Evidence of two-step deprotonation of D-mannitol in aqueous solution

Deprotonation of D-mannitol was studied in aqueous basic solutions by means of potentiometry and C-13 NMR spectroscopy. Two-step dissociation in the pH range from 12 to 13.8 was shown, and successive dissociation constants K-a1 and K-a2 were determined. In a solution with ionic strength I = 1.0 M (NaOH + NaNO3) pK(al) = 13.1 +/- 0.1 and pK(a2) = 13.8 +/- 0.2. With increasing ionic strength from 0.75 to 3.0 M, both pK(a1) and pK(a2) values decrease. Deprotonation-induced chemical shifts in pH-variable 13C NMR spectra show that the OH-groups next to internal carbon atoms C-3 and C-4 dissociate to a greater extent compared to OH-groups next to external carbon atoms C-1 and C-6. (c) 2005 Elsevier Ltd. All rights reserved.