Oxonium and quinonoid intermediates in the sulfonation of dimethoxynaphthalenes (DMONs)

Article abstract of DOI:10.1039/b008160o

Monoprotonated oxonium cations (such as 2) and quinonoid dications (such as 5) of 2,3-, 2,6- and 2,7-dimethoxynaphthalenes are described. Their role as direct substrates in sulfonation reactions is discussed together with details of the sulfonation mechanism for 2,3-dimethoxynaphthalene (2,3-DMON), which is presented as a characteristic example. Steady sulfonation rate constants of 2,6- and 2,7-DMON in 80-88% H₂SO₄ are a result of the DMON protonation equilibria. Binding energies and heats of formation were calculated for several mono- and disulfonic acids of DMONs. Substituent effects are discussed.

Full text of DOI:10.1039/b008160o

View Article Online / Journal Homepage / Table of Contents for this issue
Oxonium and quinonoid intermediates in the sulfonation of
dimethoxynaphthalenes (DMONs)
Andrzej Cisak, Dorota Kusztal and Elz˙bieta Brzezin´ska
2
Department of Analytical Chemistry, Medical University of Łódz´, 90-151 Łódz´,
Muszyn´skiego 1, Poland
Received (in Cambridge, UK) 10th October 2000, Accepted 31st January 2001
First published as an Advance Article on the web 7th March 2001
Monoprotonated oxonium cations (such as 2) and quinonoid dications (such as 5) of 2,3-, 2,6- and 2,7-
dimethoxynaphthalenes are described. Their role as direct substrates in sulfonation reactions is discussed together
with details of the sulfonation mechanism for 2,3-dimethoxynaphthalene (2,3-DMON), which is presented as a
characteristic example. Steady sulfonation rate constants of 2,6- and 2,7-DMON in 80–88% H2SO4 are a result of the
DMON protonation equilibria. Binding energies and heats of formation were calculated for several mono- and
disulfonic acids of DMONs. Substituent effects are discussed.
Introduction
Results and discussion
The study of reactive intermediates is important in the devel-
opment of concepts of modern reactivity.1 In recent years
extensive studies on the mechanistic aspects of sulfonation have
been published.2 The revival of interest into sulfonation mech-
anisms of aromatic compounds is largely due to Cerfontain and
co-workers who discovered that in aqueous sulfuric acid two
reaction mechanisms operate: at lower concentrations of the
Protonations
The presence and concentration of protonated forms were
quantitatively determined by UV–VIS spectroscopy. The spec-
tra of neutral, uncharged methoxynaphthalenes and dimeth-
oxynaphthalenes in dilute aqueous acidic solutions are alike,
with a strong absorption band at 230–240 nm (ε ca. 6 × 103 m2
molϪ1, K-band) and several weak overlapping B-bands at 250–
350 nm (ε ca. 3 × 102 m2 molϪ1). The solubilities of DMONs in
aqueous 0 to 50% H2SO4 change little and are of the order of
10Ϫ5 M. However, in 50–75% H2SO4 the solubilities increase
rapidly, although the absorption coefficients (ε) and the posi-
tions of the maxima (λmax) of the absorption bands remain very
much the same. Upon dilution with water unchanged DMONs
were obtained quantitatively. This indicated reversible proton-
ation at the methoxy oxygen outside the chromophore system
of the aromatic ring and an absence of sulfonation in this range
of H2SO4 concentrations. We applied the solubility method15
for determination of protonation constants pKs, as it permits
the detection of true acid–base equilibria before the onset of
sulfonation. Quantitative determinations of the O-protonated
forms were based on measurements of solubility15 related to the
acidity function H0 by means of the Krebs and Speakman
equation (cf. Experimental). Saturation concentrations of the
substrates were measured. This made possible the evaluation of
protonation constants pK1 for which the concentrations of the
protonated forms DMONH+ and non-protonated forms of
DMON0 are equal (half-protonation). Acidity functions are
empirical scales16,17 and therefore the pK values we obtained are
not true thermodynamic quantities.18 The symbols “pK1” and
“pK2” are used to denote the value of H0 at the first and second
half-protonations. The percentage composition of more and
less protonated forms of a given substrate could be calculated
from19 pK1 = H0 + log I1, where I1 is the ionization ratio [BH+]/
[B]. Consecutive protonations are also included: for pK2,
+
acid H3SO4 is the principal sulfonating agent, while H2S2O7
takes over above ca. 83% H2SO4.3 Uncharged substituted
benzenes and their ammonium derivatives are sulfonated by
H2S2O7 with similar slopes of rate profiles3,4 in aqueous H2SO4
but a positive charge on nitrogen near the ring retards sulfon-
ation.4 In these cases there is only one form of the substrate
that reacts with sulfuric acid: neutral (PhR) or ammonium-
substituted (PhR+), except PhNO2, which has two forms in
equilibrium:5 PhNO2 and PhNO2H+ with half-protonation in
0
99.5% H2SO4.
Phenylalkanols6 are protonated on oxygen (O-protonation)
in aqueous H2SO4.
Protonated anisole7 is the principal species present above
77% H2SO4 but it has been concluded that sulfonation will pro-
ceed predominantly via the more reactive, non-protonated
form.7,8 It has been suggested9,10 that species protonated in the
ring appear as the concentration of H2SO4 increases. Proton-
ation on C is known to occur at the most electronegative
aromatic carbon atom,11 where sulfonation also takes place
and diagnostic absorption bands appear ca. 340–350 nm for
benzenonium ions.11,12 Isomers of C6H4(OR)2 are in part
protonated on oxygen or ring-protonated at C(4) and C(6) in
concentrated aqueous sulfuric acid.13 Subsequently ring sulfon-
ation occurs at the 4- and 6-positions.13
We became interested in the basic properties of 6-methoxy-2-
naphthylpropionic acid (naproxene) and 2-methoxynaph-
thalene (neroline) and found a stepwise protonation: first on
oxygen,14a then a dication is formed14b prior to sulfonation.14c
Prior to our research only sulfonations of DMONs with SO3
in CH3NO2 have been studied.2 The aim of the present work
was to describe the properties and reactivity of dimethoxy-
naphthalenes in aqueous H2SO4.
