Aggregation effects in the reactions of 2,4-dinitrochlorobenzene with aniline in aprotic solvents

Article abstract of DOI:10.1039/a607108b

The kinetics of the reaction of 2,4-dinitrochlorobenzene with aniline have been studied in toluene and in toluene-methanol binary solvents and compared with the reactions in benzene and chloroform. The reactions in aprotic solvents exhibit a rate dependence that is third-order in amine consistent with aggregates of the aniline acting as the nucleophile. Molecular complexes between the aniline and the substrate were also detected spectrophotometrically. Additionally, an inhibitory effect of toluene complexes was observed, which is similar to the previously reported effect of benzene in the reaction of the same substrate with n-butylamine. The reaction in the presence of pyridine shows an amine dependence which indicates formation of 'mixed aggregates' between aniline and pyridine. All the above results, as well as recently reported studies of other reactions different from S_NAr, are fully interpreted within the 'dimer nucleophile' mechanism.

Full text of DOI:10.1039/a607108b

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Aggregation effects in the reactions of 2,4-dinitrochlorobenzene with
aniline in aprotic solvents
,
N. Sbarbati Nudelman,* † Cecilia E. S. Alvaro and Jeanette S. Yankelevich
Depto. de Química, Facultad de Ingeniería, Universidad Nacional del Comahue, Neuquén,
Argentina
The kinetics of the reaction of 2,4-dinitrochlorobenzene with aniline have been studied in toluene and
in toluene–methanol binary solvents and compared with the reactions in benzene and chloroform. The
reactions in aprotic solvents exhibit a rate dependence that is third-order in amine consistent with
aggregates of the aniline acting as the nucleophile. M olecular complexes between the aniline and the
substrate were also detected spectrophotometrically. Additionally, an inhibitory effect of toluene
complexes was observed, which is similar to the previously reported effect of benzene in the reaction
of the same substrate with n-butylamine. The reaction in the presence of pyridine shows an amine
dependence which indicates formation of ‘mixed aggregates’ between aniline and pyridine. All the above
results, as well as recently reported studies of other reactions different from S Ar, are fully interpreted
N
within the ‘dimer nucleophile’ mechanism.
There is active research at present to elucidate how the aggreg-
ation state of reactants can influence the mechanisms of
organic reactions. Chemists are increasingly interested not
matic amines (as nucleophile and/or as catalysts) in aromatic
solvents, as well as in binary mixtures with protic solvents. The
effects of aggregation of the nucleophile, formation of mole-
cular complexes, mixed aggregates and solvent effects are
examined.
1
only because of the fundamental aspects of organic reactions
but also because the knowledge of the influence of aggregated
2
,3
species can be constructively used in synthetic applications.
The advent of techniques that enable the study of fast reactions
4
,5
Experimental
allowed the measurement of proton affinities for strong bases
6
,7
as well as low-basicity compounds. These data are useful as a
reference for further estimations of the specific solute–solvent
interactions, specially in solvents of low permittivity.
Reagents and solvents
N-(2,4-Dinitrophenyl)aniline (mp 155–156 ЊC, lit., 155–
2
0
156 ЊC ) was prepared from 2,4-dinitrochlorobenzene (mp 52–
53 ЊC) and aniline following the procedure reported for the N-
The effect of basicity of the nucleophile on aromatic nucleo-
philic substitutions has recently been shown by the dependence
2
1
(2,4-dinitrophenyl)-2-methoxyaniline. Aniline (Fluka) was
kept overnight over potassium hydroxide, distilled over zinc
powder and then over sodium; both distillations were carried
out under nitrogen at reduced pressure. Toluene was kept over
sodium wire for several days, and distilled twice over sodium.
8
,9
8
of the rates of structurally related nucleophiles. Bordwell has
pointed out that Brønsted plots reported in the literature for
reactions in which bond formation to the nucleophile is rate-
limiting have slopes βN u that range for the most part between
2
2
21
0
.5–0.7, in particular the reactions of 2,4-dinitrochlorobenzene
Methanol and pyridine were purified by the methods
already described. Solvents were stored in special vessels which
allow delivery without air contamination.
with amines show βN u = 0.52.
