Solvolysis of o-methylbenzenediazonium tetrafluoroborate in acidic methanol-water mixtures. Further evidence for nucleophilic attack on a solvent separated aryl cation

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:8<531::AID-KIN2>3.0.CO;2-K

Rate constants for dediazoniation product formation and arenediazonium ion loss and product yields of solvolysis of o-methylbenzenediazonium tetrafluoroborate in acidic methanol-water mixtures at T = 35 °C are reported. Observed rate constants for diazonium ion loss and product formation are the same, increasing about 45% ongoing from water to methanol, and are not affected by added electrolytes like HCl, NaCl, and CuCl₂. Only three dediazoniation products are detected, o-cresol, o-chlorotoluene, and o-anisole. All data are consistent with a rate-determining step formation of an aryl cation that reacts immediately with available nucleophiles. The selectivity of the reaction toward nucleophiles, S, which can be is low and essentially constant upon changing solvent composition, suggesting that the nucleophilic attack takes place on a solvent separated aryl cation.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:8<531::AID-KIN2>3.0.CO;2-K

Solvolysis of
O-Methylbenzenediazonium
Tetrafluoroborate in Acidic
Methanol-Water Mixtures.
Further Evidence for
Nucleophilic Attack on a
Solvent Separated Aryl
Cation
R. PAZO-LLORENTE,1 M. J. SARABIA-RODRIGUEZ,1 E. GONZALEZ-ROMERO,2
C. BRAVO-DIAZ1
1Universidad de Vigo, Facultad de Ciencias, Dpto. Quimica Fisica Y Quimica Organica, As Lagoas-Marcosende, 36200
Vigo-Pontevedra, Spain
2Universidad de Vigo, Facultad de Ciencias, Dpto. Quimica Analitica Y Alimentaria, As Lagoas-Marcosende, 36200
Vigo-Pontevedra, Spain
Received 26 October 1998; accepted 11 March 1999
ABSTRACT: Rate constants for dediazoniation product formation and arenediazonium ion loss
and product yields of solvolysis of o-methylbenzenediazonium tetrafluoroborate in acidic
methanol-water mixtures at T ϭ 35ЊC are reported. Observed rate constants for diazonium ion
loss and product formation are the same, increasing about 45% ongoing from water to meth-
anol, and are not affected by added electrolytes like HCl, NaCl, and CuCl2. Only three dedi-
azoniation products are detected, o-cresol, o-chlorotoluene, and o-anisole. All data are con-
sistent with a rate-determining step formation of an aryl cation that reacts immediately with
available nucleophiles. The selectivity of the reaction toward nucleophiles, S, which can be
Correspondence to: C. Bravo-Diaz (cbravo@uvigo.es)
Contract grant sponsor: Ministerio de Educacion y Cultura
Contract grant number: DGICYT, PB94-0741
Contract grant sponsor: Xunta de Galicia
Contract grant number: XUGA 38305A94
Contract grant sponsor: Xunta de Galicia
Contract grant number: XUGA FP2P97-ED406C-32
Contract grant sponsor: Universidad de Vigo
᭧ 1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/080531-08

532
PAZO-LLORENTE ET AL.
defined by:
k
k
F
F
[Nu ]
Nu1
RϪNu1
2
S ϭ
ϭ
[Nu ]
Nu
RϪNu
1
2
2
is low and essentially constant upon changing solvent composition, suggesting that the nu-
cleophilic attack takes place on a solvent separated aryl cation. ᭧ 1999 John Wiley & Sons, Inc.
Int J Chem Kinet 31: 531–538, 1999
INTRODUCTION
[13] and microemulsions [14,15] and other macro-
molecular systems like crown ethers [16,17] and cy-
clodextrins [18]. Such systems show significant gra-
dients of polarity [19] ongoing from the bulk phase to
the core of the aggregate or show relatively hydropho-
bic microscopic environments [20] where arenedi-
azonium ions can be located [13,20]. Consequently,
they can determine the arenediazonium chemical re-
activity by governing their contact with other sub-
strates incorporated in the colloidal system [13–15]
and by governing the polarity of their immediate en-
vironment, which can be decisive for unimolecular de-
composition reactions.
