Selective hydration of alkylammonium ions in nitrobenzene

In the nitrobenzene (NB)/water system, primary to tertiary ammonium ions wi th Me, Et and n-Bu groups were distributed with dipicrylaminate (DPA(-)) an d tetraphenylborate (TPB-), and the number of water molecules being co-extr acted to NB by an alkylammonium ion (RmNH4-m+) was measured by the Karl Fis cher method. In the use of TPB- as the counter ion, the numbers of co-extra cted water molecules, mainly for primary and secondary alkylammonium ions, were somewhat smaller than those in the use of DPA(-), showing a possible i nfluence of ion-pair formation between RmNH4-m+ and TPB- in NB. Conductivit y 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 assumi ng that the ion pairs of RmNH4-m+ would have no water molecules in NB, the "true'' hydration numbers (n(h)) of the RmNH4-m+ ions in NB were successful ly obtained. The n(h) 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: n(h) = 1.64, 1.04 and 0.66, respectively, for the prim ary, secondary and tertiary ammonium ions. It was suggested that water mole cules interact directly with hydrogen atom(s) bound to the central nitrogen atom.