Kinetic studies of σ-adduct formation and nucleophilic substitution in the reactions of ethyl 2,4,6-trinitrophenyl ether, some phenyl 2,4,6- trinitrophenyl ethers, and phenyl 2,4-dinitronaphthyl ether with aniline in dimethyl sulfoxide

Article abstract of DOI:10.1139/v97-225

The reaction of ethyl 2,4,6-trinitrophenyl ether with aniline in dimethyl sulfoxide containing Dabco occurs in two stages. The first gives 5, the σ-adduct intermediate on the substitution pathway, which has been identified spectroscopically. The second yields 2,4,6-trinitrodiphenylamine, the substitution product. Kinetic studies show that proton transfer is rate limiting both in the formation of the intermediate and in its subsequent acid-catalysed decomposition. Phenoxide is a considerably better leaving group than ethoxide and the substitution reactions of phenyl 2,4,6- trinitrophenyl ethers and phenyl 2,4-dinitronaphthyl ether with aniline in DMSO occur without the accumulation of intermediates. The kinetics indicate both uncatalysed and base-catalysed pathways. The kinetic and equilibrium data for reaction of the ethyl and phenyl ethers are compared with data for σ-adduct formation from 1,3,5-trinitrobenzene and aniline.

Full text of DOI:10.1139/v97-225

627
Kinetic studies of σ-adduct formation and
nucleophilic substitution in the reactions of
ethyl 2,4,6-trinitrophenyl ether, some phenyl
2,4,6-trinitrophenyl ethers, and phenyl
2,4-dinitronaphthyl ether with aniline in
dimethyl sulfoxide
Michael R. Crampton and Ian A. Robotham
Abstract: The reaction of ethyl 2,4,6-trinitrophenyl ether with aniline in dimethyl sulfoxide containing Dabco occurs in two
stages. The first gives 5, the σ-adduct intermediate on the substitution pathway, which has been identified spectroscopically.
The second yields 2,4,6-trinitrodiphenylamine, the substitution product. Kinetic studies show that proton transfer is rate
limiting both in the formation of the intermediate and in its subsequent acid-catalysed decomposition. Phenoxide is a
considerably better leaving group than ethoxide and the substitution reactions of phenyl 2,4,6-trinitrophenyl ethers and phenyl
2,4-dinitronaphthyl ether with aniline in DMSO occur without the accumulation of intermediates. The kinetics indicate both
uncatalysed and base-catalysed pathways. The kinetic and equilibrium data for reaction of the ethyl and phenyl ethers are
compared with data for σ-adduct formation from 1,3,5-trinitrobenzene and aniline.
Key words: nucleophilic substitution, proton transfer, base catalysis, σ-adducts.
Résumé : La réaction de l’oxyde d’éthyle et de 2,4,6-trinitrophényle avec l’aniline, dans le diméthylsulfoxyde contenant du
DABCO, se produit en deux étapes. La première fournit le produit 5, l’adduit-σ intermédiaire sur la voie de la substitution,
que l’on a identifié par spectroscopie. La deuxième conduit à la 2,4,6-trinitrodiphénylamine, le produit de substitution. Des
études cinétiques ont permis de montrer que le transfert de proton est l’étape cinétiquement limitante tant pour la formation de
l’intermédiaire que pour sa décomposition acidocatalysée subséquente. L’ion phénolate est un bien meilleur groupe partant
que l’éthanolate; les réactions de substitutions des oxydes de phényle et de 2,4,6-trinitrophényle, et de l’oxyde de phényle et
de 2,4-dinitronaphtyle, avec l’aniline dans le DMSO se produisent sans accumulation d’intermédiaires. Les données
cinétiques indiquent la présence de voies réactionnelles non catalysées et d’autres catalysées par les bases. On a comparé les
données cinétiques et d’équilibre pour les réactions des oxydes d’éthyle et de phényle avec celles relatives à la formation de
l’adduit-σ à partir du 1,3,5-trinitrobenzène et de l’aniline.
Mots clés : substitution nucléophile, transfert de proton, catalyse par les bases, adduits-σ.
[Traduit par la rédaction]
copy, that reaction of the TNB–methoxide σ-adduct with ani-
line (4), or with ring-substituted anilines (5), resulted in the
formation of TNB–anilide adducts, such as 1.
