Hydroxy- and chloro-dediazoniation of 2- and 3-methylbenzenediazonium tetrafluoroborate in aqueous solution

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:1<73::AID-KIN9>3.0.CO;2-L

We have measured the rates and product yields of dediazoniation of 2- and 3-methylbenzenediazonium tetrafluoroborate in the presence and absence of electrolytes like HCl, NaCl, and CuCl₂ using a recently reported methodology that allows simultaneous determination of product concentrations and rates of product formation and, indirectly, loss of starting material. Activation parameters were also obtained: enthalpies of activation are high, and entropies of activation are positive. All results are consistent with a heterolytic mechanism involving the fragmentation of the arenediazonium ion into a very reactive phenyl cation.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:1<73::AID-KIN9>3.0.CO;2-L

Hydroxy- and Chloro-
Dediazoniation of 2- and 3-
Methylbenzenediazonium
Tetrafluoroborate in
Aqueous Solution
1
1
1
ROMAN PAZO-LLORENTE, MARIA JOSE SARABIA-RODRIGUEZ, CARLOS BRAVO-DIAZ,
2
ELISA GONZALEZ-ROMERO
1Universidad de Vigo, Facultad de Ciencias, Dpto. Quimica Fisica y Organica, As Lagoas-Marcosende, 36200 Vigo-
Pontevedra, Spain
2
Dpto. Quimica Analitica y Alimentaria
Received 6 March 1998; accepted 4 August 1998
ABSTRACT: We have measured the rates and product yields of dediazoniation of 2- and 3-
methylbenzenediazonium tetrafluoroborate in the presence and absence of electrolytes like
HCl, NaCl, and CuCl using a recently reported methodology that allows simultaneous deter-
2
mination of product concentrations and rates of product formation and, indirectly, loss of
starting material. Activation parameters were also obtained: enthalpies of activation are high,
and entropies of activation are positive. All results are consistent with a heterolytic mechanism
involving the fragmentation of the arenediazonium ion into a very reactive phenyl cation.
᭧
1999 John Wiley & Sons, Inc. Int J Chem Kinet 31 73–82, 1999
INTRODUCTION
diuretics, sulphonamides [6,7], phenols, and aryl-
amines [8]. All those methods are based on arenedi-
azonium derivatization employing reactions that occur
either at the arene nucleus (dediazoniation reactions,
Scheme 1A) or at the terminal nitrogen of the diazo-
nium group (nucleophilic addition, 1B) to form stable
compounds that can be analyzed quantitatively. The
arrow in Scheme 1 indicates a bonding model that im-
plies a C9N dative bond (synergistic C ; N ␴ bond
and C : N ␲ backdative bond) as found by theoret-
ical studies, [9] and will be used hereafter in all
schemes to represent such bond model.
Addition reactions, Scheme 1B, are the classical
route to obtain azo dyes and pigments [10]. In prin-
ciple, all aromatic and heteroaromatic diazonium salts
can be used as electrophilic reagents; its electrophil-
icity depends on substituents: electro-withdrawing
Use of arenediazonium ions as chemical trapping re-
agents has increased notably in the last years. They are
currently been used as assay to measure proteases ac-
tivities [1], to probe interfacial compositions of mi-
celles and microemulsions [2,3], to determine coun-
terion selectivities and co-ion concentrations in
micelles, [4,5] and to determine other chemicals like
Correspondence to: C. Bravo
Contract grant sponsor: Ministerio de Educaci o´ n Cultura (DGI-
CYT)
Contract grant number: PB94-0741
Contract grant sponsor: Xunta de Galicia (XUGA)
Contract grant number: 38305A94
Contract grant sponsor: Universidad de Vigo
1999 John Wiley & Sons, Inc.
᭧
CCC 0538-8066/99/010073-10

7
4
PAZO-LLORENTE ET AL.
highly reactive phenylcation that reacts rapidly and
with very low selectivity with available nucleophiles
[12]. In nonaqueous solvents, products associated with
free radicals, for example, biphenyl derivatives and
ϩ
replacement of 9N2 by 9H, are often observed,
especially when electron-withdrawing groups are
present on the arenediazonium ion [11–13].
