Rates and mechanisms of the thermal solvolytic decomposition of arenediazonium ions

Article abstract of DOI:10.1039/a905567c

Arenediazonium tetrafluoroborates have been prepared and the kinetics of solvolysis have been investigated in water, trifluoroethanol, water-trifluoroethanol mixtures, hexafluoropropan-2-ol, trifluoroacetic acid, and ethanol by a UV method. A heterolytic mechanism involving short-lived aryl cations leads to products derived from nucleophilic capture of the aryl cations by solvent or a solute. Ionic solutes in aqueous trifluoroethanol and trifluoromethoxybenzene in trifluoroethanol have no kinetic effect and neither does replacement of the tetrafluoroborate counter-ion by chloride in trifluoroethanol. Rate constants for any one compound are not very solvent dependent, the reactions generally being characterised by high enthalpies of activation and appreciably positive entropies of activation. Compounds with 4-Cl, 4-F, 4-NO₂, and 4-MeO substituents proved too unreactive for kinetic studies, but for different reasons. In ethanol, a radical reaction with characteristically different activation parameters competes with the heterolytic path and leads to hydrodediazoniation (reduction) by hydrogen atom abstraction from the CH₂ group of ethanol.

Full text of DOI:10.1039/a905567c

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Rates and mechanisms of the thermal solvolytic decomposition of
arenediazonium ions
Peter S. J. Canning, Katharine McCrudden, Howard Maskill* and Brian Sexton
Chemistry Department, University of Newcastle upon Tyne, Newcastle upon Tyne,
UK NE1 7RU
Received (in Cambridge, UK) 9th July 1999, Accepted 27th September 1999
Arenediazonium tetrafluoroborates have been prepared and the kinetics of solvolysis have been investigated in
water, trifluoroethanol, water–trifluoroethanol mixtures, hexafluoropropan-2-ol, trifluoroacetic acid, and ethanol
by a UV method. A heterolytic mechanism involving short-lived aryl cations leads to products derived from
nucleophilic capture of the aryl cations by solvent or a solute. Ionic solutes in aqueous trifluoroethanol and
trifluoromethoxybenzene in trifluoroethanol have no kinetic effect and neither does replacement of the tetra-
fluoroborate counter-ion by chloride in trifluoroethanol. Rate constants for any one compound are not very solvent
dependent, the reactions generally being characterised by high enthalpies of activation and appreciably positive
entropies of activation. Compounds with 4-Cl, 4-F, 4-NO2, and 4-MeO substituents proved too unreactive for kinetic
studies, but for different reasons. In ethanol, a radical reaction with characteristically different activation parameters
competes with the heterolytic path and leads to hydrodediazoniation (reduction) by hydrogen atom abstraction
from the CH2 group of ethanol.
mediate formed in the decomposition of arenediazonium
salts were first addressed by Taft who observed substantial
Introduction
Reactions of arenediazonium ions, ArN2ϩ, were amongst the
enhancements in the rate due to electron donating substituents
first to be studied mechanistically,1 and the diazonium group
in the meta position of benzenediazonium salts.21 He suggested
was reported to be replaceable by iodide as long ago as 1864.2 In
that this effect implied a high degree of radical-cation character
in the transition state of the dediazoniation reaction due to
1940, Hammett3 postulated a slow unimolecular heterolytic
dissociation of arenediazonium ions into aryl cations and N2,
transfer of a π electron from the benzene ring into the vacant σ
and experimental support was provided;4 Lewis5–7, Swain8–10
(sp2) orbital of the cation. The triplet electronic state was
and DeTar11,12 were amongst other early investigators of solvo-
claimed to be more stable than the corresponding singlet, but
lytic dediazoniations. Zollinger1a,13,14 has been pre-eminent
early semi-empirical and ab initio molecular orbital calculations
amongst subsequent investigators1b,15,16 who have continued to
were not supportive although results depended upon substitu-
study the mechanisms of these important reactions by kinetics,
isotope labelling studies, kinetic isotope effect measurements,
and theoretical methods.
