Surprises in a 'simple' system: 2,4-Diaminobenzenesulfonic acid

Article abstract of DOI:10.1002/hlca.200390246

The search for the polymorphic forms of 2,4-diaminobenzenesulfonic acid (DBSA), known to exist since 1880, has revealed a surprisingly rich solid-state system for such a simple molecule. A monohydrate, a dimoiric hydrate, an anhydrate and two polymorphic forms of the hydrochloride of this material have thus far been prepared. Their characterization by microscopic and thermal methods, FT-IR spectroscopy, and single-crystal structure determination are described.

Full text of DOI:10.1002/hlca.200390246

Helvetica Chimica Acta Vol. 86 (2003)
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Surprises in a −Simple× System: 2,4-Diaminobenzenesulfonic Acid
by Janice M. Rubin-Preminger and Joel Bernstein*
Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Be×er Sheva, Israel, 84105
(e-mail: yoel@bgumail.bgu.ac.il)
Dedicated to Professor Jack D. Dunitz on the occasion of his 80th birthday
The search for the polymorphic forms of 2,4-diaminobenzenesulfonic acid (DBSA), known to exist since
1880, has revealed a surprisingly rich solid-state system for such a simple molecule. A monohydrate, a dimoiric
hydrate, an anhydrate and two polymorphic forms of the hydrochloride of this material have thus far been
prepared. Their characterization by microscopic and thermal methods, FT-IR spectroscopy, and single-crystal
structure determination are described.
1. Introduction: Historical Background.
Groth×s classic collection of the
characterization of over 8000 crystalline compounds, mainly by interfacial angles, also
contains many observations of much crystal chemistry, including polymorphism [1].
One example is 2,4-diaminobenzenesulfonic acid (DBSA) originally described as
brown monoclinic plates and colorless or weakly brown triclinic prisms [2][3]. Groth
[1] also cited this compound in what is, to the best of our knowledge, the first list of
concomitant polymorphs [4]. The 2,4-diaminobenzenesulfonic acid was initially
prepared by Post and Hardtung [2] according to Scheme 1, which yielded two
dimorphic modifications. Their findings were confirmed in a thesis by Levin [5].
Scheme 1
The concomitant polymorphism observed in the 1880×s was not reported again until
1931 when Hunter and Sprung [6] studied examples of the so-called Piria reaction,
according to Scheme 2.
After purification by filtration and extraction with alcohol and H2O two distinct
modifications of 2,4-diaminobenzenesulfonic acid were obtained, described as −large
monoclinic plates and elongated prisms×.
The 2,4- and 2,5-diamino derivatives have long been used as starting materials for
the preparation of azo dyes [7][8], and more recently in the manufacture of
copolyamides [9]. However, no further mention of the polymorphism of DBSA has
been reported in the more recent literature.

