organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

, a xylylene (sometimes quinone-dimethide) is any of the constitutional isomers having the formula C₆H₄(CH₂)₂. These compounds are related to the corresponding

quinone The quinones are a class of organic compounds that are formally "derived from aromatic compounds benzene.html" ;"title="uch as benzene">uch as benzene or naphthalene] by conversion of an even number of –CH= groups into –C(=O)– groups with ...

s and quinone methides 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 In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate. The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and i ...

. Certain substituted derivatives of xylylenes are however highly stable, such as tetracyanoquinodimethane and the xylylene dichlorides.

p-Xylylene

''p''-Xylylene forms upon pyrolysis of ''p''-

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...

or, more readily, the α-substituted derivatives. ''p''-Xylylene dimerizes with moderate efficiency to give ''p''-

cyclophane In organic chemistry, a cyclophane is a hydrocarbon consisting of an aromatic unit (typically a benzene ring) and a chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic ...

: Further heating of the ''p''-cyclophane gives poly(''para''-xylylene).

o-Xylylenes

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, Quinodomethanes2

α,α'-''ortho'' Xylene dibromides have been well developed for generating o-xylyenes. For example, reaction of

tetrabromo-o-xylene α,α,α',α'-Tetrabromo-o-xylene is an organobromine compound with the formula C6H4(CHBr2)2. Three isomers of α,α,α',α'-Tetrabromoxylene exist, but the ortho derivative is most widely studied. It is an off-white solid. The compound is prep ...

(C₆H₄(CHBr₂)₂) with sodium iodide affords α,α'-dibromo-o-xylylene, which can be trapped to give naphthylene derivatives. In the absence of trapping agents, the xylylene relaxes to α,α'-dibromobenzocyclobutane: :C₆H₄(CHBr₂)₂ + 2 NaI → C₆H₄(=CHBr)₂ + 2 NaBr + I₂ :C₆H₄(=CHBr)₂ → C₆H₄(CHBr)₂

Cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity". ...

s of these o-xylylenes provides a pathway to

acene In organic chemistry, the acenes or polyacenes are a class of organic compounds and polycyclic aromatic hydrocarbons made up of benzene () rings which have been linearly fused. They follow the general molecular formula . The larger represent ...

s. The

diene In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclature ...

unit formed by the two exocyclic alkene units of the ''ortho'' isomer can serve as a

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

es. For example, reaction of α,α'-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">�⁴-1,3-butadiene At high temperatures, 1,3-butadiene">1,3-butadiene">�⁴-1,3-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 electrocyclic reaction">electrocyclic ring-opening to form ''o''-xylylenes. This and other syntheses of ''o''-xylylenes, and their subsequent dimerization by [4+4cycloaddition">+4">electrocyclic reaction">electrocyclic ring-opening to form ''o''-xylylenes. This and other syntheses of ''o''-xylylenes, and their subsequent dimerization by [4+4cycloaddition to form cycloctyl structures, were used repeatedly in the synthesis of superphane.

Electronic structure

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 A triplet is a set of three items, which may be in a specific order, or unordered. It may refer to: Science * A series of three nucleotide bases forming an element of the Genetic code * J-coupling as part of Nuclear magnetic resonance spectrosc ...

ground-state.

References

{{DEFAULTSORT:Xylylene Cyclohexadienes Hydrocarbons Vinylidene compounds