The nature and extent of solute–solvent interactions can
1
0
alter tremendously the properties of the nucleophile; the vari-
ations are usually satisfactorily correlated by some of the sev-
Ancillary spectrophotometric measurements
1
0,11
eral quantitative structure–activity relationships reported.
UV–VIS spectra of the substrate, the product, and different
mixtures of both compounds with aniline at several concen-
trations were recorded in a Shimadzu spectrophotometer. UV
spectra of control solutions of the substitution product in tolu-
ene containing different amounts of aniline were recorded, and
the extinction coefficients determined at λ = 450 nm where the
reagents are transparent under these conditions. All the solu-
tions were found to obey Beer’s law.
The term quantitative structure–property relationship (QSPR)
has been proposed for cases where a specific property, such
1
2
13,14
as basicity, is examined. It is well known
that amines
undergo autoassociation in aprotic media giving rise to aggre-
gates of various stoichiometries; the dominating aggregate is
3
usually a dimer with typical formation constants of 0.1–1.0 dm
mol .
Ϫ1
On the other hand, it is now well established that molecular
complexes may play a catalytic role in chemical transform-
Kinetic procedures
1
5,16
ations,
and their influence in S Ar reactions with amines is
N
17
Kinetic runs were performed by the methods previously
23
the subject of intense current research. Although some
authors argue that these complexes are not real intermediates
on the reaction path, increasing evidence is being accumulated
following the appearance of the reaction product at
reported,
λ = 450 nm. In all cases pseudo-first-order kinetics were
observed. Pseudo-first-order rate coefficients, k , were obtained
by the least-squares method as the slope of the correlation
1
8
ψ
1
9
regarding their role in substitution reactions.
The present work described kinetic and spectrophotometric
studies on the reaction of 2,4-dinitrochlorobenzene with aro-
Ϫ A )/A against time, where A is the optical density of
the reaction mixture measured at ‘infinity’ (more than ten
ln(A∞
t
∞
∞
half-lives); the second-order rate coefficients, k , were obtained
A
by dividing k by the amine concentrations. Rate coefficients
were reproducible to ±2%. No corrections for expansion coef-
ficients were applied to the concentration values.
ψ
†
Depto. Química Orgánica, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires, Pab. II, P. 3, 1428 Buenos Aires,
Argentina.
J. Chem. Soc., Perkin Trans. 2, 1997
2125

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a
Table 1 Reaction of 2,4-dinitrochlorobenzene, DNClB, with aniline,
B, in toluene at 40 ЊC. Pseudo-first- (k ), second- (k ) and third- (k /[B])
ψ
A
A
order rate coefficients
Ϫ6
Ϫ6
Ϫ6
k /10
s
k /10
(k /[B])/10
dm mol
ψ
A
A
Ϫ1
3
Ϫ1 Ϫ1
6
Ϫ2 Ϫ1
[
B]/
dm mol
s
s
0
1
1
2
2
3
.504
.01
.50
.01
.47
.01
0.150
0.489
1.71
3.29
5.33
9.16
0.30
0.485
1.14
1.64
2.16
3.05
0.481
0.588
0.759
0.813
0.874
1.01
a
Ϫ3
[
DNClB] = 2.5 × 10 .
Fig. 1 Overall second-order rate coefficients, k , for the reaction of
A
2
,4-dinitrochlorobenzene with aniline in toluene at 40 ЊC, as a function
Results and discussion
of [aniline]
Rate dependence with aniline concentration
The rate of reaction of 2,4-dinitrochlorobenzene, DNClB, with
aniline in toluene was determined at aniline concentrations 0.5–
25
of the partial reaction rate constants by the ‘inversion’ plot,
etc.