Interestingly, kinetic studies (6, 7) showed that this conver-
sion involved a dissociative mechanism rather than an SN2
Introduction
Whereas the reaction of 1,3,5-trinitrobenzene, TNB, with pri-
mary and secondary aliphatic amines in dimethyl sulfoxide,
DMSO, spontaneously produces σ-adducts (1–3), reaction
with aromatic amines requires the presence of a strong base.
Buncel and co-workers showed, by use of 1H NMR spectros-
Received July 30, 1997.
This paper is dedicated to Professor Erwin Buncel in
recognition of his outstanding contributions to the study of
physical-organic chemistry.
M.R. Crampton1 and I.A. Robotham. Chemistry Department,
Durham University, Durham, U.K., DH1 3LE.
displacement on the σ-adduct. It was later found that 1 may be
formed directly from TNB and aniline in DMSO (8), or in
acetonitrile (9), in the presence of a tertiary amine, B, such as
Dabco or triethylamine, to act as a proton acceptor. The
1
Author to whom correspondence may be addressed.
Telephone: +44 191 374 3130. Fax: +44 191 384 4737.
E-mail: M.R.Crampton@durham.ac.uk
Can. J. Chem. 76: 627–634 (1998)
© 1998 NRC Canada

628
Can. J. Chem. Vol. 76, 1998
Scheme 1.
Fig. 1. Visible spectra of 3 (4 × 10–5 mol dm–3) in DMSO
containing aniline (0.1 mol dm–3), Dabco (0.05 mol dm–3), and
Dabco hydrochloride (0.01 mol dm–3) taken after (a) 1 min, (b)
5 min, and (c) 3 h. The spectra show the rapid formation of 5
followed by the slower conversion to 6.
leaving-group expulsion is the overall rate-limiting step. How-
ever, the observation of base catalysis in reactions of the
phenyl aryl ethers was best explained in terms of rate-limiting
proton transfer from a zwitterionic intermediate to base (17).
Previous studies of nucleophilic substitution using aniline as
the nucleophile have shown that in reaction with methyl 2,4,6-
trinitrophenyl ether there is competition between attack at the
ring carbon atom to give a diphenylamine derivative, and at
the side-chain carbon to give picric acid (19). Reactions of
aniline with phenyl 2,4,6-trinitrophenyl ether yield the
diphenylamine derivative and are subject to base catalysis in
methanol, acetonitrile, tetrahydrofuran, ethyl acetate, and ben-
zene (20). However, there was little evidence for catalysis in
DMSO (19, 21).
Here we examine the kinetics of the reactions of 3, and of
some ring-activated phenyl aryl ethers with aniline in DMSO.
Our results show that in these substitution reactions, as in the
reaction of aniline with TNB, proton transfer may be the rate-
limiting step.
Results and discussion
Kinetic and equilibrium measurements of the reactions with
aniline were generally made in the presence of Dabco and
Dabco hydrochloride. The pKa values (22) in DMSO of the
protonated forms of the bases are, for aniline, 3.82, and for
Dabco, 9.06. Hence the value of the equilibrium constant, Ka′,
for eq. [1] is 5.8 × 10–6.
[1]
aniline + DabcoH+ º aniline H+ + Dabco
Ka(DabcoH+)
Ka′ =
= 5.8 × 10−6
+
Ka(AnilineH )
reaction involves two steps, as shown in Scheme 1. The first
step is thermodynamically unfavourable due to the weak
basicity of aniline. However, the presence of a strong base
renders the proton transfer step sufficiently favourable to allow
formation of the anionic adduct. Nevertheless, kinetic studies
(10, 11) showed that the proton transfer from the zwitterion,
2, is rate limiting, corresponding to the condition k–1 > kB[B].
When Dabco was used as the added base, a specific salt effect
involving association of protonated Dabco with chloride ions
was observed (12). There is also clear kinetic evidence for
rate-limiting proton transfer in the reactions of TNB with
aliphatic amines (13–15).
Thus the concentrations of the reagents in the reaction mix-
ture will be very similar to the stoichiometric concentrations;
relatively little protonation of aniline will occur.
Reactions of ethyl 2,4,6-trinitrophenyl ether, 3
Ultraviolet–visible spectra of 3 in DMSO containing aniline
(0.1 mol dm–3) indicated no reaction during 90 min. When the
reaction was repeated with the addition of Dabco and DabcoH+
a biphasic reaction was observed. The spectra in Fig. 1 show
that, in the first stage, a species with λmax 430 nm and 500 nm
is produced. The spectrum is typical of that for σ-adducts (2,
1
3) and H NMR experiments confirm the formation of 5, the
We have previously examined the kinetics of the reaction
of ethyl 2,4,6-trinitrophenyl ether (16), 3, and some ring-acti-
vated phenyl aryl ethers (17) with aliphatic amines in DMSO.
