engleski

Method for Estimating SN1 Rate Constants: Solvolytic Reactivity of Benzoates

Nucleofugalities of pentafluorobenzoate (PFB) and 2, 4, 6-trifluorobenzoate (TFB) leaving groups have been derived from the solvolysis rate constants of X, Y-substituted benzhydryl PFBs and TFBs measured in the series of aqueous solvents, by applying the LFER equation: log k = sf (Ef + Nf). The heterolysis rate constants of dianisylmethyl PFB and TFB, and those determined for 10 more dianisylmethyl benzoates in aqueous ethanol constitute a set of reference benzoates whose experimental ∆G‡ have been correlated with the ∆H‡ (calculated by PCM quantum-chemical method) of the model epoxy ring formation. Because of the excellent correlation (r = 0.997), the method for calculating the nucleofugalities of substituted benzoate LGs have been established, ultimately providing the method for determination of the SN1 reactivity of any benzoate in a given solvent. Using the ∆G‡ vs. ∆H‡ correlation, and taking sf based on similarity, the nucleofugality parameters for about 70 benzoates have been determined in 90%, 80% and 70% aqueous ethanol. The calculated intrinsic barriers for substituted benzoate leaving groups show that substrates producing more stabilized LGs proceed over lower intrinsic barriers. Substituents on the phenyl ring affect the solvolysis rate of benzhydryl benzoates by both field and inductive effects.

nucleofugality; reactivity; benzoate; solvolysis; benzhydryl; 2-oxyethyl; epoxidation; pcm; b3lyp

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