| Abstract
| - A series of X,Y-substituted benzhydryl phenyl carbonates 1 and X,Y-substituted benzhydryl methylcarbonates 2 were subjected to solvolysis in different methanol/water, ethanol/water, and acetone/watermixtures at 25 °C. The LFER equation, log k = sf(Ef + Nf), was used to derive the nucleofuge-specificparameters (Nf and sf) for phenyl carbonate (1LG) and methyl carbonate (2LG) leaving groups in agiven solvent in SN1 type reaction. Kinetic measurements showed that phenyl carbonates solvolyze oneorder of magnitude faster than methyl carbonates. Optimized geometries of 1LG and 2LG at B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), and MP2(full)/6-311++G(d,p) levels revealed that negative chargedelocalization in carbonate anions to all three oxygen atoms occurs due to negative hyperconjugation.Phenyl carbonate (1LG) is a better leaving group (Nf = −0.84 ± 0.07 in 80% v/v aq EtOH) than methylcarbonate 2LG (Nf = −1.84 ± 0.07 in 80% v/v aq EtOH) because of more pronounced negativehyperconjugation, which is characterized with a more elongated RO−C bond and more increasedRO−C−CO angle in 1LG than in 2LG. Calculated affinities of benzhydryl cation toward methyl andphenyl carbonate anions (ΔΔEaff = 11.7 kcal/mol at the B3LYP/6-311++G(d,p) level and ΔΔEaff = 2.7kcal/mol at the PCM-B3LYP/6-311++G(d,p) level in methanol, respectively) showed that 1LG is morestabilized than 2LG, which is in accordance with greater solvolytic reactivity of 1 than 2.
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