Temperatures and Enthalpies of Solid−Solid and Melting Transitions of the Odd-Numbered n-Alkanes C₂₁, C₂₃, C₂₅, C₂₇, and C₂₉

Very high purity samples of normal pentacosane and heptacosane are made by synthesis: the procedures for the C 25 and C 27 synthesis and the original conditions of their purification, particularly using the extraction by supercritical carbon dioxide, are described. Measurements of temperatures and enthalpies of the solid−solid transitions and of the melting transition were carried out by differential scanning calorimetry on the C 25 and C 27 synthetic samples and the odd-numbered homologous n -alkanes (from C 21 to C 29 ). The results show the importance of the effect of the impurities, particularly for the δ and γ crystal−crystal transition temperatures of C 25 and C 27 . Several current data of the literature are also shown concerning the temperatures and the enthalpies of solid−solid and melting transitions of these five n -alkanes.