Butanolysis of 4-methylbenzenediazonium ions in binary n-BuOH/H 2O mixtures and in n-BuOH/SDS/H2O reverse micelles. Effects of solvent composition, acidity and temperature on the switch between heterolytic and homolytic dediazoniation mechanisms

Article abstract of DOI:10.1039/c0ob00143k

We investigated the effects of solvent composition, acidity and temperature on the switch between heterolytic and homolytic mechanisms in the course of the butanolysis of 4-methylbenzenediazonium (4MBD) ions in binary BuOH/H ₂O mixtures and in reverse micelles, RMs, composed of n-BuOH, H ₂O and sodium dodecyl sulfate, SDS, by employing a combination of spectrometric (UV/vis) and chromatographic (HPLC) techniques. In reaction mixtures with high n-BuOH percentages, S-shaped variations of k_obs with acidity, defined hereafter as -log([HCl]), are obtained with rate enhancements of up to ～370-fold on going from -log([HCl]) = 2 to 6, with inflection points at -log[HCl] ～ 4. HPLC analyses of the reaction mixtures show that the substitution product 4-cresol, ArOH and the reduction product toluene, ArH, are formed competitively. The variation of their yields with acidity is also S-shaped, so that at high acidities (-log[HCl] < 3) only traces of ArH are detected but on lowering the acidity, the reduction product ArH becomes predominant The largest variations of k_obs and of the product yields with acidity are found in the -log[HCl] = 3-5 range, suggesting that a turnover in the dediazoniation mechanism takes place under acidic conditions. The results can be interpreted in terms of two competitive reaction pathways, one heterolytic, involving a rate-determining formation of an extremely reactive aryl cation that traps the nucleophiles available in its solvation shell leading to the formation of substitution products (D_N + A_N mechanism) and a second route where the BuOH reacts with 4MBD to yield an unstable O-adduct of the type Ar-NN-O-R (diazo ether) in a rapid pre-equilibrium step that initiates a radical process leading to the formation of the reduction product ArH (O-coupling mechanism). The results illustrate how the heterolytic and homolytic mechanisms can be switched by just changing the acidity of the solution. Kinetic analyses of the variations of k_obs with acidity at different temperatures allowed us to separate k_obs into the components for the heterolytic pathway, k_HET, and that for the homolytic one, k_HOM, to determine relevant thermodynamic parameters for both reaction pathways and for the equilibrium constant K for the formation of the O-adduct Ar-NN-O-R.

Full text of DOI:10.1039/c0ob00143k

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Butanolysis of 4-methylbenzenediazonium ions in binary n-BuOH/H2O
mixtures and in n-BuOH/SDS/H2O reverse micelles. Effects of solvent
composition, acidity and temperature on the switch between heterolytic and
homolytic dediazoniation mechanisms
Alejandra Ferna´ndez-Alonso, Ma Jose´ Pastoriza Gallego and Carlos Bravo-D´ıaz*
Received 19th May 2010, Accepted 22nd July 2010
DOI: 10.1039/c0ob00143k
We investigated the effects of solvent composition, acidity and temperature on the switch between
heterolytic and homolytic mechanisms in the course of the butanolysis of 4-methylbenzenediazonium
(4MBD) ions in binary BuOH/H2O mixtures and in reverse micelles, RMs, composed of n-BuOH, H2O
and sodium dodecyl sulfate, SDS, by employing a combination of spectrometric (UV/vis) and
chromatographic (HPLC) techniques. In reaction mixtures with high n-BuOH percentages, S-shaped
variations of kobs with acidity, defined hereafter as -log([HCl]), are obtained with rate enhancements of
up to ~370-fold on going from -log([HCl]) = 2 to 6, with inflection points at -log[HCl] ~ 4. HPLC
analyses of the reaction mixtures show that the substitution product 4-cresol, ArOH and the reduction
product toluene, ArH, are formed competitively. The variation of their yields with acidity is also
S-shaped, so that at high acidities (-log[HCl] < 3) only traces of ArH are detected but on lowering the
acidity, the reduction product ArH becomes predominant The largest variations of kobs and of the
product yields with acidity are found in the -log[HCl] = 3–5 range, suggesting that a turnover in the
dediazoniation mechanism takes place under acidic conditions. The results can be interpreted in terms
of two competitive reaction pathways, one heterolytic, involving a rate-determining formation of an
extremely reactive aryl cation that traps the nucleophiles available in its solvation shell leading to the
formation of substitution products (DN + AN mechanism) and a second route where the BuOH reacts
with 4MBD to yield an unstable O-adduct of the type Ar–N N–O–R (diazo ether) in a rapid
pre-equilibrium step that initiates a radical process leading to the formation of the reduction product
ArH (O-coupling mechanism). The results illustrate how the heterolytic and homolytic mechanisms can
be switched by just changing the acidity of the solution. Kinetic analyses of the variations of kobs with
acidity at different temperatures allowed us to separate kobs into the components for the heterolytic
pathway, kHET, and that for the homolytic one, kHOM, to determine relevant thermodynamic parameters
for both reaction pathways and for the equilibrium constant K for the formation of the O-adduct
Ar–N N–O–R.
