Add time:09/24/2019 Source:sciencedirect.com
Methylboronic acid–methylboronate interchange has been studied kinetically by the band shape analysis of 11B NMR signals. The system was analyzed as two-site exchange CH3B(OH)2⇌CH3B(OH)3 − for a self-buffered aqueous solution in pH 9.5–11.4 at 24–55°C. The exchange rate was expressed as: rate=(kex[CH3B(OH)3 −]+kX2)[CH3B(OH)2]=(kex[CH3B(OH)2]+kY2)[CH3B(OH)3 −]. The second-order rate constant, kex (dm3 mol−1 s−1) (24.2°C)=7.03×105, is independent of pH, and the kinetic parameters obtained at I=0.10 M are ΔH≠ (kJ mol−1)=17.5±1.9 and ΔS≠ (J mol−1 K−1)=−74.3±6.2. A dimeric transition state is suggested for the path similarly to the interchange between boric acid and borate. The first-order rate constants kX2 and kY2 depend on the hydroxide ion concentration indicating alternative routes for the interchange.
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