Although considerable amounts of data are avail-
able on selectivities for solvolysis in alcohol-water
mixtures, systematic studies on individual substrates
over the whole composition range are relatively rare
[7], probably because reliable results in highly aque-
ous or alcoholic mixtures are difficult to obtain be-
cause of the increased rates of solvolysis and de-
creased solubilities [12]. Suitable substrates on which
one can test the selectivity solvent dependence are ar-
enediazonium salts because they are soluble in water
and in a number of organic solvents and their decom-
position reactions can be slow enough to allow mixing
of reagents at any solvent composition. Rates of het-
erolytic dediazoniation of a number of arenediazonium
ions vary by a factor of only 9 in 19 solvents
[16,17,21].
Information concerning the reactivity-selectivity prin-
ciple can be obtained through two principal ways [1–
4]. One is to derive kinetic selectivity parameters for
the reactions by comparing their rate responses to sub-
stituents or environmental changes with rates for a
model reaction; the second one is to generate an in-
termediate in an environment where two or more re-
agents compete to form stable compounds, so that
analysis of the product mixture provides a measure of
the selectivity of the intermediate and a powerful way
to identify it. Examples of the latter are the nucleo-
philic attack on carbocations [5,6] which may occur
[1,3,4,7] on “free” carbocations, on solvent separated
ion pairs, or on “intimate” contact ion pairs. If two
nucleophiles, Nu1 and Nu2, react with the same inter-
mediate, R, with specific rate constants for nucleo-
philic attack kNu and kNu , and the two rate constants
1
2
of product formation determine the product yields, that
is, the reaction is under kinetic control, the selectivity
S of the reaction can be defined [8,9] by eq. (1), where
F stands for percent of conversion (i.e., percent yield)
to the corresponding reaction product, and nucleo-
phile concentrations are assumed to refer to the reac-
tion site.
kNu
kNu
FR Nu [Nu2]
1
Ϫ
1
Nu1
S
ϭ
ϭ
(1)
Nu2
FR Nu [Nu1]
Ϫ
2
2
Arenediazonium salts have been shown to be very
important for both dyestuff industry and synthetic
chemistry. Arenedediazoniation chemistry has been
subject to great controversy because their chemistry
varies dramatically with experimental conditions
[16,17,22,23], changing not only the mechanism of the
reactions but the relative amount of dediazoniation
products. Spontaneous solvolytic dediazoniations take
place through two main mechanisms [17], heterolytic
and homolytic. Literature reports [17] indicate that, in
the absence of electron donors, heterolytic dediazon-
iations mainly take place in solvents of low nucleo-
philicity like H2O or TFE. Meanwhile, homolytic de-
diazoniations are favored by an increase in the
nucleophilicity (HMPT, Pyridine). MeOH, EtOH, and
DMSO have been reported as borderline solvents
Selectivity values, S, for solvolysis of a number of
carbocations in aqueous alcohols are often solvent de-
pendent and it has been proposed that the solvent de-
pendence of S may be due to changes in the nature of
the ion pair leading to products [10] or because a sec-
ond molecule of solvent assists the nucleophilic attack
acting as general base [7,11]. Therefore, determination
of S values at different solution compositions [4,7] can
be used to provide evidence of the presence of cationic
intermediates and the nature of the cationic interme-
diate, complementing other studies involving added
nucleophilic traps, which have the disadvantage that
they may induce mechanistic changes [12]. These sol-
volysis studies also can be of interest to those working
on dediazoniations in colloidal systems like micelles

SOLVOLYSIS IN ACIDIC METHANOL-WATER MIXTURES
533
where both mechanisms can be observed, depending
on several factors like pH [17], substituents of the ar-
omatic ring [24,25], or even the atmosphere [25] (O2
or N2).
M HCl. The injection volume was 25␮L in all runs
and the UV detector was set at 220 nm or 275 nm.