Arenediazonium ions undergo Nα–Nβ rearrangement
ents.22 More recent high level ab initio studies confirm a singlet
ground state phenyl cation with an appreciably less stable, very
short-lived triplet.23 These theoretical studies have been com-
plemented by experimental studies of the properties and reac-
(to a degree which depends upon the nature of the arene group
tions of aryl cations generated other than by thermolysis of
and the reaction conditions) which indicated that the aryl
diazonium ions.24
cation formed in the initial heterolysis may undergo internal
The reduction of arenediazonium salts by ethanol to give
return with the nitrogen molecule.7,8,13,17 Hydrogen and nitrogen
arenes was a notable early preparative use of dediazoni-
kinetic isotope effect measurements6,8,9,18,19 provided further
ations,2,25 but yields are variable and substituent dependent;
support for the intermediacy of the aryl cation and, in particu-
decomposition of benzenediazonium salts with ethanol, for
lar, excluded the intermediacy of a benzenespirodiazirine
example, yields phenyl ethyl ether and benzene in ratios which
cation. Zollinger proposed a second steady-state intermediate
depend upon the precise experimental conditions. Electron-
on the basis of further isotopic labelling studies as shown in the
donating substituents favour the formation of aryl ethers whilst
mechanism of Scheme 1 with an ion–molecule pair intervening
electron-withdrawing groups favour reduction to arenes. The
thermal decomposition of arenediazonium tetrafluoroborates
in the absence of solvent to give fluoroarenes reported by Balz
and Schiemann is another useful dediazoniation reaction.26,27
This reaction remains an important method for the production
of fluoroarenes which continue to be increasingly important
in the manufacture of pharmaceuticals and agrochemicals.
There have been a number of subsequent developments
and alternatives28 which improve yields and safety including
use of arenediazonium hexafluoroantimonates and hexafluoro-
Scheme 1 Heterolytic mechanism of dediazoniation.
phosphates.29
between the diazonium ion and the fully dissociated aryl cation
plus nitrogen molecule.20
Questions concerning the structure of the aryl cation inter-
We have decomposed arenediazonium tetrafluoroborates in
ionising solvents of different nucleophilicities and sought to
intercept the intermediate aryl cation by fluoride donating
J. Chem. Soc., Perkin Trans. 2, 1999, 2735–2740
This journal is © The Royal Society of Chemistry 1999
2735

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Table 2 Rate constants at 25.0 ЊC and activation parameters for
XC6H4N2ϩBF4Ϫ in TFE
Table 1 Rate constants at 25.0 ЊC and activation parameters for
XC6H4N2ϩBF4Ϫ in water
Compound, X
106k25/sϪ1
∆H‡/kJ molϪ1
∆S ‡/J KϪ1 molϪ1
Compound, X
106k25/sϪ1
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
1a, Ha
53.0
6.77
240
0.27
1.82
1.31
9.69
11.3
109
109
99
128
111
134
99
37
23
16
56
16
81
1a, Ha
92.1
12.2
873
114
113
103
111
118
124
124
116
62
40
42
10
44
59
54
20
1b, 4-tert-Bu
1c, 3-MeO
1d, 3-CF3
1e, 3-Cl
1f, 3-F
1g, 3-NO2
1h, 3-CN
1b, 4-tert-Bu
1c, 3-MeO
1d, 3-CF3
1e, 3-Cl
1f, 3-F
1g, 3-NO2
1h, 3-CN
0.7
3.01
1.18
0.60
0.3
Ϫ17
Ϫ75
79
a Previously reported results,35 106k25/sϪ1 = 35.1, ∆H‡/kJ molϪ1 = 108,
∆S‡/J KϪ1 molϪ1 = 33; it was also established that EDTA had no effect
upon the rate constant at 25 ЊC.
a Ref. 35; it was also established that EDTA had no effect upon the rate
constant at 25 ЊC, and that replacement of the tetrafluoroborate (1a) by
the chloride (2) gave identical results between 25 and 55 ЊC.
agents in order to improve the yields of fluoroarenes.24,27,30,31 We
report here preparations of our substrates, kinetics results, and
mechanistic deductions regarding the rate-limiting step;
further product analysis and mechanistic considerations will be
reported separately.
