Spectroscopic studies of arenediazonium ion stability in surfactant solutions

Article abstract of DOI:10.1016/j.molliq.2015.04.002

In the article a comparative study of dediazoniation of benzenediazonium, p-bromo- and p-methylbenzenediazonium ions in surfactant solutions is presented. The emphasis is put on the effects of temperature and surfactant type on decomposition. To monitor the kinetics of dediazoniation UV-VIS spectroscopy was employed. The observed rate constants, k_obs, were obtained by fitting the absorbance-time data to the integrated first-order kinetic equation and then the activation energies were obtained from the Arrhenius plot. It was found that temperature is the main factor determining the decomposition rate of arenediazonium ions in aqueous acidic solutions (pH = 1.81). The presence of -Br and -CH₃ groups in the benzene ring makes the ions more stable in comparison with the parent ion, increasing dramatically their half-life times. The effect of surfactant on the dediazoniation was rather small, indicating that in the investigated surfactant concentration range, the effects of surfactant aggregates (single molecules, premicelles and micelles) on the dediazoniation rate constant are small.

Full text of DOI:10.1016/j.molliq.2015.04.002

MOLLIQ-04755; No of Pages 6
Journal of Molecular Liquids xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Spectroscopic studies of arenediazonium ion stability in
surfactant solutions
a,
a
a
b
Jolanta Narkiewicz-Michalek ⁎, Andrzej Sienkiewicz , Marta Szymula , Carlos Bravo-Diaz
a
Maria Curie-Sklodowska University, Faculty of Chemistry, 20-031 Lublin, Poland
Departmento de Química-Física, Universidad de Vigo, 36-200 Vigo, Spain
b
a r t i c l e i n f o
a b s t r a c t
Available online xxxx
In the article
a
comparative study of dediazoniation of benzenediazonium, p-bromo- and p-
methylbenzenediazonium ions in surfactant solutions is presented. The emphasis is put on the effects of temper-
ature and surfactant type on decomposition. To monitor the kinetics of dediazoniation UV–VIS spectroscopy was
employed. The observed rate constants, kobs, were obtained by fitting the absorbance–time data to the integrated
first-order kinetic equation and then the activation energies were obtained from the Arrhenius plot. It was found
that temperature is the main factor determining the decomposition rate of arenediazonium ions in aqueous acid-
Keywords:
Benzenediazonium ions
Surfactants
Dediazoniation kinetics
3
ic solutions (pH = 1.81). The presence of –Br and –CH groups in the benzene ring makes the ions more stable in
comparison with the parent ion, increasing dramatically their half-life times. The effect of surfactant on the
dediazoniation was rather small, indicating that in the investigated surfactant concentration range, the effects
of surfactant aggregates (single molecules, premicelles and micelles) on the dediazoniation rate constant are
small.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
In the absence of catalysts, with weakly basic nucleophiles, in aque-
ous acid and in the dark, dediazoniations mostly proceed via rate deter-
mining loss of N , generating a highly reactive phenyl cation that reacts
2
Arenediazonium salts, ArN+
X−, constitute an important class of or-
2
ganic compounds because of a wide variety of chemical processes that
they may undergo just by making slight changes in the reaction condi-
rapidly, with very low selectivity with available nucleophiles [4,16]. In
nonaqueous solvents, products associated with free radicals, for exam-
+
tions. One of the current applications exploits the unique characteristics
ple, biphenyl derivatives and replacement of N
2
by H are often ob-
+
of the reactions between ArN
2
ions and weakly basic nucleophiles
served, especially when electron-withdrawing groups are present on
the arenediazonium ion [4,17–21].
(
(
e.g., antioxidants) to probe their localization in the colloidal systems
foods, cosmetic and pharmaceutical formulations) [1–3]. One impor-
In this context heterolytic dediazoniation has been reported to occur
with ortho-, meta- and para-methylbenzenediazonium (2MBD, 3MBD,
4MBD) and para-nitrobenzenediazonium (4NBD) ions in an aqueous
medium [10,22], while other authors interpreted their results as show-
ing evidence of heterolytic and homolytic processes during the thermal
and photochemical dediazoniation of several arenediazonium ions in
trifluoroethanol and ethanol [19–21]. However, in our latest article,
we demonstrated that, at moderate pH, water can act as nucleophile
giving rise to a diazohydroxide, which eventually can decompose to
give radical products [23].
