Chain-Length Dependence of the Thermodynamic Behavior of Homologous α,ω-Disubstituted Alkanes

The chain-length dependence of the thermodynamic properties associated with the solid-to-liquid, liquid-to-gas, and solid-to-gas phase equilibria is analyzed and discussed for homologous families of linear α,ω-disubstituted alkanes, R -(CH 2 ) n - R series. A remarkable alternation on the melting properties exhibited by even and odd-numbered alkanes is clearly emphasized in their α,ω-disubstituted derivatives since the even members display increased properties due to their higher crystal packing density. The odd–even effect is also perceived in the values of Δ sub H o and Δ sub S o . Strong hydrogen bonding contributes to high boiling points and Δ vap H o values evidenced by alkane-α,ω-diols. Moreover, the anomalously low values of Δ sub H o and Δ vap H o reported for larger dicarboxylic acids suggest the formation, in the vapor phase, of hydrogen-bonded cyclic structures. Furthermore, the analysis of the Δ fus H o /Δ sub H o and Δ fus S o /Δ sub S o ratios is used to highlight the contribution of functional groups to the cohesive interaction preserved in the liquid phase after a fusion transition. The thermodynamic interpretation indicates a higher structuration in the liquid alkane-α,ω-diols and alkane-α,ω-dioic acids, which have lower ratios of Δ fus H o /Δ sub H o than corresponding n -alkanes. In addition, the thermodynamic analysis supports that hydrogen bonding in the liquid phase of alkanamines or alkane-α,ω-diamines has a significant low contribution to the overall intermolecular interactions.