The present article describes part of a recent and
on-going investigation of the effects of added electro-
lytes and other compounds on dediazoniation rates and
product yields of arenediazonium ions employing a
novel, recently reported, methodology [14] that allows
simultaneous determination of dediazoniation product
yields, rate constants for the formation of all dedi-
azoniation products, and, indirectly, the rate constant
for the disappearance of the diazonium salt. To expand
the scope of its applicability we examined the kinetics
and mechanism of the dediazoniation of 2- and 3-
methylbenzenediazonium salts (OMBD and MMBD,
respectively) in water. Few references about activation
parameters of dediazoniations in water [15] can be
found in the literature, but a number of them can be
found in other solvents [16–19], so we have also de-
termined their activation parameters.
Scheme 1 Basic representation of reactions of arenedi-
azonium ions. (A) Dediazoniation; (B) nucleophilic addi-
tion.
substituents increase their electrophilicity, meanwhile,
electrodonating substituents decrease it. Therefore, the
coupling components must be substances that possess
structures able to build up very high electron densities
at one or more carbon atoms. This requirement is usu-
ally achieved by the presence of hydroxy or amino
groups, which increases the C-nucleophilicity of the
coupling component. Dediazoniations of aromatic di-
azonium ions involve a number of mechanisms lead-
ing to a wide range of products [11,12]. They can take
place with a variety of nucleophiles, Scheme 1A,
via both spontaneous, for example, the Griess
EXPERIMENTAL
Instrumentation
Ϫ
Ϫ
Ϫ
Ϫ
(
Y ϭ Cl , Br , I ), Schieman (Y ϭ F ), and cata-
UV-VIS spectra and some kinetic experiments were
obtained 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 that in-
cluded 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) reverse-phase column (25-cm length,
Ϫ
Ϫ
lyzed, for example, the Sandmeyer (Y ϭ Cl , Br ,
CN ) reactions. They are believed [11,12] to occur by
Ϫ
either heterolytic (Scheme 2A), or, in the presence of
an electron donor, D, homolytic pathways (Scheme
2
B). In the absence of catalysts, with weakly basic
nucleophiles, in the presence of O , in aqueous acid,
2
and in the dark, dediazoniations are believed to pro-
ceed via rate determining loss of N , generating a
2
Scheme 2 Simplified dediazoniation mechanisms. (A) Heterolytic; (B) homolytic.

2
- AND 3-METHYLBENZENEDIAZONIUM TETRAFLUOROBORATE
75
4
.6-mm internal diameter, and 5-␮m particle size) us-
ing a mobile phase of 65/35 v/v MeOH/H O contain-
2
Ϫ4
ing 10 M HCl. The injection volume was 25 ␮L in
all runs, and the UV detector was set at 220 nm. pH
was measured using a previously calibrated Metrohm
7
13 pH-meter.
Materials, Reagents, and Solutions
Reagents were of the maximum purity available and
were used without further purification. Cresols, ArOH,
chlorotoluenes, ArCl, copper (II) chloride (99.999%),
and the reagents used in the preparation of diazonium
salts (as tetrafluoroborates) were purchased from Al-
drich. 2-Naphthol-6-sulfonic acid, sodium salt (2N6S)
was purchased from Pflatz & Bauer. Other materials
employed were from Riedel de Haen. All solutions
were prepared by using Milli-Q grade water.
Diazonium salt preparation was done following an
anhydrous method [20] and stored in the dark at low
temperature to minimize its decomposition. Their
stock solutions were prepared dissolving the appro-
priate amount of the diazonium salt in aqueous HCl to
minimize diazotate formation [12] and to give final
Ϫ4
concentrations of about 1 ϫ 10 M and [HCl] ϭ
Ϫ3
3
.6 ϫ 10 M. Stock solutions were generally used
immediately or within 90 min with storage in an ice
bath to minimize decomposition.
Figure 1 Beer law plot OMBD and MMBD. Experimental
conditions: [HCl] ϭ 0.01 M, ٗ ␭ ϭ 260 nm, ᭹ ␭ ϭ
3
20 nm, ᭺ ␭ ϭ 247 nm, ᭡ ␭ ϭ 310 nm.
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 eq.
