Mechanism of the benzenediazonium tetrafluoroborate thermolysis in the solid state

Article abstract of DOI:10.1016/j.tca.2006.02.002

The thermolysis of benzenediazonium tetrafluoroborate was studied by thermogravimetry in dynamic mode. The decomposition of [ArN{triple bond, long}N]⁺BF₄^- in the solid state with the formation of C₆H₅F, BF₃, C₆H₆, and N₂ starts at T > 348 K. The speed of the thermolysis was estimated gravimetrically and by infrared spectroscopy, considering the change of the intensity of the absorption band at 1498 cm^-1, which corresponds to fluorobenzene. The maximal rate of thermolysis observes at the 366.5 K. A kinetic scheme, which includes the formation of a neutral complex [C₆H₅^δ+?BF₄ ^δ-], is proposed for the thermolysis of arenediazonium tetrafluoroborate. The decomposition of the complex with the formation of free-radical intermediates explains the chain character of the thermolysis.

Full text of DOI:10.1016/j.tca.2006.02.002

Thermochimica Acta 444 (2006) 1–5
Mechanism of the benzenediazonium tetrafluoroborate
thermolysis in the solid state
a,∗
a,∗
a
Eugen P. Koval’chuk , Oleksandr V. Reshetnyak , Zoryana Ye. Kozlovs’ka ,
b
c
d
Jerzy Bła z˙ ejowski , Roman Ye. Gladyshevs’kyj , Mykola D. Obushak
a
Department of Physical and Colloid Chemistry, Ivan Franko National University of L’viv, Kyrylo and Mefodiy Str. 6, 79005 L’viv, Ukraine
b
Faculty of Chemistry, University of Gda n´ sk, J. Sobieskiego 18, 80-952 Gda n´ sk, Poland
Department of Inorganic Chemistry, Ivan Franko National University of L’viv, Kyrylo and Mefodiy Str. 6, 79005 L’viv, Ukraine
c
d
Department of Organic Chemistry, Ivan Franko National University of L’viv, Kyrylo and Mefodiy Str. 6, 79005 L’viv, Ukraine
Received 2 June 2005; received in revised form 23 January 2006; accepted 3 February 2006
Available online 23 March 2006
Abstract
The thermolysis of benzenediazonium tetrafluoroborate was studied by thermogravimetry in dynamic mode. The decomposition of
+
−
[
ArN N] BF
4
in the solid state with the formation of C
6 5 3 6 6 2
H F, BF , C H , and N starts at T > 348 K. The speed of the thermolysis was estimated
−
1
gravimetrically and by infrared spectroscopy, considering the change of the intensity of the absorption band at 1498 cm , which corresponds to
fluorobenzene. The maximal rate of thermolysis observes at the 366.5 K. A kinetic scheme, which includes the formation of a neutral complex
· · ·BF
free-radical intermediates explains the chain character of the thermolysis.
2006 Elsevier B.V. All rights reserved.
δ+
δ−
], is proposed for the thermolysis of arenediazonium tetrafluoroborate. The decomposition of the complex with the formation of
[
C
6
H
5
4
©
Keywords: Benzenediazonium tetrafluoroborate; Solid state; Thermolysis; Chain mechanism
•
1
. Introduction
˙
Ar N N → Ar + N2.
(2)
Due to the presence of the diazo group N+ N, arenediazo-
Another way to generate electrons for the reduction of DAS
consists in the radiolysis of water producing solvated electrons
nium salts (DAS) are highly reactive and are used in the synthesis
of azodyes [1], in preparative synthesis [2], and analytical meth-
ods [3]. The basic process which most completely reflects the
properties of DAS, is the dediazotization, i.e. the elimination
of free nitrogen molecule [4]. The dediazotization occurring
during the electrochemical reduction of arenediazonium salts
was studied by Elofson and Gadallah [5], who found that the
addition of one electron by the DAS cation results in the forma-
tion of a diazonium radical, which easily disproportionates with
the elimination of free nitrogen and the formation of a phenyl
radical:
[6]:
γ
−
•
•
H2O−→ e + OH + H + H2 + H2O2;
(3)
(4)
aq
+
−
•
˙
e
+ Ar N N → Ar N N → Ar + N2.
aq
The break of the C N bond in DAS can also be induced by
ultraviolet rays [7]:
+
h␯
−
•
•
Ar N NX −→ Ar + N2 + X .
(5)
−
One of the main factors that influence the stability of DAS is
temperature. The thermolysis of arenediazonium tetrafluorobo-
rate in the absence of solvent leads to the formation of ArF with
a good yield (Balz–Schiemann reaction) [8,9]:
+
+e
˙
Ar N N ꢀ Ar N N;
(1)
∗
Corresponding authors. Tel.: +380 0322 728069.
E-mail addresses: kovalch@franko.lviv.ua (E.P. Koval’chuk),
reshetniak@franko.lviv.ua (O.V. Reshetnyak).
(6)
0
040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.02.002