Materials
S values for dediazoniations can be easily calcu-
lated provided that a suitable method to determine
product yields is available. We have recently reported
a new method for monitoring dediazoniations [13,26]
that allows, within the same experiment, to obtain
product yields at any time and the rate constants for
the formation of all dediazoniation products and, in-
directly, that for the disappearance of the diazonium
ion. Here, we report part of a recent and ongoing in-
vestigation of the effects of solvents and added elec-
trolytes on the rates and mechanisms of dediazonia-
tions. The aim of the article is double: to expand the
scope of applicability of the method to solvents other
than water and to complement studies concerning the
nucleophilic attack on aryl cations. We have chosen
methanol as solvent and o-methylbenzenediazonium
tetrafluoroborate, OMBD, as substrate because it con-
forms to the requirements described above about sol-
ubility, its dediazoniation mechanism in water is
known [27,28] showing a convenient dediazoniation
rate constant value at T ϭ 35ЊC.
Few studies of heterolytic arenedediazoniations in
water-alcohol mixtures over the whole composition
range have been done [17]. The present knowledge
concerning dediazoniations in alcohols is based basi-
cally on the works of DeTar et al., Bunnet et al., and
Broxton et al., who worked mainly under alkaline con-
ditions [17]. However, relatively little work was done
under acidic conditions [16,17,29] and OMBD does
not appear to have been studied under such acidic con-
ditions [24,25,30], so we have also examined its ki-
netic behavior in the presence and absence of added
nucleophiles.
Reagents were of maximum purity available and were
used without further purification. o-cresol, ArOH, o-
chlorotoluene, ArCl, o-anisole, ArOMe, copper (II)
chloride (99.999%), and the reagents used in the prep-
aration of diazonium salts (as tetrafluoroborates) were
purchased from Aldrich. 2-Naphthol-6-sulfonic acid,
sodium salt, (2N6S) was purchased from Pfaltz &
Bauer. Other materials employed were from Riedel de
Haen. All solutions were prepared by using Milli-Q
grade water. Diazonium salts were prepared under no-
naqueous conditions [31] and were stored in the dark
at low temperature to minimize its decomposition and
recrystallyzed periodically. CuCl2 forms a number of
chlorocomplexes in solution and can be reduced to
form Cu(I), which has been reported as a catalyst in
dediazoniations [32] (Sandmeyer reaction). To prevent
reduction of Cu(II) to Cu(I) we carefully purified the
diazonium salt, avoided using reducing agents, and did
not deoxygenate the water because dissolved O2
rapidly oxidizes Cu(I) to Cu(II) in aqueous solu-
tions [33].
Methods
Kinetic data were obtained both spectrophotometri-
cally and chromatographically. Observed rate con-
stants were obtained by fitting the absorbance-time or
percent yield-time data to the integrated first-order
equation (1) using a non-linear least-squares method,
where M is the measured magnitude of the UV-VIS
absorbance or yields.
MtϪMϱ
Ln
ϭ Ϫkot
(2)
ͩ ͪ
M0ϪMϱ
EXPERIMENTAL
Instrumentation
All runs were done at T ϭ 35 Ϯ 0.1 ЊC with OMBD
as the limiting reagent. Duplicate or triplicate experi-
ments gave average deviations lower than 5%.
UV-VIS spectra and some kinetic experiments were
followed on a Beckman DU-640 UV-VIS spectropho-
tometer equipped with a thermostated cell carrier at-
tached to a computer for data storage. Product analysis
was carried out on a WATERS HPLC system, which
included a 560 pump, a 717 automatic injector, a 486
VIS-UV detector, and a computer for data storage.
Products were separated on a Microsorb-MV C-18
(Rainin) reverse phase column (25 cm length, 4.6 mm
internal diameter, and 5 ␮m particle size) using a mo-
Spectrophotometric kinetic data were obtained by
following the disappearance of diazonium salt at an
appropriate wavelength to minimize interference
mainly by chlorocuprate(II) complexes. Beer’s law
plots (not shown) for aqueous and methanolic OMBD
solutions up to 2.00 ϫ 10Ϫ4 M are linear (cc. Ն 0.999).
Stock solutions were prepared by dissolving the dia-
zonium salt in the appropriate acidic (HCl) mixture to
minimize diazotate formation [34], to give final con-
4
4
bile phase of 70/30 v/v MeOH/H2O containing 10Ϫ
centrations of about 1 ϫ 10Ϫ M and [HCl] ϭ 3.6 ϫ

534
PAZO-LLORENTE ET AL.