To overcome the solubility problem, we propose the use of
Introduction
reverse micelles as reaction media. Reverse micelles (RMs),
Solvolytic studies in mixed solvents have an advantage of providing
Scheme 1, are thermodynamically stable aggregates in which
the opportunity to vary continuously the reaction conditions
nanoscale droplets of a polar liquid, usually water, are surrounded
compared to the more abrupt changes usually found when chang-
by a surfactant layer and distributed uniformly in a nonpolar
ing the nature or position of substituents. However, solubility
continuous phase. The centre of the RMs, the so-called water pool,
problems may arise preventing systematic studies on individual
Scheme 1, provides a unique reaction site because of the “tailored”
substrates over the whole composition range. This is the case
sizes that can be achieved with these systems.1–4 The water in the
of binary water–alcohol mixtures, where the short chain alkyl
water pool may have different properties depending on the ratio
alcohols (CnH2n+1OH, n < 3) such as MeOH or EtOH, are
wo = [H2O]/[SURF], and can be used in place of polar organic
miscible with water at any proportion but alcohols with longer
solvents to carry out chemical reactions.5–7 Hence, the systems
alkyl chains (n ≥ 4) have limited solubility in water. For instance,
n-BuOH is partially soluble in water and only binary systems
containing percentages of n-BuOH ranging 0–~10% and ~90–
100% can be prepared, but pentanol is essentially insoluble at any
of the “surfactant–water–organic solvent” type, Scheme 1, find
useful application in preparative chemistry including chemical
transformations of water-insoluble substances, preparation of
nano-size particles and in bioorganic synthesis.5,8–10 RMs can
proportion.
also be considered as a model of biomembrane fragments and
can be seen as an approach to better understanding the role of
biomembrane environments in biocatalysis.11–14
The use of RMs as reaction media may have significant effects on
the chemical reactivity because such organized systems introduce
Universidad de Vigo, Facultad de Qu´ımica Dpt., Qu´ımica F´ısica, , 36200
Vigo, Spain. E-mail: cbravo@uvigo.es; Fax: 34 986 812556; Tel: 34 986
812303
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Table 1 Effect of the percentage of BuOH on the observed rate constants,
k
obs, for solvolyses of 4MBD in binary BuOH/H2O mixtures. [4MBD] =
1.2 ¥ 10-4 M, T = 50 ◦C, -log[HCl] = 2
% BuOH
104 kobs/s-1
0
3
6
90
93
96
98
2.27 0.01
2.46 0.01
2.58 0.01
7.87 0.03
7.51 0.02
6.99 0.02
7.71 0.02
same as that in pure water.42 Moreover, investigations on the
dediazoniation of structurally similar arenediazonium ions in
BuOH/SDS/H2O reverse micelles showed that they are mostly
located in the interfacial region of the reverse micelles,41,43 and
consequently 4MBD dediazoniation is expected to take place
primarily in the interfacial region of the RMs, Scheme 1, where a
mixture of water, SDS and BuOH is present.
Scheme 1 Basic representation of a reverse micelle showing the different
regions of the micellar solution: 1) water pool, 2) interface, 3) organic
phase—in the present work, n-BuOH.
new environments that may have large effects on the physical
properties of the substrate11,15–18 such as stabilization of ground or
excited states, acid–base and redox equilibria, and so forth. RMs
may also be very interesting for solvolytic studies and a number of
solvolytic reactions have been exploited to gain insights into the
properties of the RMs and those of the encapsulated water.14,19–22 In
addition, RMs allow preparation of reaction mixtures containing
a wide range of percentages of alcohol (n ≥ 4) in the system,11,23,24
and for instance, reverse micelles containing percentages of BuOH
in the range 40–90% can be easily prepared by adding different
amounts of sodium dodecyl sulfate, SDS.
Here we take advantage of the distinctive characteristics of
organized surfactant molecular assemblies and their unique solu-
bilization characteristics1,14,25–28 to expand our previous solvolytic
dediazoniation studies and, particularly, to further investigate
the switch between heterolytic and homolytic dediazoniations,
which can be achieved by just changing the acidity of the
reaction mixture.29–34 Solvolytic dediazoniations in BuOH–H2O
mixtures have not been studied over a wide composition range
so far,35 probably because of the mentioned solubility problems,
and so the use of reverse micelles allows one to overcome the
solubility problem by introducing a new reaction media and,
at the same time, the results can be compared with those in
binary BuOH/H2O mixtures to highlight the role of organized
systems in the target reaction. For this purpose, a combination of
spectroscopic (UV-VIS) and chromatographic (high performance
liquid chromatography, HPLC) techniques were used.
