieve information about
the nature of the interaction of ammonium cations with
calixarene [8] anions, when we found the effect of
macrocyclic and the interactions with p-cloud of the cal-
ixarene. In the present study, we search for the effect of the
role of the hydrogen bond interaction in the salts formed by
amine and phenols [9], using a poor p-donor such 4-ni-
trophenol. Recent investigation on salts of 4-nitrobenzoic
acid and alkanolamines [10] showed the role of hydrogen
bond interaction.
´
& Marcio Lazzarotto
marcio@iq.ufrgs.br
1
Universidade Federal do Rio Grande do Sul, Av. Bento
Gonc¸alves, 9500, Porto Alegre, RS 91501-970, Brazil
2
Inmetro SUR-RS, Av. Berlim, 627 - Sa˜o Geraldo,
Porto Alegre, RS 90240-581, Brazil
Compounds of 4-nitrophenol are known by their non-
linear optical properties [11] by the presence of electron
donor (–OH or –O-) group, and electron withdrawing
group and ammonium cations have attracted a lot of
3
Universidade Estadual de Ponta Grossa, Av. Carlos
Cavalcanti, 4748, Ponta Grossa, PR 84.030-900, Brazil
4
Embrapa Florestas, Estrada da Ribeira, KM 111,
P.O. Box 319, Colombo, PR 83.411-000, Brazil
123

M. Lazzarotto et al.
– o
o
attention by their applications as drugs [12], surfactants
[13], ionic liquids [14], lubricants [15] and materials for
electrical applications [16]. Thermal stability is critical for
some applications and allows determine thermodynamic
parameters related to heat absorption, as well the possi-
bility of side reactions. The present study seeks to establish
important variables for the thermal properties of 4-nitro-
phenolate salts with aliphatic ammonium salts.
– o
o
N+
N+
H
H
N
N+
H3CCN
R
H
H
R
H
o–
OH
Scheme 1 Synthesis of ammonium salts of 4-nitrophenolate
bands of N?–H above 3050 cm-1 and the region above
3050 cm-1 is similar for ammonium salts, with a band
above 3300 cm-1 and another between 3150 and
3250 cm-1, with the exception of ethylammonium salt,
whose infrared spectra does not show band above
3300 cm-1, and two bands at 3215 and 3100 cm-1 for N?–
H stretching. The low frequency for stretching vibration of
hydrogen bond aggregates is an evidence for strong
hydrogen bond, and we consider that the ammonium group
in ethylammonium is involved in three strong hydrogen
bonds with phenolate or nitro groups, while other, the
ammonium salts have one hydrogen bonded to nitrogen not
able to interact by hydrogen bond (Fig. 1).
Materials and methods
The alkylammonium-4-nitrophenolate salts were obtained
by adding ten times excess of alkylamine (Aldrich and
Merck) to 4-nitrophenol (Aldrich) solution (100 g L-1) in
acetonitrile. Methylamine (VETEC) and ethylamine
(VETEC) were used as aqueous solutions. Slow evapora-
tion of the solvent gave pale yellow to orange crystals,
collected and washed with hexane to remove adsorbed
amine. Salts from amines with short chains (methyl, ethyl,
propyl and tert-butyl) crystallize faster, while salts from
amines with longer chains required several days to the
1
The analysis of H-NMR of the products are in agree-
ment with the ionic nature of ammonium–phenolate
interaction, as seen for the ethylammonium-4-nitropheno-
late spectrum shown in Fig. 2, by the shift of d of the
ammonium moiety to low fields (d 3.7 and d 3.0), when
compared with the relative amine, while the chemical shift
of the protons attached to the aromatic ring of the 4-ni-
trophenolate (d 7.9 and d 7.0) were shifted to high field,
in relation with the 4-nitrophenol, mostly the proton near
the phenolate. However, the most important feature was the
absence of neutral 4-nitrophenol, reported in some crystal
structures [7, 17].
1
formation of crystal samples. Infrared and H-NMR anal-
yses are in agreement with the proposed structure for the
salts.