Therefore, it is obvious that reaction conditions need to be chosen
carefully when trying to establish the mechanisms involved in the de-
composition of such versatile compounds as arenediazonium ions.
There are many factors influencing the arenediazonium ion decomposi-
tion. Besides its molecular structure, the most important are tempera-
ture, exposure to light, type and location of substituents in the
benzene ring, the presence of catalysts and nucleophiles in the system.
The character of microenvironment (its polarity, acidity, ionic strength,
+
tant limiting factor in the application of ArN
2
for quantitative investi-
gations of a variety of chemical processes is their stability.
+
2
Reactions of arenediazonium ions, ArN , were among the first to be
studied mechanistically [4]. In 1940, Hammett [5] postulated a slow
unimolecular heterolytic dissociation of arenediazonium ions into aryl
cations and N , and experimental support was provided [6]. The studies
2
of these important reactions were continued by Zollinger [4,7,8] and
other investigators [9–12].
Dediazoniations of aromatic diazonium ions can proceed through a
number of mechanisms leading to a wide range of products [2,4]. They
can take place via both spontaneous, (for example the Griess-
Schiemann), and catalysed, (for example the Sandmeyer), reactions.
They are believed to occur by either heterolytic (A), or, in the presence
of an electron donor, homolytic (B) pathways.
⁎
Corresponding author.
E-mail address: jolanta.narkiewicz-michalek@umcs.lublin.pl (J. Narkiewicz-Michalek).
http://dx.doi.org/10.1016/j.molliq.2015.04.002
167-7322/© 2015 Elsevier B.V. All rights reserved.
0
Please cite this article as: J. Narkiewicz-Michalek, et al., Spectroscopic studies of arenediazonium ion stability in surfactant solutions, J. Mol. Liq.
2015), http://dx.doi.org/10.1016/j.molliq.2015.04.002
(

2
J. Narkiewicz-Michalek et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Scheme 1. Three types of reactions common for arenediazonium ions: A — heterolytic loss of dinitrogen in the presence of weakly basic nucleophiles, B — homolytic displacement of ni-
trogen by electron transfer from a donor molecule, C — addition to the terminal nitrogen by strongly basic nucleophiles [2].
etc.) as well as the presence of surfactant aggregates are also of some
importance [13–15,24,25].
(4BRBD) tetrafluoroborates) in order to establish the best conditions
of their use in the colloidal system structure investigations.
Determination of micellar interfacial layer composition is difficult
because this layer cannot be physically isolated and analysed separately.
Romsted et al. [26] have devised a novel method of determining con-
centrations of various nucleophiles (e.g., water, halide ions, alcohols, an-
tioxidants) at the aggregate interface using arenediazonium ions.
Product yields from the reaction of weakly basic ions and molecules
with arenediazonium ions – at least one of the reagents should be ag-
gregate bound – are proportional to the concentrations of ions and mol-
ecules at the aggregate interface. In some cases the spontaneous loss of
nitrogen by arenediazonium ions can make the application of the pro-
posed method impossible. That is why the reaction conditions should
be appropriately chosen and controlled.
2. Experimental
2.1. Materials
Reagents were of maximum available purity and were used
without further purification. HCl and NaOH, the materials em-
ployed in the preparation of the Britton–Robinson buffer and
+
−
arenediazonium tetrafluoroborates, ArN
2 4
BF , were from Fluka or
Aldrich. Benzenediazonium ions, BD, and the substituted 4-methyl-,
4MBD, and 4-bromo, 4BrBD, benzenediazonium ions were prepared
from the corresponding anilines (Sigma, Fluka) following a standard
non-aqueous procedure as described elsewhere [22]. Britton–Robison
buffer of the desired pH was prepared by mixing solutions of 0.04 M
acetic, boric and ortophosphoric acids with the appropriate amounts
Micelles are likely to affect dediazoniation reactions in two essential
ways; first by governing their contact with other substrates incorporat-
ed in the micelle, with water or with ions present as counter ions in the
micellar Stern layer; second by governing the polarity of their immedi-
+
−
of 0.2 M NaOH to get the desired pH. Stock ArN
2 4
BF solutions were
ate environment which can be decisive for unimolecular decomposition
kept in the dark at low temperature to minimize its decomposition.
