Allene Hale
   HOME

TheInfoList



OR:

In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, allenes are
organic compounds 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. The s ...
in which one
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
atom has
double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
s with each of its two adjacent carbon centres (). Allenes are classified as cumulated dienes. The parent compound of this class is
propadiene Propadiene () or allene () is the organic compound with the formula . It is the simplest allene, i.e. a compound with two adjacent carbon double bonds. As a constituent of MAPP gas, it has been used as a fuel for specialized welding. Production ...
, which is itself also called ''allene''. Compounds with an allene-type structure but with more than three carbon atoms are members of a larger class of compounds called
cumulene In organic chemistry, a cumulene is a compound having three or more ''cumulative'' (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (), which is also ...
s with bonding.


History

For many years, allenes were viewed as curiosities but thought to be synthetically useless and difficult to prepare and to work with.The Chemistry of the Allenes (vol. 1−3); Landor, S. R., Ed.; cademic Press: London, 1982. Reportedly, the first synthesis of an allene,
glutinic acid Penta-2,3-dienedioic acid (one of two chemicals called glutinic acid), is in allene-containing dicarboxylic acid. It was the first allene to be synthesized, in 1887, but the structure of it was thought to be a propyne core instead of an allene. Th ...
, was performed in an attempt to prove the non-existence of this class of compounds. The situation began to change in the 1950s, and more than 300 papers on allenes have been published in 2012 alone. These compounds are not just interesting intermediates but synthetically valuable targets themselves; for example, over 150 natural products are known with an allene or cumulene fragment.


Structure and properties


Geometry

The central carbon atom of allenes forms two
sigma bond In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of s ...
s and two
pi bond 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 orbitals ...
s. The central carbon atom is sp-hybridized, and the two terminal carbon atoms are sp2-hybridized. The bond angle formed by the three carbon atoms is 180°, indicating linear geometry for the central carbon atom. The two terminal carbon atoms are planar, and these planes are twisted 90° from each other. The structure can also be viewed as an "extended tetrahedral" with a similar shape to
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
, an analogy that is continued into the stereochemical analysis of certain derivative molecules.


Symmetry

The symmetry and isomerism of allenes has long fascinated organic chemists. For allenes with four identical substituents, there exist two twofold axes of rotation through the central carbon atom, inclined at 45° to the CH2 planes at either end of the molecule. The molecule can thus be thought of as a two-bladed
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
. A third 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 belong to the D2d
point group In geometry, a point group is a mathematical group of symmetry operations (isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every p ...
. Because of the symmetry, an unsubstituted allene has no net dipole moment, that is, it is a non-polar molecule. An allene with two different substituents on each of the two carbon atoms will be
chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...
because there will no longer be any mirror planes. The chirality of these types of allenes was first predicted in 1875 by Jacobus Henricus van 't Hoff, but not proven experimentally until 1935. Where A has a greater priority than B according to the
Cahn–Ingold–Prelog priority rules In organic chemistry, the Cahn–Ingold–Prelog (CIP) sequence rules (also the CIP priority convention; named for R.S. Cahn, C.K. Ingold, and Vladimir Prelog) are a standard process to completely and unequivocally name a stereoisomer of a ...
, the configuration of the
axial chirality Axial may refer to: * one of the anatomical directions describing relationships in an animal body * In geometry: :* a geometric term of location :* an axis of rotation * In chemistry, referring to an axial bond * a type of modal frame, in music * ...
can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the back atom, only the group of higher priority need be considered. Chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties. There are a few examples of drug molecule having an allene system in their structure.  Mycomycin, an antibiotic with tuberculostatic properties, is a typical example. This drug exhibits enantiomerism due to the presence of a suitably substituted allene system. Although the semi-localized textbook σ-π separation model describes the bonding of allene using a pair of localized orthogonal π orbitals, the full molecular orbital description of the bonding is more subtle. The symmetry-correct doubly-degenerate HOMOs of allene (adapted to the D2d point group) can either be represented by a pair of orthogonal MOs ''or'' as twisted helical linear combinations of these orthogonal MOs. The symmetry of the system and the degeneracy of these orbitals imply that both descriptions are correct (in the same way that there are infinitely many ways to depict the doubly-degenerate HOMOs and LUMOs of benzene that correspond to different choices of eigenfunctions in a two-dimensional eigenspace). However, this degeneracy is lifted in substituted allenes, and the helical picture becomes the only symmetry-correct description for the HOMO and HOMO–1 of the C2-symmetric 1,3-dimethylallene. This qualitative MO description extends to higher odd-carbon cumulenes (e.g., 1,2,3,4-pentatetraene).


