R. Davis, Edward Bromels, C. Kibby
Mar 1, 1962
Citations
1
Influential Citations
105
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Journal
Journal of the American Chemical Society
Abstract
The hydrolysis of sodium borohydride in aqueous buffer solutions at 25 deg s catalyzed by general acids. The Bronsted constant is nearly 0.9. Salt effects and solvent isotope effects were measured. A mechanism involving a ratedetermining proton transfer from a general acid onto the borohydride ion is consistent with the data of numerous investigators. One reactive intermediate was trapped by hydrolyzing sodium borohydride in aqueous trimethylamine. Trimethylamine borane was produced in 1.45 t 0.09% yield while the remainder of the borohydride gave hydrogen gas. Hydrolysis of sodium borodeuteride occurs faster than the hydrolysis of sodium borohydride. The primary inverse isotope effect, k/sub H//k/sub D/, is 0.70 plus or minus 0.02 at 25 deg . The isotope effect for the boron-hydrogen bond was calculated. Several likely models of the activated complex give values of k/sub H// k/sub D/ from 0.60 to 0.95. A simplified model predicts 0.65 if the amount of primary bond breaking is proportional to the amount of secondary bond stiffening. The possible role of steric effects on secondary hydrogen isotope effects was estimated on a static model. The results show that both normal and inverse secondary isotope effects can be explained in this manner.