# 22.2: Physical Properties of Arenes


The pleasant odors of the derivatives of many arenes is the origin of the name aromatic hydrocarbons. The arenes themselves generally are quite toxic; some are carcinogenic and inhalation of their vapors should be avoided. The volatile arenes are highly flammable and burn with a luminous sooty flame, in contrast to alkanes and alkenes, which usually burn with a bluish flame leaving little carbon residue.

The more common arenes and their physical properties are given in Table 22-1. They are less dense than water and are highly insoluble. Boiling points increase regularly with increasing molecular weight, but there is little correlation between melting point and molecular weight. The melting point is highly dependent on the symmetry of the compound; thus benzene melts $$100^\text{o}$$ higher than methylbenzene, and the more symmetrical 1,4-dimethylbenzene (para-xylene) has a higher boiling point than either the 1,2- or the 1,3-isomer. This latter fact is utilized in the separation by fractional crystallization of 1,4-dimethylbenzene from mixtures of isomers produced from petroleum.

Table 22-1: Physical Properties of Arenes