Chemical and spectral properties

Allenes differ considerably from other alkenes in terms of their chemical properties. Compared to isolated and conjugated dienes, they are considerably less stable: comparing the isomeric pentadienes, the allenic 1,2-pentadiene has a heat of formation of 33.6 kcal/mol, compared to 18.1 kcal/mol for (''E'')-1,3-pentadiene and 25.4 kcal/mol for the isolated 1,4-pentadiene. The C–H bonds of allenes are considerably weaker and more acidic compared to typical vinylic C–H bonds: the bond dissociation energy is 87.7 kcal/mol (compared to 111 kcal/mol in ethylene), while the
gas-phase acidity The proton affinity (PA, ''E''pa) of an anion or of a neutral atom or molecule is the negative of the enthalpy change in the reaction between the chemical species concerned and a proton in the gas phase: ::: A- + H+ -> HA ::: B + H+ -> BH+ ...
is 381 kcal/mol (compared to 409 kcal/mol for ethylene), making it slightly more acidic than the propargylic C–H bond of propyne (382 kcal/mol). The 13C NMR spectrum of allenes is characterized by the signal of the sp-hybridized carbon atom, resonating at a characteristic 200-220 ppm. In contrast, the sp2-hybridized carbon atoms resonate around 80 ppm in a region typical for alkyne and nitrile carbon atoms, while the protons of a CH2 group of a terminal allene resonate at around 4.5 ppm — somewhat upfield of a typical vinylic proton. Allenes possess a rich cycloaddition chemistry, including both +2and +2modes of addition, as well as undergoing formal cycloaddition processes catalyzed by transition metals. Allenes also serve as substrates for transition metal catalyzed hydrofunctionalization reactions.


Synthesis

Although allenes often require specialized syntheses, the parent allene,
propadiene Propadiene () or allene () is the organic compound with the formula . It is the simplest allene, i.e. a compound with two adjacent carbon double bonds. As a constituent of MAPP gas, it has been used as a fuel for specialized welding. Production ...
is produced industrially on a large scale as an equilibrium mixture with
methylacetylene Propyne (methylacetylene) is an alkyne with the chemical formula . It is a component of MAPD gas—along with its isomer propadiene (allene), which was commonly used in gas welding. Unlike acetylene, propyne can be safely condensed.Peter Päs ...
: :H2C=C=CH2 <=> H3C-C#CH This mixture, known as
MAPP gas MAPP gas was a trademarked name, belonging to The Linde Group, a division of the former global chemical giant Union Carbide, for a fuel gas based on a stabilized mixture of methylacetylene (propyne), propadiene and propane. The name comes from th ...
, is commercially available. At 298 K, the Δ''G°'' of this reaction is –1.9 kcal/mol, corresponding to ''K''eq = 24.7. The first allene to be synthesized was
penta-2,3-dienedioic acid Penta-2,3-dienedioic acid (one of two chemicals called glutinic acid), is in allene-containing dicarboxylic acid. It was the first allene to be synthesized, in 1887, but the structure of it was thought to be a propyne core instead of an allene. Th ...
, which was prepared by Burton and Pechmann in 1887. However, the structure was only correctly identified in 1954. Laboratory methods for the formation of allenes include: *from geminal dihalocyclopropanes and organolithium compounds (or metallic sodium or magnesium) in the
Skattebøl rearrangement The Skattebøl rearrangement is an organic reaction for converting a geminal dihalo cyclopropane to an allene using an organolithium base. This rearrangement reaction is named after its discoverer, Lars Skattebøl, Professor emeritus at the Univer ...
(
Doering–LaFlamme allene synthesis In organic chemistry, the Doering–LaFlamme allene synthesis is a reaction of alkenes that converts them to allenes by insertion of a carbon atom. This name reaction is named for William von Eggers Doering and a co-worker, who first reported it. ...
) via rearrangement of cyclopropylidene carbenes/carbenoids *from reaction of certain terminal
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 n ...
s with
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
,
copper(I) bromide Copper(I) bromide is the chemical compound with the formula CuBr. This diamagnetic solid adopts a polymeric structure akin to that for zinc sulfide. The compound is widely used in the synthesis of organic compounds and as a lasing medium in c ...
, and added base ( Crabbé–Ma allene synthesis) *from propargylic halides by SN2′ displacement by an organocuprate *from
dehydrohalogenation In chemistry, dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications. Dehydrohalogenation from alkyl halid ...
of certain dihalides *from reaction of a triphenylphosphinyl ester with an acid halide, a
Wittig reaction The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide called a Wittig reagent. Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes. Most o ...
accompanied by dehydrohalogenation *from propargylic alcohols via the
Myers allene synthesis In organic chemistry, the Myers allene synthesis is a chemical reaction that converts a propargyl alcohol into an allene by way of an arenesulfonylhydrazine as a key intermediate. This name reaction A name reaction is a chemical reaction named afte ...
protocol—a
stereospecific In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers."Overlap Con ...
process *from metalation of allene or substituted allenes with BuLi and reaction with electrophiles (RX, R3SiX, D2O, etc.) The chemistry of allenes has been reviewed in a number of books and journal articles. Some key approaches towards allenes are outlined in the following scheme: One of the older methods is the Skattebøl rearrangement (also called the Doering–Moore–Skattebøl or Doering–LaFlamme rearrangement), in which a gem-dihalocyclopropane 3 is treated with an organolithium compound (or dissolving metal) and the presumed intermediate rearranges into an allene either directly or via carbene-like species. Notably, even strained allenes can be generated by this procedure. Modifications involving leaving groups of different nature are also known. Arguably, the most convenient modern method of allene synthesis is by
sigmatropic rearrangement A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular reaction. The name ''sigmatropic'' is the result of a compounding of the long ...
of propargylic substrates. Johnson–Claisen and Ireland–Claisen rearrangements of ketene acetals 4 have been used a number of times to prepare allenic esters and acids. Reactions of vinyl ethers 5 (the Saucy–Marbet rearrangement) give allene aldehydes, while propargylic sulfenates 6 give allene sulfoxides. Allenes can also be prepared by
nucleophilic substitution In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The ...
in 9 and 10 (nucleophile Nu can be a hydride anion), 1,2-elimination from 8, proton transfer in 7, and other, less general, methods.