2
E.P. Koval’chuk et al. / Thermochimica Acta 444 (2006) 1–5
The thermolysis of DAS in solution is enhanced by an increase
of the nucleophility of the solvent or by an increase of the elec-
trophilicity of the nitrogen atom in ␤-position of DAS, which is
determined by the nature and position of the substitutes in the
phenyl ring [10]. This effect is due to the formation of a complex
with a charge transfer between a solvent molecule, which acts
as an electron donor (ED), and the nitrogen atom of DAS on
which the positive charge is localized. The resulting complex is
readily dediazotized [11]:
2. Experimental
Freshly distillated aniline (Linegal Chemicals, analytical
grade) was used for the synthesis of benzenediazonium tetraflu-
oroborate. The diazoniation of aniline was made according
to a widely accepted method, described in [9]. IR spectra of
the benzenediazonium tetrafluoroborate solid sample, pressed
into a pellet together with KBr, were obtained on a Specord
M80 spectrophotometer (Carl Zeiss Jena), scanning the region
from 4000 to 400 cm . Thermogravimetric (TG) and differen-
tial thermogravimetric (DTG) measurements were carried out
using a microbalance NETZSCH TG 209 in dynamic mode
−
1
+
+
•
•
Ar N N+ : ED → Ar N N ED → Ar + N2 + ED.
(7)
−
1
with a heating rate of 2 and 5 K min
(in the 293–393 K
The influence of the environment on the reaction of the thermal
dediazotization in acid methanol solutions was investigated by
Bunnett and co-workers [12]. In a nitrogen atmosphere the dedi-
azotization takes place homolytically according to Eqs. (1) and
and 393–523 K temperature intervals, respectively) in an inert
argon atmosphere. The weights of the samples for analysis were
3
−1
∼10 mg, the argon flow was 30 cm min , and the range of
temperature change was from 293 to 523 K. Evolution of the IR
absorption spectrum of volatile products of the benzenediazo-
nium tetrafluoroborate thermolysis in the dynamic mode (time
interval between next spectrum scanning 23 s) was recorded on
a BRUKER IFS66 FTIR spectrophotometer.
(
2), whereas under oxygen the reaction proceeds heterolytically
via the formation of an arene-cation:
+
+
Ar N N → Ar + N2.
(8)
The presence of strong electron acceptors in the phenyl ring
favors the homolytical type of reaction via the intermedi-
3. Results and discussion
ate formation of phenyl radicals. The dediazotization of, for
+
example, C6F5
N
N takes place homolytically and the one-
Typical differential and integral curves of the mass losses
(both with time and elevation of temperature) detected during
the heating of the DAS sample in the dynamic mode are shown
in Fig. 1. As it indicates results of DTG analysis (Fig. 1a), the
decrease of the DAS sample’s weight starts at T > 336 K and
maximal rate of thermolysis observes at the ≈366.5 K. DAS
thermolysis finished practically within the 37 min after the start
of sample heating (Fig. 1b), that corresponds to reached tem-
perature ≈370 K. However, the thermal decomposition of ben-
zenediazonium tetrafluoroborate starts at 348 K, while an initial
minor mass change, which displays as the maximum on DTG
curve at 344.4 K, is due, apparently, to the loss of residual water.
Previously an analogous peak was observed for the thermolysis
of polyaniline [23]. The presence of this peak at 344.4 K cannot
be associated with the simple absorption of water, because the
loss of moisture should take place already at 303 K (the absorp-
electron reduction by, for example, iodine ions can be the
source of tetrafluorinephenyl radicals [13]. The yield of fluo-
rinated arenes is considerably influenced by the nature of the
counter-ion. Hence, inthecaseoftetrafluoroborateofdiazonated
o-aminobenzoic acid, o-fluorobenzoic acid is formed with a
yield of 19% [14], whereas for hexafluorophosphates the yield
increases to 78% [15]. Substitution ofthe diazo groupby fluorine
takes place in the case of pyridine-2-o-diazonium tetrafluorobo-
rate with a yield of 50% of the fluorine derivative during boiling
in petroleum [16].
The Balz–Schiemann reaction has been used for the synthesis
of the new antibacterial pyridonecarboxylic acid enoxacine [17],
halogen derivatives of thiophenecarboxylates [18], and dimethyl
derivatives of metacyclophane [19]. Moreover, drawbacks asso-
ciated with the classic Balz–Schiemann reaction are eliminated
in a series of examples by conducting fluorodediazoniation in
ionic liquid solvents, thus opening up a new horizon for a this
process [20].
Despite the large number of studies of the dediazotization
reaction and its wide application for preparation purposes, its
mechanism is not yet explained. In particular, it is not yet known
if the reaction proceeds internally or externally spherically, the
speed of thermolysis has not been investigated, etc. Moreover,
the thermal decomposition of arenediazonium salts was stud-
ied for its solutions in most cases [10–12,21,22]. Therefore, in
the present work the thermolysis was studied by derivatography
with the determination of the gaseous products by an IR spec-
trometer for the solid samples of arenediazonium tetrafluorobo-
rates. The analysis of the products of the thermolysis as a func-
tion of time allowed us to define a chain decomposition of DAS,
to propose a reaction scheme and to explain the formation paths
of the gaseous products of the Balz–Schiemann reaction.
−1
tion energy of water molecules is 13–22 kJ mol [24]). It is
clear that water interacts with DAS molecules via intermolecu-
lar hydrogen bonds, the binding energy of which lies in the range
−
1
63–75 kJ mol [25], which explains the higher temperature.
The hydrogen bonding confirms the IR spectrum of initial
DAS in the solid state, which is shown in Fig. 2. The oscilla-
tion modes were attributed according to data of work [26]. The
valence vibrations of diazo group corresponds the absorption
−
1
−1
band with maximum at 2290 cm while the band at 1080 cm
−
can be refers to the BF4 ions. There are band at 1640, 1480
and 1420 cm , which corresponds to the vibration of ben-
−
1
−
1
zene ring. The bands at 750 and 700 cm can be attributed
to the deformation ring-puckering vibrations of C H bonds in
the monosubstituted benzene. The stretching vibrations of O H
bond under the formation of hydrogen bonds (of N· · ·HO type
−1
in our case) corresponds wide band in the 3800–2600 cm
−1
wavelength interval with a maximum at 3430 cm , however