In agreement with the classic work of Orvik and Bunnett (18),
the results for 3 indicated that substitution involves the specific
base – general acid catalysis mechanism, SB–GA, in which
intermediate on the substitution pathway. A second, much
slower, reaction gave a species with λmax 445 whose spectrum
is identical to that of 2,4,6-trinitrodiphenylamine, 6, in the re-
action medium. Since the pKa value (22) of 6 is 8.01, this
species will be present largely in its deprotonated form, 7.
The 1H NMR spectrum of 3 in [2H6]DMSO shows bands at
© 1998 NRC Canada

629
Crampton and Robotham
Scheme 2.
Table 1. Kinetic and equilibrium data for the equilibration of 3 and 5 in DMSO at 25°C.
[Aniline]/mol dm–3 [Dabco]/mol dm–3 [Dabco H+]/mol dm–3 Abs a (430 nm) K1KDabco b/dm3 mol–1 kfast/10–3 s–1 kcalc c/10–3 s–1
0.10
0.10
0.10
0.10
0.10
0.01
0.02
0.04
0.06
0.08
0.02
0.04
0.06
0.10
0.15
0.10
0.10
0.10
0.10
0.10
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.63
0.79
0.81
0.88
—
7.5
7.6
5.6
5.2
—
14
14
16
20
24
14
14
16
20
25
0.43
0.58
0.80
0.88
—
6.9
6.2
8.0
8.6
—
3.5
3.3
4.9
8.5
4.7
7.9
12
16
12
16
a Absorbance at completion of fast reaction forming 5.
Abs.[DabcoH+]
b Calculated as
.
(1.05 − Abs)[An][Dabco]
c Calculated from eq. [6] with K1kDabco = 1.17 dm6 mol–2 s–1, kDabcoH+ = 0.2 dm3 mol–1 s–1, and kAn/kDabco = 0.46.
were made in buffered solutions, Dabco with Dabco hydro-
chloride, to suppress picric acid formation.
δ 9.09 (ring hydrogens), 4.26 (q, J = 7 Hz, OCH2), and 1.35
(t, CH3). In the presence of aniline (1 equiv.) and Dabco (1
equiv.), bands due to 5 are observed at δ 8.62 (ring), 3.14 (q,
J = 7 Hz, OCH2), and 1.08 (t, CH3). It is interesting (4, 5) that
the anilide group of 5 shows a singlet at δ 5.98 attributed to
the amino hydrogen together with spin-coupled, J = 8 Hz,
bands due to ring hydrogens at δ 6.93(t), 6.52(t), and 6.35(d).
Over a period of 1 h the bands due to 5 decrease in intensity
while bands attributable to the substitution products, 7 and
ethanol, increase in intensity. A small band at δ 8.58 is also
observed, indicating the formation of some picric acid.
UV–visible measurements show that, in solutions contain-
ing Dabco but without aniline, 3 is very slowly converted to
picric acid, λmax = 380 nm. This reaction could be inhibited by
the presence of Dabco hydrochloride. The latter observation
indicates that picric acid formation is likely to be due to attack
of hydroxide, formed from adventitious water in the solvent,
at the 1-position rather than attack by Dabco at the side-chain
ethyl group. All quantitative measurements involving aniline
Our kinetic and equilibrium results are interpreted in terms
of Scheme 2, in which the zwitterion, 4, is expected to be an
intermediate present in low concentrations. The first stage of
the reaction, representing the equilibration of 3 and 5, was
monitored at 430 nm. With concentrations of aniline (An),
Dabco, and Dabco hydrochloride in large excess of the con-
centration of 3, first-order kinetics were observed. Rate con-
stants, kfast, and absorbances at completion of the equilibration
are given in Table 1. All measurements were made in the pres-
ence of DabcoH+, 0.01 mol dm–3, to maintain constant ionic
strength.
An equilibrium constant for formation of 5 may be defined
in terms of the Dabco concentration by eq. [2]. The absorbance
values in Table 1 lead to a value for K1KDabco of 7 ± 1 dm3 mol–1.
