Melting points and enthalpies of fusion of anthracene and its heteroatomic counterparts

Anthracene is a common byproduct of incomplete combustion of fossil fuels and other anthropogenic sources. Its heteroatomic counterparts, including 9-bromoanthracene, 1,5-dibromoanthracene, 9,10-dibromoanthracene, 2-chloroanthracene, 9,10-dichloroanthracene, 9-anthraldehyde, 2-anthracenecarboxylic acid, 9-anthracenecarboxylic acid, and anthraquinone, are formed through various mechanistic pathways during the combustion process. We use a differential scanning calorimeter to measure the melting points and enthalpies of fusion of these compounds. As expected, we find no correlation between molecular mass and melting point and enthalpy of fusion—rather the type, number and position of the heteroatoms substituted on the parent molecule all influence its fusion thermodynamics. A wide range of melting points is noted for the same substituents(s) at different carbon positions. This suggests that intermolecular forces, such as hydrogen bonding and steric repulsion, are significantly impacted by the position of the substituents on the linear anthracene parent molecular. In addition, different substituents at the same position further suggest that the electronegativity/polarity of a given atom strongly influences the observed fusion behavior.