3
.0 . Formation of N-(2,4-dinitrophenyl)aniline was quanti-
Molecular complexes
Formation of molecular complexes between anilines and nitro
compounds has long been known. Silber and co-workers
have thoroughly studied the formation of electron–donor–
acceptor (EDA) complexes between nitroaromatic compounds
tative and pseudo-first-order kinetics were observed throughout
the work; the pseudo-first-order rate coefficients are given in
Table 1. The second-order rate coefficients, k , were found to
increase rapidly with aniline concentration, [B], as shown in
Table 1; the plot of k vs. [B] (Fig. 1) shows a quadratic depend-
30
19,31
A
A
and aliphatic amines, and their role in S Ar reactions. The
proposed reaction scheme is shown by eqn. (4).
N
ence. If the quotient k /[B] is plotted against [B], (not shown) a
A
straight line is obtained, consistent with it being third-order in
the amine term in the kinetic law, as had been previously
observed for other aromatic nucleophilic substitutions with
KS
S
+
B
'EDA complex'
2
4
amines in aprotic solvents.
This result is striking, since most of the S Ar for which a rate
k3, B
N
k
–1
k1
dependence that is third-order in amine had previously been
reported were reactions in which the decomposition of the
'intermediate complex'
Products
(4)
1
7,25,26
zwitterionic intermediate was rate determining.
The reac-
k2
tions of 2,4-dinitrochlorobenzene with amines show a β
=
N u
2
7
0
.52, consistent with bond formation to the nucleophile being
For K [B] ӷ 1 and (k ϩ k [B]) Ӷ k the whole kinetic
Ϫ1
equation can be simplified to eqn. (5), which shows that a linear
rate limiting and an almost symmetric position of the transition
s
2
3
8
state along the reaction coordinate. On the other hand, a recent
study of reactions of DNClB with a series of nucleophiles that
7
k k K k k K [B]
spans a reactivity range of almost 10 shows a satisfactory
1
2
s
1
3
s
kA =
ϩ
(5)
9
correlation (slope 0.95 ± 0.13) with the Nϩ values. This last
kϪ1
kϪ1
correlation includes neutral as well as anionic nucleophiles, for
which doubtless the first step is rate limiting.
response between kA and [B] is expected for base-catalysed
reactions.
The present results can be interpreted in terms of the ‘dimer
nucleophile’ mechanism in which a dimeric aggregate of the
The reactions of 1,2-dinitrobenzene with aliphatic primary
amines were fully examined; the formation of EDA complexes
between reactants was not kinetically distinguishable, although
their presence was detected spectrophotometrically by a non-
2
8
amine is considered to attack the substrate in the first step
[
eqns. (1–3)]. In this mechanism, attack by the monomer of the
3
1
K
zero optical density at zero time. Nevertheless, in any case,
19
2
B
B:B
(1)
(2)
k2
Silber and co-workers assume that the formation of EDA
complexes might be responsible for a rate dependence that is
third-order in amine; the EDA complex (if formed) evolves to
the intermediate complex without requiring an additional
amine molecule.
k1
S + B:B
[SB2]
Products
k–1
k3, B
Aniline being a better donor than aliphatic amines, it was of
interest to examine the possibility of formation of EDA com-
plexes in the present case. Fig. 2 shows the plot of the absorb-
ance at 450 nm extrapolated to t = 0 for reactions carried out at
several aniline concentrations up to 3 . In spite of the fact that
the substrate is almost transparent at this wavelength, a non-
negligible absorbance was observed that increases with [anil-
ine], indicating likely formation of a complex between the
substrate and aniline. Evidence for the formation of a complex
between the product and aniline was thoroughly sought but was
not found.
