Electrochemical determination of rate constants and product yields for the spontaneous dediazoniation of p-nitrobenzenediazonium tetrafluoroborate in acidic aqueous solution

Article abstract of DOI:10.1002/(SICI)1097-4601(2000)32:7<419::AID-KIN4>3.0.CO;2-0

We examined the kinetics and mechanisms of the dediazoniation of p-nitrobenzenediazonium tetrafluoroborate in acidic aqueous solutions by employing differential pulse polarography (DPP) and differential pulse voltammetry (DPV) on a glassy carbon electrode combined with the use of a coupling reaction to quench unreacted p-nitrobenzenediazonium ion. These electrochemical techniques show an effective sensitivity and selectivity for detecting arenediazonium ions and arenedediazoniation products under the appropriate experimental conditions (pH, solvent, electrolyte), which allows simultaneous monitoring of the rates of arenediazonium ion loss and product formation and determination of product yields.

Full text of DOI:10.1002/(SICI)1097-4601(2000)32:7<419::AID-KIN4>3.0.CO;2-0

Electrochemical
Determination of Rate
Constants and Product
Yields for the Spontaneous
Dediazoniation of
p-Nitrobenzenediazonium
Tetrafluoroborate in Acidic
Aqueous Solution
a
1
2
´
1
M EMMA ROMERO-NIETO, BEATRIZ MALVIDO-HERMELO, CARLOS BRAVO-DIAZ,
´
2
ELISA GONZALEZ-ROMERO
1
Dpto. Qu ı´ mica F ı´ sica y Qu ´ı mica Org a´ nica, Universidad de Vigo, Facultad de Ciencias, 36200 Vigo-Pontevedra, Spain
Dpto. Qu ı´ mica Anal ´ı tica y Alimentaria, Universidad de Vigo, Facultad de Ciencias, 36200 Vigo-Pontevedra, Spain
2
Received 20 October 1998; accepted 3 February 2000
ABSTRACT: We have examined the kinetics and mechanisms of the dediazoniation of p-nitro-
benzenediazonium tetrafluoroborate in acidic aqueous solutions by employing differential
pulse polarography (DPP) and differential pulse voltammetry (DPV) on a glassy carbon elec-
trode combined with the use of a coupling reaction to quench unreacted p-nitrobenzenedi-
azonium ion. These electrochemical techniques show an effective sensitivity and selectivity
for detecting arenediazonium ions and arenedediazoniation products under the appropriate
experimental conditions (pH, solvent, electrolyte), which allows simultaneous monitoring of
the rates of arenediazonium ion loss and product formation and determination of product
yields. ᭧ 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 419–430, 2000
INTRODUCTION
chemistry [2] and a substantial body of knowledge
about their industrial aspects [3] and about their use
in preparative chemistry is available [4]. The mecha-
nism of arenediazonium reactions continue to be con-
troversial because they vary dramatically with exper-
imental conditions [5–8], which change the reaction
pathway and the amounts of dediazoniation products.
Arenediazonium reactions are normally divided
[6,9] into two groups (see Scheme I), those that occur
Arenediazonium salts have been shown to be very im-
portant for both the dyestuff industry [1] and synthetic
Correspondence to: Elisa Gonz a´ lez-Romero (eromero@uvigo.es)
Contract grant sponsor: DGICyT of Spain
Contract grant number: PB94-0741
Contract grant sponsor: Xunta de Galicia
Contract grant number: XUGA 38301A92 and XUGA 38305A94
᭧
2000 John Wiley & Sons, Inc.

4
20
ROMERO-NIETO ET AL.
Scheme I Possible classification of arenediazonium salts reactions, dediazoniations (1A) and addition (1B) and heterolytic
1C) and homolytic (1D) dediazoniation mechanisms.