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Scheme 2
As part of an investigation of the series of concomitant polymorphs originally given
by Groth [1], we undertook a study of the solid-state chemistry of DBSA, which proved
to be far richer than originally suspected. In the course of this investigation, we isolated
an anhydrate (DBSA), a monohydrate (DBSA1W), a dimoiric hydrate (DBSA2³3W),
and two polymorphic forms of the hydrochloride (DBSAHCl1 and DBSAHCl2) of this
material (see below, Table 3). However, in spite of considerable efforts we have not yet
succeeded in reproducing the second polymorph of the anhydrate, reported by Post and
Hartung [2] and Hunter and Sprung [6], suggesting that this might be a case of a
disappearing crystal form [10], even though Groth and/or previous authors apparently
succeeded in obtaining well developed crystals. The characterization and comparison of
the various crystal forms by optical microscopy, single-crystal and powder X-ray
diffraction, FT-IR, mass spectrometry, and thermogravimetric analysis (TGA) is
reported here.
2. Results. 2.1. Preparation of Crystal Forms. DBSA is fairly soluble in H2O,
sparingly soluble in MeOH and EtOH, and when recrystallized from these solvents, at
least three weeks are generally required to produce even poor quality crystals.
Whenever any crystal of sufficient quality (and we use this term quite loosely) for single
crystal analysis was obtained, a data set was immediately measured. In this way, we
discovered that the crystal habit is not always a good indication of crystal form [11] and
that the differences in crystal habit may not be indicative of polymorphism for this
system.
Several crystal habits of the DBSA were obtained from different crystal growth
conditions (Table 1). The crystal habits of the material are quite diverse: brown plates,
brown needles, and colorless plates, which appear concomitantly, and flattened
diamonds. The zwitterionic 4-aminobenzenesulfonic acid monohydrate is also known
to crystallize in two crystal habits −elongated monoclinic laths and rhombic plates×
[12].
Attempts to make the −m-diamidobenzenesulfonic acid× as DBSA is called by Groth
[1] initially thought to be the 3,5-substituted derivative but now known to be the 2,4-
substituted derivative
by the method suggested by Post and Hardtung [2] (see
Scheme 1), produced a beige crystalline powder, which on analysis by TGA, proved to
be the monohydrate denoted DBSA1W; the original beige powder did not contain
crystals of sufficient size for single-crystal X-ray analysis. After several days of standing
in the remaining mother liquor, a second type of crystal orange-brown prisms was
observed and was suitable for single-crystal X-ray analysis; however, the prisms proved

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Table 1. Growth Conditions for Various Crystal Habits of DBSA
Crystal description
Brown plates
Photograph
Growth conditions and comments
Saturated aqueous solution at 138 after standing
for several weeks
Diamond shaped crystals with
a flattened end
Vapor diffusion of acetone into a saturated
aqueous solution at 228; several batches of
crystals were obtained but only the first one
showed this habit; subsequent batches had the
plate-like habit
Brown needles and faintly
brown-colorless plates, con-
comitantly
Saturated aqueous solution with dil. AcOH
solution at 608, by slow evaporation
Orange-brown needles
Saturated aqueous solution in 0.5-mm-diameter
capillary tubes at 228, by slow evaporation
2
to be the 2,4-diaminobenzenesulfonic acid dimoiric hydrate (gr. ˆ ³3, i.e., three
molecules of DBSA and two molecules of H2O in the asymmetric unit) denoted
DBSA2³3W. The DBSA1W and DBSA2³3W were subsequently found to exist concom-
itantly in the sample vial.
DBSA2³3W can also be produced from saturated aqueous solution under several
different conditions (Table 2). It should be noted that only experiments conducted at
room temperature (228) produced this hydrate. All of the examples were verified by
full single crystal analysis to be the dimoiric hydrate.
The growth conditions of the two polymorphs of the 2,4-diaminobenzenesulfonic
acid hydrochloride are quite similar. DBSAHCl1 was obtained first; it was grown from
a saturated aqueous solution with dilute HCl solution at 608 by slow evaporation over
several weeks. This produced opalescent plates and prisms (Fig. 1,a). Subsequent
attempts to reproduce this experiment have thus far been unsuccessful, and may
indicate that this is another example of a disappearing crystal form [10]. However,
changing the temperature of crystallization to 358 resulted in the concomitant
crystallization of DBSAHCl1 and DBSAHCl2, exemplifying the principle that once
a polymorphic form has been obtained, it should always be possible to produce it again
although not necessarily under the same conditions [10].

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Table 2. Growth Conditions for 2,4-Diaminobenzenesulfonic Acid Dimoiric Hydrate (DBSA2³3W)
Crystal description
Brown plate
Photograph
Growth conditions
Saturated aqueous solution at 228 by slow evap-
oration
Pale brown plates
Vapor diffusion of MeOH into a saturated
aqueous solution at 228
Brown-colorless plates
Saturated aqueous solution with dil. HCl solution
at 228
Initially clear brown frag-
ments; then half the crystals
became beige and polycrystal-
line while maintaining their
original morphology, and the
remainder became dark brown
in color
Saturated aqueous solution at 228
Blocky colorless crystals
Saturated aqueous solution at 228, vial scratched
to induce kinetic crystallization
Colorless plates
Saturated aqueous solution at 228

