Comparison of diffusion coefficients of aryl carbonyls and aryl alcohols in hydroxylic solvents. Evidence that the diffusion of ketyl radicals in hydrogen-bonding solvents is not anomalous?

The diffusion coefficients of benzyl-, sec-phenethyl-, and diphenylmethyl a lcohol and the corresponding arylcarbonyls (benzaldehyde, acetophenone, and benzophenone) were measured by Taylor's dispersion method in both ethyl an d isopropyl alcohol. The experimental values are compared to published tran sient gracing measurements of the corresponding aryl ketyls (benzyl-; sec-p henethyl-, and diphenylmethyl-ketyl radical). The diffusion coefficients of the aryl alcohols are between 50 and 70% slower than the corresponding ary l ketones. The slower rate of diffusion is attributed to the capability of alcohol to participate as a hydrogen-bond donor with the solvent (ROH- - -O < (H)(R)). The ketone can only act as a weak hydrogen-bond, acceptor, lack ing acidic hydrogens to participate in hydrogen-bonding interactions with t he solvent. On the other hand, the diffusion coefficient of the aryl alcoho ls and the corresponding aryl ketyls are comparable within expected experim ental error. This work shows that the diffusion of ketyl radicals is not an omalously slow and that aryl alcohols are significantly better models than the corresponding aryl ketones for analyzing the diffusion of aryl ketyls i n both ethyl and isopropyl alcohol. The standard empirical recipes of Spern ol-Wirtz and Wilke-Chang do not adequately account for the interactions bet ween the solutes and the hydroxylic solvents ethyl and isopropyl alcohol.