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58-403: Carotenoids are pigments produced by plants and algae and play a role in light harvesting as part of the photosynthesis process. Xanthophylls are a subset of carotenoids, identified by the fact that they are oxygenated either as hydroxyl groups or as epoxide bridges. This makes them more water soluble than carotenes such as beta-carotene. Fucoxanthin is a xanthophyll that contributes more than 10% of

116-507: A carbon atom which forms four single bonds, the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp mixtures in a tetrahedral arrangement around the carbon to bond to four different atoms. Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties and are symmetrically disposed in space. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies. Chemist Linus Pauling first developed

174-407: A conjugated carbonyl group (carbon-oxygen double bond) in the polyene chain. All of these features provide fucoxanthin with powerful antioxidant activity. In macroalgal plastids, fucoxanthin acts like an antenna for light harvesting and energy transfer in the photosystem light harvesting complexes. In diatoms like Phaeodactylum tricornutum , fucoxanthin is protein-bound along with chlorophyll to form

232-401: 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

290-638: A golden-brown color, due to their high content of fucoxanthin. Generally, diatoms contain up to 4 times more fucoxanthin than seaweed, making diatoms a viable source for fucoxanthin industrially. Diatoms can be grown in controlled environments (such as photobioreactors ). Brown seaweeds are mostly grown in the open sea, often exposed to metals and metalloids. Limited studies of fucoxanthin in humans indicate low bioavailability . Allenes In organic chemistry , allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms ( R 2 C=C=CR 2 , where R

348-1108: A hydride anion), 1,2-elimination from 8 , proton transfer in 7 , and other, less general, methods. Allene itself is the most commonly used member of this family; it exists in equilibrium with propyne as a component of MAPP gas. The reactivity of substituted allenes has been well explored. 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. Protonation of allenes gives cations 11 that undergo further transformations. 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 [4+2]-, (2+1)-, and [2+2]-variants, which deliver, e.g., 12 , 14 , and 16 , respectively. [REDACTED] Another area of allene chemistry involves cyclic allenes, where

406-676: A light harvesting protein complex. Fucoxanthin is the dominant carotenoid, responsible for up to 60% of the energy transfer to chlorophyll a in diatoms When bound to protein, the absorption spectrum of fucoxanthin expands from 450-540 nm to 390-580 nm, a range that is useful in aquatic environments. Fucoxanthin is present in brown seaweeds and diatoms and was first isolated from Fucus , Dictyota , and Laminaria by Willstätter and Page in 1914. Seaweeds are commonly consumed in south-east Asia and certain countries in Europe, while diatoms are single-cell planktonic microalgae characterized by

464-459: A low d electron count , the p-orbitals are unoccupied and sd hybridisation is used to model the shape of these molecules. In some general chemistry textbooks, hybridization is presented for main group coordination number 5 and above using an "expanded octet" scheme with d-orbitals first proposed by Pauling. However, such a scheme is now considered to be incorrect in light of computational chemistry calculations. In 1990, Eric Alfred Magnusson of

522-551: 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. 8δ -benzocyclononene. This may be combined with the λ-convention for specifying nonstandard valency states, e.g. 2λ δ ,5λ δ -thieno[3,4-c]thiophene. [REDACTED]  This article incorporates text by Oleksandr Zhurakovskyi available under

580-423: A quantitative depiction of the bond formation when the molecular geometry deviates from ideal bond angles. The amount of p-character is not restricted to integer values; i.e., hybridizations like sp are also readily described. The hybridization of bond orbitals is determined by Bent's rule : "Atomic s character concentrates in orbitals directed towards electropositive substituents". For molecules with lone pairs,

638-460: A similar way. For example, ethene (C 2 H 4 ) has a double bond between the carbons. For this molecule, carbon sp hybridises, because one π (pi) bond is required for the double bond between the carbons and only three σ bonds are formed per carbon atom. In sp hybridisation the 2s orbital is mixed with only two of the three available 2p orbitals, usually denoted 2p x and 2p y . The third 2p orbital (2p z ) remains unhybridised. forming

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696-421: A substituent attached to a saturated carbon α (i.e., directly adjacent) to an allene is referred to as an allenylic substituent. While allenes have two consecutive ('cumulated') double bonds, compounds with three or more cumulated double bonds are called cumulenes . For many years, allenes were viewed as curiosities but thought to be synthetically useless and difficult to prepare and to work with. Reportedly,

754-541: A total of three sp orbitals with one remaining p orbital. In ethene, the two carbon atoms form a σ bond by overlapping one sp orbital from each carbon atom. The π bond between the carbon atoms perpendicular to the molecular plane is formed by 2p–2p overlap. Each carbon atom forms covalent C–H bonds with two hydrogens by s–sp overlap, all with 120° bond angles. The hydrogen–carbon bonds are all of equal strength and length, in agreement with experimental data. The chemical bonding in compounds such as alkynes with triple bonds