Table 3 Rate constants at 25.0 ЊC and activation parameters for
XC6H4N2ϩBF4Ϫ in trifluoroacetic acid
Compound, X
104k25/sϪ1
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
1a, H
1i, 3-Me
1c, 3-MeO
0.47
2.25
5.15
111
110
108
45
54
55
Methods
Arenediazonium tetrafluoroborates (1a–n) were made by
Table 4 Rate constants at 25.0 ЊC and activation parameters for
XC6H4N2ϩBF4Ϫ in hexafluoropropan-2-ol
Compound, X
106k25/sϪ1
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
1a, H
97.2
1730
1.7
0.01
519
116
98
73
172
108
66
32
Ϫ123
178
56
1c, 3-MeO
1d, 3-CF3
1h, 3-CN
1i, 3-Me
Table 5 Rate constants at 25.0 ЊC and activation parameters for
Starkey’s general method and yields were satisfactory;32 all were
stored below room temperature, protected from daylight, and
usually purified before use. Benzenediazonium chloride (2) was
made by a non-aqueous method and kept moist with diethyl
ether until used, then destroyed; no samples of this compound
were stored. Kinetics were investigated by our usual method of
monitoring UV absorbance with time, all data collection being
carried out automatically, and rate constants were computed by
iterative non-linear regression methods. Rates of the more
reactive compounds (1a,c,i) were investigated down to about
20 ЊC whereas the less reactive compounds (1d,g,h) were
investigated at temperatures up to 85 ЊC in water. Mean values
of first-order rate constants were obtained from at least two
results at each temperature; the rate constant at 25.0 ЊC and the
enthalpy and entropy of activation were then determined by a
computer-based version of the Eyring equation.33 The least
reactive compounds, i.e. benzenediazonium tetrafluoroborate
with 4-Cl, 4-F, and 4-NO2 substituents (1l–n), proved too
unreactive for us to investigate kinetically, and (for the same
reason) we were able to make only very limited kinetic meas-
urements with the 4-MeO compound (1k). Products were iden-
tified by comparison with authentic samples, and analysed by a
combination of UV and NMR spectroscopy, and GLC.34
XC6H4N2ϩBF4Ϫ in ethanol
Compound, X
104k25/sϪ1
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
Ϫ64
14
Ϫ20
Ϫ87
128
26
1a, H
2.32
4.09
8.44
9.41
2.62
3.63
75
97
85
64
132
100
1c, 3-MeO
1d, 3-CF3
1f, 3-F
1h, 3-CN
1i, 3-Me
Table 6 Rate constants at 25.0 ЊC and activation parameters for
XC6H4N2ϩBF4Ϫ in TFE–H2Oa
Compound,
X
Molar ratio
TFE:H2O
105k25/
∆H‡/kJ
∆S‡/J KϪ1
sϪ1
molϪ1
molϪ1
1b, 4-tert-Bu
1c, 3-MeO
1f, 3-F
1:1
1:1
1:1
1.34
38.9
13.1
3.6
108
102
126
126
26
32
67
64
1f, 3-F
95:5
a Up to 0.25 mol dmϪ3 of NaPF6, and NaClO4 had no effect, or a
Ϫ
slight rate-decreasing effect, upon the reaction of 4-MeC6H4N2ϩBF4
(1j) in 1:1 aqueous TFE at 51 ЊC.
Results
than the 4-methyl compound at 84 ЊC in water. Table 6 contains
results in aqueous trifluoroethanol (TFE); increasing con-
centrations of sodium or ammonium hexafluorophosphate
and sodium perchlorate up to 0.25 mol dmϪ3 at 51 ЊC in 1:1
aqueous TFE led to either no effect or small decreases in the
rate constant for 4-toluenediazonium tetrafluoroborate. Table 7
shows the effect of increasing concentrations of trifluoro-
methoxybenzene (TFMB) in pure TFE upon the rate constant
First-order rate constants at 25.0 ЊC (k25), ∆H‡, and ∆S‡ for
arenediazonium tetrafluoroborates in the five pure solvents are
shown in Tables 1–5. Replacement of benzenediazonium tetra-
fluoroborate (1a) by the chloride (2) in TFE led to rate con-
stants identical within experimental error at five temperatures
between 25 and 55 ЊC. Our best estimate for the reactivity of the
4-methoxy compound (1k) is that it is 250 times less reactive
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J. Chem. Soc., Perkin Trans. 2, 1999, 2735–2740

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Table 7 Rate constants for 4-MeC6H4N2ϩBF4Ϫ (1j) in TFE containing
trifluoromethoxybenzene (TFMOB) at two temperatures
tions were all carried out under normal atmospheric conditions,
consequently step (ii) is not expected to be reversible as was
observed under very high pressures of nitrogen.17] The mechan-
ism of Scheme 1 allows capture of the initial ion–molecule pair
by high concentrations of strong nucleophiles such as thio-
cyanate, which would reduce internal return and hence lead to a
rate enhancement, as has been reported.6 The reaction condi-
tions we used would not be expected to affect internal return
and we observed no second-order rate effects with any solutes.
