Formation of diazohydroxides ArN2OH in aqueous acid solution: Polarographic determination of the equilibrium constant KR for the reaction of 4-substituted arenediazonium ions with H2O

Article abstract of DOI:10.1002/poc.3194

In aqueous acid (pH <4) solutions, in the dark, and in the absence of reductants, arenediazonium ions, ArN₂⁺ decompose spontaneously through the rate-limiting formation of the extremely unstable aryl cation that reacts with any nucleophile present in its solvation shell (D _N-‰+-‰A_N mechanism). However, in weak acidic and alkaline solutions, ArN₂⁺ react with H₂O and OH^- at the terminal nitrogen to give azo adducts of the type ArN₂OH that are in equilibrium with the parent ArN₂ ⁺. The diazohydroxide, in this case an acid, is in equilibrium with its conjugate base, and diazoate ArN₂O^-. The equilibrium constant for reaction with OH^- has been determined for a limited number of ArN₂⁺ from kinetic measurements but not with H₂O (K_R). Here, we have exploited the electrochemical properties of ArN₂⁺ to determine, for the first time, the equilibrium constants K_R of formation of 4-substituted X-ArN ₂OH (Xi￡H, Me, MeO, Br, and NO₂), which can decompose in several ways including Z-E isomerization or further reaction with OH^- to give diazoate ArN₂O^-. The technique applied was differential pulse polarography, which is very selective and sensitive. The determined pK_R values are 5-6, and they are somewhat higher than those obtained for the reaction of ArN₂⁺ with alcohols ROH (pK_DE-‰=-‰3-5) under similar acidic conditions. The K_R values are not very sensitive to changes in the nature of the substituent in the aromatic ring and a linear Hammett plot with a slope of ρ-‰=-‰0.58 was obtained.

Full text of DOI:10.1002/poc.3194

Research Article
Received: 4 June 2013,
Revised: 28 July 2013,
Accepted: 31 July 2013,
Published online in Wiley Online Library: 19 September 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3194
Formation of diazohydroxides ArN2OH in
aqueous acid solution: polarographic
determination of the equilibrium constant
KR for the reaction of 4-substituted
arenediazonium ions with H2O†
Andrzej Sienkiewicza, Marta Szymulaa, Jolanta Narkiewicz-Michaleka
and Carlos Bravo-Díazb*
In aqueous acid (pH <4) solutions, in the dark, and in the absence of reductants, arenediazonium ions, ArN2+ decom-
pose spontaneously through the rate-limiting formation of the extremely unstable aryl cation that reacts with any
nucleophile present in its solvation shell (DN + AN mechanism). However, in weak acidic and alkaline solutions,
ArN+2 react with H2O and OHꢀ at the terminal nitrogen to give azo adducts of the type ArN2OH that are in equilibrium
with the parent ArN2+. The diazohydroxide, in this case an acid, is in equilibrium with its conjugate base, and
diazoate ArN2Oꢀ. The equilibrium constant for reaction with OHꢀ has been determined for a limited number of
+
ArN2 from kinetic measurements but not with H2O (KR). Here, we have exploited the electrochemical properties
+
of ArN2 to determine, for the first time, the equilibrium constants KR of formation of 4-substituted X–ArN2OH
(X¼H, Me, MeO, Br, and NO2), which can decompose in several ways including Z–E isomerization or further reaction
with OHꢀ to give diazoate ArN2Oꢀ. The technique applied was differential pulse polarography, which is very
selective and sensitive. The determined pKR values are 5–6, and they are somewhat higher than those obtained
for the reaction of ArN2+ with alcohols ROH (pKDE = 3–5) under similar acidic conditions. The KR values are not very
sensitive to changes in the nature of the substituent in the aromatic ring and a linear Hammett plot with a slope of
ρ = 0.58 was obtained. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: diazohydroxide; diazonium
of covalently bonded adducts are azo dyes (C-coupling)[3] or forma-
INTRODUCTION
Arenediazonium ions, ArN2+, are important intermediates in pre-
tion of diazo ethers (O-coupling).[14] The adducts can display
geometrical isomerism, leading to the Z-(cis, syn) and E-(trans, anti)
forms, Scheme 1, which show different stabilities and can be
parative and synthetic chemistry that became industrially signifi-
cant after Griess[1] discovered the azo-coupling reaction.[2–5] They
can be easily prepared from anilines in the presence of a suitable
nitrosating agent in both aqueous and aprotic media under mild
conditions, and their chemical structures can be easily tailored
because of the wide variety of commercially aromatic amines.