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Fig. 1. Photographs of a) DBSAHCl1 and b) DBSAHCl2
DBSAHCl2 also tends to decompose on standing for several months into a beige
powder, similar in shape and form to that of the anhydrate powder, and giving a
microscope FT-IR spectrum identical to that of the anhydrate. DBSAHCl2 was
obtained by slow evaporation from a saturated aqueous solution containing three drops
of conc. HCl solution at 458. These crystals tend to be brown cubes growing in clusters
and tend to be multiply twinned. They do not display the same opalescence as that seen
in DBSAHCl1 (Fig. 1,b).
2.2. Single-Crystal X-ray Analysis. Thus far, we have determined the single crystal
structures of the following crystal forms of 2,4-diaminobenzenesulfonic acid: DBSA,
DBSA2³3W, and two polymorphs of the DBSAHCl (Table 3). In all of the structures
obtained, the molecule is zwitterionic, resulting from a transfer of the H-atom from the
SO3H group to the para-NH2 group. The 4-aminobenzenesulfonic acid is known to exist
as a zwitterion in solution, as well as in the solid state [12][13]. There is no reason to
suppose that this should not be true of DBSA as well.
Table 3. Crystallographic Data for Various Forms of 2,4-Diaminobenzenesulfonic Acid
Anhydrate
Dimoiric hydrate
Hydrochloride
Form I
Hydrochloride
Form II
Abbreviation
Empirical formula
Crystal system
Space group
DBSA
DBSA2³3W
DBSAHCl1
C6H9ClN2O3S
orthorhombic
Pbca
DBSAHCl2
C6H9ClN2O3S
monoclinic
P21/n
C6H8N2O3S
monoclinic
P21/c
C6H8N2O3S ¥ 2³3 H2O
triclinic
≈
P1
a [ä]
b [ä]
c [ä]
a [8]
b [8]
g [8]
Volume [ä3]
7.958(2)
5.824(1)
15.476(3)
9.9611(6)
10.9142(6)
12.3683(8)
91.405(3)
109.056(2)
105.654(2)
1214.4(1)
6
13.448(1)
7.8085(9)
16.585(2)
6.391(7)
9.68(1)
14.08(1)
98.78(3)
90.55(2)
708.9(2)
4
1.756
1741.6(3)
8
1.714
0.0601
871.2(2)
4
1.713
Z
Density (calc.) [g/cm3]
1.643
R1
0.04150.0343
0.0401
A comparison of the SÀO bond lengths in the various forms of DBSA reveals no
major differences, indicating that there is no evidence of distinct single and double
bonds [14]. Also an examination of the CÀN bond lengths reveals the expected bond
‡
‡
lengths for NH3 and NH2 groups [15]. These data and the location of the NH3 H-
atoms from the difference maps (all with full occupancy) support the zwitterionic
nature of the material. This is in good agreement with the structures of 2-amino-

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benzenesulfonic acid (OTANAC) [16], 3-aminobenzenesulfonic acid (ANISAC)1)
[17], and 4-aminobenzenesulfonic acid (AFAZEM) [14] and its monohydrate
(SANACM) [12].
The ORTEP diagram of the asymmetric unit of DBSA (Fig. 2) clearly displays the
zwitterionic nature of the molecules of 2,4-diaminobenzenesulfonic acid in the various
crystal forms. The hydrochlorides possess an additional H-atom at NH2ÀC(2), with a
ClÀ counterion.
Fig. 2. ORTEP Diagram and atomic numbering of the asymmetric unit of DBSA. Note: all the crystal structures
follow the same atomic-numbering system.
The crystals of DBSA2³3W possess Z ˆ 6, i.e., with Z' ˆ 3 (Fig. 3). Z' ˆ 3 is quite rare
with only 2138 such structures out of more than 272066 entries in the November 2002
≈
release of the CSD, of which 247 are found in space group P1. Two of the molecules
tend to pack in a pairwise fashion with overlapping aromatic rings and the SO3À group
‡
and the NH3 group at C(4) of two adjacent molecules one above the other, while the
third molecule is off to one side at an angle to the plane of the two other benzene rings.
As can clearly be seen from the stereoviews of the packing diagrams (Fig. 4), the
structures of the various crystal forms are quite different. The reference molecule
(displayed as a ball-and-stick structure) has been chosen with the same orientation in
all of the structures displayed in Fig. 4, i.e., on the plane of the benzene ring with the
SO3À group on the vertical and NH2ÀC(2) pointing to the left. However, the presence
of a primary ammonium ion will bring anions closer together, by H-bonding to at least
two different anions, forcing layered sheets to form [18]. As such, we expect this
packing motif to be present in all of these structures.
1
)
REFCODE from the Cambridge Stuctural Database (CSD), November 2002 edition.