Ϫ
1
Ϫ1
yielding ⑀260 ϭ 9664 Ϯ 137 M cm
and ⑀320
ϭ
Ϫ1
Ϫ1
1
8
906 Ϯ 46 M cm (OMBD) and ⑀247 ϭ 4674 Ϯ
Ϫ
1
Ϫ1
Ϫ1
Ϫ1
2 M cm
and
⑀310 ϭ 1502 Ϯ 26 M cm
(1) using a commercial nonlinear least-squares
(MMBD).
method, where M is the measured magnitude, either
absorbance or yields. (The word “yield” will be used
to represent percent yield here and throughout the text,
except in figures and equations where we will employ
the symbol “Y”).
Preliminary HPLC experiments showed that only
two decomposition products are formed: ArOH and
ArCl. Calibration curves for converting HPLC peak
areas into concentrations were obtained simulta-
neously for all dediazoniation products, ArOH and
ArCl, by employing commercial samples dissolved in
solutions of similar composition to those used in the
HPLC analysis of dediazoniation products (see be-
low). The equations used to convert peak areas into
concentrations and their typical retention times
ln(M Ϫ M ) ϭ Ln(M Ϫ M ) Ϫ k t
(1)
t
ϱ
o
ϱ
obs
All runs were done at T ϭ 35 Ϯ 0.1ЊC, except
when investigating the influence of temperature, with
diazonium salts as the limiting reagent.
9
(tr/min) were: area ϭ 8.11 ϫ 10 [OMB9OH]
9
Spectrophotometric kinetic data were obtained by
following the disappearance of diazonium ion at an
appropriate wavelength to minimize interferences
mainly by chlorocuprate (II) complexes. The Beer’s
law plots (Fig. 1) for aqueous MMBD and OMBD
(t ϭ 6.1); area ϭ 8.07 ϫ 10 [OMB9Cl] (t ϭ
r
r
9
27.4); area ϭ 8.62 ϫ 10 [MMB9OH] (t ϭ 5.8);
and area ϭ 8.90 ϫ 10 [MMB9Cl] (t ϭ 29.0).
r
9
r
Yields of dediazoniation products were obtained from
the ratio of the dediazoniation product concentration,
[Analyte], and the initial diazonium salt concentration,
Ϫ
4
solutions up to 2.00 ϫ 10 M are linear (cc. ϭ 0.999)

7
6
PAZO-LLORENTE ET AL.
[DS], estimated by weight, eq. (2).
Influence of CuCl on k
2 obs
[
Analyte]
The effect of CuCl concentration on kobs was studied
2
Y ϭ
(2)
by keeping the total chloride ion concentration con-
[DS]
Ϫ
stant ([Cl ] ϭ 1.0 M) to hold the relative amount of
tot
Chromatographic kinetic data for all dediazoniation
chloro complexes in the medium constant [23–25].
products were obtained following a published proce-
dure [14]. Dediazoniation was quenched at convenient
times with an aliquot of a stock quenching solution
prepared by dissolving the coupling reagent 2-naph-
thol-6-sulphonic acid, sodium salt (2N6S), with an ap-
proximately 20-fold excess of 2N6S after mixing, in
water containing TRIS buffer (C ϭ 0.05 M) to give a
final pH of about pH ϭ 8 after mixing. This pH was
chosen to maximize the rate of azo dye formation
kobs values (Table I) are not affected by changing
Ϫ3
Cu(II) concentration from [CuCl ] ϭ 0–50 ϫ 10
2
M, and they are virtually identical to those obtained in
NaCl or HCl. The effect of higher CuCl concentra-
2
tions could not be studied by UV-VIS spectroscopy
because of the absorbance of Cu(II) complexes
[23,24].
[
10,21] because phenoxide ions are much more reac-
tive than their parent phenols [11], but as pH increases,
the competing reaction of arenediazonium ions with
OH to form diazotates becomes significant [21,22].
Estimates of kobs by HPLC and Quenching
Yields of ArOH, ArCl and their total (Y ϭ
T
Ϫ
YArOH ϩ YArCl) as a function of time were determined
The use of a coupling reaction to stop the dediazon-
iation reaction requires that its rate be faster than the
dediazoniation rate [14]. Auxiliary experiments done
by following azo dye formation spectrophotometri-
cally shows that under our experimental conditions the
coupling reaction is essentially over in the time of mix-
ing reagents. All samples were analyzed in duplicate,
being the relative standard deviation of the peak areas
was less than 2%.
by HPLC analysis of the products. Results in Figure
2(A) (OMBD) and 2(B) (MMBD) are representative.
