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An allene is a hydrocarbon
Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "pure" hydrocarbons, whereas other hydrocarbons with bonded com...
 in which one atom of carbon
Carbon

Carbon is a chemical element with chemical symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalence?making four electrons available to form covalent bond chemical bonds....
 is connected by 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, or between atoms and other covalent bonds....
 with two other atoms of carbon. Allene also is the common name for the parent compound of this series, propadiene.

Such pair of bonds make allenes much more reactive than other alkene
Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
s. For example, their reactivity with gaseous chlorine
Chlorine

Chlorine...
 is more like the reactivity of alkyne
Alkyne

Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, with the formula CnH2n-2. The alkynes are traditionally known as acetylenes or the acetylene series, although the name acetylene is also used to refer specifically to the simplest member of the series, known as e...
s. Allenes can be chiral
Chirality (chemistry)

The term chiral is used to describe an object that is non-Superposition on its mirror image.Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands...
.

central carbon of allene forms two sigma bond
Sigma bond

In chemistry, sigma bonds are the strongest type of covalent bond chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups....
s and two pi bond
Pi bond

In chemistry, pi bonds are covalent bond chemical bonds where two lobes of one involved electron atomic orbital overlap two lobes of the other involved electron orbital....
s.






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An allene is a hydrocarbon
Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "pure" hydrocarbons, whereas other hydrocarbons with bonded com...
 in which one atom of carbon
Carbon

Carbon is a chemical element with chemical symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalence?making four electrons available to form covalent bond chemical bonds....
 is connected by 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, or between atoms and other covalent bonds....
 with two other atoms of carbon. Allene also is the common name for the parent compound of this series, propadiene.

Such pair of bonds make allenes much more reactive than other alkene
Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
s. For example, their reactivity with gaseous chlorine
Chlorine

Chlorine...
 is more like the reactivity of alkyne
Alkyne

Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, with the formula CnH2n-2. The alkynes are traditionally known as acetylenes or the acetylene series, although the name acetylene is also used to refer specifically to the simplest member of the series, known as e...
s. Allenes can be chiral
Chirality (chemistry)

The term chiral is used to describe an object that is non-Superposition on its mirror image.Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands...
.

Geometry

Allene3d
The central carbon of allene forms two sigma bond
Sigma bond

In chemistry, sigma bonds are the strongest type of covalent bond chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups....
s and two pi bond
Pi bond

In chemistry, pi bonds are covalent bond chemical bonds where two lobes of one involved electron atomic orbital overlap two lobes of the other involved electron orbital....
s. The central carbon is sp-hybridised
Orbital hybridisation

In chemistry, hybridisation or hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties....
, and the two terminal carbons are sp2-hybridised. The bond angle formed by the three carbons is 180°, indicating linear geometry for the carbons of allene.

Symmetry

For allenes with four identical substituents, the compound will have a twofold axis of rotation through the center carbon. The axis is inclined at 45° to the CH2 planes on either side of the molecule. The molecule can thus be thought of as a two-bladed propeller
Propeller

A propeller is a type of fan which transmits power by converting rotational motion into thrust. It can be used to drive an fixed-wing aircraft, ship, or the fluid within a pump....
. Another twofold axis of rotation passes through the C=C=C bonds, and there is a mirror plane passing through both CH2 planes. Thus this class of molecules will have a D2d point group
Point group

In mathematics, a point group is a group of geometric symmetry leaving a point fixed....
.

An allene with only two substituents, A and B will be chiral
Chirality (chemistry)

The term chiral is used to describe an object that is non-Superposition on its mirror image.Human hands are perhaps the most universally recognized example of chirality: The left hand is a non-superposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands...
 because there will no longer be any mirror planes. Where A has a greater priority than B according to the Cahn-Ingold-Prelog priority rule
Cahn-Ingold-Prelog priority rule

The Robert Sidney Cahn?Christopher Kelk Ingold?Vladimir Prelog priority rules, CIP system or CIP conventions are a set of rules used in organic chemistry to name the stereoisomers of a molecule....
, the configuration can be determined by considering the top, then the bottom. For the bottom, only the group of higher priority need be considered.

Because of the symmetry, an unsubstituted allene has no dipole moment
Dipole moment

Dipole moment refers to the quality of a system to behave like a dipole. Dipole moment is the measured polarity of a polar covalent bond. It is defined as the product magnitude of charge on the atoms and the distance between the two bonded atoms....
.

Synthesis

Laboratory methods for the formation of allenes are:
  • from geminal dihalocyclopropanes and organolithium compounds in the Skattebøl rearrangement
    Skattebøl rearrangement

    The Skatteb?l rearrangement is an organic reaction for converting a geminal halogen cyclopropane to an allene using an organolithium base. This rearrangement reaction is named after its discoverer, Lars Skatteb?l, professor emeritus at the University of Oslo....
    .
  • from reaction of certain terminal alkyne
    Alkyne

    Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, with the formula CnH2n-2. The alkynes are traditionally known as acetylenes or the acetylene series, although the name acetylene is also used to refer specifically to the simplest member of the series, known as e...
    s with formaldehyde
    Formaldehyde

    Formaldehyde is a chemical compound with the chemical formula H2CO. It is the simplest aldehyde. Formaldehyde exists in several forms aside from H2CO: the cyclic trimer trioxane and the polymer Polyoxymethylene....
     copper(II) bromide
    Copper(II) bromide

    Copper bromide is a chemical compound. It is used in photographic processing as an intensifier and as a Halogenation agent in organic synthesis...
     and added base
  • from dehydrohalogenation
    Dehydrohalogenation

    Dehydrohalogenation is an organic chemistry reaction from which an alkene is obtained from an alkyl halide. It is also called the ?-Elimination reaction....
     of certain dihalides
    Halide

    A halide is a binary compound, of which one part is a halogen atom and the other part is an chemical element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound....
    .


See also

  • Compounds with three or more adjacent carbon-carbon double bonds are called cumulene
    Cumulene

    A cumulene is a chemical compound with three or more cumulative covalent bond, for example butatriene , H2C=C=C=CH2. Unlike alkanes and most alkenes, cumulenes tend to be rigid, which makes them appealing for molecular nanotechnology....
    s.
  • The allene motif is frequently encountered in carbomer
    Carbomer

    In organic chemistry, a carbomer is an expanded molecule obtained by insertion of a C2 unit in a given molecule . Carbomers differ from their templates in size but not in symmetry when each C?C single bond is replaced by at least one alkyne bond, and when a double bond is replaced by an allene bond....
    s.
  • Using a suitable catalyst (e.g. Wilkinson's catalyst
    Wilkinson's catalyst

    Wilkinson's catalyst is the common name for chlorotrisrhodium, a chemical compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use....
    ), it is possible to reduce just one of the double bonds of an allene.
  • Propadiene is one of the two main components of MAPP gas
    MAPP gas

    MAPP gas is liquefied petroleum gas mixed with methylacetylene-allene. In Australia, it is known as RazorGas and is a trademark of ELGAS....
    .


External links

  • IUPACs Gold Book
    Gold Book

    Compendium of Chemical Terminology is a book published by IUPAC containing internationally accepted definitions for terms in chemistry. Work on the first edition was initiated by Victor Gold, hence its informal name, the Gold Book....
     definition of allenes