[5]
[3]
[DabcoH+]
k1 kDabco
[2]
K1KDabco
=
=
×
+
[An] [Dabco]
k−1 kDabcoH
© 1998 NRC Canada

630
Can. J. Chem. Vol. 76, 1998
Alternatively, the equilibrium constant may be defined in
terms of the aniline concentration in eq. [3]:
of K1KDabco, and explaining why adduct formation is not ob-
served in the absence of Dabco.
[5] [AnH+]
[3]
k1
kAn
[4]
K1KAn = K1KDabco Ka′
[3]
K1KAn
=
×
=
×
[An]2
+
k−1
kAnH
The variation of kfast with values of the Dabco and aniline
concentrations is compatible only with proton transfer being
rate determining in the formation of 5. Using the condition
k–1 > kB[B] and treating 4 as a steady-state intermediate leads
to the rate expression given in eq. [5]:
Use of eq. [1] leads to eq. [4], showing that the value of
K1KAn will be many orders of magnitude lower than the value
k1
+
+
+
+
[5]
kfast
=
[An](kAn[An] + kDabco[Dabco]) + kDabcoH [DabcoH ] + kAnH [AnH ]
k−1
+
+
Here kDabco and kAn represent kB when reaction involves Dabco and aniline, respectively, while kDabcoH and kAnH represent
+
kBH for reactions with the appropriate conjugate acids.
Since KAn/KDabco has a constant value (eq. [4]) it is possible to reduce the number of variables in eq. [5] to give eq. [6]:
kAn
kAn
[An]
kDabco [Dabco]
+
+
[6]
kfast = K1kDabco[An] [Dabco] +
[An] + kDabcoH [DabcoH ] 1 +
kDabco
Excellent agreement between the calculated and observed values of kfast was obtained with K1kDabco 1.17 ± 0.10 dm6 mol–2 s–1,
3
kDabcoH 0.20 ± 0.05 dm mol–1 s–1, and kAn/kDabco 0.46 ± 0.10. The latter value indicates that aniline competes effectively with the
much stronger base Dabco in the proton transfer step, 4 º 5. This point is discussed later. It was not possible to obtain an acceptable
fit between calculated and observed values of kfast with lower values of the ratio kAn/kDabco. Combination of the values of K1kDabco
and kDabcoH+ leads to a value of K1KDabco of 5.9 dm3 mol–1, in acceptable agreement with that obtained independently from
absorbance measurements at equilibrium.
+
The slow product-forming reaction was measured at 445 nm. Values of the first-order rate constants, kslow, are reported in
Table 2. Making the assumption that 3 and 5 are in rapid equilibrium during this stage of the reaction leads to the rate expression
of eq. [7].
K1KDabco[Dabco][An]
K1KDabco[Dabco][An] + [DabcoH+]
+
+
+
+
[7]
kslow = ksub,DabcoH [DabcoH] + ksub,AnH [AnH ]
+
Here ksub,DabcoH and ksub,AnH are, respectively, the rate constants for the substitution reactions involving protonated Dabco and
+
aniline. Use of eq. [1] results in eq. [8]:
K1KDabco[Dabco][An][DabcoH+]
K1KDabco[Dabco][An] + [DabcoH+]
[Dabco]
[An]
+
+
[8]
kslow = ksub,DabcoH + ksub,AnH
Analysis of the results using the known value for K1KDabco
concentration, values of the first-order rate constant, kobs, in-
creased linearly with Dabco concentration, as shown in Fig. 3.
These results are interpreted in terms of the process shown in
Scheme 3. It is known (17, 23) that phenoxide is a considerably
better leaving group than ethoxide, by a factor of ca. 106. Our
failure to observe 10, the intermediate on the substitution path-
way, may be attributed to its rapid decomposition by loss of
phenoxide. The assumption that 9 may be treated as a steady-
state intermediate leads to the rate expression of eq. [9], where
kAn and kDabco represent kB for the respective bases.
of 7 dm3 mol–1 showed that most of the reaction flux involved
the protonated Dabco. Agreement between observed and cal-
culated values of kslow was optimized with values of 0.06 ±
3
–1
0.01 dm3 mol–1 s–1 for ksub,DabcoH and 0.005 ± 0.002 dm mol
+
s–1 for ksub,AnH Ka′. Using the known value of Ka′ leads to a
+
3
value for ksub,AnH of 860 dm mol–1 s–1.
+
Reactions of phenyl 2,4,6-trinitrophenyl ethers, 8
Reactions of 8a–c with aniline in DMSO proceeded in a single
stage. UV–visible spectra, and NMR spectra (in more concentrated
solutions), at completion of reaction were identical to those of
6, the expected substitution product, in the reaction medium.