+
+
reactions. Full understanding of the behaviour of ArN
2
requires the
The purity of ArN
spectroscopy.
2
was checked periodically by employing UV–VIS
knowledge of their location in the micelle since substrate reactivity de-
pends largely on the microenvironment of the reaction and the extent of
its penetration into the aggregate. Electrostatic considerations lead one
to presume that the most hydrophilic arenediazonium ions will be
mainly located in the bulk aqueous phase in cationic micellar systems;
meanwhile they will be located close to the micellar aggregates in the
anionic and nonionic systems. Hydrophobic arenediazonium ions are
expected to be associated to the aggregate either in nonionic or ionic
systems.
The surfactants used were: sodium dodecyl sulphate, (SDS),
hexadecyltrimethylammonium bromide (CTAB) and t-octyl phenoxy
polyethoxyethanol (TRITON X-100) all Fluka Chemie Ag and RdH
Laborchemicalien GmbH & Co. K production. All solutions were made
with Milli-Q grade water. The CMC values in the Britton–Robinson buff-
er solution, pH = 1.81, determined by surface tension measurements
were: 0.003 M for SDS, 0.0004 M for CTAB and 0.00013 M for TX-100.
The purpose of our studies was to investigate the influence
of temperature and surfactant type on the kinetics of three
arenediazonium salts decomposition (benzenediazonium (BD), 4-
methylbenzenediazonium (4MBD) and 4-bromobenzenediazonium
Fig. 1. The decrease of absorbance of BD in time, λmax = 262 nm, pH = 1.8, T = 60 °C. The
Fig. 2. BD, 4MBD and 4BrBD decomposition in the buffered aqueous solution (pH = 1.81)
dashed line denotes the absorption spectrum for phenol.
at 60 °C.
Please cite this article as: J. Narkiewicz-Michalek, et al., Spectroscopic studies of arenediazonium ion stability in surfactant solutions, J. Mol. Liq.
2015), http://dx.doi.org/10.1016/j.molliq.2015.04.002
(

J. Narkiewicz-Michalek et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
3
Table 1
Values of rate constants, kobs, and half-life times, t1/2, for BD, 4MBD and 4BrBD decompo-
sition reaction.
T/°C
BD
4MBD
4BrBD
5
−1
5
−1
5
−1
k
obs·10 /s
t
1/2/h
k
obs·10 /s
t1/2/h
k
obs·10 /s
t1/2/h
2
3
4
5
6
2
0
0
0
0
0.38
0.87
4.17
19.00
68.00
50.23
22.22
4.62
1.01
0.28
58.00
201.00
739.00
0.33
0.10
0.03
0.22
1.00
4.50
88.87
19.25
4.28
range (240–400 nm). The exemplary spectra recorded for BD in a long
time interval are shown in Fig. 1. The spectra for substituted ions (not
shown) behaved similarly, i.e., they decreased in time to very low absor-
bance values. As shown in Fig. 1 the final spectrum for BD decomposi-
tion is almost the same as for phenol. It means that the reaction
occurs according to the heterolytic mechanism and phenol is its sole
product. It follows from literature [10,24,25] that the other two
arenediazonium ions decompose according to the same heterolytic
mechanism. The absorbances of decomposition products (p-cresol
and p-bromophenol respectively) at the characteristic wavelength
(
278 nm for 4MBD and 292 nm for 4BrBD) are very low in comparison
with those of parent arenediazonium ions.
Fig. 3. 4MBD decomposition in the buffered aqueous solution (pH = 1.81) in various
temperatures.
Of the three investigated arenediazonium ions, BD appeared to be
the least stable (Fig. 2). Decomposition of the other two ions 4MBD
−
2
.2. Method
and 4BrBD was much slower, indicating the stabilizing effect of CH
and Br substituents.
