Dediazoniation of 1-naphthalenediazonium tetrafluoroborate in aqueous acid and in micellar solutions

Article abstract of DOI:10.1002/kin.20320

We have measured the rates and product yields of dediazoniation of 1-naphthalenediazonium (1ND) tetrafluoroborate in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by employing a combination of UV-vis spectroscopy and high-performance liquid chromatography (HPLC) measurements. Kinetic data were obtained by a derivatization procedure with product yields were determined by HPLC. HPLC chromatograms show that in aqueous acid and in micellar solutions only one dediazoniation product is formed in significant quantities, 1-naphthol (NOH), and the observed rate constants (k_obs) are tne same when IND loss is monitored spectrometrically and when NOH formation is monitored by HPLC. Activation parameters were obtained both in the presence and absence of SDS micellar aggregates. In both the systems, the enthalpies of activation are high and the entropies of activation are positive. The enthalpy of activation in the absence of SDS is very similar to that in the presence of SDS micelles, but the entropy of activation is lower by a factor of 4. As a consequence, SDS micelles speed up the thermal decomposition of 1 ND and increase k_obs by a factor of 1.5 when [SDS] = 0.02 M. In contrast, results obtained in the presence of complexing systems such as crown ethers and polyethers show significant stabilization of the parent arenediazonium ions. Kinetic and HPLC data are consistent with the heterolytic D_N + A_N mechanism that involves the rate-determining fragmentation of the arenediazonium ion into a very reactive phenyl cation that reacts competitively with available nucleophiles.

Full text of DOI:10.1002/kin.20320

Dediazoniation of
1-Naphthalenediazonium
Tetrafluoroborate in
Aqueous Acid and in
Micellar Solutions
CARLOS BRAVO-DIAZ, MARIA JOSE PASTORIZA-GALLEGO, SONIA LOSADA-BARREIRO,
VERONICA SANCHEZ-PAZ, ALEJANDRA FERNANDEZ-ALONSO
Departamento de Quimica Fisica, Facultad de Ciencias, Universidad de Vigo, as Lagoas-Marcosende 36200,
Vigo-Pontevedra, Spain
Received 26 July 2007; revised 3 December 2007, 11 January 2008; accepted 12 January 2008
DOI 10.1002/kin.20320
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: We have measured the rates and product yields of dediazoniation of 1-
naphthalenediazonium (1ND) tetrafluoroborate in the presence and absence of sodium do-
decyl sulfate (SDS) micellar aggregates by employing a combination of UV–vis spectroscopy
and high-performance liquid chromatography (HPLC) measurements. Kinetic data were ob-
tained by a derivatization procedure with product yields were determined by HPLC. HPLC
chromatograms show that in aqueous acid and in micellar solutions only one dediazoniation
product is formed in significant quantities, 1-naphthol (NOH), and the observed rate constants
(kobs) are the same when 1ND loss is monitored spectrometrically and when NOH formation
is monitored by HPLC. Activation parameters were obtained both in the presence and absence
of SDS micellar aggregates. In both the systems, the enthalpies of activation are high and the
entropies of activation are positive. The enthalpy of activation in the absence of SDS is very
similar to that in the presence of SDS micelles, but the entropy of activation is lower by a factor
of 4. As a consequence, SDS micelles speed up the thermal decomposition of 1ND and increase
kobs by a factor of 1.5 when [SDS] = 0.02 M. In contrast, results obtained in the presence of
complexing systems such as crown ethers and polyethers show significant stabilization of the
Correspondence to: Carlos Bravo-Diaz; e-mail: cbravo@
uvigo.es.
Contract grant sponsor: Ministerio de Educacio´n y Ciencia.
Contract grant number: CTQ2006-13969-BQU.
Contract grant sponsor: Xunta de Galicia.
Contract grant number: PGDIT06PXIB314249PR.
Contract grant sponsor: FEDER (Fondo Europeo de Desarollo
Regional).
Contract grant sponsor: Universidad de Vigo.
2008 Wiley Periodicals, Inc.
c
ꢀ

302
BRAVO-DIAZ ET AL.
parent arenediazonium ions. Kinetic and HPLC data are consistent with the heterolytic
DN + AN mechanism that involves the rate-determining fragmentation of the arenediazonium
ion into a very reactive phenyl cation that reacts competitively with available nucleophiles.
ꢀ
C
2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 301–309, 2008
INTRODUCTION
mechanism [7–9] via the rate-determining loss of N2,
generating a highly reactive phenyl cation that reacts
rapidly and with very low selectivity with available nu-
cleophiles [7–9]. In alcohol–water mixtures and some
nonaqueous solvents, products associated with free
radicals may be observed, especially under experimen-
tal conditions where formation of diazoethers is fa-
vored (low acidities and high-alcohol concentrations)
[10,11]. Formation of diazoethers also seems to be fa-
vored when electron-withdrawing groups are present
on the arenediazonium ion [7–9,11].
