Role of Water in the Hydrolysis of BH₄^-1

The rate law for the hydrolysis of BH₄^- in Me₂SO as a solvent has the form of that for a general-acid catalyzed reaction. Rate coefficients for H⁺, H₂O, and 2,6-dinitrophenol are 0.74 ± 0.15, 1.5 ± 0.2 × 10^-2, and 0.19 ± 0.4 M^-1 sec^-1, respectively. That for water is six powers of ten larger than the comparable quantity in H₂O, but that for H⁺ is six powers of ten smaller. It is postulated that proton transfer from hydronium ion to BH₄^- in water uses a water molecule as a proton relay. The H⁺ rate coefficient in Me₂SO is much smaller because Me₂SO cannot function as a relay, and because of the instability of the structure Me₂SO…H…HBH₃⁺.