The reverse micelles were prepared by using wo = [H2O]/
[SURF] = 42.7, which allows changes in the percentage of BuOH
in the system in the range 45–90% BuOH, a range that cannot be
otherwise attained. The ratio wo = 42.7 was chosen because the size,
the shape, and some other relevant structural characteristics of
the reverse micellar aggregates formed are known36,37 and because
the properties of the water inside the water pool are the same
as those of bulk water.14,19,28 4MBD was chosen as substrate
because substantial knowledge on its spontaneous dediazoniation
in a number of alcohol–water mixtures38–41 and in normal SDS
micellar systems is available.42 Previous investigations have shown
that 4MBD is largely associated with normal SDS micelles
(Ks = 1390 M-1)42 and that its dediazoniation mechanism is the
Results
1
Solvolyses of 4MBD in binary BuOH/H2O mixtures
1.1 Effects of solvent composition on the observed rate con-
stants, kobs, and on the product distribution. Observed rate con-
stants, kobs, were determined spectrophotometrically by monitor-
ing the disappearance of 4MBD as indicated in the experimental
section, Table 1. kobs values increase steadily upon increasing the
BuOH content in the reaction mixture and an increase of ~4-fold is
obtained at 98% BuOH. This modest increase in kobs on increasing
[BuOH] is in keeping with that found in previous solvolytic
dediazoniations,38,39,44 and suggests that nucleophilic attack by
solvent is not involved in the rate-limiting step of the reaction,
consistent with the well known insensitivity of dediazoniations to
solvent changes.35,39,44,45 The kobs value in the absence of BuOH,
kobs = 2.27 ¥ 10-4 s-1 is in keeping with that reported in the
lite◦rature39 for the spontaneous dediazoniation of 4MBD at T =
60 C, bearing in mind its activation energy, Ea = 112 kJ mol-1.
HPLC analyses of the reaction mixtures indicate that at low
percentages of BuOH, the major dediazoniation product is ArOH
(>95%), but its yield goes down to ~30% at 96% BuOH with
a concomitant increase in the yield of ArOBu (observed as an
increase in the corresponding peak area). Only small amounts
(<10%) of the reduction product ArH were detected.
Although ArH is a minor product under our experimental
conditions, its formation was previously detected in solvolytic de-
diazoniations carried out under acidic conditions in the absence of
reductants, and its formation was explained in terms of a compet-
itive reaction pathway leading to the formation of highly unstable
diazo ethers of the type ArN N–OR, which initiates a radical
process.40,46 The finding of small amounts of the reduction prod-
ucts ArH in the course of the butanolyses of 4MBD can, therefore,
be envisaged as evidence of the potential existence of a competing
mechanism, and this possibility was further investigated.
1.2 Effects of acidity and temperature on the observed rate
constants, kobs, and on the product distribution. The effects of
acidity on kobs were investigated by employing two representative
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Table 2 Effects of acidity on kobs values for the decomposition of 4MBD
in a 3% binary BuOH/H2O mixture. [4MBD] = 1.2 ¥ 10-4 M, T = 50 ◦
C
-log[HCl]
104 kobs/s-1
6.00
5.00
4.00
3.00
2.00
1.00
0.50
0.25
2.45 0.01
2.56 0.01
2.64 0.01
2.73 0.01
2.75 0.01
2.79 0.01
2.84 0.01
2.8 0.01
reaction mixtures (3% and 90% BuOH). When employing the 3%
BuOH reaction mixture, a change in acidity (-log([HCl] = 0.25–6)
does not change kobs significantly, Table 2; however, when using
the 90% BuOH mixture, parallel changes in acidity lead to S-
shaped variations in kobs, as illustrated in Fig. 1. Note that the
largest variations in kobs are obtained in the -log[HCl] = 3–5 range,
compared with those in the -log[HCl] = 1–3 and in the -log[HCl] =
5–6 range.
The effects of temperature on kobs were also investigated and
similar sigmoideal profiles were obtained at all temperatures
investigated as shown in Fig. 1. At a given temperature, kobs
values increase ~10-fold on lowering the acidity from -log[HCl] =
2 to -log[HCl] = 6, and the extent of this increase seems to be
independent of temperature. At a given acidity -log[HCl] > 4, kobs
increases ~4-fold on increasing the temperature from T = 45 ◦C up
to T = 60 ◦C, however, the inflection point of the sigmoidal plots
seems to be very similar at any temperature.
We also analyzed the effects of acidity on the product distribu-
tion at two selected temperatures, and similar S-shaped variations
were found, Fig. 2. At high acidity, only small amounts of
ArH are obtained, and the heterolytic products are the major
dediazoniation products; however, on lowering the acidity, a
significant increase in the yield of the reduction product ArH
is detected with a concomitant decrease in the yields of the
heterolytic ones is obtained. A change in the temperature does
not seem to have a major effect on the product distribution nor on
the inflection points of the sigmoidal profiles, Fig. 2, as was found
for the variations of kobs with acidity, Fig. 1.
Fig. 2 Illustrative plots of the variation in the yields of the heterolytic
ArOH and reduction ArH dediazoniation products with acidity and
temperature for the decomposition of 4MBD in a binary 90 : 10 (v/v)
BuOH/H2O mixture. [4MBD] = 1 ¥ 10-3 M, ꢁ 50 ◦C, ᭺ 60 ◦C.
2
Solvolyses of 4MBD in reverse BuOH/SDS/H2O micelles.