In all cases, we obtained only two dediazoniation
products: ArOH and ArCl. Only yields of ArOH were
used to calculate kobs because products are formed
competitively and because conversion to ArOH is al-
most quantitative in the absence of CuCl and is re-
2
duced only slightly by added CuCl (see later). Figure
2
2
(A) and (B) also shows the first-order plots according
We have also estimated kobs for the disappearance
of diazonium ions by adding sufficient concentrated
Ϫ4
Ϫ1
to eq. (1). kobs values were kobs ϭ 10.13 ϫ 10
s
Ϫ4
Ϫ1
(OMBD) and kobs ϭ 7.80 ϫ 10
s
(MMBD),
HCl dropwise to each volumetric flask to give a final
equal, within experimental error (Ͻ5%), to those ob-
tained spectrophotometrically.
ϩ
[
H ] Ϸ 0.2 M, and measuring the absorbance of each
solution at ␭max of the corresponding azo dyes:
␭max ϭ 529 nm (OMBD), ␭max ϭ 484 (MMBD). The
added HCl ensures that only the protonated form of
the azo dye is present, and that the measured absorb-
ance is directly proportional to the concentration of
the azo dye and, therefore, the concentration of un-
reacted diazonium salt.
Observed rate constants for loss of arenediazonium
ions were also obtained from the variation of the ab-
sorbance of the azo dye with time. We chose as rep-
resentatives the results shown in Figure 3, obtained at
different temperatures for OMBD (A) and MMBD
(B). kobs values are the same, within experimental er-
ror, to those following diazonium salt loss spectropho-
tometrically (Table I and Fig. 4) and ArOH formation
chromatographically (Fig. 2).
RESULTS
Influence of NaCl and HCl on kobs
The effects of NaCl and HCl on kobs were determined
by monitoring the decrease in absorbance of arenedi-
azonium ions with time at different NaCl and HCl con-
centrations (Table I) and fitting the kinetic data to eq.
Effect of Added Electrolytes on Product
Distribution
The effects of HCl, NaCl, and CuCl on product yields
2
(
1). Table I shows that kobs is not affected by changing
was determined by analyzing conveniently prepared
samples at infinite time, i.e., after dediazoniation was
over. Table II shows that conversion to products is
essentially quantitative. Only two products are
formed9ArOH and ArCl9ArOH being the major
product. As shown, changes in electrolyte concentra-
tion have no significant influence on product yields in
the range of concentrations employed.
NaCl concentration or the acidity of the medium in the
concentration ranges employed ([NaCl] ϭ 0–1.0 M,
[HCl] ϭ 0–1.0 M), yielding average values of
Ϫ4
Ϫ1
kobs ϭ 10.31 ϫ 10
1
ment with the ones in the literature [15] obtained by
N evolution.
s
(OMBD) and kobs ϭ 8.04 ϫ
Ϫ4
Ϫ1
0
s
(MMBD). These values are in good agree-
2

2
- AND 3-METHYLBENZENEDIAZONIUM TETRAFLUOROBORATE
77
Table I Effects of Added HCl, NaCl, and CuCl on k for Dediazoniation of OMBD and MMBD at T ϭ 35ЊC
2
obs
4
3
4
Ϫ1
Run
10 [OMBD]/M
[HCl]/M
[NaCl]/M
10 [CuCl ]/M
10 k /s
o
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
2.11
2.11
2.11
2.11
2.11
2.11
2.11
2.11
2.11
2.31
2.31
2.31
2.31
2.31
2.31
0
0.1
0.5
0.7
—
—
—
—
—
0.1
0.3
0.5
0.7
1.0
1.0
1.0
1.0
1.0
1.0
—
—
—
—
—
—
—
—
—
10.19
9.90
9.50
10.42
10.41
10.71
11.30
10.56
10.23
10.86
9.47
1.0
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
1
1
1
1
1
1
—
2.02
4.04
6.06
8.08
10.00
10.71
11.35
9.40
10.33
4
3
4
Ϫ1
Run
10 [MMBD]/M
[HCl]/M
[NaCl]/M
10 [CuCl ]/M
10 k /s
2
o
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.10
0.50
1.00
5.00
10.00
50.00
7.94
7.97
8.10
8.08
8.11
8.17
8.12
8.05
7.92
7.88
7.81
8.00
7.99
8.08
8.02
7.96
7.92
8.30
0.01
0.4
0.6
0.8
1.0
—
—
—
—
—
0.01
0.01
0.01
0.01
0.01
0.01
0.01
—
0.2
0.4
0.6
0.8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1
1
1
1
1
1
1
1
1
Influence of Temperature on kobs
Activation Parameters
:
Table III shows the activation energy and the ac-
tivation parameters obtained; values of ⌬H are rela-
tively high compared with those for bimolecular re-
actions, [26,27] and the entropic term is substantially
positive. For the sake of comparison, we have included
kobs values at one fixed temperature (T ϭ 35ЊC).
