Solvents, acceptor properties electron pair acceptance

Taft and coworkers described the formulation of three scales of solvent properties which were used to unravel and rationalize solvent effects on many types of physico-chemical properties. A tt scale of polarity/polarizabilities describes the solvent s ability to stabilize a charge or a dipole by virtue of its dielectric effect. The n values have been shown to be generally proportional to molecular dipole moments. The a scale of hydrogen bond donor acidities provides a measure of the solvent s ability to donate a proton. The jS scale of hydrogen bond acceptor basicities quantifies the solvent s ability to donate an electron pair (accept a proton). 

Acceptor number (or acceptivity), AN — is an empirical quantity for characterizing the electrophilic properties (-> Lewis acid-base theory) of a solvent A that expresses the solvent ability to accepting an electron pair of a donor atom from a solute molecule. AN is defined as the limiting value of the NMR shift, S, of the 31P atom in triethylphosphine oxide, Et3P=0, at infinite dilution in the solvent, relative to n-hexane, corrected for the diamagnetic susceptibility of the solvent, and normalized ... 

Acceptor solvents show a tendency to react with electron-pair donors, their donor properties being usually small or non-existent, and although the molecules are polar, no coordination to cations is usually detectable. On the other hand they will be able to accept electron pairs from anions, such as the halide ions, and hence will solvate such anions. 

Those photosensitive systems mentioned above consist of at least one vinyl compound which has an electron donating or accepting property. When both acceptor and donor are non-polymerizable, the system is not photosensitive. Photopolymerization of styrene is not sensitized by the ECZ-CH3CN pair. The definition of donor and acceptor is a matter of relativity. Styrene is by no means neutral, but there should be no objection to considering it as a weaker donor than VCZ or ECZ and a weaker acceptor than AN or CH8CN. Photoirradiation of AN, VCZ or styrene alone in a neutral solvent, such as benzene, or in bulk does not bring about any appreciable rate of polymerization. 

Acidic/Basic Lewis acidity asicity determines the solvent s ability to donate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this aeid ase property was proposed by Gutman (DN and AN -donor and acceptor number, respeetively) based on ealorimetrie determination. The complete proton transfer reaetion with formation of protonated ions is determined by proton affinity, gas phase aeidity, aeid or base dissoeiation constants. Both concepts differ in terms of net ehemieal reaetion. 

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