Three dimethoxynaphthalenes were chosen in which the
classically most reactive 1-position of the naphthalene ring was
left unsubstituted to see how reactive it is in sulfonation
reactions.
BH+ + H+
BH22+. We used the best general-purpose H0
solv
scale19 for 2–99% H2SO4 based on twelve nitroaniline indi-
cators.20 Thus, ionization ratios for all the protonated forms of
each substrate at any acidity H0 of 2–99% H2SO4 could be
calculated from the pK and H0 values. Our results for pK1
determinations are collected in Tables 1–3, and give plots that
are are linear with correlation factors у0.95 and slopes near
unity (Fig. 1). Fairly constant pK1 values are obtained from
538
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b008160o

View Article Online
Table 1 Determination of the first protonation constant pK1 of
2,3-DMON (Si = 2.5 × 10Ϫ5 M)
Table 4 Determination of the second protonation constant pK2 for
2,7-DMON (εmax = 23000 molϪ1 dm3 cmϪ1
)
a
b
c
a
b
c
% H2SO4
H0
log I1
pK1
% H2SO4
H0
log I1
pK1
49.78
50.36
58.60
61.00
Ϫ3.28
Ϫ3.33
Ϫ4.20
Ϫ4.50
0.53
0.48
1.16
1.51
Ϫ2.75
Ϫ2.85
Ϫ3.04
Ϫ2.99
73.51
74.95
75.63
77.18
78.52
78.63
78.95
Ϫ6.39
Ϫ6.64
Ϫ6.75
Ϫ7.00
Ϫ7.21
Ϫ7.23
Ϫ7.29
Ϫ0.68
Ϫ0.46
Ϫ0.31
Ϫ0.02
+0.35
+0.34
+0.43
Ϫ7.01
Ϫ7.10
Ϫ7.06
Ϫ7.02
Ϫ6.86
Ϫ6.89
Ϫ6.86
a H0, acidity scale according to ref. 20; Si, minimum solubility in
20–30% H2SO4; I1 and pK1, see text and Experimental. b Standard
deviation, s = 0.13.c Mean: Ϫ2.91 (45.4%).
a H0, acidity scale according to ref. 20; Si, see text and Experimental;
εmax = maximum absorption coefficient for 2,7-DMON obtained at
higher acidities, extrapolated to time t0 (preparation of solution).
b Standard deviation s = 0.10.c Mean: Ϫ7.0 (77.1%).
Table 2 Determination of the first protonation constants pK1 of
2,7-DMON (Si = 1.2 × 10Ϫ5 M)
a
b
c
% H2SO4
H0
log I1
pK1
Table 5 Determination of the second protonation constants pK2 for
2,6-DMON (εmax = 10500 molϪ1 dm3 cmϪ1
)
58.0
59.9
61.0
62.0
66.0
Ϫ4.13
Ϫ4.36
Ϫ4.50
Ϫ4.62
Ϫ5.20
0.23
0.53
0.77
0.72
1.08
Ϫ3.90
Ϫ3.83
Ϫ3.73
Ϫ3.90
Ϫ4.12
a
b
c
%H2SO4
H0
log I2
pK2
74.45
75.23
77.52
79.59
81.21
81.47
Ϫ6.56
Ϫ6.68
Ϫ7.05
Ϫ7.39
Ϫ7.67
Ϫ7.71
Ϫ1.26
Ϫ1.22
Ϫ0.80
Ϫ0.24
+0.02
Ϫ0.10
Ϫ7.82
Ϫ7.90
Ϫ7.85
Ϫ7.63
Ϫ7.65
Ϫ7.81
a H0, acidity scale according to ref. 20; Si, minimum solubility in
20–30% H2SO4; I1 and pK1, see text and Experimental. b Standard
deviation s = 0.12.c Mean: Ϫ3.90 (56.0%).
a H0, acidity scale according to ref. 20; I2 and pK2, see text and Experi-
mental; εmax, maximum absorption coefficient for 2,6-DMON obtained
at higher acidities, extrapolated to time t0 (preparation of solution).
b Standard deviation s = 0.11.c Mean: Ϫ7.8 (81.4%).
Table 3 Determination of the first protonation constant pK1 of
2,6-DMON (Si = 0.9 × 10Ϫ5 M)
a
b
c
% H2SO4
H0
log I1
pK1
68.35
70.0
71.13
74.7
Ϫ5.57
Ϫ5.82
Ϫ5.99
Ϫ6.60
Ϫ6.81
0.37
0.48
0.64
1.11
1.59
Ϫ5.20
Ϫ5.34
Ϫ5.35
Ϫ5.49
Ϫ5.22
(in 45.4% H2SO4), Ϫ3.9 for 2,7-DMON (in 56% H2SO4) and
Ϫ5.3 for 2,6-DMON (in 66.7% H2SO4). Thus, 2,3-DMON is
the strongest base, which may be attributed to theformation of
a rather stable five-membered ring with the hydrogen proton
forming a hydrogen bridge (2, Scheme 1).
76.03
a H0, acidity scale according to ref. 20; Si, minimum solubility in
20–30% H2SO4; I1 and pK1, see text and Experimental. b Standard
deviation s = 0.12.c Mean: Ϫ5.32 (66.7%).