3
10Ϫ M. Stock solutions were generally used imme-
diately or within 90 min. with storage in an ice bath
to minimize decomposition.
Preliminary HPLC experiments showed that only
three decomposition products are formed: ArOH, ArCl
and ArOMe. Linear (cc. Ͼ 0.999) calibration curves
for converting HPLC peak areas, A, into concentra-
tions were obtained for these products by employing
commercial samples. The equations used were: ␭ ϭ
275 nm (A ϭ 2.77 ϫ 109 [ArOH], A ϭ 2.19 ϫ 109
[ArOMe]); ␭ ϭ 220 nm (A ϭ 8.21 ϫ 109 [ArOH],
A ϭ 1.31 ϫ 1010 [ArOMe], A ϭ 7.10 ϫ109 [ArCl]).
Yields of dediazoniation products were obtained from
the ratio of the dediazoniation product concentration,
[Analyte], and the initial diazonium salt concentration,
that is,
Figure 1 Effect of solvent composition on the observed
Ϫ
rate constant ko. [OMBD] ϭ 1Ϫ3 ϫ 10 4 M, [HCl] ϭ 0.01
M, T ϭ 35 ЊC.
that the measured absorbance is directly proportional
to the concentration of the azo dye and therefore the
concentration of unreacted diazonium salt.
F ϭ 100 [Analyte]/[OMBD].
Chromatographic kinetic data for all dediazoniation
products were obtained following a well-established
procedure [13,18,26,35] by quenching the dediazon-
iation reaction at convenient times with an aliquot of
a stock-quenching solution which was prepared by dis-
solving 2N6S in a solution containing TRIS buffer
([TRIS] ϭ 0.05 M) to give, after mixing, final 2N6S
concentrations about 20-fold excess over that of are-
nediazonium salt and a final pH about pH ϭ 9. It has
been reported that coupling rates depend on a number
of factors like pH, temperature, and solvent. pH was
chosen to maximize the rate of azo dye formation
[17,34,36] because coupling rates change dramatically
with pH due to naphthoxide ions being much more
reactive than their protonated forms [23,37]. But as
pH increases, the competing reaction of arenediazon-
ium ions with OHϪ to form diazotates becomes sig-
nificant [34]. Rate constants for coupling reactions in
a number of solvents have been reported to be higher
than those in water [17]. The variation depends on the
nature of diazonium ion or the solvation of naphthox-
ide ions. Nevetherless, auxiliary experiments done at
different MeOH/H2O mixtures by following azo dye
formation spectrophotometrically showed that under
our experimental conditions the coupling reaction was
essentially over in the time of mixing reagents; that is,
azo dye formation is, at least, 50 times faster than the
fastest dediazoniation. This procedure also allowed to
estimate ko for the decomposition of arenediazonium
salts. After dediazoniation was over, sufficient con-
centrated HCl was added dropwise to each volumetric
flask to give final [H3Oϩ] Ϸ 0.2 M and the absorbance
RESULTS
Effects of Solvent Composition and Added
Electrolytes on the Observed Rate
Constant, ko.
The effect of solvent composition on ko was investi-
gated by changing the percentage of MeOH in the re-
action mixture. Figure 1 shows a continuous increase
in ko on going from ko ϭ 9.50 ϫ 10Ϫ4 sϪ1 (0 % MeOH)
4
1
up to ko ϭ 14.0 ϫ 10Ϫ sϪ (99.6% MeOH). The ko
value at 0% MeOH is in agreement with literature val-
4
1
ues of ko ϭ 9.6 ϫ 10Ϫ sϪ obtained by N2 evolution
[27] at pH ϭ 1.6–1.8 and with that of ko ϭ 10.3 ϫ
4
1
10Ϫ sϪ obtained spectrophotometrically [28]. The
observed increase in ko is also in agreement with pub-
lished results for other dediazoniations in acidic
MeOH [30]. The sensitivity of the OMBD dediazon-
iation to changes in solvent polarity can be shown by
means of the Winstein-Grunwald equation [38,39],
Figure 2, yielding a slope of m ϭ Ϫ0.038 and an in-
tercept of i ϭ Ϫ2.90 (cc. ϭ 0.99), consistent with the
extraordinary insensitivity of dediazoniations to me-
dium effects [17], which has been attributed [28] to
the similarity in structure and charge distribution be-
tween the parent arenediazonium ion and the corre-
sponding aryl cation.