2.1 Effects of solvent composition, acidity and temperature on
the observed rate constants, kobs, and on the product distribution.
The composition and some structural parameters of the reverse
micelles employed are indicated in Table 5. Fig. 3 illustrates
the variation of kobs with %BuOH at two selected acidities. At
high acidities (-log[HCl] = 1.7), kobs increases modestly upon
increasing the BuOH content of the system. At lower acidities
(-log[HCl] = 4.3), the obtained kobs values at a given %BuOH are
much higher than those at high acidity and the increase in kobs on
increasing the %BuOH is of ~6-fold, highlighting the enormous
effect of acidity on the reaction. These observations prompted
us to further investigate the effects of acidity on kobs and on the
product distribution.
Fig. 1 Effects of acidity and temperature on kobs for the decomposition
of 4MBD in a 90% BuOH/H2O binary mixture. [4MBD] = 1.2 ¥ 10-4 M,
ꢀ T = 45 ◦C, ꢁ T = 50 ◦C, ᭺ T = 55 ◦C, ᭹ T = 60 ◦C.
Fig. 3 Effects of the percentage of BuOH in BuOH/SDS/H2O reverse
micelles on kobs for the decomposition of 4MBD. ᭹ -log[HCl] = 1.7, ꢁ
-log[HCl] = 4.3.
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Fig. 4 Effects of acidity on kobs for the decomposition of 4MBD in reverse micelles at selected temperatures. A) T = 50 ◦C, ꢀ 45% BuOH (RM1), ᭹
61.9% BuOH (RM3), ꢁ 72.5% BuOH (RM5). B) % BuOH = 72.5, ꢀ T = 40 ◦C, ᭺ T = 45 ◦C, ꢀ T = 50 ◦C. [4MBD] ~ 1 ¥ 10-4 M.
Table 3 Values of kHET, kHOM and of the equilibrium constant K (average
values from kinetic and HPLC data), obtained by fitting the experimental
data to eqn 1, which can be derived from Scheme 3
Fig. 4 illustrates the effects of acidity on kobs for the solvolyses
of 4MBD in selected reverse micelles at constant temperature
(Fig. 4A) and in a selected reverse micelle at different temperatures
(Fig. 4B). In both cases, S-shaped variations are obtained. At
high acidities, kobs = 5.5 ¥ 10-4 s-1 (RM5, %BuOH = 72.5, T =
T/◦C
% BuOH
pK
104 kHET/s-1
104 kHOM/s-1
◦
Binary mixture
50 C) and increases ~370-fold on lowering the acidity down to
45
90
90
90
90
4.9 0.1
4.6 0.1
5.2 0.1
4.9 0.1
2.8 0.3
7.0 2.0
9.0 2.0
15.0 2.0
34
48
71
121
1
2
3
-log[HCl] = 6. When using RM1 (%BuOH = 45%) and RM3
(%BuOH = 62%), the change in kobs on lowering the acidity is
~175- and ~243-fold, respectively. Similar increases in kobs are
obt◦ained on changing the temperature from T = 40 ◦C up to
50 C, Fig. 4B, at a fixed %BuOH (RM5). Higher temperatures
were not investigated because rate constants become too high to
be measured and to minimize potential changes in the structure
of the reverse micelles due to the increase in the thermal energy.
We are aware that a change in the temperature may lead to some
change in the structural parameters of the reverse micelle. We did
not investigate this point and thus we cannot quantify the extent of
this potential change; however, if any change occurs, it must not be
kinetically significant because products are formed competitively,
and because the inflection points of the sigmoidal curves are the
same at any temperature, both in the binary mixtures and in reverse
micelles as we will show in Table 3.
50
55
60
3
Reverse micelles
40
50
45
50
50
72.5
72.5
72.5
45
5.0 0.1
4.8 0.1
5.0 0.1
4.6 0.1
4.9 0.1
4
1
670 30
1830 40
1320 70
730 80
1310 50
5.5 0.8
5.0
4.0 0.8
5.6 0.4
1
62
Fig. 5. At high acidity (-log [HCl] = 2.5) and low BuOH content,
the major dediazoniation product is ArOH. Upon increasing the
BuOH content, the yield of the reduction product ArH remains
practically constant. Note that at 90% BuOH, Fig. 5A, the yield
of ArH is the same as that obtained in binary mixtures at the same
acidity, Fig. 2. This contrasts with the variation in the yield of
ArOH, which decreases upon increasing [BuOH] because of the
competitive formation of the substitution product ArOBu.
We also analyzed the effects of the percentage of BuOH in
the RMs on the product distribution at two selected acidities,
Fig. 5 Effect of the %BuOH on product distribution for the decomposition of 4MBD in reverse micelle BuOH/SDS/H2O. log[HCl] = 2.5 (left) and
-log[HCl] = 4.3 (right). Solid lines drawn to aid the eye. ꢀ 4MBH and ᭹ 4MBOH. [4MBD] ~ 1 ¥10-3 M, T = 50 ◦C.