Their use is not restricted to organic synthesis. For instance, they
are currently exploited as starting materials in nanochemistry
and colloidal chemistry to modify carbon surfaces,[6,7] to probe
the interfacial compositions of colloidal aggregates[8,9] and to
determine the distribution of polar molecules in emulsions.[10,11]
Arenediazonium ion chemistry is very rich and complex. Some of
their most important reactions are commonly included in organic
chemistry textbooks, for example, the Sandmeyer, Gomberg–Bachman,
Pschorr, Merwin, and Heck. Nevertheless, various aspects of its
chemistry are still far from being completely understood.[12–14] Its
interconverted by acid catalyzed processes.[4,16]
One of the most widely yet complex reactions studied is the
deceptively simple combination of the terminal nitrogen with
hydroxide ions, Scheme 2(A). The reaction was discovered in
the late 19th century by Griess[1] and became the source of some
controversy for several years (1894–1912) between A. Hantzsch
and E. Bamberger[15–17] because of the structures of the
* Correspondence to: Carlos Bravo-Díaz, Facultad de Quimicas, Universidad de
Vigo, 36-200, Vigo-Pontevedra, Spain.
E-mail: cbravo@uvigo.es
†
This article is published in Journal of Physical Organic Chemistry as a special
issue on the 12th Latin American Conference on Physical Organic Chemistry
by Faruk Nome (Department of Chemistry, Federal University of Santa
Catarina, Trindade, 88040-900 Florianópolis, SC, Brazil).
+
chemistry is dominated by the electrophilic character of the –N2
a A. Sienkiewicz, M. Szymula, J. Narkiewicz-Michalek
Faculty of Chemistry, Maria Curie-Skłodowska University, Lublin, Poland
group, which behaves as a Lewis acid reacting with nucleophiles
(Lewis bases, Nuꢀ or NuH followed by a proton loss) to give cova-
lently bonded adducts, ArN2–Nu, at β-nitrogen of the ArN2+, which
is the electrophilic reactive center, Scheme 1.[2,4,14,15] The examples
b C. Bravo-Díaz
Facultad de Quimicas, Universidad de Vigo, Vigo-Pontevedra, Spain
J. Phys. Org. Chem. 2014, 27 284–289
Copyright © 2013 John Wiley & Sons, Ltd.

FORMATION OF DIAZOHYDROXIDES ArN2OH
H2O (Scheme 2(C)) can be neglected.[4] If pH is increased, the for-
ward rate constants in Scheme 2(A) became too fast to measure.
On the contrary, if pH is lowered, formation of diazohydroxide
may be incomplete. Depending upon the nature of the substitu-
ents, the reverse reaction (kꢀ1) and the Z–E isomerization
reactions can be competitive, and the rate of diazoate formation
can contribute significantly to the overall rate. As a consequence
of the aforementioned and other experimental difficulties, some
Scheme 1. Basic representation of the nucleophilic addition mechanism
leading to the formation of Ar–N=N–Nu adducts in the (Z)-configuration
and (E)-configuration. The competitive spontaneous decomposition of
ArN2+, which takes place through the rate-limiting formation of a highly
reactive aryl cation (DN + AN mechanism), is also shown, Scheme 3[4,29]
mechanistic aspects are still not known, and the individual K1
+
and K2 values were only determined for a few ArN2
.
Dediazoniations at intermediate acidities (i.e., pH4–9) are quite
different from those in aqueous acid solutions. Rate constants in-
crease with the increasing pH, and a plethora of dediazoniation
products, including diazo tars that are difficult to identify, are
obtained, showing that mechanisms other than DN + AN or the
reaction with OHꢀ occur.[4] In previous solvolytic work, we demon-
+
strated that aliphatic alcohols ROH react with ArN2 under acidic
conditions to give diazo ethers of the type E–ArN2OR.[13,14,22] For-
mation and decomposition of such adducts were observed exper-
imentally in the course of dediazoniations,[14] and they are usually
unstable and decompose homolytically leading to the formation of
reduction products. However, in some instances, they isomerize to
much more stable Z-isomer, and some of them can be isolated.[23]
Here, we studied the possibility of ArN2OH adduct formation
through nucleophilic reaction between H2O and ArN2+, Scheme 2
(C). We investigated adduct formation in aqueous solution under
the conditions where the reaction with OHꢀ [Scheme 2(A)] is
negligible. The difficulties mentioned earlier in kinetic investiga-
tions of ArN2+ in moderately acidic or alkaline solutions produc-
ing a variety of products prevent efficient and practical use of
chromatographic techniques, as in the solvolytic studies with
aliphatic alcohols,[14] led to a different experimental approach.