(
at the arene ring (dediazoniation reactions, 1A) and
those that occur at the terminal nitrogen of the diazo-
nium group (nucleophilic addition, 1B). Addition re-
actions are the classical route to obtain azo dyes and
pigments [1,5]. Dediazoniation reactions, 1A, occur
with a wide variety of nucleophiles and they are be-
lieved to take place through two main mechanisms [6],
heterolytic (1C), and homolytic (1D). Competition be-
tween or coexistence of the pathways may occur de-
pending on the substituents of the aromatic ring
generate aryl radicals [5,6], but probably the most con-
venient source of free aryl radicals is the electrochem-
ical reduction of arenediazonium salts [6,13–15] be-
cause they have been shown to be easily reducible
species. Electrochemical reduction with metal elec-
trodes such as Cu, Fe, Hg, and Ti has been used
[14,15] to generate aryl radicals that act as initiators
of polymerization [16]. When a arenediazonium ion
acquires an electron [12,14,15], it forms a diazenyl
radical that yields an aryl radical and N , as shown in
2
[10,11] and experimental conditions such as pH [6],
Scheme II. Polarographic half-wave reduction poten-
solvent [5,6], or even atmosphere [11] (O or N ). The
tials, E , for a number of arenediazonium salts have
2
2
1/2
homolytic mechanism shows [6,12] a wider variety of
pathways compared to the heterolytic one and many
of them are still not completely understood [6,12].
A large number of reducing agents can be used to
been measured in a variety of solvents [17–19]. E1/2
for benzenediazonium tetrafluoroborate in sulfolane is
ϩ0.295 V vs SCE (saturated calomel electrode), in-
creasing to ϩ0.450 V with a p-NO substituent or de-
2
Scheme II Formation of aryl radicals from reduction of arenediazonium salts.

DEDIAZONIATION OF p-NITROBENZENEDIAZONIUM TETRAFLUOROBORATE
421
creasing to ϩ0.140 V measured for the p-methoxy
benzenediazonium salt.
or to monitor trace concentrations of them in the en-
vironment around industrial areas. Their reduction
mechanisms have been described in detail [25]
(Scheme IIIB).
The reduction of a number of 4-substituted ben-
zenediazonium tetrafluoroborates has also been stud-
ied in aqueous solution [15] by employing a dropping
mercury electrode (DME). The results have some con-
fusing features. Two polarographic waves are ob-
served in potential regions of ϩ0.05V to Ϫ0.02 V (vs
Ag/AgCl), a potential where few organic compounds
are electroactive [20], and around Ϫ0.97 V to Ϫ1.03
V. Microcoulometry [14] at the DME shows that the
two waves correspond to the uptake of one electron,
yielding a diazenyl radical and four (overall) electrons,
respectively, yielding phenylhydrazine [13], as shown
in Scheme IIIA. At a specific pH [21,22], the diffusion
currents are proportional to the arenediazonium salt
concentration. Therefore, a DME electrode can be
used to determine arenediazonium salts [17] both
qualitatively and quantitatively by comparing the
measured diffusion current with a calibration curve
obtained from diffusion current-salt concentrationpro-
file.
A number of methods have been developed to mon-
itor dediazoniations [26–28]; however, little attention
has been paid to the use of electrochemical methods
[6,14,15,17,32]. Here we examined the dediazoniation
of p-nitrobenzenediazonium tetrafluoroborate, pNBD,
in aqueous solution by employing differential pulse
polarography (DPP) at a DME and differential pulse
voltammetry (DPV) on glassy carbon electrode (GCE)
combined with a recently reported method [28] for
using coupling reagents to quench the dediazoniation
process at convenient times (Scheme IV). The azo dye
formed in the quenching step, which is proportional to
the amount of unreacted arenediazonium salt, contains
electroactive groups (9NϭN9 and 9NO ), and
2
both homolytic and heterolytic pNBD dediazoniation
products, that is, p-nitrophenol or p-nitrobenzene, are
also electroactive. The net reduction reactions of these
electroactive groups are showed in Scheme IIIB and
IIIC. Thus, dediazoniation rate constants can be
obtained electrochemically by monitoring directly
arenediazonium ion loss, by monitoring dediazonia-
tion product formation, and by monitoring azo
dye formation.
