13183-20-5Relevant articles and documents
Kinetics and mechanism of the phenolysis of asymmetric diaryl carbonates
Castro, Enrique A.,Pavez, Paulina,Santos, Jose G.
, p. 4494 - 4497 (2002)
The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0°C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Broensted-type plots for the nucleophilic rate constants (kN) are linear, with slopes β = 0.48 (MPNPC), 0.67 (C1PNPC), 0.41 (MPDNPC), and 0.32 (C1PDNPC). The magnitude of these slopes and the absence of a curvature in the Broensted plot at pKa = 7.1 for the C1PNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and C1PDNPC are more reactive than MPNPC and C1PNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log kN in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log kN to the basicity of the nonleaving moiety (βnlg = -0.27) is very similar to that of the nucleofuge (βlg = -0.25).