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On lowering the acidity (-log[HCl] = 4.3), Fig. 5B, the major
dediazoniation product becomes ArH, suggesting that a change in
the dediazoniation mechanism has taken place, and its yield seems
to decrease slightly upon increasing the BuOH content.
We further investigated the effect of acidity on the product
distribution in selected RMs. Fig. 6 shows sigmoidal variations in
the product distribution on changing the acidity of the solution,
at high acidities the major dediazoniation products are the
substitution ones, and their relative yield depends on the solvent
composition; however, at lower acidities, the major dediazoniation
product is the reduction one, ArH, and its yield is independent of
the solvent composition.
content, in contrast with the small changes obtained when using
reaction mixtures with low BuOH content, Table 2. Fig. 2 displays
the S-shaped variation of the product distribution with acidity,
showing that at high acidity, low yields of the reduction product
ArH are obtained but, on lowering the acidity, ArH becomes the
main dediazoniation product, suggesting a pH-dependent change
in the reaction mechanism. In both cases, the largest variations of
kobs and product yields with acidity are found in the -log[HCl] =
2–5 range, i.e., under acidic conditions.
S-shaped variations of kobs with acidity are usually observed in
reactions of acid–base pairs where both forms are attainable and
show different reactivities.47 Under our experimental conditions,
+
only two specimens may undergo acid–base processes, the ArN2
ions and BuOH. The pKa of BuOH is ~18 and the pKa value of
4MBD has been reported to be ~11,48 so it appears unlikely that
ArN2 ions react with BuO- or with OH- ions, as in an alkaline
+
medium,49,50 under our experimental conditions.
Analogous S-shaped variations of kobs and of the product
distribution were observed in previous solvolytic dediazoniations
and were interpreted in terms of the reaction mechanism shown in
Scheme 3, which hypothesizes two competitive reaction pathways,
(i) the thermal decomposition of the solvated ArN2+ ions through
the heterolytic DN + AN mechanism, leading to the formation of
substitution products (ArOH, ArOBu) and (ii) the formation of an
adduct between ArN2+ ions and BuOH in a rapid pre-equilibrium
step followed by its rate-determining homolysis, initiating a radical
process leading to the formation of the reduction products ArH,
Scheme 3. Similar mechanisms have been employed to interpret
the reactivity of arenediazonium ions with nucleophilic molecules
bearing –OH groups such as the antioxidants ascorbic acid
(vitamin C), gallic acid and methyl gallate, and –NH2 groups such
as the amino acids serine and glycine. The assumption of a rate-
limiting decomposition of the diazo ether is also consistent with
reported results for other O-coupling reactions,35,45,51,52 and was
probed experimentally in reactions of arenediazonium ions in the
presence of cyclodextrins.53,54
Fig. 6 Effect of the [HCl] on product distribution for the decomposition
of 4MBD in reverse micelle BuOH/SDS/H2O. [4MBD] ~ 8 ¥ 10-4 M, T =
50 ◦C. ꢁ 72.5%, ᭺ 61.9% and ꢀ 45% of BuOH.
Discussion
The results of binary mixtures, Table 1, show that kobs increases
modestly on increasing %BuOH from 0 up to 98% BuOH. A rate-
limiting nucleophilic attack of BuOH would lead to a substantial
change in kobs upon changing the BuOH concentration in the
reaction mixture, which is not observed. HPLC experiments show
that only small amounts of the formation of reduction product
ArH are formed. Hence, kinetic and HPLC results are consistent
with the well-established DN + AN dediazoniation mechanism, e.g.,
a rate-determining formation a highly reactive aryl cation which
traps any available nucleophile in its solvation shell, Scheme 2.
Scheme 3 Proposed competitive ionic and radical mechanisms for the
reaction of arenediazonium ions with alcohols under acidic conditions.
Scheme 2 Basic representation of the DN + AN dediazoniation mech-
anism showing the rate-limiting formation of an extremely reactive aryl
cation Ar+ that traps any available nucleophile in its solvation shell.35,39,44,45
From Scheme 3, eqn (1) can be derived where kHET and kHOM
are the rate constants for the heterolytic decomposition of ArN2
+
Fig. 1 shows that the sigmoidal variations of kobs with acidity
(Scheme 2) and for the decomposition of the diazo ether which
are obtained when employing reaction mixtures with a high BuOH
initiates the radical mechanism, respectively, with K standing for
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Table 4 Values of the half-lives and activation energies for the heterolytic (HET) and homolytic (HOM) dediazoniation processes for some
arenediazonium ions in different solvents.