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Fig. 3. ORTEP Diagram and atomic numbering of the three molecules of 2,4-diaminobenzenesulfonic acid and
two molecules of water in the asymmetric unit of DBSA2³3W
The asymmetric units of the two polymorphic forms DBSAHCl consist of a
molecule that is not just zwitterionic but has three charges at the benzene ring, both
‡
amino NH2 groups being positive (2 NH3 and the sulfonic acid group being negative
(1 SO3 ) with a ClÀ counterion, as required by the chemistry of this salt. It should be
À
noted that there is no other structure with multiple charges at a benzene ring without
stabilization by large bulky substituents listed in the November 2002 release of the
CSD.
In the stereoview of the packing diagram of DBSA (Fig. 4,a), it can be seen that the
molecules tend to pack as two parallel rows of molecules all orientated with their
benzene rings in the plane of the paper followed by another two rows of molecules with
their long axes perpendicular to them.
The stereoview of DBSA2³3W (Fig. 4,b) also shows that the molecules pack as pairs
head to tail, with their long axes parallel and their NH2ÀC(2) pointing in the same
direction. The overlap between aromatic rings is far from complete. The pairs tend to
pack as rows with alternating rows of molecules in the plane of the paper (running
vertically) and almost perpendicular to it. The H2O molecules lie in the voids between
the rows of molecules.
In DBSAHCl1, the molecules tend to pack as parallel sheets with pairs of molecules
head to tail along the long axis of the molecule (Fig. 4,c). The NH2ÀC(2) groups point
in opposite directions. There is only partial overlap of the aromatic rings in the pairs of
molecules.
In DBSAHCl2, the molecules pack in rows parallel to the c-axis with pairing with
almost complete overlap of the aromatic rings, but with an angle of ca. 588 between
À
their SO3 groups (Fig. 4,d). Again, the NH2 groups lie on opposite sides of the
benzene rings. Alternate rows of molecules are not parallel to each other.

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2.3. Hydrogen Bonding and Graph-Set Analysis. The H-bonding in the structures is
shown in Fig. 4. Graph-set analysis [19][20] with the aid of PLUTO [21] were utilized to
characterize and compare the H-bond patterns in the four structures. A comparison of
the graph-set matrices [20] (Tables 4 and 5) of the two hydrochloride polymorphs
clearly revealed the similarities and differences in the H-bond patterns of the two
crystal forms. Three first-order H-bond motifs are common to both forms: C(8) from
the para-amino group; C(6) and S(6) from the ortho-amino group, with no matching
second-order graph sets .
The three graph sets common to the two hydrochlorides are also common to the
anhydrate. The anhydrate also shares an additional H-bond pattern with each of the
hydrochloride polymorphs: C(6) with DBSAHCl1 and R22(16) with DBSAHCl2.
Hence these patterns appear to be characteristic of the packing mode of DBSA in a
variety of structures.
Fig. 4. Stereoviews of the unit cells of a) DBSA b) DBSA2³3W, c) DBSAHCl1, and d) DBSAHCl2. The view is on
the best plane of the benzene ring with the sulfonate group on the vertical and the NH2 group at C(2) to the left;
the reference molecule is displayed as a ball-and-stick model. H-Atoms are omitted for clarity.