Rate constants for product formation at different
solvent compositions were obtained by HPLC. Results
in Figure 3, which include the corresponding first-or-
der plots, are representative. Only the variation in the
amount of ArOMe formed with time is shown because
the products are formed competitively and because
of each solution was measured at ␭
of the corre-
max
sponding azo dye [28]. The added HCl ensures that
only the protonated form of the azo dye is present and

SOLVOLYSIS IN ACIDIC METHANOL-WATER MIXTURES
535
Figure 2 Winstein-Grunwald plot for solvolysis of
OMBD. Y data taken from reference 39.
conversion of OMBD to ArOMe is almost quantitative
under these conditions. Observed rate constants for
ArOH and ArOMe formation (Fig. 3A) yield an iden-
tical value, ko ϭ 12.60 ϫ 10Ϫ sϪ , which is equal,
within experimental error, to that obtained for OMBD
loss from the variation of the absorbance of the azo
dye with time, Figure 3B.We also investigated the ef-
fect of added electrolytes like HCl, NaCl, and CuCl2
on ko at different MeOH/H2O mixtures. Table IA
shows the average values, which indicate that ko is not
affected by added electrolytes, exhibiting a similar be-
havior as that observed for other arenediazonium ions
in water [27,28].
4
1
Figure 3 A) Formation of ArOMe (᭺) with time and first
order plot (ٗ). B) Decrease in the absorbance of the azo dye
formed in the coupling step (see experimental) with time and
first order plot (ٗ). [OMBD] ϭ 1.37 ϫ 10 M, [HCl] ϭ
0.01 M, %MeOH ϭ 94.3 (by weight), T ϭ 35ЊC.
Ϫ
4
Effects of Solvent Composition and Added
Electrolytes on Product Yields and on
Selectivities.
Figure 4 shows the effects of solvent composition of
product yields. Only two dediazoniation products,
ArOH and ArOMe, are formed and quantitative con-
version to products is achieved in the entire compo-
sition range. Table IB shows the effect of added elec-
trolytes on product yields at two different solvent
compositions (average values).
solvent composition, Table I, but they appear to be
solvent dependent. The low SWCl values obtained are
similar to those reported for other diazonium ions [14].
DISCUSSION
SWA values, as defined by equation (1), are essen-
tially independent of the percentage of MeOH in the
reaction mixture, Table II, suggesting that the ratio of
rates involved in equation (1) is insensitive to solvent
composition and polarity. The average SWA value is very
similar to that reported for other dediazoniations in
butanol [14], SWA ϭ 0.28 at T ϭ 40oC, and is consistent
with the low selectivities of diazonium ions toward
different nucleophiles. The constancy of S values sug-
gests that the nucleophilic attack takes place on a sol-
vent separated ion pair, in agreement with independent
experimental evidences [17,40]. SWCl values for com-
petition between H2O and ClϪ are constant at a fixed
Chromatograms show that quantitative conversion to
products is achieved at any solvent composition (Fig.
4), and that the only dediazoniation products obtained
in significant yields are ArOH, ArOMe and, in some
experiments, ArCl (Table IB and Fig. 4). No extra-
neous peaks that might be attributed to reduction prod-
ucts formed in radical pathways like biphenyls, Ar-Ar,
toluene, Ar-H, etc, were observed. The absence of re-
duction or isomerized products is consistent with the
heterolytic mechanism.
Selectivity values (Table II) are orders of magni-
tude lower than those observed for anionic nucleo-

536
PAZO-LLORENTE ET AL.