E.P. Koval’chuk et al. / Thermochimica Acta 444 (2006) 1–5
3
Fig. 3. Dynamics of change of intensity of the IR absorption bands of the volatile
products of the thermolysis of DAS. Time intervals after the thermolysis start,
s: (1) 1439; (2) 1538; (3) 1677; (4) 1686; (5) 1736; (6) 1776; (7 and 8) 1835; (9)
2
2
053; (10) 2212; (11) 2271; (12) 2410; (13) 2519; (14) 2746; (15) 2866; (16)
995.
There are the absorption bands at 3059, 1601, 1498, 1236, 1028
−
1
and 754 cm (Fig. 4) in the IR spectrum, associated with the
oscillation modes of benzene and fluorobenzene, which were
attributed according to data compiled in a review by Pankra-
tov and Zhelezko [27]. The full dynamics of the formation of
gaseous thermolysis products can be seen from Fig. 5.
It should be noticed that, beside the gaseous products, non-
volatile products are formed, which can include dimers result-
ing from the recombination of free radicals, and also poly-
meric/oligomeric products, as observed during the electrochem-
ical reduction of diazonium salts [28]. The averaged yield of
decomposition with formation of volatile substances is 44.59
%. Therefore, for the construction of the curves of mass losses
due to the heating of the particular sample with the total mass
of 11.750 mg (Fig. 1), a mass loss equal to 5.240 mg was con-
sidered to correspond to a yield of 100 %. The composition of
the solid remainder (equal to 55.41 % of the total sample mass)
or its thermal decomposition, which takes place at temperatures
higher than 373 K, were not investigated by us.
Fig. 1. (a) Integral (—) and differential (- - -) derivatograms of a sample of
arenediazonium tetrafluoroborate; b) changes of sample mass (—) and tempera-
ture (- - -) with time (sample mass: 11.750 mg; temperature scanning rate: 2 and
−
1
5
K min before and after the 373 K, respectively).
the valence vibrations of C H bonds displays also in the same
−
1
region (maximum of band at 3080–3030 cm ). Besides, there
−
1
is the weak smeared band at 935 cm , which can be attributed
to the O H deformation vibrations.
The thermolysis of DAS is accompanied by the formation of
volatile products, and their accumulation in the gaseous phase
can be followed by measuring of the increase of intensity of their
absorption bands. During the first 22 min of the sample heating
no significant decomposition (Fig. 1b) was observed and no for-
mation of volatile compounds was detected by IR spectroscopy
−1
The intensive absorption band at 1498 cm (Fig. 4), which
corresponds to fluorobenzene, was selected as a reference and
from the change of its intensity as a function of time the speed
(Fig. 3). The changes in the IR spectrum starting between 26th
and 28th min after the beginning of sample heating (Fig. 3).
Fig. 4. Absorption IR spectrum of volatile products of the DAS sample ther-
molysis.
Fig. 2. IR spectrum of benzenediazonium tetrafluoroborate in the solid state.