k1k2K[B]
k–1
+
k1k3K[B]2
kA =
(3)
+
k2 + k3[B]
nucleophile is not excluded, but the dimer is more reactive
because of its higher donicity. 2
In eqn. (3), K = [B:B]/[B ] stands for the amine auto-
0
1
2–16
association constant, which is known to be low
and justifies
application of the rate law given by eqn. (3) to the mechanism
suggested. Apart from the observed kinetics, third-order in
amines, the ‘dimer nucleophile’ mechanism was confirmed by
several other features such as: (a) ‘abnormal’ solvent effects; (b)
effects of HBD (hydrogen-bond donor) and HBA (hydrogen-
bond acceptor) co-solvents;
cis- and trans-diaminocyclohexylamines; (d) effects of HBD
and HBA additives forming mixed aggregates; (e) calculation
An alternative interpretation for the ‘more than two’ order in
amine observed in this reaction was the formation of a mole-
cular complex substrate–amine, which would not be on the
2
2,24
(c) conformational effects of
29
20
reaction coordinate to products (Scheme 1). A similar mech-
2
8
anism was then proposed to explain the third-order dependence
2
126
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 2 Reaction of 2,4-dinitrochlorobenzene, DNClB, with aniline,
a
B, in methanol–toluene binary solvents at 40 ЊC. Second-order reaction
Ϫ5
3
Ϫ1 Ϫ1
rate coefficients, kA (10 dm mol
s ).
Ϫ5
3
(
%) Methanol
E (30)/
kcal mol
k /10 dm
mol
T
A
Ϫ1 b
Ϫ1 Ϫ1
x (MeOH)
(v/v)
s
0
0
0
0
0
0
0
1
.00
.05
.22
.40
.64
.80
.91
.00
0
2
33.9
33.9
47.9
49.4
51.2
52.8
53.1
55.5
0.052
0.113
1.09
1.58
1.86
2.85
3.04
3.11
10
20
40
60
80
100
a
Ϫ4
b
Fig. 2 Optical densities at 450 nm of the reaction mixtures of 2,4-
dinitrochlorobenzene with aniline in toluene at 40 ЊC, extrapolated to
t = 0, as a function of [aniline]
[DNClB] = 5 × 10 ; [B] = 2 . P. M. E. Mancini, A. Terenzani,
M. G. Gasparri and L. R. Vottero, J. Phys. Org. Chem., 1995, 8, 617.
1 cal = 4.184 J.
+
B
S
molecular complex (MC)
+B
+
B
k′1 k′–1
k–1
k1
intermediate
k′2
k2
intermediate
k′3, B
Products
k3, B
Scheme 1
on amine observed for systems where the decomposition of the
1
8
intermediate is rate determining.
The mechanism shown in (Scheme 1) is essentially the same
as that in eqn. (4), but the major conflict of this mechanism is
the requirement of an additional molecule of amine, associated
with the assumption that the molecular complex cannot evolve
to the intermediate.
Fig. 3 Solvent effects on the second-order rate coefficients, k , for the
reaction of 2,4-dinitrochlorobenzene with 2  aniline in toluene–
A
methanol as a function of E (30) values (see ref. 34)
T
Solvent effects
In S Ar involving amines as the nucleophile, numerous recent
binary solvents from small additions of methanol up to pure
methanol. An ‘inverse’ solvent effect is found for this reaction:
an increase in rate is observed instead of the expected decrease.
The plot of kA vs. MeOH (%) (not shown) is a curve; the
increase in rate is greater for small contents of the hydroxylic
solvent, and then asymplotically decreases.
N
studies have afforded evidence for the importance of the nature
of the solvent in determining whether the formation or the
decomposition of the zwitterionic intermediate will be the rate-
determining step and it is known that a change in solvent may
1
7
cause a change in the rate-determining step. In the reactions
of DNClB with amines it was previously observed that the rates
were higher in benzene than in methanol; the increase in rate
was ascribed to the ‘built-in solvation’ effect due to the inter-
action between the o-nitro group and the amine hydrogen, in
The E values for several binary solvents have been recently
T
3
4
reported; Fig. 3 shows the plot of kA vs. ET values for the
mixed solvents. It is evident that no correlation exists between
the rates and the parameter which is considered to be a good
measure of the polarity of the medium; therefore the observed
effect cannot be due to a non-specific bulk solvent effect.
