Condensation of 2-chloropyridinium chloride with thiourea: Structure of the resulting product and its use as a brightener in nickel plating electrolytes

The protonated form, 2-chloropyridinium chloride, was synthesized and characterized spectroscopically for the first time to eliminate the influence of the basic nitrogen center in pyridine on the stability of the resulting isothiuronium salt. This compound readily condensed with thiourea to form the corresponding isothiuronium salt. Nuclear magnetic resonance (NMR) spectroscopy ( 1 H and 13 C) revealed that the salt exists as two stable conformers differing in the orientation around the C2–SC(NH 2 ) 2 bond. These conformers are most clearly distinguished in the 13 C NMR spectra by the chemical shift of the C4 signal in the pyridine ring relative to its position in neutral 2-chloropyridine. The presence of two stable conformers, determined by an additional basic center in the α-position, may represent a general pattern for this class of compounds. The synthesized pyridinium isothiuronium chloride was investigated as a brightener in a nickel plating electrolyte. Preliminary tests using a Hull cell identified an effective concentration range of 0.10–0.45 g/L and operational current densities of 3–20 A/dm 2 . The addition of the agent at 0.40–0.45 g/L to a sulfate nickel plating electrolyte produced bright nickel coatings with high current efficiency, low porosity, and increased hardness. The microhardness of the coating was found to increase with the additive concentration.