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-]+k X2)[CH3B(OH)2]=(kex[CH 3B(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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