Helvetica Chimica Acta Vol. 86 (2003)
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Fig. 4 (cont.)
In DBSA2³3W, each of the three molecules in the asymmetric unit has one H-bond
from a H2O H-atom to the SO3À group, and one H-bond to the H2O O-atom from a H-
atom of the amino at C(4) of the benzene ring. The graph-set matrix of DBSA2³3W (not
shown) is far more complex than those of the other forms due to the presence of the
three molecules of 2,4-diaminobenzenesulfonic acid in the asymmetric unit (Fig. 4,b).
The Z' ˆ 3 situation leads to many first- and second-level entries, which are D
(discrete). The ring (R) and chain (C) patterns will appear for higher-level graph sets
[20]. There are an additional seven H-bonds between the H2O molecules and the
DBSA molecules, all of them are of the D(2) H-bond type. The additional bond is to
À
the SO3 group of the central DBSA molecule (Fig. 4,a). The NH2ÀC(2) does not
participate in H-bonding with the H2O molecules. Initially, ignoring the H-bonds for
the H2O molecules, in graph-set terms at the first level, the H-bond patterns of the three
molecules in the asymmetric unit are similar. Typical of structures with Z' > 1, there are

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Table 4. First- and Second-Level H-Bond Graph-Set Matrix of DBSAHCl1
a
b
c
d
e
a
b
c
d
e
C(8)
C22(10)
C(6)
R22(12)
R44(24)
C22(10)
C22(6)
C(6)
C22(10)
R24(8)
R44(12)
C(6)
C22(14)
C22(14)
C22(14)
R44(24)
S(6)
Table 5. First- and Second-Level H-Bond Graph-Set Matrix of DBSAHCl2
a
b
c
d
e
a
b
c
d
e
C(8)
C22(16)
R22(16)
C22(16)
C22(14)
C22(6)
C22(6)
R22(16)
C22(14)
R44(20)
C22(10)
C12(8)
C(6)
C22(16)
C22(16)
R44(28)
S(6)
a number of D patterns. In addition, two of the three molecules participate in R22(16)
patterns, while the third molecule is involved in a R22(12). At the second level, D33(13) is
a pattern common to all three molecules.
Since H-bonds are primarily electrostatic in nature [18], they may be enhanced by
the presence of charge at either the donor, the acceptor, or both atoms [22]. Thus, one
would expect the strength of such H-bonds to increase, and their length to decrease
although there has been doubt cast upon this premise [23]. In looking at the H-bond
lengths of DBSAHCl1 in Table 6, virtually all of them may be considered to be −strong×
H-bonds, being less than the sum of their van der Waals radii [22]. Consistent with that
view, these charge-assisted bond lengths in Table 6 fall at the lower end of the range of
‡
N ÀH ¥¥¥ OÀÀS bond lengths when both charged groups are at the same benzene ring
(1.81 2.92 ä for 21 cases from the November 2002 release of the CSD [21]).
‡
Table 6. H-Bond Lengths N H ¥¥¥ OÀS in the two forms of DBSAHCl
DBSAHCl1
DBSAHCl2
N(1)ÀH ¥¥¥ O(1)ÀS (internal2)
N(1)ÀH ¥¥¥ O(1)ÀS
N(1)ÀH ¥¥¥ O(2)ÀS
N(1)ÀH ¥¥¥ O(3)ÀS
N(2)ÀH ¥¥¥ O(1)ÀS
N(2)ÀH ¥¥¥ O(2)ÀS
N(2)ÀH ¥¥¥ O(3)ÀS
2.352.5
2.39
1.93
2.00
1.96
1.93
2.09
2.12
2.36
2.4. Thermal Analyses. 2.4.1. Methods. Differential scanning calorimetry (DSC),
TGA, and optical hot-stage microscopy have been used individually and in
combination to elucidate the thermal properties of the various crystal forms. The
combination of DSC and optical hot-stage microscopy has proved to be a very valuable
tool in understanding this system.