#
Experimental activation parameters were determined
by measuring kobs at different temperatures. To com-
pare values, we have also obtained activation param-
eters for p-methylbenzenediazonium ion, PMBD. Fig-
ure 4 shows the corresponding Arrhenius plots.
Activation parameters were obtained according to the
theory of absolute rates by means of eq. (3), where kB
and h are the Boltzmann and Planck constants, re-
spectively.
DISCUSSION
HPLC data (Table II and Figs. 2 and 3) show that
quantitative conversion to products is achieved, and
only two dediazoniation products are formed: ArOH
and ArCl. No other peaks that might be attributed to
⌬S# ⌬H#
kobs
k
B
Ln
ϭ Ln
ϩ
Ϫ
(3)
ͩT ͪ ͩ ͪ
h
R
RT

7
8
PAZO-LLORENTE ET AL.
literature for other arenediazonium ions like benzene-
Cl
diazonium [29], S ϭ 3, p-methylbenzene diazo-
W
Cl
nium [14], SW ϭ 1.7–3.4, or 2,4,6- trimethylben-
Cl
zene diazonium [2], SW ϭ 4. These rather constant
selectivity values support the heterolytic mechanism.
Spectrophotometric kinetic data (Table I) indicate
that dediazoniation rate is not affected by added elec-
trolytes like HCl, NaCl, or CuCl , as found for p-meth-
2
ylbenzenediazonium ion [14]. HPLC kinetic data
(Figs. 2 and 3), indicate that the rate of formation of
ArOH is the same as that of disappearance of diazo-
nium salt. All these results are consistent with a
D ϩ A mechanism being the slow step the forma-
N
N
tion of a highly reactive aryl cation intermediate,
which shows a very low selectivity towards different
nucleophiles (Scheme 2A).
Analysis of activation parameters indicate that en-
thalpies of activation are as high as in many unimo-
lecular reactions of alicyclic and aliphatic compounds
[27,30]. These values contrast with those usually
Figure 2 Variation of o- and m-cresol yields (᭺) with time
obtained from quenching experiments and ln plots (᭹).
Ϫ4
Experimental conditions: [OMBD] ϭ 3.11 ϫ 10 M,
[
1
3
HCl] ϭ 0.01 M, [NaCl] ϭ 1.0 M, T ϭ 35ЊC; [MMBD] ϭ
.0 ϫ 10 M, [HCl] ϭ 0.01 M, [NaCl] ϭ 1.0 M, T ϭ
5ЊC.