Kinetic measurements were made with aniline and with so-
lutions containing aniline, Dabco, and Dabco hydrochloride.
With these concentrations in large excess of the substrate con-
centration, first-order kinetics were observed. Data obtained
with aniline alone are shown in Fig. 2. Plots of the second-or-
der rate constant, kobs/[aniline], versus aniline concentration
were linear with positive intercepts. At a constant aniline
k1[An] (k2 + kAn[An] + kDabco[Dabco])
[9]
kobs =
k−1 + k2 + kAn[An] + kDabco[Dabco]
Our results, which provide evidence for base catalysis, in-
dicate that the condition k–1 >> k2 + kAn[An] + kDabco[Dabco]
applies so that eq. [9] reduces to eq. [10].
[10] kobs = K1[An](k2 + kAn[An] + kDabco[Dabco])
Values obtained for K1k2, K1kAn, and K1kDabco can be found
in Table 3.
© 1998 NRC Canada

631
Fig. 2. Plots of the second-order rate constant, kobs/[aniline], versus
aniline concentration for reactions of 8a, 8b, and 8c with aniline in
DMSO.
Crampton and Robotham
Table 2. Kinetic data for the formation of the product, 6, in DMSO
at 25°C.
[Aniline]/ [Dabco]/ [DabcoH+]/
mol dm–3 mol dm–3
mol dm–3
kslow/10–4 s–1 kcalc a/10–4 s–1
0.10
0.10
0.10
0.10
0.10
0.01
0.02
0.04
0.06
0.08
0.05
0.05
0.05
0.05
0.02
0.04
0.06
0.10
0.15
0.10
0.10
0.10
0.10
0.10
0.05
0.05
0.05
0.05
0.01
0.01
3.4
4.3
5.1
5.5
5.9
4.7
5.1
5.5
5.6
5.7
1.8
2.6
3.4
4.3
3.5
4.5
5.1
5.7
6.1
4.5
5.0
5.3
5.5
5.6
1.7
2.5
3.3
4.1
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.003b
0.005b
0.007b
0.010
a Calculated from eq. [8] with ksub,DabcoH+ 0.060 dm3 mol–1 s–1
,
ksub,AnH Ka′ 0.005 dm3 mol–1 s–1, and K1KDabco 7.0 dm3 mol-1.
+
b Ionic strength was maintained at 0.01 mol dm–3 using
tetrabutylammonium chloride.
Reactions of phenyl 2,4-dinitronaphthyl ether, 11
UV–visible and 1H NMR studies showed that in the presence
of aniline, or aniline with Dabco, 11 was smoothly converted
into N-phenyl-2,4-dinitronaphthylamine. Kinetic measure-
ments at 440 nm showed a single first-order process. Values of
the rate constant, kobs, are given in Table 4. Data are interpreted
in a manner similar to that given for 8 so that Scheme 3 is
applicable and eq. [10] will apply. Values calculated with K1k2
2.0 × 10–4 dm3 mol-1 s–1, K1kAn 1.1 × 10–2 dm6 mol–2 s–1, and
K1kDabco 0.24 dm6 mol–2 s–1 give excellent agreement with
Fig. 3. Plots of kobs versus [Dabco] for reactions of 8a, 8b, and 8c
with aniline (0.02 mol dm–3) in DMSO containing Dabco
hydrochloride (0.05 mol dm–3).
values of kobs
.
Comparisons
Data for reactions of 3, 8, and 11 are collected in Table 3,
where they are compared with values for the formation of 1
from TNB. The value of K1KDabco, the overall equilibrium con-
stant for formation of the adduct 5 from 3, is ca. 20 times larger
than that for formation of 1 by reaction at an unsubstituted ring
position of TNB. The major effect is likely to be an increase in
K1 due to relief of steric strain present in the parent (16, 24)
when the ethoxy group is twisted from the ring plane.
Stereoelectronic stabilization of 5 relative to 1 may also con-
tribute, as described in related systems by Buncel et al. (25).
The polar effect of the ethoxy group will be expected to result
in an increase in the value of K1 and may also give rise to a
small increase in KDabco. For comparison (16), the equilibrium
constant for formation of 12 from 3 and n-butylamine has a
value 50 times larger than that for the corresponding formation
of 13. We have not observed an adduct formed by attack of
© 1998 NRC Canada

632
Can. J. Chem. Vol. 76, 1998
Scheme 3.
