A new thiazolylazo reagent 1-[(5-benzyl-1,3-thiazol-2-yl)diazenyl]naphthalene-2-ol has been synthesized via arylation of acrolein and further diazotization and diazocoupling with 2-naphthol. The chemical structure of the compound was confirmed by IR, NMR: ¹H, ¹³C, COSY, HSQC and SALDI-MS spectral data. The DSC method was employed to determine the melting point and the enthalpy of fusion (T_fus = 159.7 °C and ΔH_fus = 28.0 kJ mol⁻¹, respectively). Thermal stability and decomposition studies of the obtained azo dye were performed in air and nitrogen atmosphere using the TG technique. The evolved gases from the heated sample were registered on FTIR spectra. The decomposition of the azo dye in air atmosphere occurred in five consecutive stages. The pK_a ionization constants of the substance were determined in ethanol–water mixture (1:1 v/v) by means of the spectroscopic method and were found to be equal to 0.41 for pK_a1 and 9.59 for pK_a2. Using the spectrophotometric procedures, the methods for determination of trace amounts of toxic metallic ions with the new reagent were characterized and limits of detection were calculated.

2‐Bromo‐2‐chloro‐3‐arylpropanenitriles can be prepared by Meerwein reaction from 2‐chloroacrylonitrile and various aryldiazonium salts under copper(II) bromide catalysis. They proved to be stable compounds which form 2‐chlorocinnnamonitriles upon treatment with bases. Reaction of the title compounds with substituted thioglycolates gave substituted 3‐aminothiophenes which have not yet been accessible by other routes. Three‐component reactions with the title compound, sodium sulfide and bromonitromethane, chloroacetonitrile, or ethyl 4‐chloroacetoacetate gave 2‐nitro‐ and 2‐cyano‐substituted 3‐aminothiophenes, and thienopyridinediones in high yields and in one single step, respectively.