As previously noted, the classical route to obtain
azo dyes are addition reactions. Azo dyes have been
well studied by modern electrochemical techniques
[23–25], which can be employed to determine their
contents in industrial products, such as food additives,
Scheme III Electrochemical reduction processes: A) arenediazonium salts in acidic aqueous solution, B) azo group, and C)
nitro group (the second step proceeds with aromatic compounds only in acidic solution).

4
22
ROMERO-NIETO ET AL.
Scheme IV Representation of the coupling reaction that was employed to quench the pNBD dediazoniation between pNBD
and 2N6S to give the azo dye 6S2NpN.
EXPERIMENTAL
Instrumentation
Materials
p-Nitrobenzenediazonium tetrafluoroborate, pNBD,
was purchased from Aldrich (97%), and recrystallized
twice with cold ether. To minimize possible decom-
position of pNBD via the Schieman [9] reaction and
by reactions with water vapor, it was stored in the dark
in a dessicator at low temperature and recrystallized
periodically. Other reagents were of the maximum pu-
rity available and were used without further purifica-
tion. p-Nitrophenol ( pNB-OH), nitrobenzene ( pNB-
H), and p-nitrochlorobenzene ( pNB-Cl) were pur-
chased from Aldrich; 2-naphthol-6-sulfonic acid, so-
dium salt (2N6S), was from Pfaltz & Ba u¨ er. Other
materials employed were from Riedel de Ha e¨ n or Pan-
reac. All solutions were prepared by using Milli-Q
Differential pulse polarographic (DPP) and differential
pulse voltammetric (DPV) measurements were ob-
tained with a Metrohm E506 Polarecord, in conjunc-
tion with the cell systems described below. For DPP
experiments, a 663 VA-Stand (Metrohm) equipped
with a water-jacketed voltammetric cell was used. The
multimode working electrode was used in the DME
mode. The three-electrode system was attached to a
glassy carbon rod (2 ϫ 65 mm) auxiliary electrode
and a Ag/AgCl (3M KCl) reference electrode. For
DPV experiments, a 10 mL cell [Model VC-2 glass
vial, Bioanalytical Systems (BAS)] with a fitted top
was used. The three-electrode system was: a glassy
carbon disk (GCE) (Model MF-2012, BAS) as the
working electrode, an Ag/AgCl (Model RE-5, BAS)
as the reference electrode, and a platinum wire (Model
MW-1032) as the auxiliary electrode. GCE electrodes
were polished with a 0.4 ␮m alumina slurry (Aldrich)
and washed thoroughly and dried before use. A scan
–
1
grade water (␬ ϭ 1.2 ␮S cm ).
The azo dye 6-sulfonate-2-naphthol-1-azo-p-nitro-
benzene, 6S2NpN, was prepared in situ following a
published procedure [28] by coupling pNBD with
2N6S in solution containing TRIS buffer to give, after
mixing, final 2N6S concentrations of about 7-fold ex-
cess over that of pNBD and a pH about 9.3. A linear
correlation (cc Ͼ 0.995) not shown, between the
amount of azo dye formed and the amount of pNBD
employed was obtained.
Ϫ
1
rate of 7.5 mVs was used for both DPP at the DME
drop time 0.4 s and pulse amplitude Ϫ80 mV) and
(
DPV (pulse amplitude ϩ80 mV) on GCE. Solutions
used in polarographic measurements were bubbled
with N gas (99.999%) for at least 10 min and kept
2
Methods
under a nitrogen atmosphere during the electrochem-
ical runs. pH was measured using a previously cali-
brated Metrohm model 744 pH-meter.
All potentials given hereafter will be relative to the
just-mentioned Ag/AgCl electrodes.
Kinetic data were obtained by employing both DPP
and DPV techniques. Observed rate constants were
calculated by fitting the peak current, ip, to the inte-
grated first-order equation 1 using a nonlinear least-

DEDIAZONIATION OF p-NITROBENZENEDIAZONIUM TETRAFLUOROBORATE
423
squares method provided by a commercial computer
program, where M is the peak current of the solution
component that is monitored.
(
M Ϫ M )
ϱ
t
Ln
ϭ Ϫkobst
(1)
(
M ϪM )
ϱ
0
Dediazoniation experiments were carried out at
6
0ЊC at two HCl concentrations, 0.01 M and 1.0 M,
with pNBD as limiting reagent. Following a published
procedure [28], the aliquots of dediazoniation reaction
were quenched periodically at convenient times with
a stock quenching solution, which was prepared by
dissolving 2N6S in a solution containing TRIS buffer
([TRIS] ϭ 0.05 M) to give final 2N6S concentrations
about 7-fold excess over that of arenediazonium salt
and a pH of about 9.3. This pH was chosen to maxi-
mize the rate of 6S2NpN azo dye formation [6,29,30]
because naphthoxide ions couple much more rapidly
than their protonated forms [8,31]. We have found that
the methodology still works at pH ϭ 12.2, thus the
Ϫ
competing diazotate formation with OH is still insig-
nificant [32]. The selected pH and buffer provide the
appropriate experimental conditions to achieve a high
selectivity (different peak potentials for each dedi-
azoniation product) and satisfactory sensitivity (favor-
able signal/noise ratio to achieve the lowest limit of
detection) from the polarograms of all products
formed during dediazoniation.
Product yields, Y, were estimated from the ratio
of the final product concentration, P, and the initial
arenediazonium salt concentration, (%)Y ϭ 100 [P]/
Ϫ
5
Figure 1 DP polarograms of pNB-OH (2.86 ϫ 10 M)
(
Ϫ
5
dashed line) and 6S2NpN azo dye (2.71 ϫ 10 M) (full
line) in TRIS/2N6S buffer (pH ϭ 9.3). Inset: Cathodic peak
currents vs concentration plots for pNB-OH (᭡) and for
6S2NpN azo dye, peak I (᭹) and peak II (᭺).
[pNBD]. Final product concentrations were obtained
from the peak current values at infinite time (22 h),
that is, when dediazonation is essentially over (98%),
employing the appropriate calibration curve (vide in-
fra).
tion curves (cc Ͼ 0.999) for converting peak heights
into concentrations were obtained by employing so-
lutions of increasing 6S2NpN concentration up to
[6S2NpN] ϭ 80␮M (inset in Fig. 1). The equations
obtained were: ip(␮A) ϭ (Ϫ0.040 Ϯ 0.006) ϩ (9.8 Ϯ
Ϫ
3
0
.1) ϫ 10 [6S2NpN] (␮M) (azo group reduction,
Preliminary DPP and DPV Experiments
peak I) and ip(␮A) ϭ (Ϫ0.023 Ϯ 0.006) ϩ (1.02 Ϯ
0.01) ϫ 10 [6S2NpN] (␮M) (nitro group reduction,
peak II).
Polarograms of pNBD in aqueous acid (HCl 0.01
M) (not shown) contain three well-defined peaks and
one shoulder. Two peaks that appear at Ϫ0.012 V
(slightly overlapped with the peak of nitro reduction)
and Ϫ0.474 V are attributed to the reduction of the
arenediazonium ion to phenylhydrazine, Ar9NH9
Ϫ
2
Residual currents in both DPP and DPV experiments
were obtained by using acidic (HCl) TRIS-2N6S so-
lutions.
Polarograms of the 6S2NpN azo dye in a TRIS/
2
N6S buffer solution show two well-defined peaks
(Fig. 1, peaks I and II) which are attributed to the
reduction of the azo group to the hydrazo intermediate
and to the resulting anilines (Scheme IIIB), peak I at
Ϫ0.415 V, in agreement with published results for
other azo dyes [25], and to the reduction of the NO2
group to hydroxyaniline intermediate, peak II at
Ϫ0.542 V (first step in Scheme IIIC). Linear calibra-
NH (Scheme IIIA) in agreement with reported results
2
for other arenediazonium salts [12,14,15,32]. The first
peak of the diazo reduction is independent of pH up
to pH ϭ 5 and, at higher pH, a peak potential shift of

4
24
ROMERO-NIETO ET AL.