Several groups investigated (and still they do) the
effects of micellar and macromolecular systems on
dediazoniations, and two major lines of research are
evident. On the one hand, the basic physical organic
aspects of the reactions have been explored [12–15].
Alternatively, new methods to estimate interfacial com-
positions of weak nucleophiles in colloidal interfaces
were developed based on the unique characteristics
of dediazoniation reactions [4–6]. To date, literature
reports indicate that micellar systems change dedia-
zoniation product distribution but do not modify sub-
stantially dediazoniation rate constants [4,16,17], but
macromolecular systems such as crown ethers alter the
rate constants significantly but not the product distri-
bution [7,18].
Arenediazonium ions are important intermediates
widely used in synthetic and azo dye chemistry. Their
peculiar redox characteristics and the variety of re-
actions that they may undergo also makes them very
useful for other purposes, such as preparation of nano-
tubes, [1,2], measurement of protease activities [3],
and probing interfacial compositions of colloidal ag-
gregates [4–6].
Reactions of arenediazonium ions may occur either
at the arene nucleus (dediazoniation reactions; Scheme
1A) or at the terminal nitrogen of the diazonium group
(nucleophilic additions; Scheme 1B), which constitute
a classical route to obtain azo dyes and pigments [7].
In principle, all aromatic and heteroaromatic diazo-
nium salts can be used as electrophilic reagents. Their
electrophilicity depends on substituents: electron-
withdrawing substituents increase their electrophilic-
ity, meanwhile electron-donating substituents decrease
it. Therefore, the coupling components must be sub-
stances that possess structures with very high electron
densities at one or more carbon atoms. This require-
ment is usually achieved by the presence of hydroxy
or amino groups, which increase the C-nucleophilicity
of the coupling component [8].
Dediazoniations of aromatic diazonium ions may
proceed by a number of mechanisms leading to a wide
range of products [7–9]. In the absence of catalysts,
in aqueous acid and in the dark, dediazoniations are
believed to proceed through a heterolytic DN + AN
Micelles are dynamic aggregates of amphiphilic
molecules that create highly anisotropic interfacial re-
gions, lining the boundary formed by the highly po-
lar aqueous and nonpolar hydrocarbon regions. Be-
cause of the formation of the interfacial region imparts
new chemical and physical properties to the system
[4,19–23], the micelle acts as a microreactor concen-
trating, separating, or diluting reactants, and micelles
often have substantial effects on the chemical reactiv-
ity [21,22,24]. Because of their unique characteristics,
micelles have been exploited as of heterogeneous en-
vironments and microenvironments on a large variety
of reactions, providing relatively simple reference sys-
tems for understanding processes that are important for
grasping the complex behavior encountered in biolog-
ical assemblies [25–29].
Here we report the effects of sodium dodecyl sulfate
(SDS) micellar aggregates on the spontaneous dedia-
zoniation of 1-naphthyldiazonium ions (1ND). Dedia-
zoniation rates were monitored both spectrometrically
and by employing a derivatization method that allows
Scheme 1 (A) Two-step DN + AN dediazoniation mecha-
nism. Rate-limiting formation of an aryl cation that is trapped
by available nucleophiles. (B) Nucleophilic addition to the
electrophilic β-nitrogen of the diazonium group.
International Journal of Chemical Kinetics DOI 10.1002/kin

DEDIAZONIATION OF 1-NAPHTHALENEDIAZONIUM TETRAFLUOROBORATE
303
simultaneous determination of the rate constants for the
formation of dediazoniation products and their yields
[30]. Although thermal decomposition of benzenedi-
azonium ions has been extensively studied under dif-
ferent conditions [7,8], very few references to the re-
actions of arenediazonium ions containing fused rings
can be found in the literature and none in the pres-
ence of micellar aggregates. For instance, Kuokkanen
et al. studied the decomposition of naphthalenediazo-
nium ions in the presence of complexing molecules
such as crown ethers and acyclic polyethers in the gas
phase and in 1,2-dichloroethane, concluding that the
naphthalenediazonium ions are stabilized by forming
a nonreactive complex with the −N+2 group inserted
into the crown cavity; the largest stability achieved
when using 21-crown-7 and its derivatives rather than
with the nonspecific adduct formed with 15-crown-5
[31,32]. For example, the observed rate constant de-
creases by a factor of ∼223 when [21-crown-7] = 4
× 10−4 M compared to that in its absence. In contrast,
our results here show that SDS micelles enhance the
spontaneous rate of dediazoniation by a factor of ∼1.5
when [SDS] > 0.01.