Here, we took advantage of the electrochemical properties of
ArN2+ and determined, for the first time, the equilibrium constant
KR for the nucleophilic attack of a water molecule toward
substituted ArN2+ in aqueous solution. Differential pulse polarogra-
phy, DPP, is a convenient technique for studying dediazoniation
mechanisms and to obtain valuable information not only about
Scheme 2. Representative equilibria involved in the reaction of ArN2+ with
OHꢀ ions and H2O. A full description of the equilibria involved can be found
in the recent review by C. Bravo-Díaz and in the references therein.[14] The
spontaneous decomposition of ArN2+ ions (Scheme 1) is much slower than
the reaction with OHꢀ or H2O and is not displayed for clarity
products. In aqueous acid solutions (pH <4), dediazoniatons
proceed almost exclusively through the DN + AN mechanism, that
is, rate-limiting formation of a highly unstable aryl cation Ar+ that
reacts immediately with nucleophiles Nuꢀ or NuH present in its
solvation shell, Scheme 1.[4,14] However, in strongly alkaline
solutions (pH >9), ArN2+ reacts with OHꢀ to give diazohydroxide,
which can further react finally yielding diazoate ArN2Oꢀ, which is
quite stable, Scheme 2(A).[4,15,16] Both diazohydroxides and
diazoates may exist as Z-isomer and E-isomer, Scheme 2(B),
but, for most ArN2OH and ArN2Oꢀ, the rates of Z–E isomerization
are orders of magnitude slower than those of the reaction
+
the parent electroactive ArN2 but also on the electrochemically
generated species.[24–28]
The purpose of this paper is twofold. First, to determine if azo
adducts of the type ArN2OH are formed from the nucleophilic
attack of H2O under acidic or weakly alkaline conditions just as
diazo ethers are formed in reaction with alcohols ROH. We deter-
mined for the first time the equilibrium constants KR for ArN2OH,
adduct formation, Scheme 2(C), for a number of 4-substituted
between the parent ArN2 and OHꢀ. For the sake of clarity, we
+
will hereafter refer only to the Z-isomers, Scheme 2(A), which
are the first formed as kinetically controlled products.[4,14–16]
+
The acid–base behavior of ArN2 was extensively studied by
Zollinger,[4] Lewis,[18] Ritchie,[19,20] Sterba[16,19,20], and others[21]
in the years 1950sꢀ1970s and was shown to be very complex
because of different equilibria and competitive reactions
involved. Here, we will summarize relevant mechanistic aspects.
Further details can be found elsewhere.[2,4,14–16] On the basis of
potentiometric measurements, Zollinger[4] and Sterba[16,19,20]
showed that the second equilibrium constant in Scheme 2(A) is
higher than the first one, that is, K2 >>> K1. As a consequence,
it was not possible to determine the individual values but only
their product. On the basis of kinetic experiments, Ritchie[19,20]
+
ArN2 with H2O. The dependence of KR on the Hammett σ
constants indicates that KR values are not sensitive to electronic
changes induced by substituents in the aromatic ring. Second,
we explored other methods besides kinetics and HPLC to
expand current knowledge about the solvolytic reactions of
+
ArN2 and formation of ArN2Nu adducts.
Previous electrochemical work shows that ArN2+ can be easily
reduced on the surface of Hg electrode, Scheme 3.[26,30,31] In
concluded that both ArN2 and ArN2OH react with OHꢀ and
+
H2O in alkaline solution but that under the experimental
conditions commonly employed in kinetic experiments to
determine the rate constants k1 and k (pH 9–12), the reaction
ꢀ1
with H2O is much slower than that with OHꢀ so that the main
operating mechanism is that in Scheme 2(A) and the reaction with
Scheme 3. Chemical processes associated with the electrochemical
+
reduction of ArN2 ions[4,29]
J. Phys. Org. Chem. 2014, 27 284–289
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