Fig. 1 Log I2 vs. ϪH0 for: 1: 2,6-DMON, data from Table 4; 2:
2,7-DMON, data from Table 5.
measurements at different acidities, the scatter being due to
experimental errors. Taken together, we consider these results as
evidence that all three DMONs are reversibly protonated on
the methoxy oxygen in aqueous H2SO4 (O-protonation) with
formation of monoprotonated species, i.e., DMON +
Scheme 1
In aqueous sulfuric acid more concentrated than about 75%
the UV–VIS spectra change significantly. In fresh solutions a
yellow absorption band appears at longer wavelength at 380–
430 nm, where in dilute acids the absorbances of DMONs are
H+
DMONH+, with pK1 equal to Ϫ2.9 for 2,3-DMON
solv
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544
539

View Article Online
the yellow band extrapolated to the time t0, when plotted
against H0 give a sigmoidal curve characteristic of acid–base
titrations,23 the turning point of which is at the pK2 value
Ϫ7.0 (in 77.1% H2SO4) for 2,7-DMON and Ϫ7.8 (in 81.4%
H2SO4) for 2,6-DMON. The pK2 values were calculated from
[εi/(εmax Ϫ εi)] = I2; pK2 = H0 + log I2, which is another form of
the Henderson equation, where εi values are ε0 values on the
sigmoidal curve and εmax is the ε0 value on the plateau and
indicates 100% protonation of the substrate. The rise of the
yellow (i.e., carbonium) band from its appearance to full height
takes place in an interval of ca. 2 H0 units in accord with the
Henderson equation.
Thus, the pK2 values for 2,6-and 2,7-DMON represent the
second protonation step with addition of a second proton to
2+
the ring (C-protonation), DMONH+ + H+
DMONH2 ,
rather than a change from O-protonation to C-protonation of
the monocations. In less than ca. 98% H2SO4 the activity of
H3O+ and of free water3 is sufficient to form and stabilize
oxonium cations. Similar results have been obtained for
naproxene and neroline.14b Dications are also present as O- and
C-protonated desoxycorticosterone24 in 97% H2SO4.
Addition of the second H+ induces a redistribution of the
conjugated double bonds and formation of a quinonoid struc-
ture such as 5 (Scheme 1) with one hydrogen added to the most
nucleophilic center. A new extended chromophore system is
formed in the quinonoid dication as indicated by a decrease (at
time t0) of the K-band of DMONs (at 230–240 nm) and a
bathochromic shift of the B-bands. Instead, an intense quinon-
oid band appears at 380–440 nm.
Fig. 2 Decay of the yellow band of 2,3-DMON in 86% H2SO4. 1:
t0 ϩ 0.3 min; 6: t0 ϩ 2.3 min; 10: t0 ϩ 10 min; t0 = time of preparation
of solution.
For 2,3-DMON the yellow band appears in 82–97% H2SO4.
It increases quickly with acidity but no plateau in ε0 vs. ϪH0
was observed because of very rapid decay of this band. Its λmax
at 427 nm indicates that it is due to a quinonoid dication
2+
DMONH2 such as 5 (Scheme 1), formed in the second
protonation step. Species 5 is a diprotic acid that contains labile
protons and is also a reaction intermediate in the hydrogen
exchange reaction. Protonations on C, although very rapid, are
slow in comparison with O-protonations. Nonetheless, the
C-protonated forms should not be regarded as complexes that
contain a saturated carbon in the naphthalene ring because the
aromatic protons are exchanged by tritium and deuterium in
80% sulfuric acid.25 This exchange is a simple electrophilic aro-
matic substitution reaction in which D+ acts as an electrophilic
reagent.25 Such exchange takes place also in the naphthalene
ring and protonation of naphthalene in superacidic solutions
leads to the formation of stable naphthalenium ions where pro-
tons are in rapid equilibrium through a 1,2-hydrogen shift.22
A
broadened ᎐CH NMR signal appears in these ions at δ 5–7
᎐
2
ppm indicating labile protons and unsaturated character of this
methylene group.22 Therefore, quinonoid forms ought to be
considered as participants in the equilibria with electrophilic
reagents present in solution, that is, with proton donors and
SO3 donors at all unsubstituted positions of the ring forming
encounter complexes7 (or π-complexes) such as 6 in Scheme 1.
Encounter complexes are much more slowly and quasi-
irreversibly transformed into σ-complexes such as 3 and 7. The
latter yield the final sulfonated products, such as 4, in largely
irreversible processes indicated by the single arrows in Scheme
1. Single arrows imply that, given sufficient time and reagent,
the sulfonation reactions proceed practically to completion.
This does not exclude very slow reverse reactions and under
mild conditions the more stable product is formed.
The position in the ring at which protonation occurs prefer-
entially is determined by the structure of the primary mono-
sulfonic derivative. It is well known that protonation occurs
at the most electronegative (that is most nucleophilic) aro-
matic carbon atom.11 Dihydroxybenzenes upon dissolution in
concentrated H2SO4 are very rapidly protonated at the same
4- and 6-positions, at which subsequently ring sulfonation
occurs.13 This is in agreement with the principle of microscopic
Fig. 3 Evolution of the UV/VIS spectra of 2,6-DMON in 82% H2SO4.
1: t0 ϩ 1.3 min; 8: t0 ϩ 180 min; t0 = time of preparation of solution.
nearly zero. Such bands are diagnostic of the presence of
carbonium ions.21 These bands are shown in Figs. 2 and 3.
Protonation of 2-methoxynaphthalene and naphthalene in
superacids and the formation of a red-coloured carbocation
(C-protonation) were observed by Olah et al.22 In aqueous
H2SO4 we observed discoloration of solutions followed by
reprecipitation of the original substances upon dilution (with
ice) immediately after dissolution and the appearance of these
bands in concentrated perchloric acid. Rapid reversibility with
changing acidity indicates a protonation equilibrium. Plots of
log ([DMONH22+]/[DMONH+]) versus H0 are linear as seen in
Fig. 1. Fairly constant values of pK2 are obtained (Tables 4 and
5) although the slopes vs. the H0 scale that we used are some-
what steeper than 1: C-protonations may be less precisely
described by this H0 function. The absorption coefficients ε0 of
540
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544

View Article Online
Table 6 Paralleled determination of decay rates kqu for the quinonoid
methoxynaphthalene are present when sulfonations take place
and may be considered as three sulfonation substrates that are
in mutual, rapid protonation equilibria:
a
bands and sulfonation rates kgrav determined gravimetrically (in minϪ1
)
H2SO4
2,6-DMON
2,7-DMON
2+
DMON0 + 2H3O+
DMONH+ + H3O+
DMONH2
80% kqu
kgrav
84% kqu
kgrav
88% kqu
kgrav
92% kqu
kgrav
—
—
2.3 × 10Ϫ3
2.3 × 10Ϫ3
2.7 × 10Ϫ3
2.2 × 10Ϫ3
2.0 × 10Ϫ3
2.5 × 10Ϫ3
—
6.02 × 10Ϫ3
5.98 × 10Ϫ3
0.013
The sulfonation rate constant k of the substrate added to the
solution is a sum of the three separate reaction rates, k0 (the
fastest sulfonation rate of neutral DMON0), the slower sulfon-
ation rate k1 of monocations DMONH+ and the slowest k2 of
dications DMONH22+. Their % concentrations (or shares) are
constant at each sulfuric acid concentration and can be
obtained from measurements at increasing acidities as
described above (cf. Table 1–5) and calculated using the H0
scale and pK1 and pK2 values.