3
−
The kinetics of arenediazonium ion decomposition was deter-
Electronic substituent effects [27] can be classified into those associ-
ated with their polarity (inductive field) and those associated with their
ability to transfer charge (resonance). The methyl group has an electro-
donating character and in 4MBD ion the aromatic ring, which separates
the methyl group and the electron-deficient site, enables direct reso-
nance interaction between these groups through the π-system
(hyperconjugation). Such a direct resonance interaction is possible
only when the groups are in the para position [10].
mined by ultraviolet spectroscopy using Specord 200, Analityk Jena,
Germany, double-beam spectrophotometer. Stoppered quartz cells
with an optical path length of 1.00 cm were used. In all experiments
+
−4
2
the ArN concentration was 1·10 M. The initial absorbance value of
solution was in the region of spectrometer linearity. All measurements
were performed in thermostated and buffered (pH = 1.81) solutions.
They were prepared just before the measurement by adding appropri-
ate amount of arenediazonium salt to the equilibrated surfactant solu-
tion. The spectra were recorded at regular time intervals to monitor
the decomposition kinetics of arenediazonium ions.
−
Contrary to the methyl group, the Br group in the 4 position with-
draws electrons by induction through the σ bonds but has an electron
donating capability through the resonance. Such behaviour contrasts
2
with other electron-withdrawing groups such for example 4-NO ,
3
. Results and discussion
which has no electron donating capability. The electron-withdrawing
Br substituent in the 4-position destabilizes the aryl cation by induc-
−
Kinetic data were obtained spectrometrically by monitoring the dis-
tion more than it destabilizes the parent arenediazonium ion, and there-
fore its spontaneous decomposition reaction is much slower than that of
+
appearance of the absorbance of ArN
2
in an appropriate wavelength
a
Fig. 4. A. Arrhenius plots (Eq. (2)) from which the activation energies E for arenediazonium ion decomposition reaction were determined. B. Plots (Eq. (3)) from which the activation
enthalpies and entropies of arenediazonium ion decomposition were estimated.
Please cite this article as: J. Narkiewicz-Michalek, et al., Spectroscopic studies of arenediazonium ion stability in surfactant solutions, J. Mol. Liq.
(
2015), http://dx.doi.org/10.1016/j.molliq.2015.04.002

4
J. Narkiewicz-Michalek et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Table 2
at a considerable rate [4,29–31]. Substituents in the aromatic ring
have a marked effect on the stability of arenediazonium ions [4]. As
shown in Table 2, the methyl group in the para position is rate retarding
compared with the value for the benzenediazonium ion. The same effect
The activation parameters of dediazoniations found from Eqs. (3) and (4).
+
ΔS (J K−1 mol−1
#
ΔH (kJ mol−1
#
ArN
BD
2
lnA
E
a
(kJ/mol)
)
)
34.9
37.1
37.4
110.2
122.9
131.4
35.7
130.0
57.3
107.5
113.8
128.7
−
is observed for the Br group.
4
4
MBD
BrBD
The next part of our study concerns the investigation of inhibiting or
promoting effects of surfactant micelles on the rate of decomposition of
+
the investigated ArN
2
ions. Diazonium ions have proved to be very sen-
−
sitive to their environment, [1,14,22,23] and thus proper understanding
of their behaviour in the micellar systems is of great importance [14–16,
BD ion, or of arenediazonium ion bearing the electron-releasing CH
group [11].
3
24,25]. Hydrophobicity of arenediazonium ion and charge of the micelle
The next factor influencing the dediazoniation reaction, investigated
by us is temperature. As follows from Fig. 3 4MBD decomposition be-
comes significantly faster when the temperature increases from 22 to
are believed to be the most important factors affecting the localization
of ions in the micellar system. Positively charged hydrophilic
arenediazonium ions do not associate with cationic micelles [32]
whereas they are incorporated into negatively charged SDS micelles
and their preferential location is very close to the micellar surface. At
the same time no evidence of tight ion-pairs formation between diazo-
nium ions and anionic micelles was found [24]. One could expect similar
weak interactions of arenediazonium ions with nonionic micelles. Ac-
cording to the Pseudophase Model [33], the observed rate constant is
given by Eq. (4).
