In organic chemistry, a xylylene (sometimes quinone-dimethide) is any of the constitutional isomers having the formula C<sub>6</sub>H<sub>4</sub>(CH<sub>2</sub>)<sub>2</sub>. These compounds are related to the corresponding quinones and quinone methides by replacement of the oxygen atoms by CH<sub>2</sub> 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, such as tetracyanoquinodimethane and the xylylene dichlorides.
p-Xylylene forms upon pyrolysis of p-xylene or, more readily, the ñ-substituted derivatives. p-Xylylene dimerizes with moderate efficiency to give p-cyclophane:
Further heating of the p-cyclophane gives poly(para-xylylene).
o-Xylylenes (o-quinodimethanes) are often generated in situ, e.g., by the pyrolysis of the corresponding sulfone. Another method involves 1,4-elimination of ortho benzylic silanes. or stannanes,
ñ,ñ'-ortho Xylene dibromides have been well developed for generating o-xylyenes. For example, reaction of tetrabromo-o-xylene (C<sub>6</sub>H<sub>4</sub>(CHBr<sub>2</sub>)<sub>2</sub>) with sodium iodide affords ñ,ñ'-dibromo-o-xylylene, which can be trapped to give naphthylene derivatives. In the absence of trapping agents, the xylylene relaxes to ñ,ñ'-dibromobenzocyclobutane:
Cycloadditions of these o-xylylenes provides a pathway to acenes.
The diene unit formed by the two exocyclic alkene units of the ortho isomer can serve as a ligand in coordination complexes. For example, reaction of ñ,ñ'-dibromo-o-xylene with iron carbonyls affords low yields of the xylylene complex Fe(CO)<sub>3</sub>[÷<sup>4</sup>-C<sub>6</sub>H<sub>4</sub>(CH<sub>2</sub>)<sub>2</sub>]. This product is structurally analogous to Fe(CO)<sub>3</sub>[÷<sup>4</sup>-1,3-butadiene].
At high temperatures, benzocyclobutenes 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.