0.012
0.010
0.014
—
a Details are described in the Experimental.
Examples of % concentrations of the three forms DMON0,
reversibility, as both protonations and sulfonations are revers-
ible electrophilic substitutions that compete for a place, or pos-
ition, at the same most nucleophilic (that is, electronegative)
unsaturated carbon atom. Thus, the reaction sequence depicted
in Scheme 1 is proposed. When the SO3H group is substituted
on the ring it imparts strong acidic properties to the sulfonated
intermediates and products, which then participate in proton-
transfer equilibria that are different to those in which the
DMON substrates take part.
2+
DMONH+ and DMONH2 at different acidities are given in
Table 7. For 2,7-DMON sulfonation proceeds in 80% H2SO4
when DMONH2 is the prevailing form (76%), while there is
2+
only 0.006% of DMON0 (cf. Tables 6 and 7). One can see that in
solutions more acidic than pK2, DMONH22+ prevails (cf. Table
7 and 5 in Scheme 1) and [DMON0] decreases much faster than
in less acidic solutions, because [DMONH+] also decreases.
Similar calculations show that in 84–97% H2SO4 the concen-
tration of DMONH22+ is almost constant at nearly 100%, while
that of DMON0 decreases to insignificant values.
Sulfonation substrates
Similar results are obtained for 2,6-DMON. Also, sulfon-
ation proceeds at a noticeable rate only in solutions in which
the quinonoid band appears. The quinonoid spectra of
In order to react with cationic substrates such as 2 or 5 the
sulfonating entities ought to be polar, but electrically neutral,
molecules, such as H2S2O7, which can be attracted by the
inhomogeneous electric field created by the positive charges of
DMONH+, or DMONH22+. Positively charged sulfonating
2+
DMONH2 are different from the spectra of the sulfonated
products so the presence of the latter could be taken into
account quantitatively (cf. Fig. 3).
Thus, the rate increase cannot be due to neutral DMON0 but
the share of DMONH+ is important, in particular, in less con-
centrated H2SO4 where the concentration of monocations such
as 2 in Scheme 1 is large and fairly constant, that is, at H0 values
between pK1 and pK2 (cf. Table 7), if sulfonations proceed in
this range. An electrophilic attack by a sulfonating species on a
+
moieties, such as H3SO4 are repelled (they do react with
neutral substrates7). Hence, in our sulfonation reactions the
overall, or net, charge of the molecule does not change when an
intermediate π- or σ-complex is formed.
Sulfonations are slow in comparison with C-protonations,
but are also less reversible. Once the sulfonic group is inserted
into the aromatic ring it is not easily removed and a sulfonated
product is formed with different acidic properties, although the
methoxy substituents remain unaffected.2,14c,23,26 Therefore, the
quinonoid forms are not stable—after very rapid formation
(<1 s) the yellow bands decrease in concentrated H2SO4 in a
pseudo-first order reaction in line with the reaction rate con-
stants of the DMON substrate as determined by parallel kinetic
spectrophotometric and gravimetric measurements (Table 6
and Experimental).
This reaction was identified as sulfonation and the products
are described in the next paragraph. An agreement between the
two sets of measurements collected in Table 6 can be expected
when the products of the reaction do not have absorption
bands at the wavelength of the photometric measurements, and
are more soluble than the substrate in the gravimetric meas-
urements. The latter is generally true for the sulfonated prod-
ucts. At lower acidities the agreement is fair: the decay rate of
the quinonoid band is also the overall reaction rate of DMON.
At high acidities the products may show absorption bands that
are absent at lower acidities. In particular, this is true for 2,7-
DMON for which discrepancies arise between photometric and
gravimetric measurements and the latter better describe the
kinetics of reaction. Of course one could subtract the inter-
fering absorbances by additional measurements at another
wavelength.
monocation then takes place. The sulfonating entities are in
equilibria such as H2S2O7 + H+
SO3 + H3SO4 .
+
Assuming usual slopes of rate profiles3 for cations4 their
sulfonation rates will rise in step with acidity,7 but in solutions
near pK2 the concentrations of DMONH+ decrease in a similar
way. This will stabilize the (large) extent of reaction due to
DMONH+ and the overall reaction rate will slow down or may
be almost constant if DMONH22+ reacts much more slowly, i.e.,
if its rate profile is shifted to higher acidities. This has been
observed for the quinonoid band of 2,6-DMON: in 78–88%
H2SO4 its decay rate is nearly constant at about 0.01 minϪ1
.
For 2,7-DMON a stabilization of the reaction rate occurs in
80–88% H2SO4 (cf. Table 6). In this range of acidities the con-
centrations of neutral forms and monocations of 2,6- and
2,7-DMON rapidly decrease, while the concentrations of the
dications sharply increase. The latter react at a noticeable rate in
more acidic solution as indicated by the rapid rise of reaction
rate in very concentrated H2SO4. For 2,7-DMON this rate is
0.13 minϪ1 in 96% H2SO4 and for 2,6-DMON ca. 0.4 minϪ1 in
97% H2SO4. Above 90% H2SO4 the reaction rate rises again and
2+
the increase in sulfonation turnover will be due to DMONH2
,
although its percentage concentration is steady at almost 100%
in >90% H2SO4 because the reaction rate increases with H0.
