Cyclobutadiene is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

with the formula . It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below 35 K. It is thought to adopt a rectangular structure.

Structure and reactivity

The compound is the prototypical antiaromatic hydrocarbon with 4 π-electrons. It is the smallest 'n'' annulene ( annulene). Its rectangular structure is the result of the

Jahn–Teller effect The Jahn–Teller effect (JT effect or JTE) is an important mechanism of spontaneous symmetry breaking in molecular and solid-state systems which has far-reaching consequences in different fields, and is responsible for a variety of phenomena in sp ...

, which distorts the molecule and lowers its symmetry, converting the

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 ...

to a singlet ground state. The electronic states of cyclobutadiene have been explored with a variety of computational methods. The rectangular structure is consistent with the existence of two different 1,2-dideutero-1,3-cyclobutadiene

valence isomer In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions. Benzene There are many valence isomers one can draw for the C6H6 formula benzene. Some were originall ...

s. This distortion indicates that the

pi electron In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbital ...

s are localized, in agreement with

Hückel's rule In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was f ...

which predicts that a π-system of 4 electrons is not

aromatic In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...

. In principle, another situation is possible. Namely, cyclobutadiene could assume an undistorted square geometry, ''if it'' ''adopts a triplet spin state''. While a theoretical possibility, the triplet form of the parent cyclobutadiene and its substituted derivatives remained elusive for decades. However, in 2017, the square triplet excited state of 1,2,3,4-tetrakis(trimethylsilyl)-1,3-cyclobutadiene was observed spectroscopically, and a singlet-triplet gap of ''E''_ST = 13.9 kcal/mol (or 0.6 eV per molecule) was measured for this compound.

Synthesis

Several cyclobutadiene derivatives have been isolated with steric bulky substituents. Orange tetrakis ( ''tert''-butyl)cyclobutadiene arises by thermolysis of its isomer tetra-''tert''-butyl tetrahedrane. Although the cyclobutadiene derivative is stable (with respect to dimerization), it decomposes upon contact with .

Trapping

Samples of cyclobutadiene are unstable since the compound dimerizes at temperatures above 35 K by a Diels-Alder reaction. By suppressing bimolecular decomposition pathways, cyclobutadiene is well-behaved. Thus it has been generated in a hemicarceplex. The inclusion compound is generated by photodecarboxylation of bicyclopyran-2-one. When released from the host–guest complex, cyclobutadiene dimerizes and then converts to

cyclooctatetraene 1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because ...

. After numerous attempts, cyclobutadiene was first generated by oxidative degradation of cyclobutadieneiron tricarbonyl with ammonium cerium(IV) nitrate. When liberated from the iron complex, cyclobutadiene

reacts ''React'' (from Spanish: ''Reacciona'') is a book by Rosa María Artal published in Spain in 2011 by Aguilar, which compiles articles by José Luis Sampedro, Baltasar Garzón, Federico Mayor Zaragoza, Javier Pérez de Albéniz, Javier López Facal ...

with electron-deficient

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no ...

s to form a Dewar benzene: : Cyclobutadiene-DewarbenzeneConversion

The Dewar benzene converts to dimethyl phthalate on heating at 90 °C. One cyclobutadiene derivative is also accessible through a +2 ycloaddition of a di-

. In this particular reaction the trapping reagent is ''2,3,4,5-tetraphenylcyclopenta-2,4-dienone'' and one of the final products (after expulsion of

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

) is a

: :

References

{{Annulenes Annulenes Antiaromatic compounds Four-membered rings

Structure and reactivity

Synthesis

Trapping

See also

References