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 (R_mNH_4−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 R_mNH_4−m⁺ and TPB⁻ in NB. Conductivity measurements were then performed to determined the association constants of the R_mNH_4−m⁺ ions with TPB⁻ ions as well as DPA⁻ in ‘‘water-saturated ’’ NB, and the concentrations of free R_mNH_4−m⁺ ions were evaluated. By assuming that the ion pairs of R_mNH_4−m⁺ would have no water molecules in NB, the ‘‘ true’’ hydration numbers (n_h) of the R_mNH_4−m⁺ ions in NB were successfully obtained. The n_h values of the R_mNH_4−m⁺ ions, being little affected by the alkyl chain length, were found to decrease with heightening the class of the R_mNH_4−m⁺ ion: n_h=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.