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2.4.2. DBSA. The peak between 185 195 8 (Fig. 5) in the DSC thermogram of
DBSA corresponds to cracking of the crystals and the evolution of a gas as observed
sequentially by hot-stage microscopy (Fig. 6). The rough nature of the peak may be
accounted for by the sequential transformation of successive microcrystalline areas.
TGA on the same material (Fig. 7) gives no indication of any weight loss at this
temperature. The only peak seen represents the decomposition of the material at and
above its reported melting point of 260 2668 [24]. It should be noted that while this
melting point is listed in the Fluka catalogue, we have not observed it, i.e., this material,
like the other DBSA derivatives, begins to decompose rather than melt at this
temperature. There is no TGA event corresponding to the peak seen at ca. 185 195 8 in
the DSC thermogram of the anhydrate, and no corresponding observation of gas
evolution at this temperature when observed under silicon oil by hot-stage microscopy.
2.4.3. DBSA2³3W. The DSC thermogram of DBSA2³3W (Fig. 8) shows a large
endothermic peak at ca. 1158. Correlation with the TGA (Fig. 9) indicates that this peak
Fig. 5. DSC Thermogram of DBSA
Fig. 6. Cracking and darkening followed by evolution of bubbles from a single crystal of DBSA. The entire
sequence (from left to right) occurs in less than 20 s.

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Fig. 7. TGA Thermogram of DBSA
corresponds to the dehydration of the material to the anhydrate, followed by
decomposition. The dehydration may be followed by hot-stage microscopy of the crystals
under silicon oil. It is characterized by a darkening of the crystals, which is followed by
rapid production of bubbles indicating the escape of H2O vapor from the crystals. That this
is seen as one peak in the DSC and TGA suggests that the two crystallographically
independent H2O molecules are bound equally well in the DBSA2³3W crystal lattice.
There appears to be an exothermic event at ca. 2608, just before the onset of
decomposition, no concurrent event was observed by optical hot-stage microscopy. The
unevenness of the decomposition peak may be caused by differences in the sizes of the
crystals in the pan and their relative amounts of exposed surface area.
The TGA thermogram (Fig. 9) indicates a ratio of 2 molecules of material to
1 molecule of H2O quite different from the 3 :2 ratio obtained from single-crystal X-
ray diffraction. However, there is evidence that the material dehydrates partially on
standing. Sample vials containing crystals of previously brown plates were seen to
contain both the brown crystals and a beige powder on standing at room temperature
for an extended period of time (Fig. 10). The beige material was found to be
amorphous on examination by X-ray diffraction, and its microscope FT-IR spectrum

Helvetica Chimica Acta Vol. 86 (2003)
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Fig. 8. DSC Thermogram of DBSA2³3W
Fig. 9. TGA Thermogram of DBSA2³3W

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differs from all of the other 2,4-diaminobenzenesulfonic acid derivatives. TGA of a
similar powder from another vial was consistent with the existence of a monohydrate of
2,4-diaminobenzenesulfonic acid, although we have not yet investigated this further.
Fig. 10. Photograph of a sample of DBSA2³3W containing both brown crystals and the beige material described in
the text
2.4.4. DBSAHCl1 and DBSAHCl2. The DSC thermograms of the two polymorphs
DBSAHCl (Fig. 11) are quite different. That of DBSAHCl1 reveals only 1
endothermic peak, at ca. 2738 on decomposition. The DH of this peak is À 125.73 kJ/
mol. This corresponds well to its behavior observed under a hot-stage microscope,
where at 271 Æ 38 the crystals begin to produce bubbles, and then at 291.58, the crystals
begin to darken while continuing to produce bubbles. However, there is no observed
thermal event in the DSC thermogram corresponding to the observed darkening in
color of the crystals at 68 Æ 28.
Fig. 11. DSC Thermograms of DBSAHCl1 (red) and DBSAHCl2 (blue)