Table 3. Comparison of data in DMSO at 25°C.
K1kAn K1kDabco
+
/
/
K1k2/
dm3 mol–1 s–1
kDabcoH
/
K1KDabco
dm3 mol–1
/
kAn/
k2 dm3 mol–1
Reactant
dm6 mol–2 s–1
dm6 mol–2 s–1
dm3 mol–1 s–1
kAn/kDabco
3
0.54
—
1.17
26
—
—
0.2
74
7
0.46
—
—
—
8
TNBa
8a
0.35
—
—
—
—
0.42
0.27
0.65
0.011
1.2
0.055
0.050
0.32
—
0.35
0.17
0.72
0.045
8b
8c
11
1.6
—
5.4
2
0.9
—
0.24
0.0002
—
55
a From ref. 12. Values quoted are in presence of tetraethylammonium chloride 0.01 mol dm–3, to correspond with data for reactant 3.
Table 4. Kinetic results for reaction of phenyl
2,4-dinitronaphthyl ether with aniline in DMSO at 25°C.
a
kcalc /
10–5 s–1
[Aniline]/
mol dm–3
[Dabco]/
mol dm–3
kobs
10–5 s–1
/
0.02
0.04
0.07
0.10
0.15
0.20
0.11
0.11
0.11
0.11
0.11
—
—
0.84
2.5
6.7
14
0.84
2.6
6.8
13
—
—
aniline at the unsubstituted 3-position of 3. This may be ex-
plained by the much lower thermodynamic stability expected
for this adduct compared with its isomer 5 or with the adduct
1 formed from TNB. Attack at the 3-position of 3 will not
relieve the steric strain present in the parent, and work with ali-
phatic amines (16) suggests that the adduct so formed would have
a stability ca. 100 times lower than that of the adduct 1 from TNB.
Buncel et al. (10, 11) showed that, in the formation of 1
from TNB and aniline, the proton transfer step is rate limiting.
Similarly in the formation of 5 our results indicate rate-limiting
proton transfer. From eqs. [1] and [4], KAn/KDabco has the value
5.8 × 10–6, showing that the equilibrium constant for the pro-
ton-transfer step is very much smaller when the reaction in-
volves aniline than when it involves Dabco. Nevertheless the
ratio of 0.46 obtained for kAn/kDabco shows that kinetically ani-
line may compete effectively with Dabco, a much stronger
base, in the proton-transfer step. This may seem surprising;
however, it should be remembered that the proton-transfer
—
29
28
—
50
48
0.01
0.02
0.03
0.05
0.10
44
42
71
68
92
95
150
280
150
280
a Calculated from eq. [10] with K1k2 2.0 × 10–4 dm3 mol–1 s–1
K1kAn 1.1 × 10–2 dm6 mol–2 s–1, and K1kDabco 0.24 dm6 mol–2 s–1
,
.
step, 4 º 5, in Scheme 2 will be thermodynamically favourable
even when the reaction involves aniline as B and the anilinium
ion as BH+. Thus it is known (5, 26) that the trinitrocyclo-
hexadienate group, even though negatively charged, is elec-
tron withdrawing relative to hydrogen. Hence 4 will be more
acidic than the anilinium ion. Values of kAn and kDabco will be
© 1998 NRC Canada

633
Crampton and Robotham
affected by steric factors rather than the relative basicities of
the amines. It is likely that the proton to be transferred from 4
will be hydrogen bonded to a DMSO molecule in the solvent
(13, 27, 28) and this, together with steric hindrance to the
approach of the reagents, will reduce values of rate constants
below the diffusion limit.