In the absence of strongly nucleophilic solutes, and for the more
reactive compounds even in the presence of such solutes, the
products of reactions in solvents other than ethanol are
exclusively by solvent derived nucleophilic substitution and
fluoroarenes; in ethanol, reduction products are also
observed.30,34,35 Ratios of trifluoroethyl ethers to phenols from
reactions in aqueous TFE and appreciable yields of fluoro-
arenes from reactions in TFE implicate a highly reactive, short-
lived, and non-discriminating aryl cation intermediate.30,36 We
conclude that simple arenediazonium ions in the polar solvents
of our investigation decompose by the mechanism of Scheme 1
proposed previously.
104k/sϪ1
100%
TFE
TFE:
TFMOB,
10:1
TFE:
TFMOB,
5:1
TFE:
TFMOB,
3:1
Temperature/
ЊC
35.0
49.5
1.82
15.9
2.43
17.9
2.34
16.3
1.73
18.0
for decomposition of 4-toluenediazonium tetrafluoroborate
(1j). In all cases, products have been identified and shown to be
fluoroarenes, substitution products derived from the solvent,
and (in the case of ethanol) reduction products; yields (relative
and absolute) were measured by GLC.34
Six of the eight k25 results in Table 1 represent a relatively
narrow range of reactivity in water (a factor of 40 between
highest and lowest) but there are two out-liers; the 3-MeO
analogue (1c) is appreciably more reactive and the 3-CF3
compound (1d) appreciably less so. However, the substantially
negative ∆S‡ values for the 3-NO2 and 3-CN compounds (1g)
and (1h), compensated in part by somewhat lower ∆H‡ values,
appear qualitatively different from the other results. For TFE
and trifluoroacetic acid (TFA) shown in Tables 2 and 3, the
3-MeO compound (1c) is again the most reactive, but now the
activation parameters for all compounds are qualitatively the
same—large ∆H‡ values and distinctly positive ∆S‡ values.
Results for the 3-CF3 and 3-CN compounds (1d) and (1h) in
hexafluoropropan-2-ol (HFIP) shown in Table 4 appear anom-
alous but for different reasons. The former has a low ∆H‡ value
and a very negative ∆S‡ value, whereas the latter has a strongly
positive (favourable) ∆S‡ value and a massively unfavourable
∆H‡ value. In Table 6 we see that the proportion of water in
TFE has little effect upon the kinetics of four compounds; the
counter-ion has no detectable effect upon the rate constant for
reaction of the parent benzenediazonium ion in TFE. Molecu-
lar and ionic solutes (Table 7 and the footnote to Table 6)
have no significant effect upon rate constants for 4-toluene-
diazonium tetrafluoroborate (1j) in TFE and aqueous TFE,
respectively. In ethanol, Table 5, the range of reactivity at 25 ЊC
is very narrow, but differences in the activation parameters and
product analyses indicate mechanistic differences (see below).
Effect of substituents
We have already reported35 that 2- and 3-alkyl substituents
are rate-enhancing in polar solvents and conclude that the
stabilising (electron-releasing) inductive effect of these groups
is greater for the aryl cation (and hence, by the Hammond Pos-
tulate,33 for the transition structure leading to its formation)
than for the arenediazonium ion precursor. This is not surpris-
ing as only little of the positive charge of the arenediazonium
ion will be delocalised into the arene ring. Correspondingly,
electron-withdrawing groups without opposing electron-
supplying resonance effects (3-nitro, 3-cyano, and 3-trifluoro-
methyl) destabilise the aryl cation more than they destabilise the
arenediazonium ion and hence are rate retarding. Groups at
position 3 which are inductively electron-withdrawing but able
to supply electron density into the aromatic ring by resonance
(halogen and methoxy) are particularly interesting; 3-Cl and 3-
F are rate retarding (comparable with 3-NO2, 3-CN, and 3-CF3)
whereas 3-MeO is strongly rate-enhancing—more so than 3-
alkyl. On the grounds that the 3-MeO is further from the
electron-deficient centre in the diazonium ion than in the aryl
cation, we can dismiss any notion that this substituent is more
destabilising for the reactant than for the transition structure.