1ND was synthesized by following an anhydrous
method [33] and was stored in the dark at low tem-
perature to minimize its decomposition. Its purity was
1
checked by UV–vis and H NMR spectroscopy. 1ND
stock solutions were prepared by dissolving the appro-
priate amount of the diazonium salt in aqueous HCl
to minimize diazotate formation [7] and to give final
concentrations of about 1 × 10−4 M and [HCl] = 3.6 ×
10−3 M. Stock solutions were generally used immedi-
ately or within 120 min with storage in an ice bath to
minimize decomposition.
Methods
Kinetic data were obtained both spectrometrically and
chromatographically. Spectrophotometric kinetic data
were obtained by monitoring the disappearance of di-
azonium ion at 365 nm. The Beer’s law−p5lot (Fig. 1)
for aqueous 1ND solutions up to 20 × 10 M is linear
(cc. = 0.999), yielding ε365 = 6100 120 M−1 cm−1
in keeping with literature data [31].
Chromatographic kinetic data were obtained fol-
lowing a procedure developed in this laboratory
[30] based on the derivatization of 1ND with
the Bratton–Marshall reagent N-(1-naphthyl)ethylene-
diamine (NED) that reacts rapidly with arenediazo-
nium ions in the acidic solution [34] yielding an stable
purple dye. Dediazoniation was quenched at conve-
nient times by adding to the reaction mixture an aliquot
of a stock quenching solution prepared by dissolving
NED, with an approximately 20-fold excess after mix-
ing with respect to that of 1ND, in an aqueous acid
([HCl] = 0.001 M) solution. The reaction mixtures
were analyzed by HPLC at room temperature within
4–5 h after dediazoniation was quenched.
EXPERIMENTAL
Instrumentation
UV–vis spectra and some kinetic experiments were ob-
tained on a Beckman DU-640 UV–vis spectrophotome-
ter equipped with a thermostated cell carrier attached
to a computer for data storage. Product analysis was
carried out on a Waters high-performance liquid chro-
matography (HPLC) system that included a 560 pump,
a 747 automatic injector, a 486 vis–UV detector, and a
computer for data storage and analysis. Products were
separated on a Microsorb-MV C-18 (Rainin, Woburn,
MA, USA) reverse phase column (25-cm length, 4.6-
mm internal diameter, and 5-μm particle size) using a
mobile phase of 65/35 v/v MeOH/H2O containing 10−4
M HCl. The injection volume was 25 μL in all runs, and
the UV detector was set at 210 nm. pH was measured
using a previously calibrated Metrohm 713 pH meter.
The use of a coupling reaction to stop the dediazoni-
ation reaction requires its rate to be faster than the de-
diazoniation rate. Auxiliary experiments carried out by
following azo dye formation spectrometrically shows
that under our experimental conditions the coupling
Materials
Reagents were of the maximum purity available and
were used as received. 1-Naphthol (NOH), naphtha-
lene (NH), and the reagents used in the preparation of
1ND (as tetrafluoroborate salt) were purchased from
Aldrich, Milwaukee, WI, USA. Other materials em-
ployed were from Riedel de Ha¨en, Seelze, Germany.
All solutions were prepared by using Milli-Q grade
water.
Figure 1 Variation of the absorbance of 1ND (λ = 365 nm)
with the concentration [HCl] = 0.01 M, T = 25◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin

304
BRAVO-DIAZ ET AL.
Table I Effect of Acidity on kobs for the Spontaneous
Dediazoniation of 1ND. [1ND] ∼ 1 × 10−4 M, T = 55◦C
reaction is essentially over in the time of mixing
reagents. The micellar effects on azo dye formation
were previously studied [34], and the experimental
conditions employed were chosen to ensure that azo
dye formation is much faster than dediazoniation. All
samples were analyzed by HPLC in duplicate or trip-
licate, and the relative standard deviation of the peak
areas was less than 2%.
[HCl] (M)
105k (s−1
)
obs
1.0
7.21 0.05
7.12 0.04
7.30 0.04
7.15 0.05
7.30 0.06
10−1
10−2
10−3
10−5
Preliminary HPLC experiments in the absence and
presence of SDS ([SDS] = 0.1 M) showed that the
main decomposition product is NOH, and only traces
of the reduction product NH were detected. Calibration
curves for converting HPLC peak areas into concentra-
tions were obtained simultaneously for the dediazonia-
tion products NOH and NH by employing commercial
samples dissolved in solutions of similar composition
to those used in the HPLC analysis of dediazoniation
products (see below). The equations used to convert
peak areas into concentrations and their typical reten-
tion times (tr (min)) were area = 8.4 × 109 [NOH]
(tr = 3.9); area = 1.06 × 1010 [NH] (tr = 8.5). Yields
of dediazoniation products (Y) were determined as a
percentage from the ratio of the dediazoniation product
concentration, [analyte], and the initial diazonium salt
concentration, [1ND0], estimated by weight (Eq. (1)).
by changing the acidity of the medium in the concen-
tration ranges employed, yielding average values of
kobs = (7.20 0.05) × 10−5 s−1. This value is in good
agreement with the reported literature values obtained
by employing different techniques [31]. The effects of
NaCl ([NaCl] = 0–1 M) on kobs were also investigated,
and no significant changes in kobs were found (data not
shown).