3
2
the zwitterionic complex. It was further reported that addition
of small amounts of ethanol to benzene causes a huge decrease
in the rates of reaction of o-nitro substituted halobenzenes with
Taking into account the increasing effect of small additions
of methanol, a specific effect must be involved instead. It is
proposed that the aromatic solvent forms complexes with the
substrate; these complexes would be less reactive than the ‘free’
reactants and should ‘decompose’ before reaction; addition of
small amounts of methanol would interfere with the complexes,
and an increase in rate is expected. To confirm this assumption,
the effect of addition of a non-protic aliphatic co-solvent, like
hexane, was examined: the rate of the reaction increases with
small additions of hexane to toluene, and the same was
observed for the reactions of DNClB with substituted ani-
1
7
piperidine.
In a thorough examination of the solvent effects in the reac-
tions of DNClB with piperidine the rate dependence with
3
3a
amine concentration was studied in twelve aprotic solvents as
3
2
well as in ten protic solvents. It was found that the reaction
does not exhibit base catalysis in any of the solvents studied;
3
3
that is, addition of piperidine is rate limiting in all the cases.
Moreover, the rates in aprotic solvents were well correlated with
1
0
the Dimroth–Reichardt E (30) values. The observation of a
T
satisfactory correlation between reactivity and the ET par-
ameter in HBA aprotic solvents was interpreted as an indic-
ation of the strong intramolecular hydrogen bonding in the
3
5
lines. A similar inhibitory effect by benzene was reported in
the reactions of the same substrate with butylamine and the
rates increased with the addition of small amounts of hexane
3
3a
intermediate complex. On the other hand, and in spite of
increased polarity, the reactivity in hydroxylic solvents is slower
than in any of the aprotic solvents studied and no correlation
1
9
to the benzene. Within the frame of the ‘dimer nucleophile’
mechanism it is easily understood that the reactant–toluene
complexes should be less reactive; the effect of methanol on the
complexes is expected to be greater than on the aniline–aniline
aggregates, due to their smaller concentrations and the weaker
interactions in those complexes. It can be observed in Table 2
that the effect of methanol becomes almost negligible for
3
3b
was observed with E values.
T
It was then of interest to examine the effect of adding small
amounts of methanol, in the reactions of DNClB with aniline.
Table 2 gives the observed second-order rate coefficients for
the reactions of DNClB with aniline in methanol–toluene
J. Chem. Soc., Perkin Trans. 2, 1997
2127

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a
Table 3 Reaction of 2,4-dinitrochlorobenzene, DNClB with aniline
in the presence of pyridine in toluene at 40 ЊC. Second-order rate
Ϫ5
3
Ϫ1 Ϫ1
coefficients, kA (10 dm mol
s ).
[
Aniline]
[
Pyridine]
/

0.5 
1 
2 
0
0
0
0
0
.0
.1
.2
.3
.5
0.0240
0.0622
0.0569
0.0725
0.0470
0.0820
0.0945
0.129
0.101
0.132
0.182
0.228
0.284
a
Ϫ4
[
DNClB] = 5 × 10 .
Fig. 5 Overall second-order rate coefficient, k , for the reaction of 2,4-
A
dinitrochlorobenzene with aniline in presence of constant [pyridine] in
toluene at 40 ЊC, as a function of [aniline]. ᭝ [pyridine] = 0; ᭿ 0.1 
pyridine; ᭺ 0.2  pyridine; ᭹ 0.3  pyridine
Fig. 4 Reaction of 2,4-dinitrochlorobenzene with aniline at 40 ЊC.
Second-order rate coefficients, k , plotted as a function of [aniline]; in
A
Ϫ3
᭹
toluene, ᭿ benzene, ᭡ chloroform. [2,4-DNClB] = 2.5 × 10

methanol contents >60%. For small contents of methanol the
release of the inhibitory effect of toluene overcomes the effect
expected for a HBD solvent on the aniline dimer; as previously
reported for MeOH > 20% the third-order dependence on
Fig. 6 Overall second-order rate coefficients, k , for the reaction of
A
2
,4-dinitrochlorobenzene with aniline in the presence of pyridine in
toluene at 40 ЊC. Influence of [pyridine] at: ᮀ 0.5  aniline, ᭺ 1 
aniline, ᭹ 2  aniline
2
2
amine no longer holds.