2,4-dinitronaphthyl ether, 11, with that for phenyl
2,4,6-trinitrophenyl ether, 8b, shows decreases in K1k2, K1kAn,
and K1kDabco by factors of 250, 24, and 7, respectively. These
changes may be attributed (2, 3) to two main factors: the de-
crease in ring activation in 11 compared to 8b, and the reduc-
tion in steric crowding at the reaction centre in the zwitterionic
It is interesting to speculate on values of rate constants for
the proton-transfer step. It has previously been estimated (15,
16) that the presence of the trinitrocyclohexadienate ring in
zwitterions such as 4 will acidify the adjacent ammonium pro-
tons by a factor of ca. 500, as indicated in eq. [11]:
[11] Ka(4)/Ka(AnH+) = 500
Use of eq. [1] allows the comparison of the relative acidities
of 4 and protonated Dabco as shown in eq. [12]:
500
5.8 × 10−6
[12] Ka (4) ⁄ Ka (DabcoH+) =
= 8.6 × 107
intermediate 14 compared to that in the corresponding inter-
mediate, 9b. The first factor will result in a decrease in the
value of K1 and hence in the value of K1k2. That the decreases
observed for K1kAn and K1kDabco are smaller than that observed
for K1k2 may be attributed to the second factor. A reduction in
steric crowding in 14 compared to 9b may allow the easier
approach of a base molecule to accept a proton from the zwit-
terionic intermediate. Hence values of kAn and kDabco may be
larger in the 2,4-dinitronaphthyl system. The kAn/kDabco ratio is
somewhat decreased in the dinitronaphthyl system and this
may indicate that the situation is being approached in which
the reduction in the electron-withdrawing ability of the ring
system renders the proton transfer from 14 to aniline thermo-
dynamically unfavourable.
+
The ratio obtained corresponds to kDabco / kDabcoH and since
kDabcoH has been found experimentally to be 0.2 dm mol–1 s–1
3
+
we obtain a value for kDabco of 1.7 × 107 dm3 mol–1 s–1. This
allows the calculation of values for kAn of 8 × 106 dm3 mol–1
s–1 and for K1 of 7 × 10–8 dm3 mol-1. Since the ratio given in
eq. [11] is not known precisely, the values calculated using it
should be regarded only as estimates.
The data in Table 3 indicate that values of rate constants for
proton transfer are larger in the formation of 1 from TNB, than
in the formation of 5. Here reaction occurs at an unsubstituted
ring position so that there is less steric congestion at the reac-
tion centre. A calculation analogous to that given above leads
here to a value for kDabco in excess of 109 dm3 mol–1 s–1.
The formation of 6 from 5 involves general acid-catalysed
expulsion of the ethoxy group. Values of the rate constants for
reaction with the anilinium ion, 860 dm3 mol–1 s–1, and Dab-
coH+, 0.06 dm3 mol–1 s–1, may be used in conjunction with
eq. [1] to calculate a Brønsted α value of 0.80.
Phenoxide is known to be a considerably better leaving
group than ethoxide (17, 23), and we did not observe interme-
diates in the substitution reactions of the phenyl ethers 8a–c.
We interpret the increases in the rate constant shown in Figs. 2
and 3 as base catalysis. Previous work (19, 21), at lower base
concentrations, did not find evidence for base catalysis. The
effects we have observed are not particularly large. However,
values calculated for K1kAn and K1kDabco for reactions of 8a–c
are similar to those found for reaction of 3, where there is
genuine base catalysis. These similarities are to be expected if
proton transfer from zwitterion, 9 or 4, to base is rate limiting.
The presence of a phenoxy group rather than an ethoxy group
at the 1-position is not expected to drastically affect the value
of K1 for formation of the zwitterion (29). Nor will this change
be expected to greatly alter the values of kAn or kDabco for the
proton-transfer step. A major difference between the phenyl
ethers, 8, and the ethyl ether, 4, is that in the former case much
of the reaction flux involves the direct uncatalysed decompo-
sition of the zwitterions, 9, by the k2 step. This step is likely to
involve intramolecular proton transfer from nitrogen to oxy-
gen coupled with carbon–oxygen bond cleavage. Leaving-
group expulsion is part of the rate-limiting step here, so that
reaction of the 4-nitrophenoxy derivative, 8c, is six times faster
than that of the phenoxy derivative, 8b (Table 3).
Experimental
Ethyl 2,4,6-trinitrophenyl ether (16), 3, phenyl 2,4,6-trini-
trophenyl ether (17), 8b, and phenyl 2,4-dinitronaphthyl ether
(17), 11, were available from previous work. The 4-substituted
phenyl 2,4,6-trinitrophenyl ethers 8a and 8c were prepared by
reaction at 45°C for 3 h of picryl chloride (1 equiv.) with
sodium hydroxide (1 equiv.) in an excess of the appropriate
4-substituted phenol containing a little water. On completion,
water was added to remove any picric acid produced and re-
crystallization from ethanol yielded 8a, mp 100°C (lit. (30) mp
103°C) and 8c, mp 163°C (lit. (30) mp 157°C). 2,4,6-Trini-
trodiphenylamine, 6, and N-phenyl-2,4-dinitronaphthylamine
were available from previous work (22). Solvent, amines, and
amine salts were prepared and (or) purified as described pre-
viously (22).