Table I Average Values of the Effect of Added Electrolytes on the Observed Dediazoniation Rate Constants (1A)
and on Dediazoniation Product Yields (1B) at Different Solvent Compositions. T ϭ 35ЊC in All Runs
1A
%MeOHa
Ϫ1
104 kO/s
104[OMBD]/M
[HCl]/M
[NaCl]/M
[CuCl2]/M
0
17
44
1.05
1.00
1.26
1.00
1.26
1.07
0.01
0.01
0.01
0.01
0.01
0.01
—
0–1.0
—
0–1.0
—
—
—
0–0.01
—
0–0.01
—
9.50b
9.63
10.67
11.14
13.68
14.10c
64.5
94.3
99.6
1B
—
%MeOHa
104[OMBD]/M
[HCl]/M
[NaCl]/M
[CuCl2]/M
FArOH
FArCl
FArOMe
SWA
SWCl
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
2
1
1
1
1
1
0b
3.3
2 ϫ 10
1 ϫ 10
1 ϫ 10
1 ϫ 10
1 ϫ 10
1 ϫ 10
1.0
1.0
0–0.1
0.4
0.6
1.0
—
0–0.01
—
—
—
—
—
0–0.01
92.10
59.80
77.82
65.25
63.38
60.82
15.20
14.29
4.12
10.03
Ͻ2
4.10
6.58
9.84
—
—
—
2.43
4.60
—
4.23
4.70
4.40
—
44.0c
44.0c
44.0b
44.0b
44.0b
94.3c
94.3c
2.72
2.72
2.72
2.72
2.72
1.63
1.63
23.74
23.56
22.16
22.66
24.80
79.62
75.07
0.40
0.33
0.34
0.36
0.34
0.32
0.34
0–0.01
0.01
—
—
—
—
a Solution composition is given by percent of MeOH by weight, calculated taking into account the densities of water and methanol51 at
T ϭ 35ЊC. Molar concentrations were calculated by ignoring the small excess volume of mixed solvents52.
b Average value from two runs (relative standard deviation Ͻ7%)
c Average value of seven runs (relative standard deviation Ͻ5%)
philes competing with water in reactions with stabi-
lized carbocations [9,41,42] and they are essentially
independent of the solvent composition. These results
are consistent with a preassociation stepwise mecha-
nism, Scheme I, in which the aryl cation, which has a
short but finite lifetime, forms a “complex” with the
nucleophile before the product is formed [43]. The se-
lectivity of OMBD towards ClϪ, SWCl, at 44% MeOH,
Table IB, is about 10 times higher than SWA. Thus for-
mation of ArCl is about 10 times more favored than
ArOMe. On the other hand, SWCl at 44% MeOH is al-
most twice its value in pure water, Table IB, decreas-
ing upon increasing polarity, consistent with the for-
mation of ion pairs with halide ions, Scheme I, which
are favored upon decreasing the polarity of the me-
dium [41].
Table II Variation of Selectivity Values (Equation 1)
with Solvent Composition
% MeOHa
SWA
8.00
12.12
16.34
20.67
25.10
26.35
34.26
43.87
48.86
53.97
59.21
64.59
70.11
75.77
81.58
87.55
0.39
0.42
0.39
0.36
0.46
0.45
0.45
0.42
0.43
0.40
0.44
0.44
0.40
0.39
0.39
0.40
Figure 4 Effect of solvent composition on dediazoniation
product yields; ᭺ ArOH, ٗ ArOMe, Total yield (ϭ FArOH
ϩ FArOMe). [OMBD] ϭ 2.747 ϫ 10 M, [HCl] ϭ 0.01 M,
T ϭ 35ЊC.
Ϫ
4
a By weight. See footnote a in Table I.

SOLVOLYSIS IN ACIDIC METHANOL-WATER MIXTURES
537
due to loss of molecular vibrations is not compensated
by the loss of entropy due to restricted motion of the
solvating molecules and there is little enthalpic assist-
ance to the dissociation, therefore dediazoniations are
not specially sensitive to solvent composition
[17,28,49,50], as found in this work. The observed
small variations in rate constants due to solvent com-
position (Fig. 1), may be attributed, consequently, to
compensating changes in the enthalpies and entropies
of activation. Determination of these parameters for
this and other dediazoniations in H2O/MeOH mixtures
is part of future work.
