Toluene

Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from a group of six atoms from the benzene molecule has been replaced by a univalent group, in this case CH3. As such, its IUPAC systematic name is methylbenzene.

Quotes

 * Methylbenzene is readily metabolized because its methyl C – H bonds are relatively weak with respect to homolytic and heterolytic cleavage. When one of these methyl hydrogens has been removed, the resulting phenylmethyl (benzyl)group, C6H5CH2, may be viewed as a benzene ring whose p system overlaps with an extra porbital located on an attached alkyl carbon. This interaction, generally called benzylic resonance,stabilizes adjacent radical, cationic, and anionic centers in much the same way that overlap of a pbond and a third porbital stabilizes 2-propenyl (allyl) intermediates (...). However, unlike allylic systems, which may undergo transformations at either terminus and give product mixtures (in the case of unsymmetrical substrates), benzylic reactivity is regioselective and occurs only at the benzylic carbon. The reason for this selectivity lies in the disruption of aromaticity that goes with attack on the benzene ring.
 * K. Peter C. Vollhardt, Neil E. Schore (2011) Organic chemistry : structure and function 6th ed. Ch. 22 : Chemistry of Benzene Substituents