6
0 °C. The effect is similar for 4BrBD and BD ions.
Solid lines in Figs. 2 and 3 represent the first order kinetics plotted
according to the equation:
−kobs t
CðtÞ ¼ C0e
ð1Þ
+
where: C is the initial concentration of ArN
0
2
, C is its concentration after
time t and kobs is the temperature dependent rate constant.
Using Eq. (1) and the non-linear fitting procedure we determined
the observed rate constants and half-life times at different tempera-
tures. They are collected in Table 1.
kobs ¼ k
þ k ꢀ K ꢀ ½Mꢁ
ð4Þ
w
M
S
1
þ K ꢀ ½Mꢁ
S
One can see that the substituted ions decompose slowly in compar-
ison with BD. For example the half-life times for 4MBD and 4BrBD are
about ten and two hundred times higher than for BD, respectively. The
obtained results are in accordance with literature data [6,10,17,19,22].
Knowing the rate constants at different temperatures, we were able
to determine experimental activation parameters. Fig. 4A shows the
corresponding Arrhenius plots:
S
where: K is the association constant, [M] is the molar concentration of
surfactant in the micelles, and the subscripts W and M refer to the aque-
ous and micellar pseudophases.
There are numerous methods of determining the association con-
stant K
S
[24,25]. Among them, the UV–VIS spectroscopy is very popular.
and k
By fitting Eq. (4) to the obtained pairs of data (kobs and [M]) K
S
M
values can be estimated. However, in the case of our systems such an
approach cannot be used. Firstly, because at low temperatures (20–
30 °C) kinetics is very slow (especially for 4MBD and 4BrBD) and thus
the reliable estimation of kobs from experimental kinetic data is difficult.
Secondly, because at the highest temperature (60 °C) investigated by us,
the shift in absorbance due to the presence of 0.01 M SDS is very small
at least for BD and 4MBD ions (Figs. 5 and 6). The changes in absorbance
due to the presence of SDS are evident only in the case of 4BrBD (Fig. 7).
The SDS micelles have a stabilizing effect and the dediazoniation pro-
cess becomes slower.
Ea 1
lnkobs ¼ lnA− R T
ð2Þ
a
where E is an activation energy, A is a pre-exponential factor and R is
the gas constant.
The activation enthalpies and entropies were obtained according to
the absolute rate theory by means of Eq. (3) (Fig. 4B):
ꢀ
ꢁ
#
#
hkobs
ΔS
ΔH
R
1
T
ln
¼
−
ð3Þ
k T
R
B
B
where k and h are the Boltzmann and Planck constants, respectively.
Table 2 shows the obtained activation energies and activation pa-
+
rameters. The activation energies for the substituted ArN
2
ions are
higher than for BD, being in agreement with their greater stability. The
pre-exponential factor for BD is smaller than the pre-exponential factors
for 4MBD and 4BrBD which have approximately the same values.
#
The values of ΔH are positive and relatively high compared with
those for bimolecular reactions, and the entropic terms are also mean-
ingfully positive [27]. These values contrast with those usually found
for bimolecular reactions, which are substantially lower because break-
ing of old bonds, which requires energy, and formation of new ones,
#
which releases energy are usually synchronous [28]. Thus, ΔH values
from Table 2 suggest a transition state that has undergone bond break-
ing which is partly compensated by bond formation.
Dediazoniation, as described in Scheme 1A, suggests that formation
of the aryl cation does not involve separation of charge but its redistri-
bution; thus, the parent arenediazonium ion and the aryl cation polarize
the solvent to a similar extent; therefore, the gain of entropy is not com-
#
pensated and dediazoniations show relatively high ΔG values, indicat-
#
ing slight solvent dependence [4]. Positive ΔS values, as we have found,
suggest that the transition state has a greater structural freedom than
the reactants. On the other hand, because ΔS# values are positive, they
compensate the large enthalpy term, making dediazoniations proceed
Fig. 5. BD decomposition in time for various SDS concentrations; pH = 1.81, T = 60 °C.
Please cite this article as: J. Narkiewicz-Michalek, et al., Spectroscopic studies of arenediazonium ion stability in surfactant solutions, J. Mol. Liq.