3
8.4 mV per pH unit is observed up to pH 8 (this
hydroxy group of pNB-OH to give p-nitrobenzoqui-
none. The anodic peak of the 6S2NpN azo dye appears
at a peak potential value lower than 0.800 V (Fig.
3). Linear (cc Ͼ 0.999) calibration curves for
converting peak heights into concentrations up to
change in the peak potential/pH slope is probably re-
lated to the behavior of pNBD as a Lewis acid [6]).
The peak at Ϫ0.089 V and the shoulder at Ϫ0.262 V
in the polarogram are attributed to the reduction of
Ϫ
4
9
NO group to corresponding aniline group [23], Ar-
[pNB-OH] ϭ 1.2 ϫ 10 were obtained (inset in
Fig. 3), yielding ip(␮A) ϭ (0.323 Ϯ 0.108) ϩ (6.93 Ϯ
2
NH . A peak potential shift of 58.6 mV per unit pH is
2
Ϫ
2
observed up to pH 10. This value is in agreement with
the classical irreversible reduction of the nitro group
following a four-electron/four-proton process [23,24]
0.15) ϫ 10 [pNB-OH] (␮M). The significant devi-
ation from linearity at higher pNB-OH concentrations
is attributed to electrode fouling by oxidation products
and in some cases to polymerization/adsorption phe-
nomena on the electrode surface [20,32,33].
(Scheme IIIC—first step). A linear relationship be-
tween ip at Ϫ0.089 V (pH ϭ 2.0) and concentration
for [pNBD] ϭ 3.04 ϫ 10 M with the equation:
Ϫ
4
Ϫ3
ip(␮A) ϭ (0.080 Ϯ 0.009) ϩ (5.45 Ϯ 0.01) ϫ 10
pNBD] (␮M) (nitro and diazonio groups reduction)
was found.
DP polarograms for a number of possible dedi-
RESULTS
[
Polargraphic Kinetic Data
azoniation products, pNB-OH, pNB-Cl, and pNB-H,
were also obtained (Figs. 1 and 2). Figure 1 (peak III
at Ϫ0.720 V) shows the p-nitrophenol DP polarogram
in TRIS/2N6S buffer solution. Its cathodic peak cur-
rent increases linearly (cc Ͼ 0.999) with [pNB-OH]
DPP observed rate constants, k , for disappearance
of the arenediazonium ion and formation of dediazon-
obs
Ϫ
4
up to [pNB-OH] ϭ 1.2 ϫ 10 M (inset in Fig. 1), and
the equation obtained for converting peak heights into
concentrations was ip(␮A) ϭ (0.274 Ϯ 0.003) ϩ (1.55
Ϫ
2
Ϯ 0.00) ϫ 10 [pNB-OH] (␮M) (nitro group reduc-
tion).
Preliminary DPP experiments indicate that the
presence of any one of the dediazoniation products,
pNB-OH, pNB-Cl, and pNB-H, leads to detectable
shifts in the peak potential values of the cathodic peaks
of the other ones. Figure 2 (full line-peak “a”) shows
the cathodic reduction peak obtained for p-nitrophenol
dissolved in a TRIS buffer solution. Upon addition of
an aliquot of a p-NBCl solution, three reduction peaks
are obtained (Fig. 2, dotted line); the peak labeled “a”,
attributed to pNB-OH, was shifted to Ϫ0.772 V and
peaks “b ” Ϫ0.544 V) and the shoulder “b ” Ϫ1.060
1
2
V) are associated with the reduction of pNB-Cl. When
pNB-H is added to the latter solution, that is, when the
three dediazoniation products investigated are present
simultaneously in solution, an additional cathodic
peak due to the reduction of the nitro group of pNB-
H (peak “c” at Ϫ0.642 V) is observed in the polaro-
gram (Fig. 2, dashed line), and again shifts in the peak
potentials of the other products are observed. There-
fore, the peak potential values can provide us with
selective information about the nature of the dedi-
azoniation products present in solution.