Clearly, the 3-methoxy substituent stabilises the aryl cation
(and hence the preceding transition structure) more than the
arenediazonium ion which indicates that, as in electrophilic
aromatic substitution,37 the electron supply through resonance
dominates electron withdrawal by induction. There must, there-
fore, be delocalisation of the positive charge of the aryl cation
to carbons able to access the stabilising electron density of the
3-methoxy substituent. Any electron supply by resonance from
3-halo substituents, however, is more than cancelled by electron
withdrawal through induction, and such substituents remain
rate retarding (again, as in electrophilic aromatic substitution).
As reported previously,35 4-alkyl groups are rate-retarding;
consequently, their cation-stabilising effect upon the diazonium
ion must be greater than upon the aryl cation. In view of their
greater distance from the cationic site of the diazonium ion of
4-alkyl than 3-alkyl, the mechanism of the stabilising effect of
4-alkyl is not principally induction. We also see by comparison
of our present and earlier studies35 that the 4-tert-butyl group is
consistently less rate-retarding than the 4-methyl group. The 4-
tert-butyl group is more polarisable than methyl which confirms
that induction is not the principal diazonium ion-stabilising
mechanism of 4-alkyl groups and implicates hyperconjugation,
the C–C effect being less than the C–H effect. There is earlier
evidence supporting hyperconjugative stabilization of the aryl
cation formed in dediazoniation9 and otherwise.38 Groups
Discussion
Solvolytic dediazoniation reactions of arenediazonium salts are
typically characterised by entropies of activation appreciably
more positive than those for SN1 reactions of aliphatic sub-
strates, and have considerably larger enthalpies of acti-
vation.10,12,35 These activation parameters have been interpreted
as evidence for the rate-limiting step being either the initial
heterolysis of the C–N bond of the arenediazonium ion, or the
subsequent separation of the initially formed ion–molecule
pair, i.e. steps (i) or (ii) in Scheme 1. The increase in entropy due
to the impending departure of the nitrogen molecule in the
formation of the rate-limiting transition structure is not can-
celled out by increased solvation, hence the positive ∆S‡ values.
The absence of significantly enhanced solvation as substrate
becomes transition structure also means there is little assistance
to the enthalpic cost of the heterolysis, hence the large ∆H‡
values. We have no explanation at the moment, however, for the
very few reactions in polar media which have activation param-
eters significantly different, i.e. of 1g in water, 1d in HFIP, and
1h in water and HFIP but note that the two reactions in HFIP
are very slow and results from them may be less reliable. Earlier
isotopic labelling studies showed that the initial ionization is
reversible7,8,17 in which case the separation of the ion–molecule
pair, step (ii) in Scheme 1, is the rate-limiting step. [Our reac-
J. Chem. Soc., Perkin Trans. 2, 1999, 2735–2740
2737

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was the least reactive of the set. We also note that the activation
parameters in EtOH do not follow the normal pattern.
Product analytical evidence confirmed that reactions in
EtOH are qualitatively different from the reactions in the more
ionising media;30,34 products of hydrodediazoniation, i.e. reduc-
tion products, are formed in appreciable yields in ethanol. Our
observation of faster rates and higher yields for hydrodediazo-
niation with electron-withdrawing substituents is compatible
with intrusion of a radical mechanism for reactions in ethanol.
This is in agreement with an earlier report that thermal
homolytic dediazoniation is favoured over heterolytic by
increasing the electrophilicity of the β nitrogen atom of the
diazonium ion, e.g. by suitable electron-withdrawing
substituents in the aromatic ring.18 Previously, both heterolytic
and radical mechanisms have been provided for the reduction
reaction in methanol, the former by DeTar42 with hydride
transfer as the crucial step but this has not been widely upheld.