A. SIENKIEWICZ ET AL.
+
aprotic solvents such as CH3CN, the reduction of ArN2 is very
Instrumentation
simple, and only one single, broad, one electron wave is
observed at the potentials close to 0 V versus saturated calomel
electrode[6,7] showing that the aryl radical is formed directly. In
aqueous solution, two polarographic waves are observed,
Scheme 3.[26] The first corresponds to the transfer of one
electron to yield the arenediazenyl radical, ArN•2, showing a
polarographic wave usually located in the potential region
+0.05 V to ꢀ0.1 V (versus Ag/AgCl), a potential where a few
organic compounds are electroactive, Scheme 3(A).[32] The
second reduction step, Scheme 3(B), is usually detected at
potentials around ꢀ0.5 V to ꢀ0.70 V involving three electrons
and three protons leading to phenylhydrazine.[30] The formation
of the aryl radicals in aqueous solution was confirmed by
coulometry on a mercury pool and in acetonitrile (ACN) in the
presence of a spin trap.[7] Figure 1 illustrates the polarographic
waves of the first reduction peak of benzenediazonium ions
(BD) at selected acidities.
pH values were determined from potentiometric measurements by
employing a previously calibrated Elmetron CP-505 pH meter.
The DPP measurements were obtained with a computer controlled
μAUTOLAB type III (Metrohm, Herisau, Switzerland) attached to a VA-stand
(Mtm anko model M 165) equipped with a water jacketed voltammetric cell.
Polarograms were recorded in a computer by employing the General
Purpose Electrochemical Software GPES v. 4.9 (Eco Chemie, Belgium).
The multimode working electrode was used in the DME mode. The
three-electrode system was composed of a hanging mercury electrode,
a platinum rod auxiliary electrode, and an Ag/AgCl (3M KCl) reference
electrode. All solutions used in the polarographic measurements were
bubbled with N2 gas (99.99%) for 10–15 min and kept under a N2 atm
throughout the measurements. All potentials given hereafter will be
relative to the aforementioned Ag/AgCl electrode.
X–ArN2+ is thermally unstable and decompose spontaneously in aqueous
acid solution (DN + AN mechanism, Scheme 1), with half lives ranging from
hours to days depending on the position and nature of the substituents in
+
the aromatic ring.[4] At T =25°C, 4-substituted X–ArN2 are quite stable,[4]
and no significant decomposition takes place in the time scale of a typical
electrochemical experiment (0.5–3 min).
EXPERIMENTAL
Materials
RESULTS AND DISCUSSION
Reagents were of the maximum purity available and used as is. HCl, NaOH,
the materials employed in the preparation of the Britton–Robinson buffer
and arenediazonium tetrafluoroborates, X–ArN2+ BF4ꢀ, were from Fluka or
Aldrich. BD and the substituted 4-methyl-benzenediazonium ion,
4-methoxy-benzenediazonium ion, and 4-bromo-benzenediazonium ions
were prepared from the corresponding anilines (Sigma, Fluka, St. Louis,
Mo, USA) following a standard non-aqueous procedure as described
elsewhere.[33] 4-nitrobenzenediazonium tetrafluoroborate was purchased
from Aldrich (97%) and recrystallized twice from cold ether.
Previous investigations on the electrochemical behavior of
X–ArN2+ in aqueous solution show that the chemical process as-
+
sociated with the electrochemical reduction of X–ArN2 is irre-
versible.[26] When protons are involved, as in Scheme 2(C),
electron uptake may occur before or after the possible proton
transfer, and it is very convenient to obtain some insights into
the characteristics of the electrochemical processes and their
time sequences for correct interpretation of the results. Thus,
we first analyzed variations of the peak currents with tempera-
ture and concentration.
All solutions were made with Milli-Q grade water. Britton–Robison
buffer of the desired pH was prepared by mixing solutions of 0.04 M
acetic, boric, and orthophosphoric acids with the appropriatꢀe amounts
+
Figure 2(A) is illustrative and shows the variation of ip with T
for 4-bromo-benzenediazonium at two pH values. Similar plots
of 0.2 M NaOH to obtain the desired pH. Stock X–ArN2 BF4 solutions
were kept in the dark at low temperature to minimize its decompo-
+
+
were obtained for the other ArN2 investigated. The slopes of
sition. The purity of X–ArN2 was checked periodically by employing
UV–vis spectroscopy.
