N-(3-Sulfopropyl)aniline (SPA, also AnPS) is an organic compound with the molecular formula C9H13NO3S. It is an aniline derivative in which the nitrogen atom bears a 3-propane sulfonic acid group (–(CH2)3–SO3H), introduced by ring opening of 1,3-propanesultone. SPA is primarily used as a monomer in the synthesis of poly(N-3-sulfopropylaniline) (PSPA), a water-soluble, self-doped member of the polyaniline family of conducting polymers.
Synthesis SPA is prepared by the ring-opening addition of 1,3-propanesultone to aniline under mild conditions, without catalyst or inert atmosphere protection. The reaction is carried out by adding the sultone dropwise to excess aniline at room temperature; the product precipitates as a white solid, which is isolated by filtration, washed with acetone, and purified by recrystallization from hot methanol. Typical reported yields are 87–95%. An alternative procedure uses acetonitrile as solvent at 100 °C for 11 hours, with the product recrystallized from ethanol (yield ~87%). Ethanol has also been used as solvent at 60 °C for 24 hours. The use of excess aniline suppresses bis-alkylation at the nitrogen atom.
Spectroscopic characterization
NMR spectroscopy In D2O, the 1H NMR spectrum of SPA shows signals attributed to five distinct proton environments. Aromatic protons appear at δ 7.59 (t, 2H, Ar-H), 7.53 (t, 1H, Ar-H), and 7.46 (d, 2H, Ar-H); aliphatic protons appear at δ 3.60 (t, 2H, –NH–CH2–), 3.03 (t, 2H, –CH2–SO3H), and 2.19 (q, 2H, central –CH2–), consistent with the monosubstituted structure. The 13C NMR spectrum (D2O) shows signals at δ 134.5, 130.4, 129.9, 122.3 (aromatic carbons), 50.2, 47.7, and 20.9 (aliphatic carbons).
Infrared spectroscopy The FT-IR spectrum (KBr pellet) shows characteristic absorptions at 1594 and 1472 cm−1 (aromatic C=C stretching), 1168 and 1045 cm−1 (O=S=O asymmetric and symmetric stretching), and 3400–3500 cm−1 (N–H stretching). The S/N atomic molar ratio of 1, confirmed by X-ray photoelectron spectroscopy, is consistent with a single sulfonate group per nitrogen atom.
UV–visible spectroscopy In 1.2 M HCl, SPA shows UV absorption at λ = 208 nm (ε = 4218 M−1·cm−1) and λ = 254 nm (ε = 217 M−1·cm−1). The absorption at 254 nm is characteristic of the phenyl chromophore of monosubstituted anilines.
Mass spectrometry FAB high-resolution mass spectrometry gives m/z 214.65 (M+), in agreement with the calculated monoisotopic mass for C9H13NO3S.
Polymerization SPA undergoes oxidative polymerization in aqueous solution using ammonium persulfate as oxidant to give poly(N-3-sulfopropylaniline) (PSPA). The polymerization proceeds in pure water without added acid, as the acidification produced by the persulfate reduction is sufficient to initiate and sustain the reaction. The resulting polymer is soluble in water and is self-doped by its own pendant sulfonate groups. SPA also undergoes electrochemical polymerization on electrode surfaces in acidic aqueous media. Sequential electropolymerization of aniline followed by SPA, and direct copolymerization from mixed solutions, have been reported as routes to films with improved electroactivity at neutral pH relative to unmodified polyaniline.
Applications
Precursor to conducting polymers SPA is the principal monomer for the synthesis of PSPA and its copolymers with aniline, phenylenediamine isomers, and other monomers. These materials are used in electrochemical sensors, biosensors, and electrode modification.
Electrode modification Films of poly(aniline-co-SPA) deposited by sequential or co-electropolymerization on glassy carbon electrodes function as amperometric sensors for ascorbate at physiological pH, with detection limits reported as low as 2.2 µM. Dopamine and uric acid do not interfere under the reported conditions.
Nanocomposites SPA can be intercalated into vanadium pentoxide (V2O5) xerogel and polymerized in situ by V2O5 acting as oxidant in the presence of air, yielding PSPA/V2O5 nanocomposites with enhanced electrical conductivity compared to either component alone. Such composites have been investigated as cathode materials for rechargeable lithium batteries.
Redox flow batteries PSPA derived from SPA has been investigated as an aqueous catholyte in aqueous organic redox flow batteries, exploiting the polymer's water solubility, electroactivity across a wide pH range, and the reversible redox chemistry of the polyaniline backbone.
See also Polyaniline Conducting polymers 1,3-Propanesultone Poly-N-(3-sulfopropyl)aniline
References