1H NMR spectra were recorded using Varian-200 XL or
VXR-400 spectrometers. UV–visible spectra and kinetic
measurements were made with Beckman Lambda 2 or Applied
Photophysics SX.17 MV stopped-flow spectrophotometers at
25°C. Reported rate constants are the means of several deter-
minations and are precise to ±5%.
Acknowledgements
We thank the Engineering and Physical Sciences Research
Council (EPSRC) for a studentship and for a research grant to
purchase the stopped-flow spectrophotometers. Mrs. J. Say is
thanked for assistance with the NMR measurements.
Comparison of data for reaction of phenyl
© 1998 NRC Canada

634
Can. J. Chem. Vol. 76, 1998
16. M.R. Crampton and P.J. Routledge. J. Chem. Soc. Perkin Trans.
2, 573 (1984).
References
1. M.R. Crampton and V. Gold. J. Chem. Soc. B, 23 (1967).
2. E. Buncel, M.R. Crampton, M.J. Strauss, and F. Terrier. Electron
deficient aromatic- and heteroaromatic-base interactions. El-
sevier, Amsterdam. 1984.
3. F. Terrier. Nucleophilic aromatic displacement. VCH, New York.
1991.
4. E. Buncel and J.G.K. Webb. Can. J. Chem. 50, 129 (1972).
5. E. Buncel and J.G.K. Webb. Can. J. Chem. 52, 630 (1974).
6. E. Buncel and J.G.K. Webb. Tetrahedron Lett. 4417 (1976).
7. E. Buncel, J.G.K. Webb, and J.F. Wiltshire. J. Am. Chem. Soc.
99, 4429 (1977).
17. R. Chamberlin and M.R. Crampton. J. Chem. Soc. Perkin Trans.
2, 1831 (1995).
18. J.A. Orvik and J.F. Bunnett. J. Am. Chem. Soc. 92, 2417 (1970).
19. J. Hirst, G. Hussain, and I. Onyido. J. Chem. Soc. Perkin Trans.
2, 397 (1986).
20. O. Banjoko and P. Otiono. J. Chem. Soc. Perkin Trans. 2, 399
(1981).
21. T.A. Emokpae, P.U. Uwakwe, and J. Hirst. J. Chem. Soc. Perkin
Trans. 2, 125 (1993).
22. M.R. Crampton and I.A. Robotham. J. Chem. Res. (S), 22 (1997).
8. E. Buncel and H.W. Leung. J. Chem. Soc. Chem. Commun. 19
(1975).
23. C.F. Bernasconi and M.C. Muller. J. Am. Chem. Soc. 100, 5530
(1978).
9. T.O. Bamkole, J. Hirst, and I. Onyido. J. Chem. Soc. Perkin
Trans. 2, 1201 (1981).
10. E. Buncel and W. Eggimann. J. Am. Chem. Soc. 99, 5958 (1977).
11. E. Buncel, W. Eggimann, and H.W. Leung. J. Chem. Soc. Chem.
Commun. 55 (1977).
24. C.M. Gramaccioli, R. Destro, and M. Simonetta. Acta Crystal-
logr. Sect. B: Struct. Crystallogr. Cryst. Chem. B24, 129 (1968).
25. E. Buncel, J.M. Dust, and F. Terrier. Chem. Rev. 95, 2261 (1995).
26. M.R. Crampton. J. Chem. Soc. B, 2112 (1971).
27. M.R. Crampton and S.D. Lord. J. Chem. Soc. Perkin Trans. 2, 369
(1997).
12. E. Buncel and W. Eggimann. J. Chem. Soc. Perkin Trans. 2, 673
(1978).
28. C.F. Bernasconi and F. Terrier. J. Am. Chem. Soc. 97, 7458
(1975).
13. C.F. Bernasconi, M.C Muller, and P. Schmid. J. Org. Chem. 44,
3189 (1979).
29. R. Chamberlin, M.R. Crampton, and R.L. Knight. J. Chem. Res.
(S), 444 (1993).
14. C.F. Bernasconi. Acc. Chem. Res. 11, 147 (1978).
15. M.R. Crampton and B. Gibson. J. Chem. Soc. Perkin Trans. 2,
533 (1981).
30. K. Okon. Rocz. Chem. 32, 213 (1958); 32, 218 (1958).
© 1998 NRC Canada