Thermogravimetric (TG) curves were performed using
the Shimadzu DTG-60H equipment. The conditions were:
heating temperature 30–300 °C, rate of 10 °C min-1
,
nitrogen flow of 100 mL min-1 and mass of
3.16 0.23 mg in aluminum crucible. The calibration of
the instrument was evaluated with standard calcium oxalate
monohydrate. Infrared analyses were performed in a IR
1
Prestige 21-Shimadzu and H-NMR in a Bruker Fourier
300 MHz.
Results and discussion
Addition of the amine to the solution of 4-nitrophenol in
acetonitrile results in the formation of the ammonium salt
and 4-nitrophenolate, observed by the appearance of yel-
low to orange color in the solution. The methyl, ethyl and
tert-butylamine salts precipitate rapidly, while the salts of
propyl, butyl, pentyl, hexyl and dodecylamine are more
soluble in acetonitrile and formed solid samples after par-
tial evaporation of the solvent. The yields were variable
and higher for methyl, ethyl and t-butyl (around 80%) and
lower for propyl, butyl, pentyl, hexyl and dodecyl
(50–20%) (Scheme 1).
Infrared analysis of the solid materials revealed that the
presence of bands from symmetrical and unsymmetrical
stretching vibration of the nitro group at 1550–1475 cm-1
and of the aromatic ring 1590 cm-1 and the stretching
Frequency/cm–1
Fig. 1 Infrared spectra of methylammonium and ethylammonium
4-nitrophenolate salts
123

Thermal analysis of salts from 4-nitrophenol and aliphatic amines
Fig. 2 1H-NMR of
ethylammonium-4-
nitrophenolate in D2O
10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Chemical shift (δ)
The solid samples obtained from 4-nitrophenol and
aliphatic amines were submitted to thermal analysis,
demonstrating total mass loss in the range of 293–500 K,
as shown in Fig. 3. The curves of TGA obtained from the
ammonium salts revealed two endothermic events, with the
exception of tert-butylammonium salt, that showed three
ones.
secondary amines [17], but the mass loss remains insuffi-
cient to support such hypothesis. Otherwise, the use of
large excesses of amine prevents such stoichiometry. The
results obtained revealed a maximum in the melting tem-
perature for ethylammonium salt, and we can attribute this
result to the strong hydrogen bond interactions shown by
the infrared spectrum. The sequence shows a drop in this
temperature until the minimum at 322 K for butylammo-
nium salt, followed by similar values around 348 K, as
seen in Fig. 3. For the TGA, it was not possible define a
plateau between both endothermic events for amines with
chains longer than ethyl, and the first endothermic event
marks the start of mass loss, which proceeds continuously
until the total loss achieved around 493 K (Fig. 4).
This nonlinear effect behavior for the temperature of the
first endothermic event should result from the contribution
of different aspects related to the ammonium–phenolate
interaction and related to amine departure from the solid
phase. Such effects may be: ammonium acidity (or amine
basicity), amine boiling point, entropy effects and other
effects related to crystalline packaging. While the pre-
dominance of enthalpic factors leads to an increase in
amine with the chain length, the predominance of the
entropy leads to the opposite direction.
The first event occurred in different temperatures
according to the chain length, and for methylamine and
ethylamine salts, it coincides with the loss of mass
observed by DTG curves. The losses of mass were about 18
and 25% of the initial mass, for methylammonium and
ethylammonium salts, respectively, and match a 1:1 amine:
phenol stoichiometry. These variations could be attributed
to the loss of methylamine and ethylamine with 1:1 stoi-
chiometry of amine/phenol. The loss of mass is associated
with an endothermic peak shown by DTA curves at 387 K
and 410 K for methyl and ethylamine. The increase in the
leaving temperature of the amine at 299 K follows the
trend of boiling temperature increase of 22 K.
This synchrony between the endothermic event and the
loss of mass is an evidence to define the association
between the proton transfer from the ammonium cation to
the phenolate and the release of the amine from the solid.