HPLC chromatograms show that NOH is the main
dediazoniation product and, as indicated before, kobs
for NOH formation was determined at different acidi-
ties and temperatures by monitoring the variation of
its yield as a function of time. Figure 2 is a repre-
sentative and shows that product formation follows a
first-order kinetics, with kobs = (7.1 0.1) × 10−5 s−1
,
a value equal to that determined by monitoring
1ND loss spectrometrically at the same temperature
(Table I). Figure 2 also shows that final NOH yield is
more than 90%, that is, quantitative conversion of 1ND
to NOH is achieved.
[Analyte]
Y = 100
(1)
[1ND0]
Observed rate constants were obtained by fitting the
absorbance–time or yield–time data to the integrated
first order (Eq. (2)), using a nonlinear least-squares
method provided by a commercial computer program
(GraFit 5.0), where M is the measured magnitude, ei-
ther absorbance or yields. All runs were done at T =
55 0.1◦C, except when investigating the influence of
temperature or otherwise indicated.
Activation parameters were determined by measur-
ing kobs at different temperatures according to the the-
ory of absolute rates by means of Eq. (3), where kB
and h are the Boltzmann and Planck constants, respec-
tively. Figure 3 shows the corresponding linear plot,
and the determined activation parameters are indicated
Mt − M
∞
ln
= −kobs
t
(2)
M0 − M
∞
RESULTS
Dediazoniation of 1ND in the Absence
of SDS
Effects of Acidity and Temperature. The effects of
HCl on kobs were determined by monitoring the de-
crease in the absorbance of 1ND with time at different
HCl concentrations at T = 55◦C and by fitting the
kinetic data to Eq. (2). In all runs, the first-order ki-
netics was obtained for more than 3t1/2. Results are
displayed in Table I, showing that kobs is not affected
Figure 2 Representative example of the determination of
k
for product formation and log plot. Yields calculated
obs
by employing Eq. (2). [1ND] ∼2 × 10−4, [HCl] = 0.01 M,
T = 55◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin

DEDIAZONIATION OF 1-NAPHTHALENEDIAZONIUM TETRAFLUOROBORATE
305
Figure 3 Effects of temperature on k
according to
obs
Eq. (3). [1ND] ∼ 1 × 10−4 M, [HCl] = 0.01 M.
in Table II; the values of ꢀH‡ are relatively high com-
pared with those for bimolecular reactions [35–37],
and the entropic term is clearly positive. For the sake
of comparisons, we have included in Table II the ac-
tivation parameters obtained for some representative
benzenediazonium ions.
ꢀ
ꢁ
ꢀ
ꢁ
kobs
T
kB
h
ꢀS‡
R
ꢀH‡
RT
ln
= ln
+
−
(3)
Figure 4 (A) Effects of SDS on kobs. Dn is the micellized
surfactant, defined as Dn = [SDS] – CMC. The solid line
was calculated by employing Eq. (5) with CMC = 0.002 M.
[1ND] = 2 × 10−4 M, [HCl] = 0.01 M, T = 55◦C. (B)
Variation in the percentage of NOH formed with time and
log plot according to Eq. (1) in the presence of SDS micellar
aggregates. [1ND] = 1 × 10−4 M, [SDS] = 0.01 M, [HCl] =
0.01 M, and T = 55◦C.
SDS Micellar Effects on Dediazoniation of 1ND. The
effects of SDS on kobs were obtained by running the
reaction at different [SDS]. Figure 4A shows that, upon
increasing [SDS], kobs increases following a saturation
kinetics pattern, reaching a plateau region at [SDS]
> 0.01 M with kobs = 9.0 × 10−4s−1, approximately
1.5 times its value in the absence of SDS. Although
Table II Activation Parameters in Aqueous and in Micellar Systems for the Thermal Decomposition of 1ND. For the
Sake of Comparisons, Activation Data for Selected Benzenediazonium Salts Are Also Included
XC6H4N+2 BF−4
Ea (kJ/mol)
ꢀH‡ (kJ/mol)
ꢀS‡ (J/mol)
105k25 C (s−1
)
◦
X = Ha
113
–
–
108.2
109.5
112.4
116
118
115
109
99
111
106
107
110
113
116
118
119
37
16
16
5.30
24.0
0.18
23.2
18.3
3.45
7.20
7.33
9.3
X = 3-MeOa
X = 3-Cla
X = Mea
X = 2-Mea
X = 3-Mea
1NDb
40.4
43.0
21.6
38
50
153 13
154
2
1
2
6
1NDc
1NDd,e
1
1
1NDd,f
116 0.4
8
9.5
a Data from Crossley et al. [55] and Maskill et al. [54].
b This work, in the absence of SDS.
c Data estimated from that reported by Kuokkanen et al. [31] at T = 25◦C taking into consideration the activation energy.
d This work, in SDS micellar systems.
e [HCl] = 0.01 M.
f
[HCl] = 0.1 M.
