Radialene
Encyclopedia
[n]Radialenes are alicyclic organic compound
s containing n cross-conjugated
exocyclic double bond
s. The double bonds are commonly alkene
groups but those with a carbonyl
(C=O) group are also called radialenes. For some members the unsubstituted parent radialenes are elusive but many substituted derivatives are known.
Radialenes are related to open-chain dendralenes
and also to compounds like butadiene and benzene
which also consist of a ring of sp2 hydridized carbon atoms.
Radialenes are investigated in organic chemistry
for their unusual properties and reactivity but have not ventured outside the laboratory. Reported uses are as experimental building blocks for novel organic conductors and ferromagnets. The first radialene called hexaethylidencyclohexane was synthesised in 1961
with all carbon atoms in the same plane. This is verified experimentally in hexamethyl[3]radialene and cyclobutanetetraonetetrakis(hydrazone). Decamethyl[5]radialene has a twist envelope geometry with C2 symmetry while a chair conformation is calculated for [6]radialene and found experimentally for hexa-(ethylidene)cyclohexane
Due to their specific pi-electron distributions, hydrocarbons such as perylene
and triphenylene
are not considered radialenes. One study describes a [6]radialene composed of thiophene
units :
This compound is reported as planar with D3h symmetry
(X-ray diffraction) but not aromatic: the carbon-carbon bond
length
s are unusually long (145 pm vs. 140 pm for benzene
) and the calculated NICS value is close to zero.
of cis,trans,cis-tetra(bromomethyl)cyclobutane with sodium methoxide
in ethanol
Hydrogenation
with platinum on carbon
gives cis,cis,cis-tetramethylcyclobutane in accordance with the proposed structure. On standing in air at room temperature
the compound accepts oxygen and polymerizes
.
Only substituted [6]radialenes exist as stable compounds. Stable derivatives are the hexamethyl substituted, dodecamethyl substituted and hexabromo substituted radialene.
A trisalkoxy-substituted radialene has also been also reported, the central ring adopting a non-planar twist-boat conformation:
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s containing n cross-conjugated
Cross-conjugation
Cross-conjugation is a special type of conjugation in a molecule, when in a set of three Pi bonds only two pi-bonds interact with each other by conjugation, the third one is excluded from interaction...
exocyclic double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
s. The double bonds are commonly alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
groups but those with a carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
(C=O) group are also called radialenes. For some members the unsubstituted parent radialenes are elusive but many substituted derivatives are known.
Radialenes are related to open-chain dendralenes
Dendralenes
A dendralene is a discrete acyclic cross-conjugated polyene. The simplest dendralene is buta-1,3-diene or [2]dendralene followed by [3]dendralene , [4]dendralene and [5]dendralene and so forth. [2]dendralene is the only one not cross-conjugated.The name dendralene is pulled together from the...
and also to compounds like butadiene and benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
which also consist of a ring of sp2 hydridized carbon atoms.
Radialenes are investigated in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
for their unusual properties and reactivity but have not ventured outside the laboratory. Reported uses are as experimental building blocks for novel organic conductors and ferromagnets. The first radialene called hexaethylidencyclohexane was synthesised in 1961
Conformation
[3] and [4]radialenes are expected to have a planar molecular geometryMolecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
with all carbon atoms in the same plane. This is verified experimentally in hexamethyl[3]radialene and cyclobutanetetraonetetrakis(hydrazone). Decamethyl[5]radialene has a twist envelope geometry with C2 symmetry while a chair conformation is calculated for [6]radialene and found experimentally for hexa-(ethylidene)cyclohexane
Due to their specific pi-electron distributions, hydrocarbons such as perylene
Perylene
Perylene or perilene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12, occurring as a brown solid. It or its derivatives may be carcinogenic, and it is considered to be a hazardous pollutant. In cell membrane cytochemistry, perylene is used as a fluorescent lipid probe...
and triphenylene
Triphenylene
In chemistry, the organic compound triphenylene is a flat polycyclic aromatic hydrocarbon consisting of four fused benzene rings. Triphenylene can be isolated from coal tar, but it is also made synthetically using benzyn chemistry. One molecule of triphenylene has delocalized 18-π-electron systems...
are not considered radialenes. One study describes a [6]radialene composed of thiophene
Thiophene
Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene...
units :
This compound is reported as planar with D3h symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
(X-ray diffraction) but not aromatic: the carbon-carbon bond
Carbon-carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is said to be formed between one hybridized orbital from each...
length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
s are unusually long (145 pm vs. 140 pm for benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
) and the calculated NICS value is close to zero.
Synthesis and properties
The parent [3],[4],[5] and [6]radialenes polymerize when in contact with oxygen.[4]Radialenes
The unsubstituted [4]radialene has been prepared in an elimination reactionElimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...
of cis,trans,cis-tetra(bromomethyl)cyclobutane with sodium methoxide
Sodium methoxide
Sodium methoxide is a chemical compound, with formula CH3ONa. This colourless solid, which is formed by the deprotonation of methanol, is a widely used reagent in industry and the laboratory...
in ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
Hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
with platinum on carbon
Platinum on carbon
Platinum on carbon is a material comprising metallic platinum dispersed on finely divided particles of carbon, used as a catalyst in chemical reactions....
gives cis,cis,cis-tetramethylcyclobutane in accordance with the proposed structure. On standing in air at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
the compound accepts oxygen and polymerizes
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
.
[6]Radialenes
The parent [6]radialene is unstable and polymerises immediately on formation. It has been synthesised from 1,5,9-cyclododecatriene, 1,3,6-tri(chloromethyl)mesitylene and tricyclobutabenzeneTricyclobutabenzene
Tricyclobutabenzene is an aromatic hydrocarbon consisting of a benzene core with three cyclobutane rings fused onto it. This compound and related compounds are studied in the laboratory because they are often display unusual conformations and because of their unusual reactivity...
Only substituted [6]radialenes exist as stable compounds. Stable derivatives are the hexamethyl substituted, dodecamethyl substituted and hexabromo substituted radialene.
A trisalkoxy-substituted radialene has also been also reported, the central ring adopting a non-planar twist-boat conformation: