Physical Properties of Substituted Benzenes

The physical properties of substituted benzenes resemble those of alkanes and alkenes of similar shape and molecular weight. Table 13.1 collects some physical properties for a number of common substituted benzenes. Notice the effects of symmetry. -Xylene melts at a much higher temperature than the ortho or meta isomer, for example. Many para isomers have high melting points, and crystallization can sometimes be used as a means of separating the para regioisomer from the others. 

TABLE 13.1 Physical Properties of Some Substituted Benzenes  

You have learned that aromatic compounds are particularly stable. This stabili-  

Another important physical property of aromatic compounds is their involvement in n stacking. These flat molecules lie easily on top of each other, and this property is a critical feature of enzyme structure-activity and in DNA double helix formation. 

Aromatic compounds are called arenes and the term for an arene with an open valence is aryl. A general abbreviation for aryl is Ar, in analogy to the R used for a general alkyl species. 

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The physical properties of substituted benzenes vary depending on the nature of the substituent. Alkylbenzenes, like other hydrocarbons, are nonpolar and thus have lower boUing points than benzenes with polar substituents such as phenol, aniline, and benzoic acid. The melting points of substituted benzenes depend on whether or not their molecules can be packed close together. Benzene, which has no substituents and is flat, can pack its molecules very closely, giving it a considerably higher melting point than many substituted benzenes. 

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