The solvent effect on the amine concentration dependence
was then examined in other aprotic solvents. Fig. 4 shows the
plot of kA vs. [aniline] in toluene, benzene and chloroform.
shows the overall reaction picture which involves the monomer,
the dimer and the mixed aggregate.
The whole kinetic expression that takes into account the
various reaction pathways, as well as the simplifications that
can apply to limiting situations are fully discussed in ref. 17.
2
0
20
It can be observed that in the three solvents the behaviour is
parabolic; a similar solvent effect was observed in the reactions
of aniline with 2,4-dinitrofluorobenzene, where also the ‘dimer
The general expression for k can be reduced to the condensed
A
2
9
nucleophile’ mechanism was fully confirmed. On the other
hand, the observed lack of sensitivity to a change in the aprotic
solvent (Fig. 4) justifies the analysis of the effect of [aniline] in
Fig. 1 purely as a matter of the composition of the activated
complex.
eqn. (7), where B and P stand for aniline and pyridine, respect-
ively. Several reactions were carried out to test eqn. (6) and the
four constants could be determined which are given in eqn. (7).
2
k
= k
[B] ϩ k
[B] ϩ k
[B][P] ϩ k
[P]
(6)
A
a
b
c
d
2
Effect of a HBA additive
k
A
= 0.057[B] ϩ 0.003[B] ϩ 0.140[B][P] ϩ 0.09[P] (7)
Overwhelming evidence of the participation of mixed aggre-
gates between the amine nucleophile and HBA additives on
Table 4 gives the observed kA for different [aniline] and
[pyridine] as well as the kA calculated from eqn. (7). Experi-
ments carried out in the range [aniline] = 0.5–2.0 , in the pres-
ence of [pyridine] from 0.1 to 0.5 , showed that eqn. (8) holds
in the whole range of concentrations studied. It can be
1
7,19,36
the rates of S Ar has been accumulated in recent years.
N
The enhancement effect of the HBA additives is intrinsically
1
7
involved in the ‘dimer nucleophile’ mechanism, and it is
also interpreted by the mono-hetero-conjugate mechanism
2
6
developed by Hirst. In fact, both mechanisms can be con-
sidered as part of the same spectrum.
observed that catalysis by the stronger base pyridine (k =
d
0.090) is more important than by aniline (k = 0.057), while
a
The influence of pyridine was studied by measuring the rate
dependence with the aniline concentration at various fixed
reaction with the ‘mixed aggregate HBA–nucleophile’ (in which
the nucleophile is H-bonded to a stronger base) is more import-
[
pyridine]; the second-order rate coefficients are given in
ant (k = 0.140) than with the self-associated nucleophile
c
Table 3. It can be observed in Fig. 5 that the quadratic depend-
ence of k on [aniline] holds at every fixed [pyridine], even in the
(k = 0.003), as expected on the basis of the ‘dimer nucleophile’
b
mechanism. As shown in Table 4, the second-order rate co-
efficients calculated by eqn. (7) agree satisfactorily with the
experimental kA.
A
presence of [pyridine] as high as 0.5 . On the other hand, when
the reactions were carried out in the presence of fixed amounts
of aniline, the observed second-order rate coefficients were pro-
portional to [pyridine]. Fig. 6 shows the linear dependence of
kA with [pyridine] at fixed [aniline].
Within the frame of the ‘dimer nucleophile’ mechanism,
these results are interpreted as an indication of the formation
of mixed aggregates between aniline and pyridine; Scheme 2
Although it has been generally accepted that base catalysis
should not be expected in the S Ar reactions with chloro-nitro
N
compounds since chloride ion is a good nucleofuge, genuine
base catalysis has previously been reported for a few reactions
1
9,37
of DNClB in non-polar aprotic solvents.
