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. Hard alkoxide ion preferentially abstracts a hard proton, so initiating E2 elimination, whereas soft nucleophilic malonate carbanion preferentially displaces soft bromide ion nucleofuge. (Note that the pKa values of the conjugate acids, ethanol and diethyl malonate, a similar, around 16.) 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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