Xylylene
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. Certain substituted derivatives of xylylenes are however highly stable, an example being tetracyanoquinodimethane.
p-Xylylene forms upon pyrolysis of p-xylene or, more readily, the α-substituted derivatives (see equation). Upon condensation, p-xylylene dimerizes with moderate efficiency to give p-cyclophane:
Further heating of the p-cyclophane gives poly(para-xylylene).
The reaction of α,α'-dibromo-o-xylene with iron carbonyls affords low yields of the xylylene complex Fe(CO)3[η4-C6H4(CH2)2]. This complex is similar to Fe(CO)3[η4-1,3-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.
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