Protonation near to the aromatic ring retards sulfonation also
in the case of 2-naphthol26 in 91–95% H2SO4; 2-naphthol has a
strong quinonoid band at λ 385 nm.14c
2,3-DMON (pK1 = Ϫ2.9; pK2 = ca. Ϫ9.0) is different as its
monocation is the principal substrate for sulfonation in 62–92%
H2SO4. Again, the concentration of neutral 2,3-DMON rapidly
Parallel kinetic measurements for 2,3-DMON could not be
made because of very rapid decay of its quinonoid band: it is
0.4 minϪ1 in 86% H2SO4 and ca. 7 minϪ1 97% H2SO4. These high
rate constants may be due to sulfonation at positions 5 and 6 far
from the positively charged oxonium methoxy groups. Their
negative effect on the sulfonation rate is therefore reduced.
As discussed in the Protonations section, three forms of each
2+
becomes insignificant while the share of 2,3-DMONH2 is
important or prevalent in very concentrated H2SO4. Thus, our
results indicate that both mono- and dications of DMONs are
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544
541

View Article Online
Table 7 Fractions of DMON0, DMONH+ and DMONH22+ as a function of the H0, pK1 and pK2 valuesa
% 2,7-DMON0
% 2,7-DMONH+
% 2,7-DMONH2
2+
% H2SO4
H0
0.0
56.0
68.0
77.1
80.0
84.0
90.0
97.0
7.0
100
50
2
0.04
0.006
5 × 10Ϫ4
8 × 10Ϫ6
3 × 10Ϫ8
0
50
95
50
0
0.04
3
50
76
93.1
99.0
99.94
Ϫ3.9
Ϫ5.5
Ϫ7.0
Ϫ7.5
Ϫ8.1
Ϫ9.0
Ϫ10.2
24
6.9
1.0
0.06
2ϩ
% H2SO4
H0
% 2,6-DMON0
% 2,6-DMONH
% 2,6-DMONH2
0.0
66.7
75.0
82.0
90.0
97.0
7.0
Ϫ5.3
Ϫ6.6
Ϫ7.8
Ϫ9.0
Ϫ10.2
100
49.9
4.5
0
49.9
90
49.9
6
0.4
0
0.2
5.5
49.9
94
0.2
1 × 10Ϫ3
5 × 10Ϫ6
99.6
% 2,3-DMON0
% 2,3-DMONHϩ
% 2,3-DMONH2
2ϩ
% H2SO4
H0
0
7.0
Ϫ2.9
Ϫ9.0
100
0
50
50
6.3
0
45.4
90.0
97.0
50
4 × 10Ϫ5
50
4 × 10Ϫ5
3 × 10Ϫ7
Ϫ10.2
93.7
a Examples: 2,7-DMON, pK1 = Ϫ3.9, pK2 = Ϫ7.0; 2,6-DMON, pK1 = Ϫ5.3; pK2 = Ϫ7.8; 2,3-DMON, pK1 = Ϫ2.9; pK2 = ca. Ϫ9.0. Data from Tables
1–5 are used in the transformed Henderson equation: I = [BH+]/[B]; %BH+ = 100I/(I + 1); %B = 100%Ϫ%BH+; log I1 = log ([DMONH+]/
[DMON0]) = pK1ϪH0; log I2 = log ([DMONH22+]/[DMONH+]) = pK2ϪH0. In solutions more acidic than pK2 the concentration at time t0 of neutral
DMON0 as a function of H0 decreases at twice the rate of that of DMONH+, because DMONH+ also decreases.
the direct substrates in sulfonation and not the neutral DMON
molecules as the latter are protonated when sulfonations
proceed.
the yellow band is absent. The first SO3 group is meta-
positioned with formation of 2,6-DMON-4-monosulfonic acid
(2,6-DMON-4-S). The second SO3 substituent is inserted at
position 7. Only one 2,6-DMON-4,7-disulfonic acid (2,6-
DMON-4,7-S2) is obtained.
Sulfonation products
Sulfonation of 2,3-DMON in 62–80% H2SO4 only occurs for
monocation 2 at a relatively slow rate, which enabled us to pre-
pare monosulfo-2,3-DMON free of its disulfo derivative. The
latter is formed in 91–97% H2SO4. Barium salts of 2,3-DMON-
monosulfonic acid contained two isomers in the proportion 1:2
of the 2,3-DMON-5-monosulfonic salt (2,3-DMON-5-S) and
2,3-DMON-6-monosulfonic salt (2,3-DMON-6-S), respect-
ively. Only 2,3-DMON-5,7-disulfonic acid (2,3-DMON-5,7-S2)
is obtained after exhaustive sulfonation of 2,3-DMON. Here
positions 5,7 and 6,8 are equivalent, which means that in this
case the –SO3 group directs the second substituent to a meta
position.
Sulfonation of 2,7-DMON proceeds via O-protonated and
C-protonated intermediates with ortho-substitution at position
3 (or 6). 2,7-DMON-3-Monosulfonic acid (2,7-DMON-3-S) is
formed. Further sulfonation proceeds in 87–97% H2SO4 with
formation of 2,7-DMON-3,6-disulfonic acid (2,7-DMON-3,6-
S2). In our substrates the β-positions in the naphthalene ring are
more reactive than the α-positions.
Mild sulfonation conditions were applied to obtain the first, or
early, stable products. The relative ease of sulfonation of aro-
matic ethers (and phenols) can be explained by the interaction
of the lone pair of n-electrons remaining on methoxy oxygen
with the naphthalene ring, which retains much of its nucleo-
philic and aromatic properties in protonated ethers.