the linear plots, expressed as the percentage of variation (usually
denoted as the half temperature coefficient, w), yield the values
of w = (0.77 0.12)% °Cꢀ1 at pH= 2.37 and w = (0.25 0.04)% °Cꢀ1
at pH = 4.7. Diffusion currents increase linearly with the
temperature, with the w values much lower than those of kinetic
currents, which usually range from 5 to 20% °Cꢀ1 [34]
Therefore,
.
the calculated w values suggest that the rate of transport of
+
ArN2 from the bulk solution to the surface of the electrode is
determined by its diffusion, in accordance with the previous
electrochemical results,[25,26] and the corresponding peak currents
are a measure of [ArN2+ ] in the bulk solution. Figure 2(B) is repre-
sentative and shows that the diffusion currents are proportional to
[ArN2+] according to the Ilkovic equation. Thus, the first reduction
+
peak of ArN2 is irreversible, and the variation of ip values with
[ArN2+] is linear at least up to [ArN2+] ~10ꢀ4 M, in accordance with
the literature results.[26]
Figure 1 shows that the peak potentials Ep of the first
+
reduction peak of ArN2 depend on the pH of the medium.
Figure 3 is illustrative and shows the variation of Ep with pH
for two representatives X–ArN2+. Two linear segments with a
break point at pH 5–6 are obtained. The slope of the first
segment, S1, is very low or negligible as shown in Table 1, indi-
cating that the Ep values are pH-independent up to ~ pH = 5–6.
However, the values of the slopes of the second linear segment,
S2, are much higher and negative, Table 1, that is, upon the
Figure 1. Representative polarograms of benzenediazonium ions
obtained at different pH values. For the sake of clarity, only the first
reduction peak is displayed
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2014, 27 284–289

FORMATION OF DIAZOHYDROXIDES ArN2OH
increasing pH, the Ep values are shifted in a linear fashion
toward more negative values.
The results in Fig. 3 are qualitatively consistent with the
+
hydrolysis of ArN2 in dynamic equilibrium and compared with
a slow rate electron transfer, Scheme 4,[34] bearing in mind that
+
Scheme 4. Dynamic equilibrium reaction of ArN2 with H2O to give
+
the formation of ArN2OH through the reaction of ArN2 with
ArN2OH and a proton and its electrochemical reduction (first reduction peak)
OHꢀ [Scheme 2(A)] is negligible from pH 2 to 10.[4]
+
When [H+] >>> KR, ArN2 is the dominant species in solu-
tion, that is, [ArN2+ ] >>> [ArN2OH], and the concentration of
ArN2+ reduction. However, at higher pH, ArN2+and ArN2OH are
in dynamic equilibrium, and the rate of proton transfer is much
faster than the rate of electron transfer to ArN2+ (or ArN2(OH2)+.
Ep values, which are measured under conditions of dynamic
equilibrium, depend (Nernst equation) on both [ArN2+] and
[ArN2OH] and thus on the values of equilibrium constant KR
(Scheme 4) and [H+ ]. At a given pH, Ep values do not depend
on [ArN2+] (contrary to ip); however, Ep values depend on pH.
The dependence of Ep on pH shown in Fig. 3 is typical of acid–
base systems in dynamic equilibrium, Scheme 4.[34,35] For such
a system, the peak current for the irreversible electrodic process
is given by Eqn 1[34]
ArN2OH is negligible, and hydrogen ions are not involved in
Â
þÃ
ꢀ
ip ¼ Bke ArN2
(1)
where B is the product of a number of constants that includes,
Figure 2. (A) Effects of temperature on ip for the first reduction peak of
4BrBD at pH = 2.37 (α) and pH=4.7 (β), [4BrBD] ≈4× 10ꢀ5 M. (B) Variation
of the peak current of the first reduction peak of 4MeOBD. Experimental
conditions are as follows: Britton–Robinson Buffer (0.04 M) pH = 2, ΔU = 1.25
V, vscan =0.041V sꢀ1
among others, the number of electrons n involved and the
ꢀ
Faraday constant F, ke is the heterogeneous rate constant for
the electrode process, which is given by Eqn 2, and [ArN2+] is the
concentration of the oxidized form at the surface of the electrode.
βnF
ꢀ
ð
E ꢀE0
ke ¼ ke0e
(2)
RT ð
Þ
p
ꢀ
In Eqn 2, k0e is the heterogeneous rate constant of the
electrode process at the standard potential E0, β is the transfer
coefficient, n is the number of electrons involved, F is the
Faraday constant, and Ep is the measured peak potential.