A radical mechanism proposed by DeTar42,43 and others,44,45
following much earlier suggestions by Hey and by Waters,46 is
generally accepted. We have now shown that when 3-nitro- and
3-fluorobenzenediazonium tetrafluoroborate are decomposed
in 1,1-dideuterioethanol, deuterium is incorporated to give
3-deuterionitrobenzene and 3-deuteriofluorobenzene34 as
required by Scheme 2. Zollinger also reported that addition of
pyridine to TFE for the dediazoniation of benzenediazonium
tetrafluoroborate increased the overall rate and yields of the
products of homolytic decomposition, concluding that increas-
ing the nucleophilicity of the solvent itself, as well as intro-
ducing nucleophilic solutes, promotes homolysis of diazonium
ions.47 On this basis, the electron donor in Scheme 2 is probably
Fig. 1 Resonance stabilisation of the 4-methoxybenzenediazonium
ion.
which are better able conjugatively to stabilise the arenediazo-
nium ion, e.g. N- or O-bonded substituents or cyclopropyl and
vinyl, are predicted to be more strongly rate retarding. This is
observed in the present results for the 4-methoxy group and
wholly consistent with resonance stabilisation of the arene-
diazonium ion illustrated in Fig. 1 and proposed earlier by
Hughes39 and Swain.10
Nitro, cyano, and trifluoromethyl substituents at position 4
are rate retarding so (as at position 3) they destabilise the aryl
cation by induction more than they destabilise the arenediazo-
nium ion. Halogens, like methoxy, are inductively electron-
withdrawing but electron-donating by resonance. We have
observed the former effect to be greater upon the aryl cation
than upon the arenediazonium cation so, by this mechanism, 4-
halo substituents are expected to be rate retarding. The
electron-donating resonance effect of 4-halo groups will also
be rate retarding because (as with 4-methoxy, Fig. 1) it select-
ively stabilises the arenediazonium ion; these compounds
were, in fact, too unreactive for us to be able to investigate
kinetically.
As discussed above, we have observed appreciable effects of
substituents upon rates of dediazoniation, and qualitative
inductive, resonance, and hyperconjugative effects have been
invoked to account for them. Attempts to fit the results to a
two-parameter Hammett type correlation failed. More sophis-
ticated approaches using multiparameter equations would
doubtless allow improved correlations, and might allow better
resolution of effects between resonance, inductive, and (for 2-
substituents) steric effects.9,40,41 This was not a primary object-
ive of the present investigation, however, and a course we have
not taken.
eϪ
ϩ
ؒ
ArN2
ArN2
ؒ
ؒ
ArN2
Ar ϩ N2
ؒ
Ar ϩ CH3CL2OH
ArL ϩ CH CLOH
˙
3
ϩ
ϩ
ؒ
CH CLOH ϩ ArN
ArN2 ϩ CH3CLO H
˙
3
2
CH3CLOϩH
CH3CLO ϩ Hϩ
Scheme 2 Radical mechanism for reduction of arenediazonium ions
by ethanol.
Effect of solutes
Increasing concentrations of sodium and ammonium hexa-
fluorophosphate and sodium perchlorate have no appreciable
effect upon the rate constant for decomposition of 4-toluene-
diazonium tetrafluoroborate in aqueous trifluoroethanol. The
former, a potential fluoride donor, modestly increases the yield
of fluoroarene.34 Trifluoromethoxybenzene is also a potential
fluoride donor but, as Table 7 shows, even high proportions in
TFE have no measurable effect upon the rate of decomposition
of 4-toluenediazonium tetrafluoroborate. These results support
the mechanistic proposal that the decomposition of the arene-
diazonium ion in polar media is unimolecular; the effects of
solutes upon the product distributions are due to involvement
in product-forming steps subsequent to the rate-limiting step,
Scheme 1.
the solvent, ethanol. It remains now to quantify by further
product analysis the relative extents of the the parallel hetero-
lytic and homolytic paths for individual arenediazonium salts in
ethanol, and to investigate systematically which other solvents
allow the homolytic reductive reaction path.
Experimental
All starting materials were purchased from either Aldrich or
Fluorochem and were of reagent grade purity. Trifluoroethanol
(TFE) purchased from Fluorochem was distilled and stored
over 4 Å molecular sieves, and water was distilled using all-glass
equipment. Other solvents were laboratory grade and purified
1
by standard methods. H NMR spectra were recorded on a
Effect of solvent
Bruker WP200 (200 MHz) spectrometer, the solvent being
CDCl3 unless otherwise stated.