Figure 2 DP polarograms of solutions of pNB-OH (full
line), pNB-OH ϩ pNB-Cl (dotted line) and pNB-OH ϩ
pNB-Cl ϩ pNB-H (dashed line) in TRIS buffer (pH ϭ 9.3).
Because both 6S2NpN and pNB-OH contain elec-
tro-oxidable groups ϪOH), DPV voltammograms
were obtained for these products. Figure 3 shows the
anodic peak at ϩ1.130 V due to the oxidation of the
Ϫ
5
Ϫ5
[
pNB-OH] ϭ 1.2 ϫ10 M, [pNB-Cl] ϭ 5.4 ϫ 10 M and
Ϫ
5
[pNB-H] ϭ 8.2 ϫ 10 M. See text for full description of
polarograms.

DEDIAZONIATION OF p-NITROBENZENEDIAZONIUM TETRAFLUOROBORATE
425
OH were also obtained from the variation of peak cur-
rent due to the reduction of its nitro group (peak III at
Ϫ
5
Ϫ1
Ϫ0.744 V, Fig. 6). A kobs value 3.18 ϫ 10
s
was
obtained, which is equal, within experimental error, to
those obtained following azo dye disappearance (Ta-
ble I).
Figure 7 shows the polarograms of quenched so-
lutions obtained at different times when dediazonia-
tion is carried out in 1 M HCl. Peaks I and III are
associated with the reduction of the azo group of the
6
S2NpN azo dye and the reduction of the NO group
2
of the pNB-OH. Peak II, associated with the reduc-
tion of the NO group of the 6S2NpN azo dye, over-
2
laps with that of pNB-Cl; therefore, peak II is not as
suitable as the others to obtain kobs values. Conse-
quently, kobs values were obtained employing only data
from peaks I and III (Fig. 8). Average k values,
obs
Ϫ
5
Ϫ
1
Ϫ
5
Ϫ1
3
.49 ϫ 10
s
(peak I) and 3.03 ϫ 10
s
(peak
III) were obtained, which are equal, within experi-
mental error, to those obtained at 0.01 M HCl
(Table I).
Figure 3 Anodic peak current vs. concentration plot for
pNB-OH at GCE in TRIS/2N6S buffer (pH ϭ 9.3) and the
corresponding DP voltammograms.
iation products in 0.01 M HCl aqueous solutions were
obtained from polarograms of quenched solutions.
Figure 4 shows representative polarograms obtained at
different times. Three peaks are observed with an ex-
cellent resolution. Heights of peaks I (at Ϫ0.415 V)
and II (at Ϫ0.542 V), which are associated with the
reduction of the azo and nitro groups of the 6S2NpN
azo dye, respectively, decrease with time while the
growth of peak III (at Ϫ0.744 V) is due to reduction
of the nitro group of pNB-OH. No peaks attributable
to pNB-Cl or pNB-H were detected.
Figure 5, chosen as representative, shows the vari-
ation of the peak current, ip, of peaks I (5A) and II
(5B) with time and their corresponding first-order
plots. The rate constants for dediazoniation, k
ϭ
s
obs
Ϫ
5
Ϫ
1
Ϫ
5
Ϫ1
3
.39 ϫ 10
s
(peak I) and kobs ϭ 3.39 ϫ 10
Figure 4 DP polarograms in TRIS/2N6S buffer (pH ϭ
.3) as a function of time for disappearance of 6S2NpN
(peaks I and II) and for formation of pNB-OH (peak III).
(
peak II) in 0.01 M HCl at 60ЊC, are in excellent agree-
9
ment with literature values obtained employing other
techniques (Table I). kobs values for formation of pNB-
Ϫ
4
[pNBD] ϭ 1.81 ϫ 10 M, [HCl] ϭ 0.01 M, T ϭ 60ЊC.