812-414: Is H or some organyl group ). Allenes are classified as cumulated dienes . The parent compound of this class is propadiene ( H 2 C=C=CH 2 ), which is itself also called allene . A group of the structure R 2 C=C=CR− is called allenyl , while a substituent attached to an allene is referred to as an allenic substituent (R is H or some alkyl group). In analogy to allylic and propargylic ,

870-411: Is sp-hybridized , and the two terminal carbon atoms are sp -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 , an analogy that is continued into

928-420: 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 C 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 sp -hybridized carbon atoms resonate around 80 ppm in a region typical for alkyne and nitrile carbon atoms, while

986-431: Is about 102° which implies the presence of some orbital hybridisation. The carbon atom can also bond to four hydrogen atoms in methane by an excitation (or promotion) of an electron from the doubly occupied 2s orbital to the empty 2p orbital, producing four singly occupied orbitals. The energy released by the formation of two additional bonds more than compensates for the excitation energy required, energetically favoring

1044-511: Is by sigmatropic rearrangement 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 9 and 10 (nucleophile Nu can be

1102-522: Is considered an effective heuristic for rationalizing the structures of organic compounds . It gives a simple orbital picture equivalent to Lewis structures . Hybridisation theory is an integral part of organic chemistry , one of the most compelling examples being Baldwin's rules . For drawing reaction mechanisms sometimes a classical bonding picture is needed with two atoms sharing two electrons. Hybridisation theory explains bonding in alkenes and methane. The amount of p character or s character, which

1160-539: Is decided mainly by orbital hybridisation, can be used to reliably predict molecular properties such as acidity or basicity. Orbitals are a model representation of the behavior of electrons within molecules. In the case of simple hybridization, this approximation is based on atomic orbitals , similar to those obtained for the hydrogen atom, the only neutral atom for which the Schrödinger equation can be solved exactly. In heavier atoms, such as carbon, nitrogen, and oxygen,

1218-484: Is explained by sp hybridization. In this model, the 2s orbital is mixed with only one of the three p orbitals, resulting in two sp orbitals and two remaining p orbitals. The chemical bonding in acetylene (ethyne) (C 2 H 2 ) consists of sp–sp overlap between the two carbon atoms forming a σ bond and two additional π bonds formed by p–p overlap. Each carbon also bonds to hydrogen in a σ s–sp overlap at 180° angles. Hybridisation helps to explain molecule shape , since

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1276-440: Is that it incorrectly predicts the ultraviolet photoelectron spectra of many molecules. While this is true if Koopmans' theorem is applied to localized hybrids, quantum mechanics requires that the (in this case ionized) wavefunction obey the symmetry of the molecule which implies resonance in valence bond theory. For example, in methane, the ionised states (CH 4 ) can be constructed out of four resonance structures attributing

1334-440: Is the tetrahedral bond angle of 109°28' (around 109.5°). Pauling supposed that in the presence of four hydrogen atoms, the s and p orbitals form four equivalent combinations which he called hybrid orbitals. Each hybrid is denoted sp to indicate its composition, and is directed along one of the four C–H bonds. This concept was developed for such simple chemical systems, but the approach was later applied more widely, and today it

1392-512: Is used to model the shape of these molecules. As the valence orbitals of transition metals are the five d, one s and three p orbitals with the corresponding 18-electron rule , sp d hybridisation is used to model the shape of these molecules. These molecules tend to have multiple shapes corresponding to the same hybridization due to the different d-orbitals involved. A square planar complex has one unoccupied p-orbital and hence has 16 valence electrons. In certain transition metal complexes with

1450-476: The CC BY 2.5 license. Orbital hybridization#sp2 hybrids This is an accepted version of this page In chemistry , orbital hybridisation (or hybridization ) is the concept of mixing atomic orbitals to form new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory . For example, in

1508-487: The University of New South Wales published a paper definitively excluding the role of d-orbital hybridisation in bonding in hypervalent compounds of second-row ( period 3 ) elements, ending a point of contention and confusion. Part of the confusion originates from the fact that d-functions are essential in the basis sets used to describe these compounds (or else unreasonably high energies and distorted geometries result). Also,

1566-412: The electron density associated with an orbital is proportional to the square of the wavefunction, the ratio of p-character to s-character is λ = 3. The p character or the weight of the p component is N λ = 3/4. Hybridisation describes the bonding of atoms from an atom's point of view. For a tetrahedrally coordinated carbon (e.g., methane CH 4 ), the carbon should have 4 orbitals directed towards

1624-520: The octet rule . While the simple model of orbital hybridisation is commonly used to explain molecular shape, hybridisation is used differently when computed in modern valence bond programs. Specifically, hybridisation is not determined a priori but is instead variationally optimized to find the lowest energy solution and then reported. This means that all artificial constraints, specifically two constraints, on orbital hybridisation are lifted: This means that in practice, hybrid orbitals do not conform to