Helvetica Chimica Acta Vol. 86 (2003)
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DBSAHCl2 displays two endothermic peaks (onset temperatures at ca. 958 and ca.
2148, resp.) before the split decomposition peak at the same temperature as that of
DBSAHCl1. The lower-temperature peak has a DH of 2.79 kJ/mol and is consistent
with a darkening of the crystals observed by hot-stage microscopy. The second, broader
peak has a DH of 7.57 kJ/mol and does not correspond to any phenomenon observed by
hot-stage microscopy. The double-peaked endotherm at ca. 2808 has a smaller DH
(À 80.25kJ/mol) than that of DBSAHCl1 ( À125.73 kJ/mol); it corresponds well to the
observed darkening and onset of decomposition observed by hot-stage microscopy at
278.28. From the sequence of the appearance of the two crystal forms, DBSAHCl2 is
expected to be more stable than DBSAHCl1, in accordance with Ostwald×s rule of
stages.
2.5. FT-IR Spectroscopy. Due to the known tendency [2][6] of DBSA to crystallize
concomitantly and because of the variety of observed crystal habits, bulk analysis
proved uncertain. All the FT-IR spectra were measured by microscope FT-IR as this
allowed the analysis of single crystals of material, as well as possibly different crystal
forms resulting from the hot-stage experiments.
The FT-IR spectrum of DBSA2³3W displays the characteristic broad peak at
3350 cmÀ1 for H2O (Fig. 12), with several peaks of asymmetrically substituted aromatic
rings (1455, 1592 cmÀ1) and peaks of the SO3À (615and 1233 cm À1) and NH3 groups
‡
(1646 cmÀ1).
Fig. 12. FT-IR Spectra, run by means of an FT-IR microscope, of DBSA2³3W (pink) and DBSA (blue). The
intensities of these spectra are normalized to facilitate comparison.
The FT-IR spectrum of DBSA is quite different from that of DBSA2³3W, as would
be expected from their crystal structures. For example, the absence of the broad peak

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assigned to H2O at 3350 cmÀ1, and the addition of two peaks instead assigned to NH2
stretching, as well as a large peak at 3074 cmÀ1 for an asymmetrically substituted
aromatic ring, and several smaller ones at 2674 and 2594 cmÀ1 for NH3 , 1182 and
‡
1018 cmÀ1 for SO3À and 730 cmÀ1 for an NH2 group. However, there are also a number
of peaks at similar positions as a result of the same molecule×s presence in both
structures, such as the peaks at 2890, 1486, and 1447 cmÀ1 (CH stretch and bend),
1300 cmÀ1 (NH2), and 1106 cmÀ1 (asymmetrically substituted aromatic ring).
The FT-IR spectra of the two polymorphs of DBSAHCl (Fig. 13) share a number of
features, especially at lower wavelengths, although the overall shapes of the spectra are
quite different. There are a number of peaks, which can be used to unequivocally
identify each polymorph (Table 7).
Fig. 13. FT-IR Spectra of DBSAHCl1 (pink) and DBSAHCl2 (blue)
Table 7. Comparison of Peak Positions and Assignments [25][26] Characteristic of the DBSAHCl Polymorphs
DBSAHCl1
DBSAHCl2
nÄ/cmÀ1
Assignment
nÄ/cmÀ1
Assignment
716.0
796.9
amino group
2 neighboring aromatic CH groups
788.7
937.2
2 neighboring aromatic CH groups
asymmetrically substituted aromatic
asymmetrically substituted aromatic
R-SO3
CH stretch
amino group
1005.1
1991.6
3321.1
3395.8
3447.1
1023.5
1904.4
asymmetrically substituted aromatic
R-SO3
3371.1
3468.7
amino group
NH stretch
NH stretch
Discussion. We have described and characterized a number of crystal forms of
2,4-diaminobenzenesulfonic acid and of its hydrochloride salt. Originally described as
an example of concomitant crystallization of polymorphs, this system apparently has