4
E.P. Koval’chuk et al. / Thermochimica Acta 444 (2006) 1–5
Fig. 5. Evolution of IR absorption spectrum of the volatile thermolysis products
when keeping the DAS sample (interval between spectrum scans: 23 s).
Fig. 6. Change of intensity of the absorption band at 1498 cm 1 during the
−
thermolysis of a DAS sample vs. time.
of the thermolysis was estimated (Fig. 6). The intensity of band
−1
at 1498 cm increases after the ∼1540 s of thermolysis. Up
to 1740 s the process flows at a constant speed, but is from this
moment on self-accelerated. After 2200 s the decomposition and
the resulting elimination of gaseous compounds ends, and the
intensities of the absorption bands vanish to zero. Such changes
of speed are typical for chain reactions. We believe that at the
initial stage of the thermolysis of DAS (between 25 and 26 min,
when sample destruction is initiated after the water elimination
BF3, and C H F are eliminated in small quantities, and the
6
5
formation of ion pairs is conditioned by the kinetics of the
competing reaction (6). After the accumulation of a sufficient
quantity of ion pairs (10), the latter decompose with the forma-
tion of BF3 and two free-radical particles:
(Fig. 1), and 27–28 min, when the spectra of volatile products
were obtained (Fig. 6)) the heterolytical decomposition of the
benzenediazo-cation with the formation of a benzene-cation and
the elimination of a nitrogen molecule (whose rotation-vibration
spectrum is forbidden in IR spectroscopy) takes place:
(
11)
The phenyl radical and the fluorine atom attack the initial arene-
diazonium salt:
(9)
The benzene and tetrafluoroborate ions are at distance where the
forces of electrostatic attraction do not allow them to separate,
which may lead to the formation of ion pairs like:
(12)
(10)
(
13)
The probability of the formation of such complexes increases as
thethermolysisin the solid state progresses. An ionic transport in
glass-like electrolytes takes place by ions from primary formed
ionic pairs. For example, during the dissolution of silver iodine
in silver phosphate an ionic quadruple is formed:
(14)
(
15)
which in external electric field dissociates to ions that carry
the charges and provide the electric conductivity of solid elec-
trolytes [29].
During the initial (induction) period, the basic reaction (9)
results in the accumulation of ion pairs (10). At the same time
The chain starts by the reaction (11). The transformation with
the participation of free-radical particles and the formation of at
least one product of the reactions (12)–(15) is the continuation of
the kinetic chain. The interaction of phenyl radicals and fluorine

E.P. Koval’chuk et al. / Thermochimica Acta 444 (2006) 1–5
5
radicals:
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. Conclusions
The thermolysis of arenediazonium tetrafluoroborate in the
solid state is not affected by the interaction of intermediates
with the solvent, as observed for the decomposition in solution.
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(
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(
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(
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[