For the acceler-
ation effect observed in the reaction of DNClB with piperidine
2
128
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
Scheme 2
a
Table 4 Reaction of 2,4-dinitrochlorobenzene, DNClB with aniline
in the presence of pyridine, in toluene at 40 ЊC. Experimental and
law for S Ar. Even in a recent re-examination of the reaction of
N
18
2
,4-DNFB with n-butylamine in toluene, Forlani and Bosi
Ϫ5
3
Ϫ1 Ϫ1
calculated second-order rate coefficients, kA (10 dm mol
s ).
explain the observed second order in amine (a classical example
1
7,30,39
Ϫ5
Ϫ5
of base catalysis in aprotic solvents)
by the formation of a
kA (exptl.)/10
kA (calc.)/10
dm mol
3
Ϫ1 Ϫ1
3
Ϫ1 Ϫ1
similar substrate–catalyst ‘molecular complex’, and propose
that detachment of the nucleofuge occurs in a rapid step. This is
in conflict with the overwhelming evidence that decomposition
[
Pyridine]/
[Aniline]/
dm mol
s
s
0
0
0
0
0
0
0
0
0
0
0
0
0
.0
.1
.2
.3
.0
.1
.2
.3
.0
.1
.2
.3
.5
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
0.024
0.062
0.057
0.073
0.047
0.082
0.095
0.129
0.101
0.133
0.183
0.229
0.284
0.027
0.044
0.060
0.077
0.054
0.077
0.101
0.125
0.102
0.140
0.179
0.217
0.292
of the zwitterionic intermediate is rate determining in the reac-
17,30,38
tions of fluoronitrobenzenes in aprotic solvents,
which has
recently been confirmed by the elegant determination of a fluor-
ine kinetic isotope effect in the reaction of 2,4-dinitrofluoro-
4
0
benzene with piperidine in tetrahydrofuran solution.
The ‘amine dimer’ acting without dissociation in reactions of
amines in non-polar aprotic solvents, has also recently been
proposed for other reactions apart from S Ar. Thus, in the
N
bimolecular substitution reactions 1of amines with: (a)
4
hydroximoyl chlorides in benzene, (b) 1,1,1-trichloro-4-
4
2
methoxy-pent-3-en-2-one in toluene and benzene, (c) addition
a
Ϫ4
43
[
DNClB] = 5.0 × 10 .
to trans-(2-furyl)nitroethylene, a third-order in amine kinetic
law has been observed. In all these cases, the proposed reaction
route is a reversible nucleophilic attack by the amine dimer
followed by the base-catalysed transformation of the inter-
mediate into the product. These results allow us to conclude
that the dimer nucleophile mechanism is well established not
in hexane, Silber and co-workers have proposed a mechanism in
which the second molecule of amine acts as a ‘bifunctional
catalyst within aggregates of conveniently charged dipolar
1
9
species’. A similar explanation, called ‘catalysis of catalysis’,
3
8
had been offered before. Both interpretations are based on the
existence of aggregates of the nuceophile in non-polar aprotic
solvents, and their involvement in the reaction coordinate; the
observed results in the present work are consistent with such
aggregates operating as the nucleophiles in the first step.
Ar but also for other nucleophilic reactions with
only for SN
amines in non-polar aprotic solvents.
Acknowledgements
The alternative explanation for the greater than second-order
kinetic dependence on amine of the same reaction offered by
Financial support from the Universidad Nacional del
Comahue; the Universidad de Buenos Aires and the National
Research Council (CONICET) of Argentina is gratefully
acknowledged.
2
0
Forlani and co-workers who assumed that the kinetic law was
due to the formation of molecular complexes is conceptually
conflicting. Formation of these complexes is well accepted, but
its role in the mechanism is not kinetically distinguishable
regarding the amine dependence. Although many authors have
observed the formation of those complexes in different systems
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Paper 6/07108B
Received 18th October 1996
Accepted 11th April 1997
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©
Copyright 1997 by the Royal Society of Chemistry
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130
J. Chem. Soc., Perkin Trans. 2, 1997