Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 773
In the author’s opinion, it seems that further usage of the empirical parameters should
more return to the originally intended purpose, emphasizing more the qualitative aspects
rather than to devote too much effort to multilinear regression analyses based on parameters
quoted to two decimal places. Admittedly, such a scheme may nevertheless still be used to
get some insight concerning the nature of some individual solvent effect as in the recent
case of an unprecedented positive wavelength shift in the solvatochromism of an
aminobenzodifuranone. 199
The physical approach, though still in its infancy, has been helping us to see the success
of the phenomenological approach in a new light. Accordingly, the reason for this well
documented and appreciated success can be traced back to the following features
• The molecular structure and the molecular size of many common solvents are
relatively similar. The majority belongs to the so-called select solvents having a
single dominant bond dipole, which, in addition, is typically hard, viz. an O- or
N-donor. For example, if also soft donors (e.g., sulfur) had been employed to a
larger extent, no general donor strength scale could have been devised. Likewise,
we have seen that solvents other than the select ones complicate the issue.
• As it runs like a thread through the present treatment, various cancellations and
competitions (enthalpy/entropy, repulsion/attraction, etc.) appear to be conspiring
to make molecular behavior in complex fluids seemingly simple.
Notwithstanding this, the phenomenological approach will remain a venerable cornerstone
in the development of unraveling solvent effects. Only time will tell whether a new
generation of solvent indices will arise from the physical approach.
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