The stoichiometric concentration of the oxidized form is
Â
þÃ
ArN2
¼ ½ArN2þꢁ þ ½ArN2OHꢁ
T
and using the equilibrium constant KR, Eqn 3, the measured peak
Figure 3. Plots of the variation of Ep of the first reduction peak of 4MBD (A)
and 4BrBD (B) with pH
potential is given by Eqn 4,[34] where A is a constant.
Table 1. Slopes S1 and S2 and standard deviations of the straight lines at low (<5) pH and high (>6) pH, respectively. The S1 values
can be considered statistically 0 because the value of their standard deviation is similar or higher than the mean value. The
equilibrium constant KR values for the nucleophilic addition of water to X–ArN2+, Scheme 2(C), is determined from the intersection
of the two straight lines, as shown in Figure 3. σp values are collected from H. Maskill.[37] For the sake of comparison, the
equilibrium constants KDE for the formation of diazo ethers ArN2OR from the reaction with alkanols ROH are also displayed
+
ArN2
σp
103 S1
102 S2
pKR
pKDE
BD
0.00
ꢀ0.17
ꢀ0.28
0.23
ꢀ(1.3 0.8)
ꢀ(0.7 0.6)
ꢀ(0.2 0.1)
(0.3 0.4)
ꢀ(5.1 0.6)
ꢀ(4.6 0.2)
ꢀ(3.5 0.1)
ꢀ(4.3 0.1)
ꢀ(14 3)
5.70
5.72
5.80
5.20
5.24
—
3.6–5.3a
—
4MBD
4MeOBD
4BrBD
4NBD
3.5b
0.78
ꢀ(0.6 1)
4.18c
aref.[13,38]
.
bref.[22]
.
.
cref.[39]
J. Phys. Org. Chem. 2014, 27 284–289
Copyright © 2013 John Wiley & Sons, Ltd.
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A. SIENKIEWICZ ET AL.
½ArN2OHꢁ½Hþꢁ
Â
þÃ
KR
¼
(3)
(4)
ArN2
Scheme 5. Proposed reaction mechanism for the formation and decom-
position of diazo ethers under acidic conditions
RT
½Hþꢁ
Ep ¼ A þ In
nF KR þ ½Hþꢁ
When [H+] >> KR, Eqn 4 simplifies to Eqn 5, showing that,
at low pH, the slope of the variation of Ep with pH should
be 0 (i.e., Ep values are pH independent) as shown in Fig. 3.
Ep ¼ A
(5)
When [H+] <<KR, Eqn 4 can be rearranged into Eqn 6, which
predicts that, upon the increasing pH, the peak potentials
become more negative as illustrated in Fig. 3.
RT
Ep ¼ A- InKR ꢀ
nF
2:3RT
nF
pH
(6)
Note that Eqn 6 predicts that the variation of Ep values with
increasing pH is linear with a negative S2 slope and eventually,
Ep values may become negative as shown in Fig. 3.
At the intersection point of the two straight lines in Fig. 3(A,B),
the Ep values are the same and thus, according to Eqn 6, the
equilibrium constant KR, Scheme 4, is numerically equal to the
pH at that point (i.e., pKR = pH). The determined pKR values are
collected in Table 1. The obtained KR values indicate that
electronic changes induced by substituents are very small as
illustrated in Fig. 4, where the Hammett plot is displayed. The
slope of the linear plot is rather small, ρ = 0.58, but positive,
suggestive of an increase in the electron density on the atom
adjacent to the ring during the reaction.[36] That is, the posi-
tive charge on the diazonium group becomes neutral in
diazohydroxide.
Figure 4. Effects of substituents on the formation of diazohydroxides
according to Scheme 4; σp values are from H. Maskill[37]
reinforce the idea that the formation of diazo ether adducts of
the type Ar–N=N–O–X is general because diazo ethers are also
formed in reactions with alcohols other than alkanols under
+
acidic conditions.[14] For instance, ArN2 reacts with molecules
bearing OH groups in their molecular structure such as gallic acid
(3,4,5-tryhydroxybenzoic acid) and some of their derivatives,[14]
+
ascorbic acid (vitamin C), and cyclodextrins.[12,25] ArN2 reacts
with alkanols ROH (MeOH, EtOH, and BuOH) to give the
corresponding adduct ArN2OR, Scheme 1, and because
deprotonation is not possible, the ArN2OR either isomerizes to
much more stable E-diazo ether or decomposes homolytically
in the rate-determining step leading to the formation of
reduction products. Some representative values for the equilib-
rium constant KDE, are shown in Table 1. The KDE values are not very
different from those obtained in aqueous solution, but because the
KDE values are only known for a limited number of ArN2+, no
general comparisons can be performed, and more work on their
formation and decomposition reactions is warranted.