Substituent
4-Me
Solvent composition
t1/2 (HET)/sa
t1/2 (HOM)/sa
Ea (HET)/kJ mol-1
Ea (HOM)/kJ mol-1
H2Ob
3450
1770
1971
359
1000
1200
~ 83000
15687
3648
~ 180000
—
—
—
—
35
144
4
—
1375
1824
—
2390
2980
110
119
109
116
103
—
119
—
—
—
—
—
77
74
76
—
—
—
—
71
74
70–30 TFE-H2Oc
99.5–0.5 MeOH–H2Ob
70–30 EtOH–H2Od
90–10 BuOH–H2Oe
72.5 BuOHf
4-NO2
4-Br
H2Og
80–20 MeOH–H2Oh
95–5 MeOH–H2Oh
H2Og,i
~ 100
~ 100
~ 100
25–75 MeOH–H2Oi
75–25 MeOH–H2Oi
—
a T = 50 ◦C. b From ref. 39. c From ref. 55. d From ref. 63. e This work, binary mixture. f This work, reverse micelles. g From ref. 64. h From ref. 46. i From
ref. 55.
the equilibrium constant of the formation of the diazo ether,
Scheme 3.
kHET [H+ ]+ kHOM K[BuOH]
(1)
kobs
=
K[BuOH]+[H+ ]
Eqn (1) is typical of processes where an S-shaped dependence
of kobs with -log[H+] is observed (where [H+] represents the
concentration of protonated solvent molecules). From eqn (1),
and by considering limits, we find that when [H+] ꢀ K[BuOH],
kobs ª kHET, i.e., the reaction proceeds wholly through the DN
+
AN mechanism and only heterolytic products are obtained. On
the other hand, when [H+] ꢁ K[BuOH], kobs ª kHOM, i.e. the
reaction proceeds wholly through the O-diazo ether and formation
of reduction products is favored. The solid lines in Fig. 1, 2, 4 and 6
were obtained by fitting the experimental data to eqn (1) by means
of a non-linear least-squares method provided by the GraFit 5.0.5
computer program, and the average pK value, as well as those for
kHET and kHOM, are listed in Table 3.
The variation in equilibrium constant K for the formation of
the diazo ether, Table 3, does not show a clear tendency upon
changing the temperature, and by considering their average value,
the free Gibbs energy for the formation of the diazo ether can
be obtained, DG ª 30 kJ mol-1, indicating that the formation of
the diazo ethers is not a spontaneous process. On the other hand,
at any temperature kHOM ꢀ kHET in both binary mixtures and
reverse micelles, in line with previous reports40,46,50,55 but, curiously
enough, the kHOM values in the binary mixtures are much lower
than those in the reverse micelles. Rate enhancements of the rate
of decomposition of diazo ethers (i.e., kHOM) have been previously
reported in the presence of normal micelles56–58 and thus our results
may not be surprising, but the present data can not provide a
convincing explanation for this observation because nothing is
known about the micellar effects on the reactivity and stability of
such adducts in spite of the fact that the bond-rotating mechanism
to transform the Z-isomer into the much more stable E-derivative
has been described in aqueous solution.59
Fig. 7 Variation of ln(kHET) (᭹) and ln(kHOM) (ꢁ) with 1/T according to
the Arrhenius equation. Data from Table 3.
homolytic process, Ea (HOM) = 75 9 kJ mol-1 (binary mixture)
is equal to that obtained in reverse micelles, Ea (HOM) = 77
6 kJ mol-1, and is substantially lower than the activation energy
obtained for the heterolytic process. Curiously enough, the Ea
(HOM) value for the butanolyses and ethanolyses of 4MBD are
the same and are equal to that obtained for the methanolysis
of 4-bromobenzenediazonium ions in MeOH–H2O mixtures, Ea
(HOM) = 74 10 kJ mol-1, suggesting that the activation energies
for the homolytic breakdown of diazo ethers are not very sensitive
to both the nature of the substituents in the aromatic ring and the
solvent, Table 4.
It may be instructive to compare the present results with
those obtained in other solvolytic dediazoniations. Substituents
on the aromatic ring have a marked effect on the stability of
arenediazonium ions, and their effects are not understandable in
terms of the Hammett equation but can be interpreted in terms of
a dual substituent parameter separating the resonance (associated
with the ability to transfer charge) and inductive (associated with
the polarity) effects. Electron withdrawing substituents such as 4-
NO2 or 4-Br destabilize the aryl cation by induction more than
they destabilize the parent arenediazonium ions, and therefore
its spontaneous decomposition is much lower than that of the
parent, or of arenediazonium ions bearing electron-releasing
groups such as 4-Me; compare for instance the half-life value
The activation parameters for the heterolytic and homolytic
pathways can be obtained by fitting the kHET and kHOM values in
Table 3 to the Arrhenius equation, Fig. 7. The estimated value for
the heterolytic process, Ea (HET) = 103 10 kJ mol-1, is in keeping
with those reported in the literature.60 The activation energy for the
◦
for decomposition of 4MBD in H2O at T = 60 C, t1/2 = 3450 s,
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with that for the decomposition of 4-nitrobenzenediazonium ions
at the same temperature, t1/2 ~ 83000 s (Table 4). However,
since arenediazonium ions decompose spontaneously through
the formation of an extremely reactive aryl cation (DN + AN
mechanism, Scheme 2), the kHET values are not very sensitive to
changes in solvent composition as shown in this and in previous
dediazoniation reports.35,45,48
pathway and the competitive radical mechanism comprising the
formation of an unstable diazo ether that further decomposes.
