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 alcohol and the corresponding arylcarbonyls (benzaldehyde, acetophenone, and benzophenone) were measured by Taylor's dispersion method in both ethyl and isopropyl alcohol. The experimental values are compared to published transient gracing measurements of the corresponding aryl ketyls (benzyl-; sec-phenethyl-, and diphenylmethyl-ketyl radical). The diffusion coefficients of the aryl alcohols are between 50 and 70% slower than the corresponding aryl 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, lacking acidic hydrogens to participate in hydrogen-bonding interactions with the solvent. On the other hand, the diffusion coefficient of the aryl alcohols and the corresponding aryl ketyls are comparable within expected experimental error. This work shows that the diffusion of ketyl radicals is not anomalously slow and that aryl alcohols are significantly better models than the corresponding aryl ketones for analyzing the diffusion of aryl ketyls in both ethyl and isopropyl alcohol. The standard empirical recipes of Spernol-Wirtz and Wilke-Chang do not adequately account for the interactions between the solutes and the hydroxylic solvents ethyl and isopropyl alcohol.