In such solutions an electrophilic attack by a sulfonating
entity on the relatively electronegative ring carbon atom of the
oxonium monocation leads to the formation of the monosul-
fonic product. In the process the DMONH+ ions are removed
from the solution and the concentrations of the more and less
2+
protonated forms of the substrate (i.e., of DMONH2 and
DMON0) also decrease proportionally as indicated by the I
values, which are constants at each H0 value. If the quinonoid
band of DMONH22+ is present, it will decrease in step with the
sulfonation rate.
However, with rising acidity in 92–97% H2SO4 the initial con-
centration of the oxonium cations decreases rapidly (cf. Table
7) while the sulfonation rate, concentration of DMONH22+ and
the decay of the quinonoid band increase sharply. This means
that the dications are also sulfonated, in particular, at acidities
where their concentration approaches 100%. Otherwise a
levelling or decline of sulfonation rates would be observed. The
dications are C-protonated on the ring carbon atom, which
decreases the sulfonation rate, but with increasing percentage of
H2SO4 the sulfonation potential of the solution increases and
the reaction rate also increases. The sulfonation potential of
solution can be represented by log aH S O , which has been
Both O-protonated monocations as well as O- and C-proton-
ated dications of monosulfonic products can be substrates in
further sulfonations of DMONs. This is indicated by the
quinonoid spectra23 of 2,7-DMON-3-S and the absence of such
bands29 for 2,6-DMON-4-S.
Sulfonic acids such as 4 in Scheme 1 are strong acids and
dissociate in solutions of lower acidity.
2
2
7
Conclusions
evaluated3,4b as a function of % H2SO4 and is related via the %
H2SO4 content to the H0 scale.20 This sequence of reactions
would apply also to other methoxynaphthalenes.
In the case of 2,6-DMON the products have a UV absorption
spectrum rather similar to that of the parent 2,6-DMON, but
a) In our research we found that 2,3-, 2,6- and 2,7-DMONs are
reversibly protonated in concentrated H2SO4. The pK1 and pK2
values have been determined and the results are presented in
Table 7.
542
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544

View Article Online
Table 8 Binding energies (BE) and heats of formation (HF) (in kcal molϪ1) of mono- and disulfonic acids of 2,3-, 2,6- and 2,7-DMONa
2,3-DMON
HF
BE
2,6-DMON
HF
BE
2,7-DMON
HF
BE
1-S
5-S
6-S
Ϫ145.84
Ϫ150.43
Ϫ151.83
Ϫ3185.94
Ϫ3190.53
Ϫ3191.92
1-S
3-S
4-S
Ϫ148.90
Ϫ147.81
Ϫ149.31
Ϫ3188.99
Ϫ3187.91
Ϫ3189.41
1-S
3-S
4-S
Ϫ148.80
Ϫ148.96
Ϫ149.42
Ϫ3188.90
Ϫ3189.06
3189.52
5,1-S2
5,7-S2
5,8-S2
5,4-S2
5,6-S2
6,1-S2
6,4-S2
6,7-S2
Ϫ262.69
Ϫ268.18
Ϫ260.65
Ϫ184.79
Ϫ257.54
Ϫ264.54
Ϫ255.57
Ϫ262.35
Ϫ3547.86
Ϫ3553.36
Ϫ3545.83
Ϫ3469.96
Ϫ3542.72
Ϫ3549.72
Ϫ3540.75
Ϫ3547.52
4,1-S2
4,7-S2
4,8-S2
4,3-S2
4,5-S2
Ϫ258.71
Ϫ266.63
Ϫ261.81
Ϫ253.02
Ϫ191.28
Ϫ3543.88
Ϫ3551.81
Ϫ3546.99
Ϫ3538.20
Ϫ3476.46
3,1-S2
3,6-S2
3,8-S2
3,5-S2
Ϫ262.06
Ϫ265.51
Ϫ261.82
Ϫ254.42
Ϫ260.72
Ϫ3547.24
Ϫ3550.69
Ϫ3547.00
Ϫ3539.60
Ϫ3545.90
a Isomers obtained in this work are in italic type. The sulfonic group is abbreviated as S preceded by a number indicating its position on the
naphthalene ring.
b) The sulfonation rate constants are, within experimental error,
the same as the decay rate constants of the quinonoid dications.
These results are collected in Table 6.
suspension was stirred. Every 1 or 2 hours the spectra of the
solutions were recorded in quartz cuvettes. A shallow minimum
of solubility, Si, was found in 20–30% H2SO4. The increase of
solubility, SH, was attributed to the formation of a protonated
form if a plateau of solubility was reached (sometimes the next
day) and if, after ice was added, the concentration returned to
Si. We used an extended form of the Krebs and Speakman
equation27,28
c) As a result of protonation the neutral DMON0 molecules
disappear from solutions in which sulfonation proceeds and
therefore are not the direct substrates of sulfonation.
d) The immediate substrates of sulfonation in acids of medium
strength are the respective monocations of DMON and at
acidities higher than pK2 these are the dications of DMON.
e) Barium or sodium salts of mono- and disulfonic acids of 2,3-
and 2,6-DMON have been isolated and their structure was
established by NMR spectroscopy. All the products we have
prepared and identified are printed in italic type in Table 8.
f) Further sulfonation of dimethoxynaphthalenes proceeds on
the naphthalene ring with the fewer substituents. At room tem-
peratures in aqueous H2SO4 up to 97% only a second SO3 group
is substituted.
pK1 = H0 + log[(SH/Si) Ϫ 1]
(1)
equivalent to the standard (Henderson) equation, which
describes the acid–base titration curve, but adapted to the range
of the H0 acidity scale, ϪH0 = pK1 Ϫ log I; Si is the concen-
tration of the uncharged form B of the DMON. BH+ is the
protonated form DMONH+; SH is the solubility measured at
higher acid concentrations and is the sum of B and BH+.
In order to clarify the observed substituent effects theoretical
calculations of binding energies (BE) and heats of formation
(HF in kcal molϪ1) were made.