From propylammonium salt, it is no longer possible to
observe a singular mass variation related to this endother-
mic event. The curve becomes very smooth, and the mass
loss begins its descent around 373 K until the total loss of
mass at 500 K. The mass lost nearby the first endothermic
event corresponds to a maximum of 5% for propylamine,
which cannot be attributed to the loss of one molecule of
amine. Another suggested possibility was a stoichiometry
of 4-nitrophenol: amine 2:1, already known for salts of
The factor from the ammonium acidity is related with
the proton transfer between the ammonium cation and
phenol, and can be obtained from the relation with the
Gibbs free energy and the difference of pKa (DpKa = pKa
ammonium - pKa phenol).
DG ¼ À2:303RTDpKa
Since the pKa is nearly constant for all amines used in
this experiment and DpKa is nearby constant in the linear
123

M. Lazzarotto et al.
420
40%
6 μV
400
380
360
340
320
300
Dodecylamine
Hexylamine
0
2
4
6
8
10
12
Chain length in carbons
Pentylamine
Butylamine
Fig. 4 Variation of temperature of the first endothermic event with
the length chain of the amine
Table 1 Boiling point (b.p.) of
amines
Amine
b.p./K
Methylamine
Ethylamine
267
289
321
318
351
377
405
520
1-Propylamine
tert-butylamine
1-Butylamine
1-Pentylamine
1-Hexylamine
1-Dodecylamine
Propylamine
Ethylamine
from longer chains should result as an increase in the
entropic factor related to amine vaporization, similar to
alkanes with different aliphatic chains [18], whereas the
packing effect is related with the correct geometry to find
the minimal energy in the solid phase and is hard to predict
and find correlations.
endo
Methylamine
300
350
400
450
500
550
Temperature/K
We proposed a simple model to rationalize the results,
based in the early evidences characterized by ther-
mochromism phenomenon. Ammonium cations and 4-ni-
trophenolate interact by electrostatic and hydrogen bonding
through the phenoxide and nitro groups with the positive
hydrogen of the ammonium group along the O-–H?–N
axis. When the temperature reaches the first endothermic
event, the solid melts, and the ammonium cation
Fig. 3 TG/DTA curves for salts of 4-nitrophenol with aliphatic
amines
amine series, we concluded that the free energy associated
with the transfer of the proton is practically the same in the
series. This effect alone should result in values of tem-
perature for the events constants.
The second effect to be considered is the boiling point of
amines, shown in Table 1, that increases for larger chain
lengths by the increase in the molecular mass of the amine,
and once that the proton transfer regenerates the amine, the
contribution of this effect should be important for the
profile of thermal analysis.
–o
o
– o
o
N +
N+
H
H
N
N+
heat
H
R
H
R
H
o –
Another effect is the entropy related with translation
modes by vaporization, as well the variation of vibrational
and rotational modes from the aliphatic chain. The effect
OH
Scheme 2 Proton transfer of the ammonium cation to 4-nitropheno-
late promoted by heat
123

Thermal analysis of salts from 4-nitrophenol and aliphatic amines
transfers back the proton to the phenolate, resulting in the
original amine and 4-nitrophenol (Scheme 2).
not result in large entropy variation because the rest of the
chain already has some rotational freedom in the ionic
form, and there is no additional entropic gain with the
change from the ionic form to molecular form. Early
reports for the melting point of symmetrical quaternary
ammonium salts exhibit a sudden drop with the increase in
the alkyl chain length [19].
Both methylamine and ethylamine have lighter chains
and lower boiling points and leaves the solid when their
salts melt, whereas for other amines with heavier chains,
the loss of mass corresponding to the reverse proton
transfer occurs after the melting of the salts. The amine
remains in the liquid phase, but it is loosely bonded and
vaporized by further heating. Amines with longer chains
also have higher boiling points, and the variation of mass
after the aggregation change results in a slower loss of
mass corresponding to the departure of the amine with high
molecular weight, which explains the variations in TG
analysis.
