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In 1962, Edwards and Pearson (the latter of HSAB theory) introduced the phrase alpha effect for this anomaly. He offered the suggestion that the effect was caused by a transition state (TS) stabilization effect: on entering the TS the free electron pair on the nucleophile moves away from the nucleus, causing a partial positive charge which can be stabilized by an adjacent lone pair as for ...
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are Lewis bases. Nucleophilic describes the affinity of a nucleophile to bond with positively charged ...
The electron pair (:) from the nucleophile (Nuc) attacks the substrate (R−LG) and bonds with it. Simultaneously, the leaving group (LG) departs with an electron pair. The principal product in this case is R−Nuc. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged.
A later paper by Edwards and Pearson, following research done by Jencks and Carriuolo in 1960 [8] [9] led to the discovery of an additional factor in nucleophilic reactivity, which Edwards and Pearson called the alpha effect, [7] where nucleophiles with a lone pair of electrons on an atom adjacent to the nucleophilic center have enhanced ...
The Cieplak effect relies on the stabilizing interaction of mixing full and empty orbitals to delocalize electrons, known as hyperconjugation. [2] When the highest occupied molecular orbital of one system and the lowest unoccupied molecular orbital of another system have comparable energies and spatial overlap, the electrons can delocalize and sink into a lower energy level.
Contour plot of electron density Laplacian values in the plane of the heterocycle for a model NacNac-supported aluminium(I) molecule, depicting electron density accumulation and the nucleophilic aluminium(I) lone pair. Plot adapted from the work of Cimpoesu and coworkers. [15]
[2] [3] Hence, the observed for nucleophilic attack appears to be influenced primarily by the energetics of the HOMO-LUMO overlap of the nucleophile-electrophile pair in the systems studied—see the Bürgi–Dunitz article, and the related inorganic chemistry concept of the angular overlap model (AOM) [b] [11] [c] [d] [12] [13] —which leads ...
The discovery and categorization of heterolytic bond fission was clearly dependent on the discovery and categorization of the chemical bond. In 1916, chemist Gilbert N. Lewis developed the concept of the electron-pair bond, in which two atoms share one to six electrons, thus forming the single electron bond, a single bond, a double bond, or a triple bond. [3]