All solvents were commercial products of high purity and used as received. The m-phenylenediamine (99% purity) used in this paper was purchased from Sinopharm Chemical Reagent Co., Ltd. NMR spectra were recorded on a Bruker ALX 400 Plus spectrometer operating at 400 MHz for ¹H NMR. Infrared spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer with use of pressed KBr pellets. Electronic absorption spectra were obtained on a Shimadzu UV-2600 spectrophotometer. Positive-ion ESI mass spectra were recorded with Thermo ScientificTM Q ExactiveTM Focus mass spectrometer.

In a 250 mL three-necked flask, 1,2-dichloroethane (138 g, 1.4 mol) was added, m-phenylenediamine (32 g, 0.3 mol) was added, stirring was done to dissolve it, and sulfur trioxide (48 g, 0.6 mol) was added slowly under stirring condition, and the temperature was raised to 60˚C, the reaction was carried out for 10 hours. The reaction was stopped, and solids were generated, filtered, washed, and dried to obtain the white 2,4-diaminobenzenesulfonic acid, and then it is recrystallized to obtain compound 1, and compound 1 is characterized for testing. Yield: 95% (53.58 g), ¹H NMR (400 MHz, DMSO-d₆) δ 7.44 (d, J = 8.3 Hz, 1H), 6.48 (d, J = 2.1 Hz, 1H), 6.44 (dd, J = 8.3, 2.1 Hz, 1H). IR spectrum (KBr disc, cm⁻¹): υ(N−H) 3383, 3327, υ(C−H) 3058, υ(C−N) 1302, 1008. MS (ESI): m/z = 189.0330 [M+H]⁺.

The reaction of m-phenylenediamine and chlorosulfonic acid was referred to the method of literature [14] , the specific reaction steps: in a 500 mL three-necked flask add 1,2-dichlorobenzene (191 g, 1.3 mol), and then add m-phenylenediamine (30 g, 0.28 mol), stirring to dissolve, heating to 110˚C to react for 1 hour; and then reduce the temperature to 70˚C, with a constant pressure buretting funnel to add a slow drop of chlorosulfonic acid (45 g, 0.39 mol) , the end of the drop, and continue the react for 2 hours, measure the end point, not reached, continue to heat for 2 hours, measure the end point, the end point reached (m-phenylenediamine content: 0.05%).

After treatment, add 40 mL water to the flask to remove excess chlorosulphonic acid, then add about 1000 mL water (water temperature 85˚C) and dilute it to 2000 mL beaker, stir for 0.5 hour, then place it in the separator funnel, let it stand overnight. The next day, it was found that there was obvious stratification, the upper water phase was stationary on the upper part of the refrigerator for a period of time, there was crystal precipitation, centrifugal filtration, to produce compound 2: Yield: 95% (26 g).

Intensity data were collected on a Bruker SMART APEX 2000 CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K. The collected frames were processed with the software SAINT [15] . The data were corrected for absorption using the program SADABS [16] . Structures were solved by the direct methods and refined by full-matrix least-squares on F2 using the SHELXTL software package [17] [18] . All non-hydrogen atoms expect for the lattice solvent molecules were refined anisotropically. The positions of all hydrogen atoms were generated geometrically (C_sp3−H = 0.96 Å), assigned isotropic thermal parameters and allowed to ride on their respective parent carbon atoms before the final cycle of least-squares refinement.

The equation for the monosulfonation reaction is shown in Figure 1. In 1,2-dichloroethane, the direct reaction of m-phenylenediamine with SO₃ was performed in one step to obtain the monosulfonated compound 1. The reaction mechanism is shown in Figure 2, m-phenylenediamine directly onto two sulfonic acid groups, after a period of reaction, a water molecule and sulfuric acid molecule fall off to get monosulfonated compound 1. The equation for the dual sulfonation reaction is shown in Figure 3, m-phenylenediamine was first incorporated into the solvent 1,2-dichloroethane, then chlorosulfonic acid was added, and after a period of reaction, filtration was carried out, and the bissulfonated compound 2 was obtained by spinning. The reaction mechanism is shown in Figure 4, the chlorosulfonic acid method is fast, and it will react quickly with aniline compounds to produce chloroaniline compounds, mainly because the molecular structure of chlorosulfonic acid is very active, easy to lose electrons, forming a new chemical bond, that is, the sulfonic acid group (-HSO₃) in the chlorosulfonic acid molecule will replace the hydrogen atoms on the aniline molecule. m-Phenylenediamine and chlorosulfonic acid react rapidly, with two sulfonic acid groups (-HSO₃) replacing hydrogen atoms on the carbon atoms of the m-phenylenediamine ring almost simultaneously. The reaction mechanism between m-phenylenediamine and chlorosulfonic acid is shown in Figure 4. Since m-phenylenediamine could not be directly reacted with normal sulfuric acid, the reaction was performed by mixing m-phenylenediamine with concentrated sulfuric acid and stirring, after the stirring was finished it was left at room temperature and after some time it was found that a crystal compound 3 appeared.