International Journal of Chemical Kinetics DOI 10.1002/kin

306
BRAVO-DIAZ ET AL.
the increase in kobs is modest, the results contrast with
those obtained for dediazoniations in the presence of
crown ethers and acyclic polyethers, where a signif-
icant stabilization of the parent diazonium ion is ob-
served [7,31,32].
Figure 4B shows that NOH formation follows a
first-order kinetics with kobs = (1.01 0.05) × 10−3
Scheme 2 Proposed stepwise dediazoniation mechanism
showing the formation of a ion–molecule complex and a
solvent separated ion–molecule pair [53,54].
s
−1, a value equal to that obtained spectrometrically
by monitoring 1ND loss at the same [SDS] (Fig. 4A).
Activation parameters were determined in the presence
of SDS by measuring kobs at different temperatures by
means of Eq. (3). Table II lists the determined activa-
tion parameters.
The obtained values for the activation parameters
(Table II) are consistent with the proposed preasso-
ciation stepwise mechanism like the one shown in
Scheme 2, in which the aryl cation has a short but
finite lifetime, and that has been employed to describe
product distribution in terms of ion–molecule or ion–
nucleophile pairs [41,42]. Nucleophilic attack may oc-
cur on “free” carbocations, contact ion–molecule pairs
or contact ion–solvent pairs. Rate constants for nu-
cleophilic attack on carbocations have been reported
to be close to the diffusion control limits, ∼109 to
DISCUSSION
HPLC data in the absence of SDS show that 1ND is
transformed quantitatively to NOH (Fig. 2). In the pres-
ence of SDS (Fig. 4B), NOH is again the main dedia-
zoniation product but conversion to NOH is less than
quantitative. This phenomenon has already been ob-
served in dediazoniations taking place in SDS micellar
systems [38,39] and is attributed to a competing reac-
tion between the arenediazonium ions and the sulfate
head groups of the surfactant, yielding the correspond-
ing aryl alkyl sulfate, which is unstable and further
hydrolyzes leading to the formation of aryl sulfates
(ArOSO−3 ), which are not detected by reversed-phase
HPLC because of its ionic nature [12]. Because no
other peaks that might be attributed to radical path-
ways, i.e., NH, were observed in the chromatograms,
we assumed, as Cuccovia et al. [38] and Bravo et al.
[39] did in the past, that the formation of the unknown
product is a competitive reaction with an unidentified
component, which leads to a reduction in the total ob-
served yields, which are calculated on the basis of the
initial PNBD concentration.
HPLC and spectrophotometric kinetic data (Table I
and Figs. 2 and 4) show that kobs for 1ND loss is the
same as that for NOH formation in the presence or
in the absence of SDS micelles. The determined kobs
values are independent of acidity (Table I) and of
the nucleophile (Cl−) concentration. All kinetic and
HPLC evidence are consistent with a mechanism in
which the formation of an intermediate in the slow
step is followed by further reaction with nucleophiles
leading to formation of products [40], that is, consis-
tent with 1ND undergoing decomposition through a
rate-limiting formation of a highly reactive aryl cation
that further reacts with any nucleophile available in its
solvation shell, i.e., the commonly accepted heterolytic
DN + AN mechanism (Scheme 2A).
1013 M−1 −1[43]. With lifetimes in the nanosecond
s
order of magnitude, aryl cations do not have time to
diffuse away and a hypothetical preassociation of the
nucleophile with the precursor would not account for
much of the trapping, in a similar fashion than other
extremely short-lived carbocations behave [44]. Thus,
in the absence of specific interactions, arenediazonium
ions will decompose mainly through the solvated aryl
cations, i.e., upon unimolecular removal of N2, one nu-
cleophile molecule of the first coordination shell will
form the corresponding dediazoniation product. Con-
sistent with this interpretation, Scheme 2 reflects that
the formation of the aryl cation does not involve the
separation of charge, but its redistribution and con-
sequently no significant reorganization of the coordi-
nation shell is expected upon formation of the corre-
sponding aryl cation.
The relatively high ꢀH‡ values found suggest
a transition state that has undergone bond breaking
with little compensating bond making as indicated in
Scheme 2; the enthalpy of activation is as high as in
many unimolecular reactions of alicyclic and aliphatic
compounds [36,37], in clear contrast with typical val-
ues usually found for bimolecular reactions, which are
substantially lower because the breaking of old bonds,
which requires energy, and the formation of the new
ones, which releases energy, are highly concerted and
usually synchronous [36].