It may be illustrative to compare the nucleophilic reactions of
+
ArN2 with aliphatic alcohols ROH, Scheme 5, and those with
water (HOH). On the basis of the nucleophilicity of the solvents,
one would expect less homolytic products in water than in alco-
hols, but this is not the case. Dediazoniations in weakly alkaline
aqueous solutions produce a complex mixture of dediazoniation
products formed (sometimes diazo tars); however, in alcohols,
the number of products rarely exceeds three or four and diazo
tars seldom form.[4]
The reactions of ArN2+ with the alkoxide ions, ROꢀ, lead to the
formation of diazo ethers of the type Ar–N=N–O–R,[14] Scheme 1,
which are structurally similar compounds to those obtained from
the reaction with OHꢀ. However, the kinetics and mechanism of
these O-coupling reactions are quite different because the
primary Z-diazoether product cannot further react with the other
ROꢀ (or OHꢀ) ion, that is, deprotonation is not possible as in the
case of diazohydroxides, Scheme 2(A). Alkoxide ions are much
less solvated in alcohols than the OHꢀ ions in water, and
because the relative permittivity of the alcohols is substantially
lower than that of water, the reaction of ArN2+ with ROꢀ is much
faster than that with OHꢀ, the reverse reaction is much slower,
and the equilibrium is shifted toward the O-adducts.[4]
CONCLUSIONS
In conclusion, we have been able to show that ArN2OH adducts are
+
formed from nucleophilic attack of reaction of H2O with X–ArN2
under acidic conditions and to estimate, for the first time, the equi-
librium constant KR for the formation of diazohydroxides ArN2OH
formed by the nucleophilic attack of H2O at a number of 4-
substituted ArN2+ by employing DPP. The KR values, obtained from
variations of the peak potential of the first reduction peak with pH,
range 5–6 and are rather insensitive to substituent effect.
As discussed before, the spectroscopic or chromatographic
characterization of the ArN2OH adducts is difficult, and we only
know about their formation because of the variations in the peak
potential of X–ArN2+ with pH. However, the results obtained here
Differential pulse polarography has been shown to be very use-
ful to investigate chemical behavior of ArN2+ in aqueous solution.
Compared with the methods used in the past to determine the
ionization constants of ArN2+ (mainly spectrometric – stopped flow
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J. Phys. Org. Chem. 2014, 27 284–289

FORMATION OF DIAZOHYDROXIDES ArN2OH
[6] J. Pinson, F. Podvorika, Chem. Soc. Rev. 2005, 34, 429.
[7] J. Pinson, in Aryl Diazonium Salts (Ed.: M. M. Chehimi), Wiley-VCH
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[8] L. S. Romsted, in Reactions and Synthesis in Surfactant Systems
(Ed.: J. Texter), Marcel-Dekker, NY, 2001.
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Biomol. Chem. 2010, 8, 5304.
– and potentiometric), DPP has the advantage of its high selectivity
and sensitivity, allowing to overcome some of the major problems
found in the past. In addition, the electrochemical experiments can
be performed in a short period making that the spontaneous
+
decomposition of ArN2 be negligible. Moreover, DPP has also
been shown to be very useful in kinetic studies and valuable infor-
mation not only on the parent ArN2+ but also on the electrochem-
ically generated aryl radicals Ar•, for instance, the association
constants of Ar• with sodium dodecyl sulfate micelles and cyclo-
dextrins were obtained by employing this technique.[24,25]
[14] C. Bravo Díaz, in Diazohydroxides, Diazoethers and Related Species, in
"Patai’s Chemistry of Functional Groups: The Chemistry of Hydroxyl-
amines, Oximes and Hydroxamic Acids", Zvi Rappoport, J. F. Liebman,
Eds.,Vol 2, p. 853., J. Wiley & Sons, Chichester, UK, 2011.
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(Ed.: S. Patai), J. Wiley & Sons, NY, 1978.
