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推 frankykung:大感謝^^ 01/03 07:20
Relative nucleophilicities of halide ions in polar aprotic solvents are quite
different from those in polar protic solvents
A guiding principle:
the freer the nucleophile, the greater its nucleophilicity
Polar aprotic solvents (e.g., DMSO, acetone, acetonitrile, DMF) are very
effective in solvating cations, but not nearly so effective in solvating anions.
Because anions are only poorly solvated, they participate readily in SN
reactions, and nucleophilicity parallels basicity: F- > Cl- > Br- > I-
Polar protic solvents (e.g., water, methanol) anions are highly solvated by
hydrogen bonding with the solvent the more concentrated the negative charge of
the anion, the more tightly it is held in a solvent shell the nucleophile must
be at least partially removed from its solvent shell to participate in SN reactions
because F- is most tightly solvated and I- the least, nucleophilicity is
I- > Br- > Cl- > F-
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