Xylylene comprises two isomeric organic compounds with the formula C6H4(CH2)2. These compounds are related to the corresponding quinones by replacement of the oxygen atoms by CH2 groups. ortho- and para-xylylene are best known, although neither is stable in solid or liquid form. The meta form is a diradical. Certain substituted derivatives of xylylenes are however highly stable, an example being tetracyanoquinodimethane.
Further heating of the p-cyclophane gives poly(para-xylylene).
The diene unit formed by the two exocyclic alkene units of the ortho isomer can act as a ligand in coordination complexes. For example, reaction of ?,?'-dibromo-o-xylene with iron carbonyls affords low yields of the xylylene complex Fe(CO)3[?4-C6H4(CH2)2]. This product is structurally analogous to Fe(CO)3[?4-1,3-butadiene].
At high temperatures, benzocyclobutenes can undergo electrocyclic ring-opening to form o-xylylenes. This and other syntheses of o-xylylenes, and their subsequent dimerization by [4+4] cycloaddition to form cycloctyl structures, were used repeatedly in the synthesis of superphane.
Despite the observed chemistry of para-xylylene (i.e. its rapid polymerization to poly-p-xylylene), which suggests the compound exists as a diradical, physical evidence unanimously concludes that the lowest electronic state of p-xylylene is a closed shell singlet. Additionally, several computational methods confirm this assignment. Conversely, meta-xylylene is a non-Kekulé molecule that has a triplet ground-state.