Three crystal structures have been confirmed by x-ray crystallography. The views of the molecular structures of compounds 1, 2 and 3 are shown in Figures 5-11, respectively. In compound 1, the benzene ring is connected to two NH₂

and two HSO₃-sulfonic acid groups. In compound 2, the benzene ring is connected to two NH₂ functional groups as well as one HSO₃-sulfonic acid group, while in compound 3, the two sulfonic acid groups are present in the free form. The average S-O length of these products range from 1.453(3) - 1.461(4) Å, which are similar to those in the previous report (1.4454(13) - 1.4762(10) Å) [19] . The average C-N length in compounds 1, 2 and 3 is 1.413(2) - 1.462(6) Å. The bond angles of O-S-O are 108.7(2)˚, 112.52(10)˚, 111.33(10)˚. Selected bond lengths and angles for compounds 1-3 are collected in Table 1 for comparison. The crystallographic data of the three crystals are shown in Appendix (Table A1).

The ¹H NMR spectrum of 2,4-diaminobenzenesulfonic acid compound 1 is shown in Figure 12. It can be obtained by nuclear magnetic resonance hydrogen

spectra that the chemical shift of 6 - 7 ppm belongs to the characteristic peak in the aromatic region. A total of three groups of peaks were found, among which 7.44 and 6.48 ppm were double peaks, and the signal belonged to the characteristic peak on the hydrocarbon adjacent to the benzene ring. There were two double peaks at the 6.44 ppm signal, which were cracked due to the influence of the connection of amino groups by the adjacent carbon. In summary, the characteristic peak in the aromatic region of the product was in line with that of the product. In addition, the nuclear magnetic carbon spectrum further verified the structure of the compound, and the peaks at 109.06, 110.49, 120.10, 129.21, 139.27, 140.31 ppm in the Figure 13 corresponded to the peaks of carbon in six different environments on the benzene ring.

The infrared spectrum of compound 1 is shown in Figure 14; the spectral bands near 3383 cm⁻¹ and 3327 cm⁻¹ are N-H asymmetric stretching vibration peaks and symmetric stretching vibration peaks of the amino group in aromatic

primary amine. At 3058 cm⁻¹ is the aromatic ring C-H stretching vibration peak. The absorption peak of sulfonyl SO 2 − is around 1187 cm⁻¹. The C-N stretching vibration peak on the aromatic ring is at 1302 cm⁻¹. The out-of-plane bending vibration absorption peak of two adjacent hydrogens on the benzene ring is at 812 cm⁻¹. The out-of-plane bending vibration absorption peak of three adjacent hydrogen on the benzene ring is at 679 cm⁻¹. The upward absorption of the amino group is a hill-like wide peak near 1018 - 861 cm⁻¹, so it overlaps with some characteristic peaks belonging to the benzene ring near this band, and the sulfonyl group attached to the benzene ring also has a certain influence on some characteristic peaks on the benzene ring.

The ultraviolet spectra of compound 1 in water and DMSO solvents display an intense absorption band at around 300 nm as shown in Figure 15, the peak near 300 nm is the absorption peak on the benzene ring, which has a red-shift compared with the absorption peak on the general benzene ring. This is because the benzene ring of 2,4-diaminobenzenesulfonic acid is connected with two auxochromic group N and S atoms. The interaction between N and S atoms and the benzene ring makes p-π conjugated and expands the range of electron activity. The absorption peak on the benzene ring moves towards the long wave direction, and the absorption intensity is also increased. The polarity of different solvents is different, and the position of the absorption peak, absorption intensity and spectral shape are also different. As shown in the figure, when the polar solvent increases, it can move n→π* transition to the short-wave direction, because the polarity of the ground state is large, and a strong hydrogen bond can be formed between non-bonding electrons (n electrons) and polar solvents. The ground state energy reduction is greater than the energy reduced by the interaction between the anti-bond orbital and the polar solvent, so the energy required for the transition becomes larger, so the absorption wavelength shifts short.

The mass spectrum of compound 1 is shown in Figure 16, it shows that the excimer ion peak m/z 189.0330 [M+H] + (calcd [M+H]⁺, m/z 189.0256) has a molecular weight of 188. The molecular formula is C₆H₈N₂O₃S when combined with the results of ¹H NMR.