Free Energy of Solution for Tetramethylstannane in Hydroxylic Solvents

Abstract

A re-examination based upon published data for the standard free energy of solution of tetramethylstannane in thirty-two pure solvents was undertaken in order to account for the inadequacy of scaled particle theories in predicting the thermodynamic quantities associated with the dissolution of the solute in hydroxylic solvents. The Kamlet-Abboud-Taft linear free energy treatment of solute-solvent interactions was applied to this chemical system. Contrary to the usual assumptions about the idealized solute behavior for tetramethyltin(IV), there is evidence for a significant and systematic perturbation of the free energy of interaction because of hydrogen bond donor effects when water and the alcohols are the solvents. Like nearly all tin(IV) compounds of the type SnX4, the gaseous tetramethylstannane molecule has an ideal tetrahedral geometry as a result of sp3 bonding about the central tin atom. The tin-carbon bond distance of 2.134 A is nearly equal to the van der Waals radius for the tin atom (2.17 A); and, as a spherically symmetrical molecule in which the Sn atom is shielded from its surroundings by the outer methyl carbons, it is quite unlikely that outer 5d orbitals of Sn(IV) will be utilized to form specific solvates when tetramethylstannane is dissolved in polar solvents (Zubieta and Zuckerman, 1978). Thus, tetramethyltin(IV) would be expected to behave as an ideal uncharged solute species which is comparable structurally to the large quaternary ammonium and arsonium ions but without the complicating influences upon solubility and the thermodynamics of solution arising from ion-solvent electrostatic and dipolar interactions. The standard free energy of solution (AG,) as well as the enthalpy and entropy of solution have been measured for reaction 1 (Abraham and Nasehzadeh, 1981) in a variety of both hydrogen bonding and aprotic solvents (S) at room temperature.