Solvolytic unimolecular reactions can show positive
or negative ꢀS‡ values. Positive ꢀS‡ values as those
found in this work (Table II) suggest that the transition
state has a greater structural freedom than reactants, in
line with the reported positive volumes of activation,
International Journal of Chemical Kinetics DOI 10.1002/kin

DEDIAZONIATION OF 1-NAPHTHALENEDIAZONIUM TETRAFLUOROBORATE
307
ꢀV ‡, for a number on dediazoniations in different sol-
vents [45–48], and contrast with those for bimolecular
reactions such as SN2 and cycloadditions [49], which
are largely negative and typically ranging between −40
and −160 J mol−1 deg−1. Because the parent arenedia-
zonium ion and the aryl cation polarize their neighbor-
ing solvent molecules to a similar extent (Scheme 2),
the gain of entropy due to the loss of molecular vibra-
tions is not compensated by the loss of entropy due to
the restricted motion of the solvating molecules.
In the presence of micelles, ꢀH‡ is about the same
than in its absence but ꢀS‡ is about 3–4 times higher.
This could be because 1ND may be partially solvated
in the transition state by the surfactant molecules, and
because the micellar Stern layer has a lower polar-
ity than the bulk water [22]. The increase in the ꢀS‡
value compared to that in the absence of SDS decreases
ꢀG‡, making 1ND dediazoniation to proceed faster
than in pure water. As a consequence, Fig. 4A shows
that in the presence of SDS micelles, the rate constant
for the thermal decomposition of 1ND increases mod-
estly. Such a micellar effect can be interpreted quanti-
tatively in terms of the pseudophase model [19,22,50]
(Scheme 2), which assumes that the micelles are uni-
formly distributed in the aqueous phase and act as a
separate phase, the so-called pseudophase in which the
reaction takes place.
Micellar aggregates can modify chemical reactivity
essentially in two ways, namely (I) by governing the
concentrations of reactants within the micellar pseu-
dophase and (II) by governing the polarity of their im-
mediate vicinity [22,23]. Since 1ND ions are cations
with a hydrophobic part, they are distributed between
the aqueous and micellar pseudophases (Scheme 3).
Assuming that the micellized surfactant is at thermal
equilibrium with solutes throughout the reaction, the
rate of the reaction will be the sum of the rates in each
pseudophase (Eq. (4)).
where the subscripts w and m stand for the water and
micellar pseudophases, respectively. By considering
the corresponding mass balances, Eq. (5) can be de-
rived, where Ks stands for the association constant of
1ND to the micellar aggregate and Dn represents, as in-
dicated before, the concentration of micellized surfac-
tant, which is defined as Dn = [SDST] – CMC (where
CMC is the critical micelle concentration) [19,22].
kw + kmKsDn
kobs
=
(5)
1 + KsDn
The solid line in Fig. 4A was drawn by fitting the
(kobs, Dn) pairs of data to Eq. (5). Values of Ks = 290
30 M−1 and km = (9.0
0.1) × 10−4s−1 were
obtained.
The Ks value is very similar to that obtained
for the association of some benzenediazonium ions,
for instance, Ks = 250 M−1 and Ks = 300 M−1
for 2-methylbenzenediazonium [17] and 4-nitro-
benzenediazonium ions [39], respectively. Such a value
indicates that even at low-surfactant concentrations
(above the CMC), a substantial fraction of 1ND ions is
incorporated into the micellar aggregate, for instance,
when [SDS] = 0.05 M, more than 90% of the total
1NBD is incorporated into the micellar aggregate. The
obtained km value is about 1.5 times higher than kw,
and the increase is a consequence of the increasing
entropy of activation (Table II).
CONCLUSIONS
The present kinetic study suggests that, in aque-
ous acid solution, the thermal decomposition of 1-
naphthalenediazonium salts in the absence or presence
of SDS micellar aggregates occurs by an SN1-like reac-
tion mechanism, with the formation of a highly reactive
1-naphthyl cation in the slow rate-determining step of
the heterolytic decomposition of the uncomplexed ion
or the complex. Subsequent fast product-determining
reactions with nucleophiles (counterion, solvent, SDS,
etc.) give the products. Activation parameters provide
further support for such a mechanism because of the
relatively high enthalpy and entropy values found.
HPLC data show that in aqueous acid the only sig-
nificant reaction is that with water to give NOH. In the
presence of SDS micelles, 1ND is incorporated into the
micellar aggregates and a value of Ks = 290 M−1 for
the association constant was estimated showing that
a significant fraction of 1ND is incorporated into the
micellar pseudophase at low-surfactant concentrations,
where it undergoes thermal decomposition in the Stern
layer with a kobs value approximately 1.5 times higher
v = kw[1NDw] + km[1NDm]
(4)
Scheme 3 Proposed reaction mechanism in the presence
of SDS micellar aggregates.