[(SH/Si) Ϫ 1] = [(BH+ + B)/B] Ϫ 1 = (BH+/B) = I (2)
In Table 8 are the results calculated after geometry optimis-
ation by AM1 semiempirical methods (a HyperChem® pro-
gram) for all possible monosulfonic acids and those disulfonic
acids that are derived from obtained monosulfonic acids. It was
found that the mono- and disulfonic acids we obtained have the
lowest values of binding energies and heats of formation for a
given DMON substrate; only 2,7-DMON-3-S differs slightly.
This may explain the site of substitution of the first and the
second SO3 group and the formation of the products observed
by us. The calculated reactivities of various naphthalene ring
positions in the neutral primary substrates appear to be less
significant, if the presence of protonated forms of DMONs is
considered.
%B = 100% Ϫ %BH+ [%BH+ = 100 I(I + 1)]
(3)
Parallel determinations of decay rates kqu for the quinonoid
band and sulfonation rates kgrav were achieved gravimetrically:
A 26–80 mg sample of a DMON was weighed in a 100 ml
conical flask with a glass stopper on an analytical balance and
40–70 ml of H2SO4 of known concentration was added and the
solution was stirred. Small samples of DMONs and larger vol-
umes of the weaker acids were used to obtain rapid and com-
plete dissolution (time t0). Final concentrations of DMONs
were 0.002–0.01 M. Just after dissolution 0.1–0.2 ml of the
(coloured) solution were transferred to a 10 ml flask and filled
to the mark with another portion of the same acid. After mix-
ing, the spectra of this diluted solution were taken in a cyclic
mode. The reactions are of (pseudo)-first order and the decay
Experimental
rates kqu of the quinonoid bands were obtained from kqu
=
Experimental procedures and methods including evaluation of
results were similar for all three title compounds. The products
we have isolated have been identified before in non-aqueous
solutions by means of NMR by others.2 The structures of our
products are also based principally on NMR spectra, but the
UV spectra were considered as the β-bands confirm the pres-
ence of a naphthalene ring. Also elemental analyses of all
products for C and H were obtained together with some miner-
alizations in 72% HClO4 if the carbon content was lower than
calculated and the presence of inorganic salts was suspected.
[ln(A0 Ϫ A∞)]/(At Ϫ A∞)]/t. A0 is the absorbance extrapolated to
time t0, A∞ is the (small) final absorbances; t is time measured
from t0.
Simultaneously at the same temperature the reaction in the
conical flask proceeded under stirring. After 90–350 min the
reaction was stopped by pouring the solution on to a weighed
amount of ice, the flask was rinsed with ice-cold water. In the
combined solutions the percentage of H2SO4 was in the Si
region of DMON, between 20 and 35%. The solution immedi-
ately turned white, and a white precipitate formed. This was left
overnight at room temperature. Then the precipitated DMON
was slowly filtered on a sintered-glass crucible No.4, washed
with cold water, dried at 55 ЊC and weighed.
Determination of the first protonation constants
Pulverized DMON (ca. 0.2 g) was placed in a 50 or 100 ml
conical flask with a glass stopper, 15–25 ml of H2SO4 of a
concentration determined by alkalimetry were added and the
The reaction rate kgrav = ln[(initial weight of DMON)/(final
weight of DMON)]/t.
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544
543

View Article Online
1
2,3-DMON was prepared from 2,3-dihydroxynaphthalene by
standard methylation with dimethyl sulfate. The product was
purified by double crystallisation from ethyl alcohol. Melting
point 116 ЊC (uncorrected). Calculated: C, 76.52; H, 6.38;
found: C, 76.51; H, 6.42%. 1H NMR: δ 3.87 (s, 2 × OCH3), 7.22
(s, H1, H4), 7.27 (m, H6, H7), 7.66 (m, H5, H8).
an ATI Unicam UV4 spectrophotometer and the H NMR
with a Bruker AC 200 and a DRX 500 spectrometer; pH meas-
urements were made with a Radiometer PHM-80 instrument.
Acknowledgements
2,6- and 2,7-DMON of 99% purity were obtained from
Sigma Chemical Co., and were used as supplied: 2,6-DMON
This work has been supported by grant no. 502–13-517(177)
from the Medical University of Łódz.
1
3
mp 153 ЊC; H NMR: δ 3.83 (s, 2 × OCH3), 7.12 (dd, J = 8.9
Hz, 4J = 2.3 Hz, H3, H7), 7.25 (d, 4J = 2.3 Hz, H1, H5), 7.71 (d,
3J = 8.9 Hz, H4, H8). 2,7-DMON mp 138 ЊC; 1H NMR: δ 3.84
(s, 2 × OCH3), 6.97 (dd, 3J = 8.9 Hz, 4J = 2.0 Hz, H3, H6), 7.22
(d, 4J = 2.0 Hz, H1, H8), 7.71 (d, 3J = 8.9 Hz, H4, H5).
References
1 G. Pandey, K. S. Sesha Poleswara Rao, D. K. Palit and J. P. Mittal,
J. Org. Chem., 1998, 63, 3968.
2 H. R. W. Ansink, E. J. de Graff, E. Zelveder and H. Cerfontain,
Recl. Trav. Chim. Pays-Bas, 1992, 111, 499.
3 C. W. F. Kort and H. Cerfontain, Recl. Trav. Chim. Pays-Bas, 1968,
87, 24.
Preparation of monosulfonic salts
1 g of DMON was dissolved at room temperature in 20 g of
80% H2SO4. The solution was stirred for 18 hours, then 50 g of
ice were added with stirring. Unreacted DMON precipitated
out, was filtered off and identified. The filtrate was neutralized
stepwise with Ba(OH)2·8H2O. BaSO4 was filtered off and
washed with distilled water. Then BaSO4 was discarded, the
filtrates were combined and extracted with diethyl ether to
remove traces of DMON and were evaporated to dryness. The
product was extracted with ethyl alcohol from the residue. After
evaporation [C10H5(OCH3)2SO3]2Ba (ca. 0.4 g) was obtained,
purity 95% according to elemental analysis. (2,3-DMON-5-
S)2Ba was enriched by repeated extraction with methanol of the
4 (a) K. Lammersma and H. Cerfontain, J. Chem. Soc., Perkin Trans.