4
26
ROMERO-NIETO ET AL.
variation in peak heights, ip (␮A), for the anodic peak
of p-nitrophenol with time and the corresponding ln
plot. From the slope of the ln plots, an average k
obs
Ϫ
5 Ϫ1
value of 3.18 ϫ 10
s
was obtained, which is equal,
within the experimental error, with those obtained by
DPP (Table I).
Estimation of Product Yields
DPP and DPV experiments show that the major de-
diazoniation product is pNB-OH. Yields of pNB-OH
can be estimated from the corresponding peak current
values at infinite time, that is, when dediazoniation is
essentially over (98%). In 0.01 M HCl, conversion to
products is almost quantitative, with pNB-OH (Y ϭ
Ϫ
9
3), as major product. Upon increasing Cl concen-
tration, pNB-OH yield decreases with a concomitant
increase in pNB-Cl; Y( pNB-OH) ϭ 82 and Y( pNB-
Cl) ϭ 10 at 1.0 M HCl. These results agree with those
reported for pNBD [34] determined by HPLC
(Table I).
DISCUSSION
The stability of arenediazonium ions depends strongly
on both the position and nature of the substituents on
the aromatic ring [6,35]. Most meta- and para-substit-
Figure 5 Variation of 6S2NpN peak currents with time
(
᭺) and ln plot (᭹). (A) peak I and (B) peak II in Figure 4.
uents, including electron-withdrawing (like p-NO2)
Ϫ
4
[pNBD] ϭ 1.81 ϫ 10 M, [HCl] ϭ 0.01 M, T ϭ 60ЊC.
and electron-donating groups (like 9CH ) show a
3
marked stabilizing effect. Consequently, all dediazon-
iation experiments were carried out at T ϭ 60ЊC to
speed the reaction [34]. Direct DPP monitoring of
pNBD loss by using any of its reduction peaks was
not possible. At the temperature employed, large var-
iations in the dropping time were observed, probably
Voltammetric Kinetic Data
DPV kinetic data were obtained by monitoring pNB-
OH formation following the oxidation of its hydroxy
group. Figure 9, chosen as representative, shows the
Table I
HCl ϭ 1.0 M
HCl ϭ 0.01 M
5
Ϫ
1
5
Ϫ1
Methods
10 k (s )
YpNB-OH
YpNB-Cl
10 k (s )
YpNB-OH
obs
obs
DPP (peak I)a
DPP (peak II)a
DPP (peak III)
DPVb
3.49
3.03
3.39
3.39
3.18
3.18
3.37
3.68
3.42
3.30
b
82
10
92
93
a,c
UV-VIS
UV-VIS
3.47
3.67
3.34
b,c
c
HPLC
83
12.5
94
d
N Evolution
2
Pressure changesc
3.22
a
Azo dye formation
pNB-OH formation
Values from ref. 34 and references therein
Value from ref. 36
b
c
d

DEDIAZONIATION OF p-NITROBENZENEDIAZONIUM TETRAFLUOROBORATE
427
Figure 6 Variation of peak current with time for pNB-OH formation (᭺) and ln plot (᭹). [pNBD] ϭ 1.81 ϫ 10 4 M,
Ϫ
[HCl] ϭ 0.01 M, T ϭ 60ЊC.
due to high Hg volatility and convection currents in
the electrode. Lower temperatures allow direct moni-
toring of arenediazonium ion loss by DPP [32], but
slow pNBD dediazoniation significantly [36] (E ϭ
A
Ϫ1
1
24 kJ mol ).
DPP experiments show that only two dediazonia-
tion products are formed, pNB-OH and pNB-Cl. No
extraneous peaks with significant heights that might
be associated with reduction products like p-nitroben-
zene were detected, consistent with the heterolytic
mechanism [6]. Kinetic DPP and DPV data indicate
that the rate of formation of pNB-OH is the same as
that of loss of the arenediazonium ion, and that both
observed rate constants are virtually unaffected by
Ϫ
changes in the acidity or in Cl concentration. These
results are consistent with the D ϩ A mechanism,
N
N
that is, formation of a highly reactive aryl cation that
shows a very low selectivity toward nucleophiles like
Ϫ
Cl compared with water and are in agreement with
published results [34,36] for pNBD dediazoniation ob-
tained employing other techniques (Table I).
