A GENERALIZED SOLVENT ACIDITY SCALE - THE SOLVATOCHROMISM OF O-TERT-BUTYLSTILBAZOLIUM BETAINE DYE AND ITS HOMOMORPH O,O'-DI-TERT-BUTYLSTILBAZOLIUM BETAINE DYE

A total of 121 organic solvents were studied and classified by means o f the solvatochromic comparison method of Kamlet and Taft, to give a s olvent acidity scale for hydrogen-bond donor solvents based on the par ameter SA. This parameter can readily be evaluated from the visible sp ectrum of a suitable basic probe (o-tert-butylstilbazolium betaine dye , TBSB) and its non-basic homomorph (o,o'-di-tert-butylstilbazolium be taine dye, DTBSB). The proposed scale has many advantages when compare d to the more widely used acidity scales such as that based on Gutmann 's acceptor number (AN) and the Taft-Kamlet alpha scale. Whilst data f or the proposed scale are derived from electronic transitions only, th ey do provide an accurate description for solvent acidity effects in s pectroscopy and other chemical areas, including thermodynamics and kin etics.