Changes in temperature do not seem to have any effect on the
product distribution or on the equilibrium constant for the for-
mation of the diazo ether, but do change both the homolytic kHOM
and heterolytic kHET rate constants allowing the determination
of the corresponding activation parameters and, as expected, the
activation energy for the heterolytic mechanism is much higher
than that for the homolytic process. The activation energy for
the heterolytic process Ea (HET), is very similar to that found in
previous reports but that for the homolytic process, Ea (HOM),
is much lower and seems to be independent of the nature of the
substituents and of the alcohol employed.
Finally, it may be worth noting the utility of reverse micelles
to study solvolytic reactions allowing the achievement of alcohol
concentrations in the system which otherwise cannot be attained
because of solubility or incompatibility problems which are
frequently encountered in organic chemistry. To overcome the
reagent incompatibility, reverse micelles (and microemulsions)
have been proved to be useful as reaction media and their use
expands the range of solvents where these reactions can be studied
and may open new mechanistic possibilities because of their
unique properties.27,28,35 In the present case, we have overcome
the solubility problem between BuOH and water and were able
to study a solvolytic dediazoniation reaction whose mechanism
can be easily modulated from a heterolytic or ionic mechanism
to a homolytic or radical one by just changing the acidity of the
solution, allowing the study of the factors that control the turnover
of the mechanism of the reaction.
+
4-Alkyl substituents in the aromatic ring make ArN2 ions less
prone to decomposition through homolytic pathways, as opposed
to the, for example, 4-NO2 or 4-Br- derivatives, and the formation
of the O-adduct is only detected under relatively basic conditions,
and its formation is not dependent on solvent composition and
temperature. This observation contrasts with those observed in the
methanolyses of 4-NO2 and 4-Br-arenediazonium ions, where the
formation of the diazo ethers depends on both pH and MeOH
concentration: at low percentages of ROH, the corresponding
diazo ethers are formed because of the equilibrium constant for
their formation is high (and consequently the reaction proceeds
through the radical mechanism, Scheme 3) but its formation is
not so favored at high %ROH in spite of the much higher [ROH]
because of the medium effect on K, and in these situations the ionic
and radical mechanisms become competitive (Table 4). However,
once the diazo ether is formed, it decomposes, initiating a radical
process whose activation energy seems to be independent of the
nature of the substituent in the aromatic ring (Table 4).
Previous studies on the formation of diazo ethers under alkaline
conditions suggest that diazo ethers are initially formed in a
highly unstable, kinetically controlled, Z-configuration which then
may undergo subsequent isomerization to the thermodynamically
stable E-isomers, which can be isolated in some instances,61 or
eventually may give rise to homolytic rupture of the bonds
providing the initiation of a radical process.35,40,46,55,58,62,63 This bond
rotating mechanism to transform the Z- into the E-isomer has
been described for Sandmeyer hydroxylations and chlorination
reactions.59
In conclusion, we have been able to investigate the formation
and decomposition of transient diazo ethers in the course of the
butanolysis of 4MBD by employing binary BuOH/H2O mixtures
and nanostructured systems such as the reverse micelles, which
permit one to overcome the solubility problems of BuOH in water.
The reaction also provides an effective method to prepare large
amounts of the reduction product ArH under relatively mild con-
ditions, hence providing a simple, effective and practical method
for replacing aromatic amino groups by hydrogen, representing an
improved alternative to current literature methods.35,48,53,65,66
In reaction mixtures with low BuOH content, a change in the
acidity does not change kobs values and only heterolytic dediazo-
niation products are obtained. The results are consistent with the
DN + AN mechanism, i.e., a rate-limiting formation of an extremely
reactive aryl cation that further reacts with available nucleophiles
in the solvation shell. However, in reaction mixtures containing
a high percentage of BuOH (either BuOH/H2O mixtures or
BuOH/SDS/H2O reverse micelles), S-shaped variations in both
kobs and in the product distribution with acidity were observed,
with inflection points close to -log[HCl] = 4. At low acidities,
substantial amounts of the reduction product ArH were detected,
indicating a change in the reaction mechanism from the heterolytic
to the homolytic one. Results can be interpreted in terms of two
competitive mechanisms (Scheme 3); the heterolytic DN + AN
Experimental
Instrumentation
UV-Vis spectra and kinetics experiments were monitored on an
Agilent 8453 spectrophotometer equipped with a thermostatted
cell carrier attached to a Julabo F12-ED thermostat and a
computer for data storage and manipulation. Product analysis
was carried out on a WATERS HPLC system equipped with a
model 600 quaternary pump, a model 717 automatic injector,
a model 2487 dual-l absorbance detector and a computer for
data storage. Products were separated with a Microsorb-MV C-18
(Rainin) reversed-phase column (25 cm length, 4.6 mm internal
diameter and 5 mm particle size) with a mobile phase of MeOH–
H2O (50 : 50 v/v), containing 10-4 M HCl. The injection volume
was 25 ml in all runs and the UV detector was set at 220 and
280 nm.
