Basic General Properties
Solubility: Aromatic hydrocarbons are low-polarity organic compounds, insoluble in water, but soluble in non-polar organic solvents such as diethyl ether, carbon tetrachloride, and petroleum ether. Liquid aromatic hydrocarbons are also excellent organic solvents.
Density: Most aromatic hydrocarbons have a density less than water. Common compounds like benzene, toluene, and xylene have densities in the range of 0.86-0.88 g/cm³. Polycyclic aromatic hydrocarbons have slightly higher densities, but are still mostly less than 1 g/cm³.
Physical State: At room temperature, low molecular weight monocyclic aromatic hydrocarbons (such as benzene, toluene, and xylene) are mostly colorless liquids with a characteristic aromatic odor; high molecular weight polycyclic aromatic hydrocarbons are colorless or white solids. Most aromatic hydrocarbons have some degree of toxicity.
Melting and Boiling Point Patterns
Boiling Point Variation: As the relative molecular mass of aromatic hydrocarbons increases, the boiling point monotonically increases. For isomers, the stronger the polarity, the higher the boiling point. For example, the boiling point order of xylene is o-xylene > m-xylene > para-xylene. This is because stronger polarity means stronger intermolecular van der Waals forces.
Melting Point Variation: Melting point is not only related to relative molecular mass but also directly related to molecular symmetry: the better the symmetry, the more tightly packed the molecules, the stronger the intermolecular forces, and the higher the melting point. For example, benzene's melting point is abnormally higher than toluene's, and para-xylene's melting point is much higher than o-xylene and m-xylene (para-xylene melting point 13℃, o-xylene -25℃, m-xylene -48℃).