SOLVATOCHROMISM OF STERICALLY HINDERED STILBAZOLIUM BETAINES AND ITS RELATIONSHIP TO REICHARDT E(T)(30) SCALE - THE PROBLEM OF THE MEASUREMENT OF THE POLARITY VS THE ACIDITY OF ALCOHOLS

The hydrogen bond acidity of twenty primary, secondary, and tertiary alkanols has been measured by the solvatochromic method by using three stilbazolium betaines: stilbazolium betaine itself (SB), o-tert-butyl-substituted (TBSB), and o,o'-di-tert-butyl-substituted SB (DTBSB). The spectral differences between two of these betaines and in particular the DELTA' values which correspond to SB and DTBSB, have been compared with Reichardt's E(T)(30) values. Excluding sterically hindered alcohols, both scales are linearly related. For normal alkanols, the DELTA' values have been compared with the absorbance of the stretching of the OH band both free and associated with N,N-dimethylacetamide (DMA). There is a linear relationship between DELTA' and In [(A(v(OH)associated)/(A(v(OH)free)] which proves that DELTA' is a measure of the hydrogen-bond donor (HBD) acidity of alcohols. Contrary to some expectations, the effect of alkyl substituents shows a slow attenuation with increasing number of carbon atoms in the chain.