Helvetica Chimica Acta Vol. 86 (2003)
3053
quite a rich crystal chemistry, some of which still remains to be investigated. For
instance, we have not yet succeeded in isolating and characterizing the second form of
DBSA reported variously by Post and Hardtung [2][3], Groth [1], and Hunter and
Sprung [6]. The investigation reported here was considerably aided by the ability to
rapidly screen crystalline samples on the single crystal diffractometer combined with
optical microscopy employing a hot stage. These methods require minimal sample
manipulation and provide a great deal of information; indeed once a crystal is mounted
on the diffractometer and proves to be different from other samples, the full structure
can be obtained in a matter of hours. The examination of many similarly appearing
crystals by single-crystal analysis has previously been shown to reveal several multiple
polymorphic structures [27]. It was this investigative approach that led to the discovery
of a number of the forms.
The variety exhibited by this −simple× system suggests re-investigating in greater
detail a number of other systems for which the older literature indicates a richness of
behavior. For example, investigations of the crystalline behavior of 2,5-diaminoben-
zenesulfonic acid and of the family of aminobenzenesulfonic acids are already
underway with preliminary results indicating diversity similar to that reported here.
Experimental Part
Synthesis. DBSA was initially prepared (Scheme 1) by exposure of meta-nitroaniline (ˆ 3-nitrobenzene-
amine) to 4 times the quantity of vitriolic acid preheated to 1758 in a melting tube. Application of 1 quantity of
sulfuric acid at 1608 for 1 h led to sulfonation to produce 2-amino-4-nitrobenzenesulfonic acid.
The finely powdered 2-amino-4-nitrobenzenesulfonic acid was then reduced by treatment with Sn and conc.
HCl soln. to produce colorless crystals of 2,4-diaminobenzenesulfonic acid. This material was separated from
the Sn by H2S through evaporation from the liquid to yield two dimorphic modifications a beige powder and
orange-brown prisms.
We attempted to prepare 3,5-diaminobenzenesulfonic acid by sulfonation of benzene-1,3-diamine. A solid
was precipitated from the soln. by the addition of AcOEt. The crystals were washed several times with AcOEt
and then dried in a vacuum flask. The material was redissolved in a minimal amount of H2O containing activated
carbon. The soln. was then filtered to remove the activated carbon and crystals of the 2,4-diaminobenzene-
sulfonic acid (DBSA) were obtained.
Thereafter, DBSA was obtained commercially from Fluka and used without further purification.
Single-Crystal X-ray Diffraction. All data were collected at r.t. by means of a SMART-6000K diffractometer
and MoKa radiation with a graphite monochromator. All atoms (including the H-atoms) were located from
difference maps, confirming the zwitterionic nature of the molecules. The reported H-atoms were refined
resulting in normal thermal parameters, and some shortening of the NÀH bonds which were not corrected. The
data were reduced by SAINT [28], solved with SHELXS [29], and then refined in SHELXTL [30].
Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as deposit numbers CCDC 205715 205718. Copies of the data
can be obtained, free of charge, on application to the CCDC, 12 Union Rd, Cambridge CB2IEZ UK (fax:
‡44 (1223) 336 033; e-mail: deposit@ccdc.cam.ac.uk. Structure solution and refinement were carried out
routinely. In all four structures the ratio of observed reflections to parameters was between 8.0 and 9.0.
Differential Scanning Calorimetry (DSC). All DSC measurements were performed using −Rheometric
Scientific Plus V v5.42× software on a Polymer-Laboratories PL-DSC differential-scanning calorimeter. The
reported measurements were all run with a heating rate of 108/min and an ambient cooling rate, in sealed Al
pans.
Thermogravimetric Analysis (TGA). All measurements were run on a Mettler-Toledo STAR system at a
heating rate of 108/min under N2 with a flow rate of 200 ml/min.
FT-IR Microscopy. FT-IR Microscopy was performed with a Bruker spectrometer, by means of an Equinox
55 connected to an IRScope II with −Opus× software.

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Helvetica Chimica Acta Vol. 86 (2003)
We are grateful to the US Israel Binational Science Foundation (Jerusalem), the Israel Science Foundation,
and the Kreitman Foundation for partial funding of this research, to Prof. Menachem Kaftory of the Haifa
Technion for the use of his DSC machine, to Ms. Jenny Zeroni for running the TGA measurements, and to Dr.
Vitaly Erukimovitch for assistance in obtaining the IR-microscopic data.
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Received March 11, 2003