The concept of a unified molecular model is proposed for the directed synthesis of chromogenic analytical reagents for the mercapto group in ("thiol-thiol") substitution reactions (SH reagents). The unified molecular model of the SH reagent includes an aromatic chromophore-containing thio fragment covalently bound by an ordinary bond on a sulfide sulfur atom to an electrophilic, relatively soft one [in terms of the principle of hard and soft acids and bases (the HSAB principle)] reaction center. The characteristics of the analytical effect of the SH reagent are determined by the structure of the thio fragment. The required change in the reactivity and selectivity of the SH reagent are achieved mainly by varying the softness (stiffness) of the reaction center. Spectral characteristics of analytical effects, reactivity and selectivity of SH reagents (obtained earlier and synthesized for the first time in this work) – hydrophobic aromatic thio (dithio) compounds: thiocyanates, symmetric disulfides, and thiomercuric compounds – were studied in an aprotic dipolar DMF medium. In support of the concept of the unified molecular model for compounds containing identical thiofragments it was established: - spectral characteristics of the analytical effect of all three types of compounds (thiocyanates, disulfides and thiomercuric compounds) are the same in the DMF medium. - in accordance with the principle of HSAB, the reactivity increases in the sequence thiocyanate < disulfide <<thiomercuric compound, which corresponds to an increase in the softness of the reaction center in the series: -С << - S-<< -Hg-. The universality of the concept of the unified molecular model follows from consideration of the spectral and kinetic characteristics not only of Ellman reagent, but as well of thio(dithio)azo compounds, thiocyanate and thiomercuric compound containing the same 3-carboxy, 4-nitrophenylthio fragment.
HYDROPHOBIC AROMATIC THIO (DITHIO)COMPOUNDS AS CHROMOGENIC ANALYTICAL REAGENTS FOR PHOTOMETRIC DETERMINATION OF THIOLS BY MERCAPTO GROUP II. A UNIFIED MOLECULAR MODEL OF THE ANALYTICAL REAGENT