Ϫ4
radical pathways, i.e., Ar-Ar, Ar-N ϭ N-Ar, Ar-H,
were detected. The absence of reduction or isomerized
products is consistent with the heterolytic mechanism
(Scheme 2A). Yields of dediazoniation products (Ta-
ble II) remain essentially constant on changing elec-
trolyte concentrations within the concentration range
employed, suggesting a very low selectivity. The se-
lectivity of arenediazonium ions towards nucleophiles
Ϫ
like H O and Cl can be defined as
2
(
YArCl)[H O]
2
SWCl
ϭ
(4)
Ϫ
(YArOH)[Cl ]
Figure 3 Typical plots of the variation of absorbance
᭺) for formation of the azo dye (quenching experiments)
vs. time and ln plots (᭹). Experimental conditions:
(
For OMBD and MMBD, using [H O] ϭ 54.5 M in
2
Cl
1
2
.0 M NaCl [28] selectivity values are S
.7(1.0 M NaCl) and SW ϭ 2.6(1.0 M NaCl), respec-
ϭ
Ϫ4
W
[OMBD] ϭ 3.11 ϫ 10 M, [HCl] ϭ 0.01 M, [NaCl] ϭ
Cl
Ϫ4
1.0 M, T ϭ 35ЊC; [MMBD] ϭ 1.0 ϫ 10 M, [HCl] ϭ
0.01 M, [NaCl] ϭ 1.0 M, T ϭ 30ЊC.
tively. These values are very similar to the ones in the

2
- AND 3-METHYLBENZENEDIAZONIUM TETRAFLUOROBORATE
79
#
tribute significantly to ⌬G . Solvation is energy re-
#
leasing; consequently, ⌬H values for those unimo-
lecular reactions are not usually high, and they are
particularly solvent dependent. Dediazoniation, as de-
scribed in Scheme 2A, suggest that formation of the
aryl cation does not involve separation of charge but
its redistribution; thus, the parent arenediazonium ion
and the aryl cation polarizes the solvent to a similar
extent; therefore, the gain of entropy is not compen-
sated and dediazoniations show relatively high ⌬H#
values, showing lower solvent dependence [12] than
those unimolecular reactions: in 19 solvents [33] the
rates of heterolytic dediazoniation vary by a factor of
only 9.
Solvolytic unimolecular reactions can exhibit both
#
#
positive and negative ⌬S values [31,34]. Positive ⌬S
values, as we have found, suggest that the transition
state has a greater structural freedom than reactants, in
line with the reported positive volumes of activation,
#
⌬
V , for a number of dediazoniations in different sol-
vents [18,35–37], providing support for the D ϩ A
n
n
#
mechanism. On the other hand, because ⌬S values are
positive, they compensate the large enthalpy term,
making dediazoniations proceed at considerable rate
compared with other unimolecular reactions, as found
for a number of dediazoniations [12,17–19].
Substituents in the aromatic ring have marked ef-
fects on the stability of arenediazonium ions [11,12].
Substituent effects are not understandable on the basis
of the Hammett equation, [12,29,38] because most
para and meta substituents, including electronwith-
Figure 4 Arrhenius plot for dediazoniations of OMBD
(᭺), MMBD (᭹), and PMBD (ٗ).
drawing (like p ϭ NO ) and electron-donating groups
2
(
like 9N(CH ) , 9CH ) show a marked stabilizing
3 2 3
found for bimolecular reactions, which are substan-
tially lower because the breaking of old bonds, which
requires energy, and the formation of the new ones,
which releases energy, are highly concerted and usu-
ally synchronous, [27] thus, ⌬H values suggest a tran-
sition state that has undergone bond breaking with
little compensating bond making.
effect, but they fit the Swain-Lupton [39] equation,
which separates the resonance and inductive effects
[9,12,40]. As shown in Table III, the effects of the
methyl groups on dediazoniation rates are surprising,
though modest. In the ortho and meta positions,
methyl is rate enhancing; in the para position it is rate
retarding compared with the value for benzenediazon-
ium ion.
#
Entropies of activation are positive as in some un-
imolecular S 1 reactions and contrast with those for
Electronic substituent effects [9] can be classified
into those associated with their polarity (inductive,
field) and those associated with their ability to transfer
charge (resonance). The methyl group behaves as an
electron-releasing group in its interaction with an elec-
tron-deficient site, which is able to attract electron den-
sity from the polarizable alkyl group through the ␴
bonds [27,30], an effect that decreases with distance.