Only a modest solvent effect upon rate constants is anticipated
for the mechanism of Scheme 1 since the rate-limiting step,
regardless of whether it is (i) or (ii), involves conversion of one
cation into another, i.e. there is no change in the charge type,
only a charge redistribution. This is generally observed; rate
constants for the parent compound at 25 ЊC in H2O, TFE,
HFIP, and TFA are all within a factor of 2. Thus, with the
exception of ethanol, the solvent effects are wholly accom-
modated by the mechanism in Scheme 1 and warrant little
further discussion. The first kinetic indication that reactions in
EtOH were different is the effect of substituents upon k25; 3-Me
and 3-MeO were found to be rate enhancing as expected, but so
were 3-F, 3-CN, and 3-NO2 which are strongly rate retarding in
other solvents. In EtOH, therefore, the parent diazonium salt
Preparations
Arenediazonium salts were all prepared by the same general
method (see below for benzenediazonium tetrafluoroborate)
and stored in the dark at Ϫ5 ЊC; they all decomposed upon
melting.
Benzenediazonium tetrafluoroborate (1a). Aniline (7.2 g, 76
mmol) was added to a stirred solution of tetrafluoroboric acid
(20 cm3, 48% wt) and water (20 cm3). The solution was placed in
an ice bath and stirred as a cold aqueous solution of sodium
nitrite (5.34 g, 76 mmol) in water (5 cm3) was added dropwise.
The reaction mixture was stirred as the temperature rose to
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J. Chem. Soc., Perkin Trans. 2, 1999, 2735–2740

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room temperature, then filtered under reduced pressure to sep-
arate the precipitate which was washed with dilute aqueous
tetrafluoroboric acid, ethanol, and diethyl ether, then dried
under vacuum. The product was purified by dissolution in
acetone followed by precipitation with diethyl ether. This was
carried out three times to remove colouration (5.95 g, 41%,
mp = 105–110 ЊC, lit.,45 108–110 ЊC).
Isopentyl nitrite. A solution of sodium nitrite (8.3 g) in water
(34 cm3) was added to a solution of water (2.5 cm3), concen-
trated sulfuric acid (3.1 cm3), and isopentyl alcohol (10.0 g)
cooled in ice. The reaction mixture was allowed to stand for 1.5
h as the temperature rose to room temperature, then crystalline
sodium sulfate was removed by filtration. The upper yellow
isopentyl nitrite was separated, washed with a solution of
sodium hydrogen carbonate (0.1 g) and sodium chloride (1.3 g)
in water (5 cm3), dried (MgSO4), and filtered (10.6 g, 80%, δH:
1.0 (d, 6H), 1.6 (m, 1H), 1.7 (m, 2H), 4.6 (t, 2H)).
4-tert-Butylbenzenediazonium tetrafluoroborate (1b). The
crude product was recrystallised from ethanol to give white
crystals (55%, mp = 100–102 ЊC, lit.,27 99–102 ЊC).
Benzenediazonium chloride (2).4 A saturated solution of
hydrogen chloride in ethanol (0.375 cm3) was added to aniline
hydrochloride (2.63 g, 20.3 mmol) in ethanol (20 cm3). This
solution was cooled in ice and isopentyl nitrite (3.00 g, 25.6
mmol) was added. The mixture was allowed to stand for 5–10
min at room temperature then the product was precipitated by
the gradual addition of diethyl ether. The crystals were filtered
under reduced pressure and washed with a 1:1 mixture of
ethanol–ether followed by ether (10 cm3). Care was taken to
ensure that the crystals were kept moist with ether; δH (d6-
DMSO): 3.3 (m, 2H), 4.2 (m, 2H), 5.2 (d, 1H); λm (ethanol): 267
nm.
3-Methoxybenzenediazonium tetrafluoroborate (1c). The
crude precipitate was a dark red solid which was purified by
recrystallisation from acetonitrile–ether three times to give a
pale yellow powder (83%, mp = 85–87 ЊC, lit.,45 87–88 ЊC).
3-(Trifluoromethyl)benzenediazonium tetrafluoroborate (1d).
The product was a white powder which was purified by dis-
solution in acetonitrile followed by precipitation using diethyl
ether (58.2%, mp = 146–147 ЊC, lit.,48 148 ЊC).