The salt with tert-butylamine displayed a unique curve
with three endothermic events. The first occurs at 355 K,
followed by another at 397 K and the last at 485 K. The
value of mass loss matches with the decomposition of the
tert-butylammonium into isobutene and ammonium, which
remains in solid phase. This result was not expected, but
can be explained by the relative stability of the tert-butyl
cation, formed by the breaking of the ammonium–carbon
bond, followed by proton transfer from the tert-butyl cation
to ammonia, resulting in isobutene and the ammonium salt
of 4-nitrophenol. This proposal is likely an E1-type reac-
tion promoted by heat. Identical results were reported [20]
by thermal decomposition of tert-butylamine promoted by
HBr (Scheme 4, Fig. 5).
The profile of the melting temperature seems almost
erratic. First, we observe an increase from methyl to ethyl,
followed by a drop until to reach a minimum for buty-
lamine, followed by a new increase until a plateau after
hexylamine.
In order to explain the profile, we propose the change in
the form of interaction between 4-nitrophenol and amine
from ionic to hydrogen-bonded in liquid phase, resulting in
loose aggregates. The hydrogen-bonded amine gains
additional vibrational and rotational modes with the phase
change and the entropy variation depends of the length
chain of the amines, whereas changes in enthalpy promoted
by the proton transfer should be almost the same for all
amines. However, the highest value for the temperature of
proton transfer for ethylammonium salt is attributed to
strong hydrogen bonds observed by infrared spectrum of
this compound (Scheme 3).
The last step is the complete vaporization of the mass
left over nearly 485 K and is coincident with the complete
loss of mass for all samples analyzed. This is an evidence
that the ammonium cation or the amine does not have
influence in this event, and this event was attributed to the
decomposition of 4-nitrophenol, already reported in such
temperature [21]. The range of the loss of mass matches the
boiling point of nitrobenzene, although we could not devise
nor find any reliable mechanism to yield nitrobenzene from
4-nitrophenol.
The entropy variation would raise the maximum for
butylamine, and further increase in the carbonic chain does
Scheme 3 Proton
transfer pathway of ammonium
salt to 4-nitrophenolate
–
O
+
–
O
+
–
O
O N
O
N
+
O
N
–
O
O
–
O
H
H
H
+
N
H
N
+
H
H
N
H
H
O
H
O
+
–
O
N
+
N O
–
+
–
N
O
O
–
O
–
O
–
O
4–nitrophenolate ammonium
electostatic interaction
+ hydrogen bond
proton transfer
4–nitophenol amine
hydrogen–bond interaction
123

M. Lazzarotto et al.
Scheme 4 Proposed reaction
for decomposition of tert-
butylammonium
–
–
O
+
O
O
O
+
N
N
CH3
4-nitrophenolate salt
+
NH4
CH2
+
H3N
CH3
CH3
H3C
CH3
–
–
O
O
tert–butilammonium
isobutene
ammonium–4–nitrophenolate
4–nitrophenolate
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5
100
80
0
– 5
– 10
– 15
60
40
20
endo
0
– 20
573
423
Temperature/K
323
373
473
523
Fig. 5 TG/DTA curve of salt of tert-butylamine and 4-nitrophenol
Conclusions
Thermal analysis of the 4-nitrophenol salts with linear ali-
phatic amines reveals two endothermic processes with mass
loss:, a) fusion of the salt followed by proton transfer from
the ammonium cation to 4-nitrophenolate anion; for methy-
lammonium and ethylammonium salt the proton transfer
occurs simultaneously with fusion, while for other amines the
protons transfer occurs slowly in higher temperatures; b) the
second endothermic peak is attributed to vaporization of
nitrobenzene, formed by decomposition of nitrophenol.
There is no linear behavior between the temperature of
the amine output and the carbon chain length, and this was
attributed to contribution from the variation of boiling
point of the amines and variation in the entropic contri-
bution with the increase in the length of chain of amines. In
addition, tert-butylamine has one more endothermic peak,
attributed to the breaking of the C–N? bond and formation
of isobutene and ammonium.
Acknowledgements The authors are grateful to Embrapa Florestas,
CAPES and CNPq.
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thermal stability of quaternary ammonium cations in organic
electrolytes for electrochemical double layer capacitors. Phys
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