On the other hand, if the methyl group is conjugated
to an electron-deficient site through an aromatic sys-
tem, then the electron-deficient site can attract charge
from the carbon–hydrogen bonds of the methyl group
through the ␲ system [31] (hyperconjugation). Direct
N
bimolecular reactions such as S 2 and cycloadditions
N
[
31], which are largely negative, typically ranging be-
Ϫ1
Ϫ1
tween Ϫ40 and Ϫ160 J mol deg . In a number of
solvolytic unimolecular reactions [30,32], a neutral
molecule dissociates to give an ion- pair; thus, the
transition state shows a significant charge separation
that significantly polarizes the solvent molecules com-
pared with the parent substrate. Molecular vibrations
of the substrate are lost, i.e., the system gains entropy,
which is compensated by the loss of entropy due to
restricted motion of the solvating molecules; therefore,
in the overall process the entropy term does not con-

8
0
PAZO-LLORENTE ET AL.
Table II Effects of Added HCl, NaCl, and CuCl on Dediazoniation Product Yields
2
04[OMBD]/M
[HCl]/M
[NaCl]/M
10 [CuCl ]/M
2
YOMB-OH
YOMB-Cl
1
2
3
3
3
3
3
3
3
3
3
3
3
3
.3
.3
.3
.3
.3
.3
.3
.3
.3
.3
.3
.3
0.02
0.02
0.02
0.02
0.30
0.50
0.70
1.00
0.02
0.02
0.02
0.02
0.3
0.5
0.7
1.0
—
—
—
—
1.0
1.0
1.0
1.0
—
—
—
—
—
—
—
—
4.0
6.0
8.0
10.0
95.67
92.41
90.93
90.41
98.62
96.44
95.80
90.67
93.42
94.93
95.00
95.22
2.8
3.6
2.4
4.4
—
2.5
3.2
4.1
4.1
3.9
3.9
4.0
04[MMBD]/M
[HCl]/M
[NaCl]/M
10 [CuCl ]/M
3
YMMB-OH
YMMB-Cl
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
0.01
0.01
0.01
0.01
0.01
0.01
0.04
0.06
0.08
0.01
0.01
0.01
0.01
0.2
0.4
0.6
0.8
1.0
—
—
—
—
1.0
1.0
1.0
1.0
—
—
—
—
—
—
—
—
—
7.0
9.0
10.0
30.0
94.83
94.01
91.08
89.90
88.90
95.30
96.40
97.90
96.34
92.60
90.50
92.47
93.05
—
2.2
2.7
3.5
4.3
—
—
—
—
4.9
6.2
4.3
4.5
resonance interaction with the reaction site takes place
than the parent methylbenzenediazonium ion (Scheme
2A). These arguments are consistent with the observed
kinetic behavior of other para or meta-substituted ar-
enediazonium ions with ␲-donating capability like
9OH (␴ ϭ Ϫ1.25 [41]) or 9OCH3 (␴ ϭ
Ϫ0.61 [41]): experimental rate constant values for de-
ϩ
only in the para position [31]; thus, high negative ␴
ϩ
values are reported for the para-methyl group (␴ ϭ
ϩ
Ϫ0.25 [41] or ␴ ϭ Ϫ0.31 [31]) compared with
ϩ
ϩ
ϩ
those for meta-methyl group (␴ ϭ Ϫ0.06 [31]).
Therefore, para-methyl stabilizes more the substrate
(p-methylbenzenediazonium ion) than the intermedi-
diazoniations with para 9OH or 9OCH substitu-
3
ate aryl cation, meanwhile the ortho and meta methyl
ents are lower than those obtained when substituents
groups stabilize more the corresponding aryl cation
are placed in the meta position [15,35]. These results
Table III Activation Parameters for Benzenediazonium Ion and Its Three Methyl Derivatives and Rate Constants at
a Fixed Temperature (T ϭ 35ЊC)
Ta Range/ЊC
4
Ϫ1
Ϫ
15
Ϫ1
Ϫ1
#
Ϫ1
⌬S /J mol K 1
#
Ϫ
1
Ϫ
10 k /s
10 A/s
EA/kJ mol
⌬H /kJ mol
3
5
BDa
2.0
10.31
8.04
0.23
(3.5)
2.1 (3.5)
2.8 (1.3)
1.7 (0.7)
(113)
108.2 (108)
109.5 (107)
112.4 (114)
OMBD
MMBD
PMBD
26.5–41.0
26.5–41.0
40.0–60.0
106 Ϯ 6
107 Ϯ 5
110 Ϯ 4
40.4 Ϯ 10
43.0 Ϯ 13
21.6 Ϯ 8
a
BD stands for benzenediazonium. Values for BD and those in parentheses are taken from ref. 15.