International Journal of Chemical Kinetics DOI 10.1002/kin

308
BRAVO-DIAZ ET AL.
than that in aqueous acid solution. HPLC data show that
NOH is the major dediazoniation product, in keeping
with the current view of micelles as water-permeated
structures. The SDS micellar induced an increase in
kobs in contrast with that observed for methylbenzene-
diazonium ions in anionic micellar systems [17] but
confirm that micellar effects on the rate of spontaneous
dediazoniations are modest.
In cationic micelles such as cetyltrimethylammo-
nium halides (CTAX), kobs should not be affected by in-
creasing [CTAX] because hydrophilic ArN+2 ions such
as the methyl derivatives are likely to be excluded from
the Stern layer [51,52]; however, the hydrophobic ones
such as 16-hexadecylbenzenediazonium ions are com-
pletely associated with CTAX micelles [4,12,38], but
they are not water soluble and thus no comparisons
between kobs values in the absence and in the presence
of surfactant can be made.
14. Costas-Costas, U.; Bravo-D´ıaz, C.; Gonza´lez-Romero,
E. Langmuir 2003, 19, 5197–5203.
15. Costas-Costas, U.; Bravo-D´ıaz, C.; Gonza´lez-Romero,
E. Langmuir 2004, 20, 1631–1638.
16. Bravo-D´ıaz, C.; Gonza´lez-Romero, E. In Current Topics
in Colloid & Interface Science, Trivandrum, India, 2001;
pp. 57–83.
17. Bravo-Diaz, C.; Soengas-Fernandez, M.; Rodriguez-
Sarabia, M. J.; Gonzalez-Romero, E. Langmuir 1998,
14, 5098.
18. Bartsch, R. A. In The Chemistry of Triple-Bonded Func-
tional Groups; Wiley: New York, 1983; pp. 889–915.
19. Romsted, L. S. In Surfactants in Solution; Mittal, K.
L.; Lindman, J. (Eds.); Plenum Press: New York, 1984;
pp. 1015–1068.
20. Luisi, P. L.; Straub, B. E. Reverse Micelles: Biological
and Technological Relevance of Amphiphilic Structures
in Apolar Media; Plenum Press: New York, 1984.
21. Bunton, C. A.; Savelli, G. Adv Phys Org Chem 1986,
22, 213.
22. Savelli, G.; Germani, R.; Brinchi, L. In Reactions
and Synthesis in Surfactant Systems; Texter, J. (Ed.);
Marcel-Dekker: New York, 2001; p. 175
23. Lin, J. M.; Masaaki, Y. Trends Anal Chem 2003, 22, 99.
24. Bunton, C. A.; Romsted, L. S. In Handbook of Mi-
croemulsion Science and Technology; Kumar, P.; Mittal,
K. L. (Eds.); Marcel Dekker: New York, 1999; pp. 457–
483.
Sonia Losada-Barreiro and Veronica Sanchez-Paz thank the
Ministerio de Educacio´n y Ciencia, Spain, for research-
training grants.
BIBLIOGRAPHY
1. Bahr, J.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.;
Smalley, R. E.; Tour, J. M. J. Am Chem Soc 2001, 123,
6536.
2. Pinson, J.; Podvorika, F. Chem Soc Rev 2005, 34, 429.
3. Lee, M. J.; Anstee, J. H. Anal Biochem 1994, 218, 480–
482.
25. Fendler, J. Membrane Mimetic Chemistry; Wiley: New
York, 1982.
26. Pelizzetti, E.; Pramauro, E. Anal Chim Acta 1985,
169, 1.
27. Moroi, Y. Micelles: Theoretical and Applied Aspects;
Plenum Press: New York, 1992.
4. Romsted, L. S. In Reactions and Synthethis in Surfactant
Systems; Texter J. (Ed.); Marcel Dekker: New York,
2001; pp. 265–294.
5. Gunaseelan, K.; Romsted, L. S.; Gonza´lez-Romero, E.;
Bravo-D´ıaz C. Langmuir 2004, 20, 3047–3055.
6. Gunaseelan, K.; Romsted, L. S.; Pastoriza-Gallego, M.
J.; Gonza´lez-Romero, E.; Bravo-D´ıaz C. Adv Colloid
Interf Sci 2006, 123–126, 303–311.
28. Russling, J. F. In Electroanalytical Chemistry; Vol. 18;
Marcel Dekker: New York, 1994; pp. 2–80.
29. Sawada, K.; Ueda, M. J. Chem TechnolBiotechnol2004,
79, 369.
30. Bravo-D´ıaz, C.; Gonza´lez-Romero, E. J. Chromatog A
2003, 989, 221–229.