3-Chlorobenzenediazonium tetrafluoroborate (1e). The pale
pink product was purified by dissolution in acetonitrile and
precipitation with diethyl ether (95%, mp = 144–146 ЊC, lit.,45
146–148 ЊC).
Solvolytic media
Densities of solvents were used to calculate the volumes of each
solvent that were required to produce the correct mixtures.
Small volumes were delivered using a Gilson Pipetman.
3-Fluorobenzenediazonium tetrafluoroborate (1f). Colourless
crystals of the pure compound were obtained either by
recrystallisation from aqueous methanol or by dissolution in
acetonitrile and precipation by diethyl ether (52%, mp = 154–
156 ЊC, lit.,49 154.5–156 ЊC).
Kinetics
UV spectral analysis was carried out either on a Pye Unicam
SP8-300 or a Cecil 5000 series (5502 model) UV–VIS spectro-
photometer and 1 cm path-length quartz cells were used.
Spectrophotometer cell compartments were thermostatted by
circulating water. The absorbance at or close to λmax was
recorded automatically at fixed time intervals for at least 3 half
lives. The absorbance–time data were fitted to a single exponen-
tial decay equation by an iterative method to give the rate con-
stant. Depending on the rate of reaction, up to four cells could
be studied at one time and average rate constants from at least
two determinations at each of several measured temperatures
over a 25–30 Њ range between about 20 and 85 ЊC were obtained.
Standard deviations on individual rate constants were generally
about 1% and reproducibility about 5%. Rate constants at
25.0 ЊC, ∆H‡, and ∆S‡ were calculated using the Eyring
equation, correlation coefficients on the Eyring plots being
never less than 0.997 and usually greater than 0.999. The esti-
mated uncertainties are ca. 20% on k25, ca. 6 kJ molϪ1 on ∆H‡,
and ca. 12 J KϪ1 molϪ1 on ∆S‡.
3-Nitrobenzenediazonium tetrafluoroborate (1g). The crude
product was a light brown solid which was purified by recrystal-
lisation from acetone–ether (89%, mp = 168–170 ЊC, lit.,50
165 ЊC).
3-Cyanobenzenediazonium tetrafluoroborate (1h). The crude
slightly off-white product was recrystallised twice from
acetonitrile–ether to give a white powder (82%, mp = 152–157
ЊC, lit.,51 155–156 ЊC).
3-Methylbenzenediazonium tetrafluoroborate (1i). The crude
red solid product was purified by recrystallisation from aceto-
nitrile–ether three times to give a white powder (78%,
mp = 101–102 ЊC, lit.,45 97–101 ЊC).
4-Methylbenzenediazonium tetrafluoroborate (1j). Recrystal-
lisation of the product was from aqueous methanol to give a
white solid (28%, mp = 109–111 ЊC, lit.,45 109–111 ЊC).
4-Methoxybenzenediazonium tetrafluoroborate (1k). White
crystals of the pure compound were prepared by dissolution of
the brown crude product in acetonitrile and precipitation with
diethyl ether (78%, mp = 140–141 ЊC, lit.,45 142 ЊC).
Acknowledgements
We thank EPSRC for a research grant and studentships to
P. S. J. C., K. McC. and B. S.; we also thank Zeneca (formerly
ICI) for financial support, and Drs J. H. Atherton and D. J.
Moody of Zeneca for helpful discussions.
4-Chlorobenzenediazonium tetrafluoroborate (1l). White
crystals were obtained directly (91%, mp = 131–133 ЊC, lit.,52
134 ЊC).
References
4-Fluorobenzenediazonium tetrafluoroborate (1m). White
crystals of the pure compound were prepared by dissolution of
the colourless crude product in acetonitrile and precipitation
with diethyl ether (49%, mp = 152–154 ЊC, lit.,26 154.5 ЊC).
1 (a) H. Zollinger, in Diazo and Azo Chemistry; aliphatic and aromatic
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in acetonitrile and precipitation with diethyl ether (96%,
mp = 156–157 ЊC, lit.,45 157–158 ЊC).
J. Chem. Soc., Perkin Trans. 2, 1999, 2735–2740
2739

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J. Chem. Soc., Perkin Trans. 2, 1999, 2735–2740