Our results show that the formation of diazo adducts with
neutral nucleophiles is much more common than expected, and
our results should also contribute to obtain a substantial body of
knowledge to propose a unified, general, picture about the
formation and decomposition of diazo ethers with neutral
nucleophile. The determination of the equilibrium constants for
the formation of adducts of the type ArN=N–OR under acidic
conditions is recent in the time scale of the ArN2+ chemistry, and
further work to determine the corresponding equilibrium
constants and to analyze their dependence with the nature and
position of the substituents in the aromatic ring is warranted.
The results are of some importance to the azo dye and pigment
industries[3] because in electrophilic substitution reactions, it is the
most basic form of the nucleophilic substrate (i.e., the phenoxide
ion or the free aromatic amine), the one which gives rise to the
highest rates of substitution, and thus the formation of
diazohydroxides may be competitive. They are also of some
[16] V. Sterba, in The Chemistry of Diazonium and Diazo Compounds,
(Ed.: S. Patai), J. Wiley & Sons, Bristol. UK, 1978.
[17] H. Zollinger, Accounts Chem. Res. 1973, 6, 335.
[18] E. S. Lewis, H. Suhr, Chem. Ber. 1958, 91, 2350.
[19] C. D. Ritchie, D. J. Wright, J. Am. Chem. Soc. 1971, 93, 6574.
[20] C. D. Ritchie, D. J. Wright, J. Am. Chem. Soc. 1971, 93, 2425.
[21] P. D. Goodman, T. J. Kemp, J. Chem Soc. Perkin Trans. II 1981, 1221.
[22] A. Fernández-Alonso, C. Bravo-Diaz, Org. Biomol. Chem. 2008, 6, 4004.
[23] M. P. Doyle, C. L. Nesloney, M. S. Shanklin, C. A. Marsh, K. C. Brown,
J. Org. Chem. 1989, 54, 3785.
[24] E. González-Romero, M. B. Fernández-Calvar, C. Bravo-Díaz,
Langmuir, 2002, 18, 10311.
[25] E. González-Romero, B. Malvido-Hermelo, C. Bravo-Díaz, Langmuir
2002, 18, 46.
[26] C. Bravo-Díaz, E. González-Romero, Electroanalysis 2003, 15, 303.
[27] C. Bravo-Díaz, E. González-Romero, in "Current Topics in Electrochem-
istry", Vol. 9, Research Trends, Trivandrum, India, 2003.
[28] M. J. Pastoriza-Gallego, S. Losada-Barreiro, C. Bravo Díaz, J. Phys. Org.
Chem. 2012, 25, 908.
+
importance to explore novel applications of ArN2 in colloidal,
organic, and in nanochemistry, where most of the reactions of
ArN2+ are carried out in aqueous or low polarity solvents, and the
grafting of surfaces from ArN2Nu adducts is being explored.[7]
[29] C. Galli, Chem. Rev. 1988, 88, 765.
[30] A. J. Fry, in The Chemistry of Diazo and Diazonium Groups (Ed.: S. Patai),
J. Wiley & Sons, N.Y., 1978.
[31] H. Viertler, V. L. Pardini, R. R. Vargas, in The Chemistry of Triple-Bonded
Functional Groups, Supplement C. (Ed.: S. Patai), J. Wiley & Sons, N.Y., 1994.
[32] D. T. Sawyer, A. Sobkowiak, J. L. Roberts, Electrochemistry for
Chemists, 2nd edn, J. Wiley & Sons, N.Y., 1995.
[33] M. C. Garcia-Meijide, C. Bravo-Diaz, L. S. Romsted, Int. J. Chem. Kinet.
1998, 30, 31.
[34] P. Zuman, in Organic Polarography (Eds.: P. Zuman, C. L. Perrin), J.
Wiley & Sons, N.Y., 1969.
Acknowledgement
C. B-D thank Prof. Frank Hegarty (University College, Dublin) for
extremely fruitful discussions. Financial support from the following
agencies is acknowledged: Xunta de Galicia (10TAL314003PR),
Ministerio de Educación y Ciencia (CTQ2006-13969-BQU), Fondo
Europeo de Desarrollo Regional (FEDER) and Universidad de Vigo.
[35] P. Zuman, FABAD J. Pharm. Sci. 2006, 31, 97.
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J. Phys. Org. Chem. 2014, 27 284–289
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