2015), http://dx.doi.org/10.1016/j.molliq.2015.04.002
(

J. Narkiewicz-Michalek et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
5
Fig. 8. 4MBD decomposition in time for various SDS concentrations; pH = 1.81, T = 22 °C.
Fig. 6. 4MBD decomposition in time for various SDS concentrations; pH = 1.81, T = 60 °C.
At room temperature (T = 22 °C), when the reaction is slow, the ef-
fect of surfactant is more distinct. In Fig. 8 the kinetic curves for 4MBD
decomposition in the aqueous SDS solutions are shown. When small
amounts of surfactant are added to the solution, the dediazoniation re-
action becomes faster, then on further increasing surfactant concentra-
tion, the process slows down and at the concentrations exceeding the
CMC it becomes even slower than in the absence of SDS. For CTAB and
TX-100 the similar behaviour was observed but the effect of slowing
down of the decomposition reaction near the CMC was less visible.
In Fig. 9 the plots of kobs vs. surfactant concentration are presented
for 4MBD in the TX-100, CTAB and SDS solutions at T = 22 °C and
SDS. The analogical effect of surfactant concentration we observed for
ascorbic acid oxidation in the solutions of anionic, cationic and nonionic
surfactants [34]. Irrespective of the surfactant head group charge, the ef-
fect was highest in the vicinity of CMC.
The observed effect of low surfactant concentration on the
arenediazonium ions decomposition is unexpected and for the time
being we cannot propose a simple explanation. In the case of cationic
and nonionic surfactant molecules the interaction with positively
charged arenediazonium ions is weak. In the case of negatively charged
SDS one could expect electrostatic attraction but as follows from con-
ductivity measurements [24], the arenediazonium ions employed in
this work do not form ion pairs with SDS.
6
0 °C. As can be expected the higher temperature the higher values of
kobs. At the lower temperature a maximum of rate constants is observed
in the vicinity of CMC and than kobs decreases to the value characteristic
for the aqueous solution without surfactant or even lower in the case of
We are not aware of any literature report discussing the changes in
the values of dediazoniation rate constants at low surfactant concentra-
tions. In our opinion, the observation needs more attention and further
studies are currently carried out and will be part of future reports.
Fig. 9. 4MBD decomposition in time for various TX-100, CTAB and SDS concentrations;
pH = 1.81, (—) T = 22 °C and (− − −) T = 60 °C.
Fig. 7. 4BrBD decomposition in time for various SDS concentrations; pH = 1.81, T = 60 °C.
Please cite this article as: J. Narkiewicz-Michalek, et al., Spectroscopic studies of arenediazonium ion stability in surfactant solutions, J. Mol. Liq.
2015), http://dx.doi.org/10.1016/j.molliq.2015.04.002
(

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J. Narkiewicz-Michalek et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
[
[
[
[
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Chem. Kinet. 31 (1999) 73–82.
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muir 15 (1999) 2823–2828.
3
.1. Conclusions
Analysis of the obtained results led us to the conclusion that
dediazoniations in the acidic aqueous solutions (pH b 2) and in the acid-
ic solutions containing surfactants proceed via heterolytic mechanism,
i.e., the rate-determining formation of a very reactive aryl cation that
traps weakly basic nucleophiles competitively. The final absorption
spectra of the investigated ions corresponded to the products of hetero-
lytic dediazoniation. This conclusion is in accordance with the earlier re-
sults showing that no products associated with radical pathways (like
toluene or biphenyls) are formed under the aqueous acidic conditions.
The substituted arenediazonium ions are more stable and at room tem-
perature they have enough high half-life times to be safely used in the
studies of O-coupling reactions in the colloidal systems.
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London, 1985.
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Fernandez-Alonso, Int. J. Chem. Kinet. 40 (2008) 301–309.
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3
[
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ics. The noticeable effect is observed only at premicellar surfactant
concentrations.
3
[
[
[
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Please cite this article as: J. Narkiewicz-Michalek, et al., Spectroscopic studies of arenediazonium ion stability in surfactant solutions, J. Mol. Liq.
2015), http://dx.doi.org/10.1016/j.molliq.2015.04.002
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