2
- AND 3-METHYLBENZENEDIAZONIUM TETRAFLUOROBORATE
81
are also in agreement with a recent theoretical elec-
6. Esteve-Romero, J. S.; Simo-Alfonso, E. F.; Garcia-Al-
varez-Coque, M. C.; Ramis-Ramos, G. Talanta 1993,
40, 1711.
. Simo-Alfonso, E. F.; Ramis-Ramos, G.; Garcia-Alva-
rez-Coque, M. C.; Esteve-Romero, J. S. J. Chromatogr
B 1995, 670, 183.
tron-density analyses [9] that indicates that a para
ϩ
9
NH2 group (␴ ϭ Ϫ1.61 [41]) enhances the
7
8
C9N bonding.
. Esteve-Romero, J. S.; Alvarez Rodriguez, L.; Garcia Al-
varez-Coque, M. C.; Ramis-Ramos, G. Analyst 1994,
CONCLUSIONS
1
19, 1381.
Rates of solvolysis of two diazonium ions have been
measured in water over a range of temperatures. Our
results show that dediazoniation of OMBD and
MMBD proceed heterolyticly, because only two prod-
ucts are formed, ArOH and ArCl, and because of the
absence of products associated with the radical path-
way. Observed rate constants for product formation
are the same as the rate constants for the decomposi-
tion of diazonium salt. Both product yields and rate
9. Glaser, R.; Horan, C. J.; Zollinger, H. Angew Chem Int
Eng Ed 1997, 36, 2210.
0. Zollinger, H. Color Chemistry; VCH; Utrech, The Neth-
erlands, 1991.
1. Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Com-
pounds; 3rd ed.; Edward Arnold; London, 1985.
2. Zollinger, H. Diazo Chemistry I, Aromatic and Heter-
oaromatic Compounds; VCH: Wein, 1994.
3. Zollinger, H. Dediazoniations of Arenediazonium Ions
and Related Compounds; J Wiley & Sons: New York,
1
1
1
1
constants are essentially independent of acidity (up to
1
983.
Ϫ
1
1
.0 M) and nucleophile concentration (up to [Cl ] ϭ
1
4. Garcia-Meijide, M. C.; Bravo-Diaz, C.; Romsted, L. S.
Int J Chem Kinet 1998, 30, 31.
.0 M) in agreement with literature values determined
for other diazonium ions. The low selectivity of the
15. Crossley, M. L.; Kienle, R. H.; Benbrook, C. H. J Am
Chem Soc 1940, 62, 1400.
Ϫ
reaction toward Cl vs. H O is consistent with the for-
2
mation of a highly reactive aryl cation intermediate.
The effects of methyl groups are different: p-methyl
decrease the rate of dediazoniation meanwhile o- and
m-methyl enhances it compared with the value for
benzenediazonium ion. Enthalpies of activation are
high, and entropies of activation are positive, making
dediazoniations proceed at a considerable rate com-
pared with typical unimolecular reactions.
16. DeTar, D. F.; Kosuge, T. J Am Chem Soc 1958, 80,
6
072.
1
7. Ishida, K.; Kobori, N.; Kobayashi, M.; Minato, H. Bull
Chem Soc Jpn 1970, 43, 285.
1
1
2
2
8. Kuokkanen, T. Finn Chem Lett 1981, 52.
9. Kuokkanen, T. Finn Chem Lett 1981, 81.
0. Doyle, M. P., Bryker, W. J. J Org Chem 1979, 44, 1572.
1. Zollinger, H.; Wittwer, C. Helv Chim Acta 1952, 35,
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209.
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2
2. Sterva, V. Diazonium-Diazo Equilibrium; J. Wiley &
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3. Schwing-Weill, M. J. Bull Soc Chim France 1973, 3,
This article is dedicated to Prof. Dr. Julia Franco-Hern a´ ndez.
Thank you, Julia. This work was supported by the following
institutions: Ministerio de Educaci o´ n y Cultura (DGICYT,
PB94-0741), Xunta de Galicia (XUGA 38305A94), and
Universidad de Vigo. We also want to thank reviewers for
improving the manuscript with their comments and sugges-
tions.
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