1682-438: The 4 hydrogen atoms. Carbon's ground state configuration is 1s 2s 2p or more easily read: This diagram suggests that the carbon atom could use its two singly occupied p-type orbitals to form two covalent bonds with two hydrogen atoms in a methylene (CH 2 ) molecule, with a hypothetical bond angle of 90° corresponding to the angle between two p orbitals on the same atom. However the true H–C–H angle in singlet methylene

1740-493: The C=C=C bonds, and there is a mirror plane passing through both CH 2 planes. Thus this class of molecules belong to the D 2d point group . 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 because there will no longer be any mirror planes. The chirality of these types of allenes

1798-409: The allene double bonds exist in a ring. This approach was applied to the total synthesis of lissodendoric acid A. Cyclic allenes also participate in metal-mediated processes. and the generation of DNA-encoded libraries using cyclic allene intermediates. Numerous natural products contain the allene functional group. Noteworthy are the pigments fucoxanthin and peridinin . Little is known about

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1856-430: The angles between bonds are approximately equal to the angles between hybrid orbitals. This is in contrast to valence shell electron-pair repulsion (VSEPR) theory , which can be used to predict molecular geometry based on empirical rules rather than on valence-bond or orbital theories. As the valence orbitals of main group elements are the one s and three p orbitals with the corresponding octet rule , sp hybridization

1914-487: The atomic orbitals used are the 2s and 2p orbitals, similar to excited state orbitals for hydrogen. Hybrid orbitals are assumed to be mixtures of atomic orbitals, superimposed on each other in various proportions. For example, in methane , the C hybrid orbital which forms each carbon – hydrogen bond consists of 25% s character and 75% p character and is thus described as sp (read as s-p-three ) hybridised. Quantum mechanics describes this hybrid as an sp wavefunction of

1972-472: The biosynthesis, although it is conjectured that they are often generated from alkyne precursors. Allenes serve as ligands in organometallic chemistry . A typical complex is Pt( η -allene)(PPh 3 ) 2 . Ni(0) reagents catalyze the cyclooligomerization of allene. Using a suitable catalyst (e.g. Wilkinson's catalyst ), it is possible to reduce just one of the double bonds of an allene. Many rings or ring systems are known by semisystematic names that assume

2030-466: 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 D 2d 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

2088-606: The bonding orbitals are isovalent sp hybrids. For example, the two bond-forming hybrid orbitals of oxygen in water can be described as sp to give the interorbital angle of 104.5°. This means that they have 20% s character and 80% p character and does not imply that a hybrid orbital is formed from one s and four p orbitals on oxygen since the 2p subshell of oxygen only contains three p orbitals. Hybridisation of s and p orbitals to form effective sp hybrids requires that they have comparable radial extent. While 2p orbitals are on average less than 10% larger than 2s, in part attributable to

2146-458: The calculated p/s ratio is approximately 3 consistent with "ideal" sp hybridisation, whereas for silane , SiH 4 , the p/s ratio is closer to 2. A similar trend is seen for the other 2p elements. Substitution of fluorine for hydrogen further decreases the p/s ratio. The 2p elements exhibit near ideal hybridisation with orthogonal hybrid orbitals. For heavier p block elements this assumption of orthogonality cannot be justified. These deviations from

2204-419: 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

2262-418: The contribution of the d-function to the molecular wavefunction is large. These facts were incorrectly interpreted to mean that d-orbitals must be involved in bonding. In light of computational chemistry , a better treatment would be to invoke sigma bond resonance in addition to hybridisation, which implies that each resonance structure has its own hybridisation scheme. All resonance structures must obey

2320-406: The delocalized orbitals of molecular orbital theory by an appropriate mathematical transformation. For molecules in the ground state, this transformation of the orbitals leaves the total many-electron wave function unchanged. The hybrid orbital description of the ground state is, therefore equivalent to the delocalized orbital description for ground state total energy and electron density, as well as

2378-500: The ejected electron to each of the four sp orbitals. A linear combination of these four structures, conserving the number of structures, leads to a triply degenerate T 2 state and an A 1 state. The difference in energy between each ionized state and the ground state would be ionization energy , which yields two values in agreement with experimental results. Bonding orbitals formed from hybrid atomic orbitals may be considered as localized molecular orbitals, which can be formed from

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2436-492: The estimated total production of carotenoids in nature. It is an accessory pigment found in the chloroplasts of many brown macroalgae, such as Fucus spp ., and the golden-brown unicellular microalgae, the diatoms. It absorbs blue and green light at bandwidth 450-540 nm, imparting a brownish-olive color to algae. Fucoxanthin has a highly unique structure that contains both an epoxide bond and hydroxyl groups along with an allenic bond (two adjacent carbon-carbon double bonds) and

2494-545: The first synthesis of an allene, glutinic acid , 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. The central carbon atom of allenes forms two sigma bonds and two pi bonds . The central carbon atom

2552-456: The form N ( s + 3 p σ ) {\displaystyle N(s+{\sqrt {3}}p\sigma )} , where N is a normalisation constant (here 1/2) and pσ is a p orbital directed along the C-H axis to form a sigma bond . The ratio of coefficients (denoted λ in general) is 3 {\displaystyle \color {blue}{\sqrt {3}}} in this example. Since

2610-709: The formation of four C-H bonds. According to quantum mechanics the lowest energy is obtained if the four bonds are equivalent, which requires that they are formed from equivalent orbitals on the carbon. A set of four equivalent orbitals can be obtained that are linear combinations of the valence-shell (core orbitals are almost never involved in bonding) s and p wave functions, which are the four sp hybrids. In CH 4 , four sp hybrid orbitals are overlapped by hydrogen 1s orbitals, yielding four σ (sigma) bonds (that is, four single covalent bonds) of equal length and strength. The following : translates into : Other carbon compounds and other molecules may be explained in

2668-503: The hybridisation theory in 1931 to explain the structure of simple molecules such as methane (CH 4 ) using atomic orbitals . Pauling pointed out that a carbon atom forms four bonds by using one s and three p orbitals, so that "it might be inferred" that a carbon atom would form three bonds at right angles (using p orbitals) and a fourth weaker bond using the s orbital in some arbitrary direction. In reality, methane has four C–H bonds of equivalent strength. The angle between any two bonds

2726-464: The ideal hybridisation were termed hybridisation defects by Kutzelnigg . However, computational VB groups such as Gerratt, Cooper and Raimondi (SCVB) as well as Shaik and Hiberty (VBSCF) go a step further to argue that even for model molecules such as methane, ethylene and acetylene, the hybrid orbitals are already defective and nonorthogonal, with hybridisations such as sp instead of sp for methane. One misconception concerning orbital hybridization

2784-418: 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

2842-420: The lack of a radial node in 2p orbitals, 3p orbitals which have one radial node, exceed the 3s orbitals by 20–33%. The difference in extent of s and p orbitals increases further down a group. The hybridisation of atoms in chemical bonds can be analysed by considering localised molecular orbitals, for example using natural localised molecular orbitals in a natural bond orbital (NBO) scheme. In methane , CH 4 ,

2900-410: The molecular geometry that corresponds to the minimum total energy value. Molecules with multiple bonds or multiple lone pairs can have orbitals represented in terms of sigma and pi symmetry or equivalent orbitals. Different valence bond methods use either of the two representations, which have mathematically equivalent total many-electron wave functions and are related by a unitary transformation of

2958-401: The parent allene, propadiene is produced industrially on a large scale as an equilibrium mixture with propyne : This mixture, known as MAPP gas , 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 , which was prepared by Burton and Pechmann in 1887. However,

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3016-475: The protons of a CH 2 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 [4+2] and [2+2] modes of addition, as well as undergoing formal cycloaddition processes catalyzed by transition metals. Allenes also serve as substrates for transition metal catalyzed hydrofunctionalization reactions. Although allenes often require specialized syntheses,

3074-723: 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 C 2 -symmetric 1,3-dimethylallene. This qualitative MO description extends to higher odd-carbon cumulenes (e.g., 1,2,3,4-pentatetraene). Allenes differ considerably from other alkenes in terms of their chemical properties. Compared to isolated and conjugated dienes, they are considerably less stable: comparing

3132-402: The set of occupied molecular orbitals. For multiple bonds, the sigma-pi representation is the predominant one compared to the equivalent orbital ( bent bond ) representation. In contrast, for multiple lone pairs, most textbooks use the equivalent orbital representation. However, the sigma-pi representation is also used, such as by Weinhold and Landis within the context of natural bond orbitals ,

3190-423: The simple ideas commonly taught and thus in scientific computational papers are simply referred to as sp , sp d or sd hybrids to express their nature instead of more specific integer values. Although ideal hybrid orbitals can be useful, in reality, most bonds require orbitals of intermediate character. This requires an extension to include flexible weightings of atomic orbitals of each type (s, p, d) and allows for

3248-486: The stereochemical analysis of certain derivative molecules. 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 CH 2 planes at either end of the molecule. The molecule can thus be thought of as a two-bladed propeller . A third twofold axis of rotation passes through

3306-577: The structure was only correctly identified in 1954. Laboratory methods for the formation of allenes include: 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: [REDACTED] One of the older methods is the Skattebøl rearrangement (also called the Doering–Moore–Skattebøl or Doering–LaFlamme rearrangement), in which

3364-569: 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 , the configuration of the axial chirality 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

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