Cyclobutadiene
Encyclopedia
Cyclobutadiene is the smallest [n]-annulene
([4]-annulene), an extremely unstable hydrocarbon
having a lifetime shorter than five seconds in the free state. It has chemical formula
44 and a rectangular
structure verified by infrared studies. This is in contrast to the square
geometry predicted by simple Hückel theory. Though it has alternating single and double bonds
, it fails Hückel's rule
, because its ring has 4 π-electrons, and 4 is not twice an odd number. Some cyclobutadiene-metal compounds are stable because the metal atom provides 2 more electrons to the system.
The pi electron energy of cyclobutadiene is higher than that of its open-chain counterpart, 1,3-butadiene, and it is therefore said to be anti-aromatic rather than aromatic. Recent studies on cyclobutadiene show that it has a rectangular structure as opposed to a square structure and two different 1,2-dideuterio-1,3-cyclobutadiene stereoisomers. This indicates that the pi electrons are localized and therefore not considered to be antiaromatic. However it is far from stable, it is highly reactive and has a very short lifetime. Cyclobutadiene dimerizes by a Diels-Alder reaction
at 35 K. The monomeric form has been studied at higher temperatures by trapping with matrix isolation
in a noble gas
.
, although he could not isolate it. Cyclobutadiene can be generated through degradation from a cyclobutadiene metal
compound for example 443 with ammonium cerium(IV) nitrate
. This cyclobutadieneiron tricarbonyl
complex was prepared from 49 and cis-dichlorocyclobutene in a double dehydrohalogenation
.
Cyclobutadiene when liberated from the iron complex reacts with electron-deficient alkyne
s to a Dewar benzene
:
The Dewar benzene converts to dimethyl phthalate
on heating at 90°C.
One cyclobutadiene derivative is also accessible through a [2+2]cycloaddition
of a di-alkyne
. 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
) is a cyclooctatetraene
:
Annulene
Annulenes are completely conjugated monocyclic hydrocarbons. They have the general formula CnHn or CnHn+1...
([4]-annulene), an extremely unstable hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
having a lifetime shorter than five seconds in the free state. It has chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
44 and a rectangular
Rectangle
In Euclidean plane geometry, a rectangle is any quadrilateral with four right angles. The term "oblong" is occasionally used to refer to a non-square rectangle...
structure verified by infrared studies. This is in contrast to the square
Square (geometry)
In geometry, a square is a regular quadrilateral. This means that it has four equal sides and four equal angles...
geometry predicted by simple Hückel theory. Though it has alternating single and double bonds
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
, it fails Hückel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
, because its ring has 4 π-electrons, and 4 is not twice an odd number. Some cyclobutadiene-metal compounds are stable because the metal atom provides 2 more electrons to the system.
The pi electron energy of cyclobutadiene is higher than that of its open-chain counterpart, 1,3-butadiene, and it is therefore said to be anti-aromatic rather than aromatic. Recent studies on cyclobutadiene show that it has a rectangular structure as opposed to a square structure and two different 1,2-dideuterio-1,3-cyclobutadiene stereoisomers. This indicates that the pi electrons are localized and therefore not considered to be antiaromatic. However it is far from stable, it is highly reactive and has a very short lifetime. Cyclobutadiene dimerizes by a Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
at 35 K. The monomeric form has been studied at higher temperatures by trapping with matrix isolation
Matrix Isolation
Matrix isolation is an experimental technique used in chemistry and physics which generally involves a material being trapped within an unreactive matrix. A host matrix is a continuous solid phase in which guest particles are embedded. The guest is said to be isolated within the host matrix...
in a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
.
Synthesis
After numerous attempts, it was first synthesized in 1965 by Rowland Pettit of the University of TexasUniversity of Texas at Austin
The University of Texas at Austin is a state research university located in Austin, Texas, USA, and is the flagship institution of the The University of Texas System. Founded in 1883, its campus is located approximately from the Texas State Capitol in Austin...
, although he could not isolate it. Cyclobutadiene can be generated through degradation from a cyclobutadiene metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
compound for example 443 with ammonium cerium(IV) nitrate
Ammonium cerium(IV) nitrate
Ceric ammonium nitrate is the inorganic compound with the formula 2Ce6. This orange-red, water-soluble cerium salt is widely used as an oxidising agent in organic synthesis and as a standard oxidant in quantitative analysis....
. This cyclobutadieneiron tricarbonyl
Cyclobutadieneiron tricarbonyl
Cyclobutadieneiron tricarbonyl or Fe3 is an organometallic complex of cyclobutadiene and an iron metal carbonyl. The chemical compound is used in organic chemistry as a precursor for cyclobutadiene...
complex was prepared from 49 and cis-dichlorocyclobutene in a double dehydrohalogenation
Dehydrohalogenation
Dehydrohalogenation is an organic reaction from which an alkene is obtained from an alkyl halide . It is also called a β-Elimination reaction and is a type of elimination reaction....
.
Cyclobutadiene when liberated from the iron complex reacts with electron-deficient alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
s to a Dewar benzene
Dewar benzene
Dewar benzene or bicyclo[2.2.0]hexa-2,5-diene is a bicyclic isomer of benzene with the molecular formula C6H6. The compound is named after James Dewar who included this structure in a list of possible C6H6 structures in 1867....
:
The Dewar benzene converts to dimethyl phthalate
Dimethyl phthalate
Dimethyl phthalate is a phthalate with molecular formula 2. It is the methyl ester of phthalic acid.Dimethyl phthalate is an ectoparasiticide and has many other uses, including in solid rocket propellants, plastics, and insect repellents....
on heating at 90°C.
One cyclobutadiene derivative is also accessible through a [2+2]cycloaddition
Cycloaddition
A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...
of a di-alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
. 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
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
) is a cyclooctatetraene
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
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