Silver(I) activated quaternization of triethylamine by alkyl iodides: Kinetic model and stereoelectronic effects

A kinetic study of the silver(I) activated homogeneous and heterogeneous quaternization of coordinated triethylamine by butyl iodide in toluene was performed. Silver iodide was used as solid catalyst. The experimental data was analysed resorting to the multistep kinetic model previously proposed by Santos and Barbosa for the same reaction with ethyl iodide. The solution quaternization rate constant, kQ and the surface quaternization parameter k′QbEt3NAgNO3k′QbEt3NAgNO3 were obtained together with estimates of the adsorption coefficients, bBuI and bEtI, for the alkyl iodides on the solid. The presence of a competitive solution reaction between silver nitrate and the alkyl halide was confirmed and its solution rate constant, kC, calculated.The data shows silver(I) coordination lead to large increases in the surface and solution amine quaternization rates. A comparison with results previously obtained for the same reaction with ethyl iodide allowed further understanding about the effect of coordinated silver nitrate ion pairs on the quaternization processes. Silver dual role as an electrophilic activator and as a structural promoter was scrutinized through a comparison of rate constants ratios with increasing alkyl chain length. Clear evidence of an electrophilic activation progressively smeared by stereochemical packing constraints in the solution and superficial transition states was obtained.

Graphical abstractSilver dual role as electrophilic activator and structural promoter was scrutinized using a multistep kinetic model comprising thermodynamic and kinetic steps. The quantitative evaluation of kinetic and surface parameters, for the homogeneous and heterogeneous pathways, provided clear evidence of electrophilic activation and catalysis, smeared by stereochemical packing constraints with increasing alkyl iodide chain length.Figure optionsDownload full-size imageDownload high-quality image (132 K)Download as PowerPoint slide