Reagents and materials
Reagents were of the maximum purity available and used without
further purification. 4-Cresol (ArOH), toluene (ArH) and the
surfactant sodium dodecyl sulfate (SDS) were purchased from
Fluka. BuOH was of HPLC grade from Merck and Acros
Organics. Other chemicals were from Riedel de Ha¨en. All solutions
were prepared by using Milli-Q grade water. The HCl solutions
were prepared by dilution from concentrated commercial HCl
and their concentrations were determined by potentiometric
measurements.
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Table 5 Composition and some relevant structural parameters of the
prepared RMs. rm stands for the macroscopic density of the reverse
micellar solution, N stands for the aggregation number, rwp is the radius of
the water pool, rc is the radius of the cell and rc - rwp indicates the thickness
of the interfacial region where the surfactant and the alcohol are located.
Further details on their preparation and how the structural parameters
were determined can be found elsewhere.70
4-Methylbenzenediazonium, 4MBD, tetrafluoroborate was pre-
pared by employing a non-aqueous procedure as indicated
elsewhere67 and was purified three times from CH3CN/cold ether;
it was stored in the dark at low temperatures to minimize its
decomposition and was recrystallized periodically. The UV-Vis
spectrum of 4MBD aqueous in acidic condition show a band
centred at l = 278 nm. The Beer’s law plot (not shown) up to 1.5 ¥
[SDS]/ rm/
10-4 M is linear, yielding e278 = (146 1) ¥ 102 M-1 cm-1 and e300
(59 1) ¥ 102 M-1 cm-1, in keeping with published data.60
=
˚ ˚ ˚
rwp/A rc/A rc - rwp/A
RM % H2O % BuOH [BuOH] g ml-1
N
1
2
3
4
5
6
7
8
39.9
31.9
27.7
23.4
20
14.4
10.9
7.4
45
56
0.086
0.056
0.043
0.033
0.027
0.017
0.013
0.008
0.92
0.89
0.88
0.86
0.85
0.84
0.84
0.83
33 23.3 33.4 10.1
26 21.8 34.4 12.6
24 20.0 33.4 13.4
16 18.6 33.4 14.8
13 17.3 33.0 15.7
11 15.8 33.8 18.0
Methods
61.9
67.8
72.5
80.1
84.9
89.8
Kinetic data were obtained from spectrophotometric UV-Vis
experiments. In all runs, first order kinetics were found for more
than 3 half-lives and the observed rate constants, kobs, were
9
8
15.4 36.2 20.8
14.7 39.6 24.9
+
obtained by fitting the variation of the absorbance due to ArN2
loss (l = 278 nm) with time to the integrated first-order eqn (2)
using a nonlinear least squares method provided by a commercial
computer program (GraFit 5.0). Solvolytic reactions were carried
out at T = 50 ◦C except where the effects of temperature
were investigated. This temperature was chosen because previous
investigations60 indicated that convenient rate constants can be
achieved.
Binary mixtures were prepared by mixing the appropriate
amounts of BuOH and water, and molar concentrations were
calculated by ignoring the small excess volume of mixed solvents.68
Reverse micellar solutions were prepared by employing a fixed
wo = [H2O]/[SDS] = 42.7 ratio according to the phase diagram
given by Jobe et al.,69 allowing variations in the percentage of
BuOH from 45% to 90%. RMs were prepared by mixing the
appropriate amounts of SDS, H2O and BuOH (by weight) to
get the desired compositions, Table 5, which also shows some
relevant, previously determined, structural parameters.36,37,41,70 The
percentage of BuOH in the reaction mixture is given by volume
throughout the text.
(A∞ − A )
t
ln
= −kobst
(2)
(A∞ − A )
0
Dediazoniation product distributions were determined at room
temperature by analyzing the reaction mixtures once the dedi-
azoniation reaction was finished, i.e., at infinite time. For this
purpose, a number of reaction mixtures of the desired composition
containing all reagents except 4MBD were prepared in volumetric
flasks and thermostated at for about 20 min. Then, aliquots of
a freshly prepared 4MBD stock solution (typically 20 mL) were
added to the volumetric flasks and left to react for at least 5 h.
After the reaction was completed, the solutions were cooled to
room temperature and aliquots of these solutions were transferred
to HPLC vials and analyzed in triplicate.
Chromatograms showed, in addition to the front peak, three
chromatographic peaks. Two of them were unambiguously identi-
fied as 4-cresol, ArOH, and toluene, ArH, by means of spiking
experiments with authentic samples. The third one was tenta-
tively associated with the substitution product 4-butyl-tolyl-eter,
ArOBu, based on our experience in previous solvolytic work and
because its peak area increases upon increasing [BuOH]. ArOBu
is not a commercial product, it was not synthesized, and its yield
was not therefore determined. The absence of this product yield
produces a systematic error in the total yields. However, this
error does not affect the kinetic results because dediazoniation
products are formed competitively (see results). Linear calibration
curves (correlation coefficient > 0.999) for converting HPLC
peak areas into concentrations were obtained for ArOH and
ArH by employing commercial samples dissolved in MeOH. The
percentage of a given analyte was determined by means of eqn
(3), where [4MBD]0 stands for the initial concentration of 4MBD,
which was estimated by weight as described elsewhere.
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