Selective hydration of alkylammonium ions in nitrobenzene
Abstract
In the nitrobenzene (NB)/water system, primary to tertiary ammonium ions with Me, Et and n-Bu groups were distributed with dipicrylaminate (DPA−) and tetraphenylborate (TPB−), and the number of water molecules being co-extracted to NB by an alkylammonium ion (RmNH4−m+) was measured by the Karl Fischer method. In the use of TPB− as the counter ion, the numbers of co-extracted water molecules, mainly for primary and secondary alkylammonium ions, were somewhat smaller than those in the use of DPA−, showing a possible influence of ion-pair formation between RmNH4−m+ and TPB− in NB. Conductivity measurements were then performed to determined the association constants of the RmNH4−m+ ions with TPB− ions as well as DPA− in ‘‘water-saturated ’’ NB, and the concentrations of free RmNH4−m+ ions were evaluated. By assuming that the ion pairs of RmNH4−m+ would have no water molecules in NB, the ‘‘ true’’ hydration numbers (nh) of the RmNH4−m+ ions in NB were successfully obtained. The nh values of the RmNH4−m+ ions, being little affected by the alkyl chain length, were found to decrease with heightening the class of the RmNH4−m+ ion: nh=1.64, 1.04 and 0.66, respectively, for the primary, secondary and tertiary ammonium ions. It was suggested that water molecules interact directly with hydrogen atom(s) bound to the central nitrogen atom.