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[摘要]：Nucleophilic substitution is ubiquitous in chem. and well studied. Nucleophilicity and leaving-group ability have been related to various reactant properties, such as electronegativity, size, polarizability, and others. Yet, the state-of-the-art is to some extent still phenomenol. Here, we try to arrive at a straightforward, causal relationship between the reactants' electronic structure and their SN2 reactivity. To this end, we have explored the potential energy surfaces of the backside as well as front-side SN2 reactions of X- + CH3Y with X, Y = F, Cl, Br, and I, using relativistic d. functional theory (DFT) at ZORA-OLYP/TZ2P. These explorations provide us with a consistent overview of trends, over a wide range of reactivities and pathways, which were analyzed using the activation strain model of chem. reactivity. A clear picture emerges from these analyses: nucleophilicity is detd. by the electron-donor capability of the nucleophile (i.e., energy and shape of the X- np AO), and leaving-group ability derives directly from carbon-leaving group (C-Y) bond strength. |
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