The Relationship Between Molecular Symmetry and Physicochemical Properties Involving Boiling and Melting of Organic Compounds.

Abstract

The reliable estimation of phase transition physicochemical properties such as boilingand melting points can be valuable when designing compounds with desired physicochemical properties. Thisstudy explores the role of external rotational symmetry in determining boiling and melting points of select organiccompounds. Using experimental data from the literature, the entropies of boiling and fusion were obtained for 541compounds. The statistical significance of external rotational symmetry number on entropies of phase change wasdetermined by using multiple linear regression. In addition, a series of aliphatic hydrocarbons, polysubstitutedbenzenes, and di-substituted napthalenes are used as examples to demonstrate the role of external symmetry ontransition temperature. The results reveal that symmetry is not well correlated with boiling point but isstatistically significant in melting point. The lack of correlation between the boiling point and thesymmetry number reflects the fact that molecules have a high degree of rotational freedom in both the liquid andthe vapor. On the other hand, the strong relationship between symmetry and melting point reflects the fact thatmolecules are rotationally restricted in the crystal but not in the liquid. Since the symmetry number is equal to thenumber of ways that the molecule can be properly oriented for incorporation into the crystal lattice, it is a significantdeterminant of the melting point.