Study of Complex Formation of n-Alkylammonium Cations by Dibenzo-18-crown-6, Dibenzo-21-crown-7, and Dibenzo-24-crown-8 in Acetonitrile, Nitromethane and Nitrobenzene Solvents and Their Binary Mixtures Using Conductometric Method

The complex formation of ammonium and four n-alkylammonium perchlorate salts with dibenzo-18-crown-6 (DB18C6), dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) was studied by conductometry in acetonitrile (AN), nitrobenzene (NB) and nitromethane (NM) solvents at different temperatures and their binary mixtures at 25°C. The stability constants of the resulting 1:1 complexes were determined and found to decrease in the order: DB21C7 > DB18C6 > DB24C8. Number of hydrogen atoms on the ammonium group available for bonding to the crown ethers was the most important factor in determining complex stability constant.The influence of other factors such as the size of the macrocyclic cavity, steric effect of the alkyl group attached to the + NH3 group, and the Gutmann donicity of the solvents on stability of the complexes were studied. In all cases, the stability constants of the resulted complexes in solvents used varied in the order: NB > NM> AN. Standard enthalpies and entropies of the complex formation were obtained from the temperature dependence of the stability constants. Obtained thermodynamic parameters were sensitive to the type of solvent. The enthalpy - entropy compensation effect was investigated in general complexation of cations with crown ethers.