31. Kuokkanen, T.; Palokangas, J.; Talvensaari, M. J. Phys
Org Chem 2000, 13, 452.
7. Zollinger, H. Diazo Chemistry I, Aromatic and Het-
eroaromatic Compounds; VCH: Weinheim, Germany,
1994.
8. Hegarty, A. F. In The Chemistry of Diazonium and Diazo
Compounds; Patai, S. (Ed.); Wiley: New York, 1978;
pp. 511–591.
9. Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Com-
pounds; Edward Arnold: Baltimore, MD, 1985.
10. Pazo-Llorente, R.; Bravo-D´ıaz, C.; Gonza´lez-Romero,
E. Eur J Org Chem 2004, 2004, 3221.
11. Pazo-Llorente, R.; Maskill, H.; Bravo-D´ıaz, C.;
Gonza´lez-Romero, E. Eur J Org Chem 2006, 2006, 2201.
12. Cuccovia, I. M.; da Silva, M. A.; Ferraz, H. M. C.;
Pliego, J. R.; Riveros, J. M.; Chaimovich, H. J. Chem
Soc, Perkin Trans 2000, 2, 1896.
32. Kuokkanen, T.; Palokangas, J.; Talvensaari, M. J Phys
Org Chem 2000, 13, 452.
33. Garcia-Meijide, M. C.; Bravo-Diaz, C.; Romsted, L. S.
Int J Chem Kinet 1998, 30, 31–39.
34. Pazo-Llorente, R.; Rodr´ıguez-Menacho, M. C.;
Gonza´lez-Romero, E.; Bravo-D´ıaz, C. J. Colloid Interf
Sci 2002, 248, 169–175.
35. Isaacs, N. Physical Organic Chemistry; Longman Sci-
entific & Technicals: Harlow, UK, 1995.
36. Maskill, H. The Physical Basis of Organic Chemistry;
Oxford University Press: Oxford, UK, 1995.
37. Lowry, T. H.; Richardson, K. S. Mechanism and The-
ory in Organic Chemistry; Harper-Collins: New York,
1987.
38. Cuccovia, I. M.; Agostinho-Neto, A.; Wendel, C. M.
A.; Chaimovich, H.; Romsted, L. S. Langmuir 1997, 13,
5032.
13. Costas Costas, U.; Bravo-D´ıaz C.; Gonza´lez-Romero,
E. Langmuir 2005, 21, 10983–10991.
International Journal of Chemical Kinetics DOI 10.1002/kin

DEDIAZONIATION OF 1-NAPHTHALENEDIAZONIUM TETRAFLUOROBORATE
309
39. Bravo-D´ıaz, C.; Romero-Nieto, M. E.; Gonzalez-
48. Kuokkanen, T. Finn Chem Lett 1981, 52.
Romero, E. Langmuir 2000, 16, 42–48.
40. Espenson, J. H. Chemical Kinetics and Reaction Mech-
anisms, 2nd ed.; McGraw-Hill: New York, 1995.
41. Romsted, L. S.; Zhang, J.; Zhuang, L. J. Am Chem Soc
1998, 120, 10046.
42. Pazo-Llorente, R.; Gonza´lez-Romero, E.; Bravo-D´ıaz,
C. Int J Chem Kinet 2000, 32, 210.
43. Steenken, S.; Ashokkumar, M.; Maruthamuthu, P.;
McClelland, R. A. J Am Chem Soc 1998, 120, 11925.
44. Finnenman, J. I.; Fishbein, J. C. J Am Chem Soc 1995,
117, 4228.
49. Pross, A. Theoretical & Physical Principles of Organic
Reactivity; Wiley: New York, 1995.
50. Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S.
Acc Chem Res 1991, 24, 357.
51. Cuccovia, I. M.; da Silva, I. N.; Chaimovich, H.; Rom-
sted, L. S. Langmuir 1997, 13, 647.
52. Pazo-Llorente, R.; Bravo-D´ıaz, C.; Gonza´lez-Romero,
E. Langmuir 2003, 19, 9142.
53. Pazo-Llorente, R.; Bravo-D´ıaz, C.; Gonza´lez-Romero,
E. Eur J Org Chem 2003, 2003, 3421.
54. Canning, P. S. J.; McCrudden, K.; Maskill, H.; Sexton,
B. J. Chem Soc, Perkin Trans 2 1999, 12, 2735.
55. Crossley, M. L.; Kienle, R. H.; Benbrook, C. H. J. Am
Chem Soc 1940, 62, 1400–1404.
45. Brower, K. R. J Am Chem Soc 1960, 82, 4535–4537.
46. Kuokkanen, T. Finn Chem Lett 1980, 189.
47. Kuokkanen, T. Finn Chem Lett 1980, 192.
International Journal of Chemical Kinetics DOI 10.1002/kin