According to Pearsons hard soft [Lewis] acid base principle, hard Lewis acids have a strong affinity for hard Lewis bases and soft Lewis acids have a strong affinity for soft Lewis bases. Enolate ions have a hard alkoxide ion end which preferentially displaces alkyl-Nfg(hard) complexes, such as oxonium ions, and a soft nucleophilic carbanion end displaces soft nucleofuges (such as iodide ion) from alkyl-Nfg(soft) complexes.Note that while hard vs soft comparisons can be made for carefully selected pairs of species and/or reactive centres, the HSAB analysis is NOT general. For more information look in the Chemogenesis
webbook section on Pearson's
HSAB principle.
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