Scheme I Dediazoniation Mechanism Showing the Step-
wise Formation of an Ion-Molecule Complex, a Solvent-
Separated Ion Pair or “Free” Aryl Cation.
Kinetic data show that ko increases modestly, about
45%, ongoing from 0% MeOH up to 100% MeOH
(Fig. 1). Dediazoniation, as represented in Scheme I,
involves a reversible cleavage of the C–N bond to
give an ion-molecule pair [17,22], ArϩN2, within a sol-
vent shell, followed by reversible formation of a sol-
vent-separated or “free” aryl cation. These intermedi-
ates have been postulated from the results of several
studies which include isotopically labeled N2 rear-
rangements in different solvents [40] and kinetic ex-
periments under N2 pressure [44]. Those intermediates
can react with nucleophiles to yield the corresponding
dediazoniation products. Thus, the dediazoniation rate
is determined by the contributions of the rates through
these reaction channels, Scheme I. A rate-limiting nu-
cleophilic attack would lead to a strong dependence of
the rate constant with solvent composition, which is
not observed for OMBD (Fig. 1), so although we can
not completely rule out a small extent of reaction
through this channel, dediazoniation takes place
mainly through the rate-limiting formation of the aryl
cation. Consistent with this interpretation, we have ob-
served that at fixed solvent composition, ko for for-
mation of ArOMe is the same as that for loss of dia-
zonium salt (Fig. 2), and neither rate is affected by
changes in acidity of the medium or in nucleophile
concentration, Table I. Cu(II) has a negligible influ-
ence on ko as expected from the changes in the nature
of the predominant copper(II) chlorocomplexes on
changing the solvent composition: the main Cu(II)
chlorocomplexes present in aqueous solutions contain-
CONCLUSIONS
Our results show that dediazoniation of OMBD in
MeOH/H2O mixtures proceeds via heterolytic path be-
cause of the absence of products which can be asso-
ciated with radical pathways. The major dediazonia-
tion products are o-cresol, ArOH, and o-anisole,
ArOMe, and when ClϪ ions are present a small amount
of o-chlorotoluene, ArCl is formed. ko values increase
with increasing MeOH percentage up to about 45% in
99.6% MeOH. Rate constants for product formation
are the same as that for diazonium ion loss and the
two rate constants are essentially independent of added
electrolytes at any solvent composition. Selectivity
values are solvent independent, suggesting a nucleo-
philic attack on a solvent separated aryl cation inter-
mediate. All evidence is therefore consistent with a
DN ϩ AN mechanism, that is, a rate-determining step
involving the formation of a highly reactive solvent-
separated aryl cation that reacts immediately, with
very low selectivity, with available nucleophiles.
The methodology employed has been successfully
applied to dediazoniations in methanol/water
mixtures, allowing simultaneous monitoring of con-
centration and determination of rates constants for
product formation and loss of starting material and to
provide reliable selectivity values. This information
can not be obtained solely by VIS-UV spectroscopy
or by chromatographic analysis of dediazoniation
products, which is basic to understanding the mecha-
nisms of the dediazoniations.
ing ClϪ ions [45,46], are CuCl3Ϫ and CuCl4 Ϫ. Mean-
2
while, in pure methanol [47], CuClϩ and CuCl2 are the
predominant ones.
Activation parameters for dediazoniation of the
three methylbenzenediazonium ions in water have
been reported [28]: enthalpies of activation are as high
as in many unimolecular reactions [41,48], and entro-
pies of activation are clearly positive. Dediazoniation,
as described in Scheme I, does not involve separation
of charge but its redistribution. Thus, the parent are-
nediazonium ion and the aryl cation polarizes the sol-
vent in a similar extent. Therefore, the gain of entropy
Financial support from the following institutions is acknowl-
edged: Ministerio de Educacion y Cultura (DGICYT, PB94-
0741), Xunta de Galicia (XUGA 38305A94) and Universi-
dad de Vigo. One of us, M. J. S. wishes to thank Xunta de
Galicia for a training research grant (XUGA FP2P97-
ED406C-32).

538
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