The acceptor number — A quantitative empirical parameter for the electrophilic properties of solvents

The electrophilic properties of 34 solvents have been characterized by the Acceptor Number (AN) which has been derived from³¹P-n.m.r. measurements of triethylphosphine oxide dissolved in the respective solvents. Relationships are found between the acceptor numbers and theZ-values,E _T-values andY-values, as well as the free energies of solvation of anions and the redox potentials of the hexacyanoferrate(III)—hexacyanoferrrate(II) system in different solvents. The new parameter provides—together with the donor number—a useful guide in choosing the most appropriate solvent for a given reaction.