The selectivity of the arenediazonium ion toward
Ϫ
Cl and H O is defined by equation 2:
2
(
%YArCl)[H O]
2
SWCl
ϭ
(2)
Ϫ
(%YArOH)[Cl ]
Figure 7 DP polarograms in TRIS/2N6S buffer (pH ϭ
.3), for disappearance of 6S2NpN (peaks I and II) and for-
mation of pNB-OH (peak III) at increasingly longer intervals
As noted before, product yields can be estimated
from the peak current values at infinite time, i.e., when
dediazoniation is essentially over (98%) and a selec-
9
of time.

4
28
ROMERO-NIETO ET AL.
tivity value of SWCl ϭ 6.6 ([H O] ϭ 54.5 M in 1.0 M
2
NaCl [37]) is obtained. This low value is comparable
with those reported for other arenediazonium ions
[27,28,35], which range from 2 to 10 and are ionic
strength dependent, and with that obtained by HPLC
Cl
under similar experimental conditions [34], SW
ϭ
5
.4. The difference in the selectivity values obtained
probably stems from the slight variations in the ex-
perimental conditions and from variations in the se-
lectivity with ionic strength [27].
Our results show that dediazoniations can be mon-
itored by electrochemical methods. Differential pulse
techniques provide an effective sensitivity useful for
trace measurements of electroactive species, at lower
concentrations of products or derivative products, im-
proving detection limits when compared with those
employing UV-VIS detection. Furthermore, electro-
chemical measurements are very selective because a
substantial number of both cathodic and anodic peaks
can be used to obtain dediazoniation rate constants and
to get estimates of product yields. Peak potential val-
ues also provide information about the nature of the
dediazoniation products present in solution, providing
information similar to that obtained by chromato-
graphic techniques with UV-VIS detection [34,35,38].
As a result, three possible dediazoniation products
and the azo dye formed can be measured simultane-
Figure 8 (A) Variation of peak current (᭺) of 6S2NpN
peak I in Fig. 7) with time and ln plot (᭹). (B) Variation
Ϫ
7
ously at concentrations levels down to 5 ϫ 10 –1 ϫ
(
Ϫ8
1
0
M, using relatively inexpensive instrumentation.
of peak current (᭺) for formation of pNB-OH (peak III in
Fig. 7) with time and ln plot (᭹). Experimental conditions:
Application of all these techniques to dediazoniations
in other solvents, where both heterolytic and homo-
Ϫ
4
[
pNBD] ϭ 1.81 ϫ 10 M, [HCl] ϭ 1.0 M, T ϭ 60ЊC.
Figure 9 Typical plot for the formation of pNB-OH obtained by measuring the variation of its anodic peak current with time
Ϫ
4
(
᭡) and ln plot (᭝). [pNBD] ϭ 1.81 ϫ 10 M, [HCl] ϭ 0.01 M, T ϭ 60ЊC.

DEDIAZONIATION OF p-NITROBENZENEDIAZONIUM TETRAFLUOROBORATE
429
lytic mechanisms can coexist, and metal catalyzed de-
diazoniations is in progress in our lab.
8. Sykes, P. A Guidebook to Mechanism in Organic
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9
. Hegarty, A. F. Kinetics and Mechanisms of Reactions
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CONCLUSIONS
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1
977, 42, 643.
We have shown that electrochemical methods can be
employed to monitor dediazoniation reactions, includ-
ing simultaneous estimations of dediazoniation prod-
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and arenediazonium ion loss. Our results are consistent
with the D ϩ A heterolytic mechanism, that is, rate-
1
1
1
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N
N
determining formation of an aryl cation that reacts im-
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This work was supported by the following institutions:
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(
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