Use and occurrence


Use

The reactivity of allenes is rich and opens wide possibilities for discovery. The two π-bonds are located at the 90° angle to each other, and thus require a reagent to approach from somewhat different directions. With an appropriate substitution pattern, allenes exhibit axial chirality as predicted by van’t Hoff as early as 1875. Such compounds are now extensively investigated. Protonation of allenes gives cations 11 that undergo further transformations.56 Reactions with soft electrophiles (e.g. Br+) deliver positively charged onium ions 13. Transition-metal-catalysed reactions proceed via allylic intermediates 15 and have attracted significant interest in recent years. Numerous cycloadditions are also known, including +2, (2+1)-, and +2variants, which deliver, e.g., 12, 14, and 16, respectively.


Occurrence

Numerous natural products contain the allene functional group. Noteworthy are the pigments
fucoxanthin Fucoxanthin is a xanthophyll, with formula C42H58O6. It is found as an accessory pigment in the chloroplasts of brown algae and most other heterokonts, giving them a brown or olive-green color. Fucoxanthin absorbs light primarily in the blue-green ...
and
peridinin Peridinin is a light-harvesting apocarotenoid, a pigment associated with chlorophyll and found in the peridinin-chlorophyll-protein (PCP) light-harvesting complex in dinoflagellates, best studied in ''Amphidinium carterae''. Biological significan ...
. Little is known about the biosynthesis, although it is conjectured that they are often generated from alkyne precursors. Allenes serve as ligands in
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
. A typical complex is Pt( η2-allene)(PPh3)2. Ni(0) reagents catalyze the cyclooligomerization of allene. Using a suitable catalyst (e.g.
Wilkinson's catalyst Wilkinson's catalyst is the common name for chloridotris(triphenylphosphine)rhodium(I), a coordination complex of rhodium with the formula hCl(PPh3)3(Ph = phenyl). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as ...
), it is possible to reduce just one of the double bonds of an allene.


Delta convention

Many rings or ring systems are known by semisystematic names that assume a maximum number of noncumulative bonds. To unambiguously specify derivatives that include cumulated bonds (and hence fewer hydrogen atoms than would be expected from the skeleton), a lowercase delta may be used with a subscript indicating the number of cumulated double bonds from that atom, e.g. 8δ2-benzocyclononene. This may be combined with the λ-convention for specifying nonstandard valency states, e.g. 2λ4δ2,5λ4δ2-thieno ,4-chiophene.


See also

*Compounds with three or more adjacent carbon–carbon double bonds are called
cumulene In organic chemistry, a cumulene is a compound having three or more ''cumulative'' (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (), which is also ...
s.


References


Further reading

* Brummond, Kay M. (editor).
Allene chemistry
' (special thematic issue). ''
Beilstein Journal of Organic Chemistry The ''Beilstein Journal of Organic Chemistry'' is a peer-reviewed open-access scientific journal established in 2005. It is published by the Beilstein Institute for the Advancement of Chemical Sciences, a German non-profit foundation. The editor-in ...
'' 7: 394–943.


External links


Synthesis of allenes
{{Authority control * Jacobus Henricus van 't Hoff