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 matter, organic materials, i.e., matter in its various forms that contain ...

, a xylylene (sometimes quinone-dimethide) is any of the

constitutional isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement of bonds) between them. The ...

s 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 methide A quinone methide is a type of conjugated organic compound that contain a cyclohexadiene with a carbonyl and an exocyclic methylidene or extended alkene unit. It is analogous to a quinone, but having one of the double bonded oxygens replaced with ...

s 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 chemical species, molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate energy level, degenerate. The term "diradical" is mainly used to describe organic compounds, where most ...

. Certain substituted derivatives of xylylenes are however highly stable, such as

tetracyanoquinodimethane Tetracyanoquinodimethane (TCNQ) is an organic compound with the chemical formula . It is an orange crystalline solid. This cyanocarbon, a relative of para-quinone, is an electron acceptor that is used to prepare charge transfer salts, which ar ...

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 su ...

or, more readily, the α-substituted derivatives. ''p''-Xylylene

dimerizes In chemistry, dimerization is the process of joining two identical or similar molecular entities by bonds. The resulting bonds can be either strong or weak. Many symmetrical chemical species are described as dimers, even when the monomer is u ...

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 Catenation, chain that forms a bridge (chemical), bridge between two non-adjacent positions of the aromatic ring. More complex der ...

: 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 pre ...

(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 hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

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 ( ); also 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 nome ...

unit formed by the two

exocyclic In organic chemistry, an alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached. Cyc ...

alkene units of the ''ortho'' isomer can serve as a

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

es. For example, reaction of α,α'-dibromo-''o''-xylene with

iron carbonyl There are three homoleptic iron carbonyl compounds: * The monomeric iron pentacarbonyl Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula . Under standard conditions Fe( CO)5 is a free-flowing, straw-colored liquid wit ...

s 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 A non-Kekulé molecule is a conjugated system, conjugated hydrocarbon that cannot be assigned a classical Kekulé structure. Since non-Kekulé molecules have two or more formal charges or radical (chemistry), radical centers, their Spin (physics ...

that has a triplet ground-state.

References

{{DEFAULTSORT:Xylylene Cyclohexadienes Hydrocarbons Vinylidene compounds