2, 1980, 28; (b) R. Bregman and H. Cerfontain, J. Chem. Soc., Perkin
Trans. 2, 1980, 33.
5 C. N. C. Deno, R. W. Gaugler and T. Schulze, J. Org. Chem., 1969,
31, 1968.
6 A. Koeberg-Telder and H. Cerfontain, J. Chem. Soc., Perkin Trans.
2, 1976, 1776.
7 H. Cerfontain, H. J. A. Lambrechts, Z. R. W. Schaasberg-Nienhuis,
R. G. Coombes, P. Hadjigeorgiou and G. P. Tucker, J. Chem. Soc.,
Perkin Trans. 2, 1985, 659.
8 B. G. Rainey, J. Am. Chem. Soc., 1966, 88, 5358.
9 K. Yates and H. Wai, Can. J. Chem., 1965, 43, 2131.
10 A. J. Kresge and L. E. Hakka, J. Am. Chem. Soc., 1966, 88, 3868.
11 T. Birchall, A. N. Bourns, R. J. Gillespie and P. J. Smith, Can. J.
Chem., 1964, 42, 1433.
12 A. J. Kresge, Y. Chiang and L. E. Hakka, J. Am. Chem. Soc., 1971,
93, 6167.
1
dry ethanol extract. H NMR: δ 3.83 and 3.91 (s, 2 × OCH3),
7.17 (s, H1), 7.32 (m, H7), 7.76 (d, H6), 7.88 (dd, H8), 7.90
(s, H4). (2,3-DMON-6-S)2Ba 1H NMR: δ 3.77 and 3.78 (s,
2 × OCH3), 6.99 (s, H1), 7.09 (s, H4), 7.57 (dd, H7), 7.64 (d,
H8), 8.01 (s, H5).
13 H. Cerfontain and A. Koeberg-Telder, Recl. Trav. Chim. Pays-Bas,
1988, 107, 543.
14 (a) A. Cisak and H. Gorzadek, Bull. Pol. Acad. Sci., Chem., 1988,
˛
36, 1; (b) A. Cisak and H. Gorzadek, Bull. Pol. Acad. Sci., Chem.,
˛
Preparation of disulfonic salts
1989, 37, 99; (c) A. Cisak and H. Gorzadek, Bull. Pol. Acad. Sci.,
˛
Chem., 1989, 37, 177.
1 g of DMON was dissolved with stirring at room temperature
in 20 g of 94% H2SO4. After 20 h the homogeneous solution
was diluted with 50 g of ice and worked up as described above
for the monosulfonic salt. About 0.5 g of 2,3-DMON-5,7 S2Ba
15 C. N. C. Deno and J. O. Turner, J. Org. Chem., 1966, 31, 1969.
16 J. F. Wójcik, J. Phys. Chem., 1985, 89, 1748.
17 Commission on Symbols, Terminology, and Units, Pure Appl.
Chem., 1979, 51, 30.
18 K. Schwabe, pH-Messung, Akademie Verlag, Berlin, 1980, p. 13 and
Table 2.
19 R. A. Cox and K. Yates, Can. J. Chem., 1983, 61, 2225.
20 C. D. Johnson, A. R. Katritzky and S. A. Shapiro, J. Am. Chem.
Soc., 1969, 91, 6654.
21 G. A. Olah and P. v. R. Schleyer, Carbonium Ions, vol. I, Wiley
Interscience, New York, 1973, p. 207.
22 G. A. Olah, G. A Mateescu and Y. K. Mo, J. Am. Chem. Soc., 1973,
95, 1865.
23 A. Cisak and D. Kusztal, Pol. J. Chem., 1997, 71, 40.
24 A. Streitwieser, Jr. and C. H. Heathcock, Organische Chemie, Verlag
Chemie, Weinheim, 1980, p. 696.
25 T. Miura and M. Kimura, Chem. Pharm. Bull., 1986, 34, 1561.
26 P. Beltrame, G. Bottaccio, P. Carniti and G. Felicioli, Ind. Eng.
Chem. Res., 1992, 31, 787.
27 A. Albert and E. P. Serjeant, Determination of Acidity Constants,
ch. 6, Chapman and Hall, London, 1971.
1
was obtained, 95–97% purity. H NMR: δ 3.79 and 3.98 (s,
2 × OCH3), 7.31 (s, H1), 7.98 (s, H4), 8.31 and 8.35 (d, 4J = 1.2
Hz, H6, H8). Mono- and disulfonic acids and salts of 2,6-
DMON were prepared by similar procedures29 to those for
2,3-DMON. 2,6-DMON-4-S: δ 3.78 and 3.85 (s, 2 × OCH3),
7.08 (m, H7), 7.25 (s, H5), 7.55 (s, H1), 7.65 (d, 4J = 1.8Hz, H8),
8.15 (s, H3). 2,6-DMON-4,7-S2: δ 3.76 (s, OCH3-C2), 4.01 (s,
OCH3-C6), 7.56 (s, H1), 8.15–8.25 (m, H3, H5, H8).
The spectra of the sulfonic salts of 2,7-DMON have been
published23 and the 1H NMR assignments are confirmed by the
1H NMR spectrum of the sodium salt of 2,7-dihydroxy-
naphthalene-3,6-disulfonic acid: δ 4.72 (s, 2 × OH), 7.11 (s, H1,
H8), 8.25 (s, H4, H5).
The preparation of mono- and disulfonates of 2,7-DMON is
described in our earlier paper.23
28 A. Cisak, Pol. J. Chem., 1982, 56, 1599.
29 A. Cisak, D. Kusztal and E. Brzezin´ ska, Acta Pol. Pharm., 2000, 57,
299.
All measurements and reactions were performed at room
temperature (that is, 22–24 ЊC). UV spectra were recorded with
544
J. Chem. Soc., Perkin Trans. 2, 2001, 538–544