In chemistry, a carbene is a highly reactive organic molecule containing a carbon atom with six valence electrons and having the general formula: R¹R²C: (two substituents and two electrons). Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 In Chemistry, valence, also known as valency or valency number, is a measure of the number of Chemical bonds formed by the Atoms ^[1] There are two kinds of carbenes: singlets and triplets. The carbon atoms of singlets are sp²-hybridised, with one empty p-orbital crossing the plane containing R¹, R², and the free electron pair. -->In Chemistry lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs Such molecules are generally very short lived, although persistent carbenes are known. A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive

The prototypical carbene is H₂C: also called methylene. Methylene is the chemical species R2C named after Methane, in which two of the carbon atom's valence electrons form no bonds One well studied carbene is Cl₂C:, or dichlorocarbene, which can be generated in situ from chloroform and a strong base. Dichlorocarbene is a Carbene commonly encountered in Organic chemistry. In situ (ɪn siːˈtuː is a Latin phrase meaning in the place. Chloroform, also known as trichloromethane and methyl trichloride, is a Chemical compound with formula C[[Hydrogen H]] Cl In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and

Contents 1 Structure 2 Reactivity 3 Carbenes and carbene ligands in organometallic chemistry 4 Generation of Carbenes 5 See also 6 References

Structure

Generally there are two types of carbenes; singlet or triplet carbenes. A diradical in Organic chemistry is a molecular species with two Electrons occupying two Degenerate Molecular orbitals (MO. A diradical in Organic chemistry is a molecular species with two Electrons occupying two Degenerate Molecular orbitals (MO. Singlet carbenes have a pair of electrons and an sp² hybrid structure. -->In Chemistry Triplet carbenes have two unpaired electrons. They may be either sp² hybrid or linear sp hybrid. Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur atoms, and dihalocarbenes.

Carbenes are called singlet or triplet depending on the electronic spins they possess. In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin Triplet carbenes are paramagnetic and may be observed by electron spin resonance spectroscopy if they persist long enough. Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field Electron paramagnetic resonance (EPR or electron spin resonance (ESR Spectroscopy is a technique for studying Chemical species that have one or more unpaired The total spin of singlet carbenes is zero while that of triplet carbenes is one (in units of ). Bond angles are 125-140° for triplet methylene and 102° for singlet methylene (as determined by EPR). Methylene is the chemical species R2C named after Methane, in which two of the carbon atom's valence electrons form no bonds Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.

For simple hydrocarbons, triplet carbenes usually have energies 8 kcal/mol (33 kJ/mol) lower than singlet carbenes (see also Hund's rule of Maximum Multiplicity), thus, in general, triplet is the more stable state (the ground state) and singlet is the excited state species. This article is about the unit of energy For its use in Nutrition and Food labelling regulations, see the article on Food energy. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity Part of the list of Hund's rules, Hund's Rule of Maximum Multiplicity (1925 is an observational rule of atomic chemistry discovered by Friedrich Hund. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy Excitation is an elevation in energy level above an arbitrary baseline energy state Substituents that can donate electron pairs may stabilize the singlet state by delocalizing the pair into an empty p-orbital. In Organic chemistry, a substituent is an atom or group of atoms substituted in place of a Hydrogen atom on the Parent chain of a Hydrocarbon lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs If the energy of the singlet state is sufficiently reduced it will actually become the ground state. No viable strategies exist for triplet stabilization. The carbene called 9-fluorenylidene has been shown to be a rapidly equilibrating mixture of singlet and triplet states with an approximately 1. In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change 1 kcal/mol (4. 6 kJ/mol) energy difference. ^[2]. It is however debatable whether diaryl carbenes such as the fluorene carbene are true carbenes because the electrons can delocalize to such an extent that they become in fact biradicals. Fluorene, or 9H-fluorene, is a Polycyclic aromatic hydrocarbon. A non-Kekulé molecule is a conjugated Hydrocarbon that cannot be assigned classical Kekulé structures In silico experiments suggest that triplet carbenes can be stabilized with electropositive groups such as trifluorosilyl groups ^[3]. In Silico is the second full-length studio album by Australian Drum and bass band Pendulum, released in Australia and Electropositivity is a measure of an element's ability to donate Electrons, and therefore form positive Ions.

Reactivity

Singlet and triplet carbenes do not demonstrate the same reactivity. Singlet carbenes generally participate in cheletropic reactions as either electrophiles or nucleophiles. In Organic chemistry, an electrocyclic reaction is a type of Pericyclic Rearrangement reaction where the net result is one Pi bond being converted In Chemistry, an electrophile (literally electron-lover) is a Reagent attracted to Electrons that participates in a chemical reaction by accepting In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to Singlet carbene with its unfilled p-orbital should be electrophilic. Triplet carbenes should be considered to be diradicals, and participate in stepwise radical additions. In Chemistry, radicals (often referred to as free radicals) are atoms molecules or ions with Unpaired electrons on an otherwise Open shell Triplet carbenes have to go through an intermediate with two unpaired electrons whereas singlet carbene can react in a single concerted step. A reaction intermediate or an intermediate is a Molecular entity that is formed from the reactants (or preceding intermediates and reacts further to give the directly In Chemistry, a concerted reaction is a Chemical reaction in which all bond breaking and bond making occurs in a single step Addition of singlet carbenes to olefinic double bonds is more stereoselective than that of triplet carbenes. In Organic chemistry, an alkene, olefin, or olefine is an unsaturated Chemical compound containing at least one Carbon Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved.

Reactions of singlet methylene are stereospecific while those of triplet methylene are not. For instance the reaction of methylene generated from photolysis of diazomethane with cis-2-butene and trans-2-butene is stereospecific which proves that in this reaction methylene is a singlet. Photodissociation, photolysis, or photodecomposition is a Chemical reaction in which a Chemical compound is broken down by Photons Diazomethane is the chemical compound CH2N2 In the pure form at room temperature it is a Yellow Gas, but it is almost universally used In Chemistry, stereospecificity is the property of a Chemical reaction that yields different stereoisomeric reaction products from two stereoisomeric ^[4]

Reactivity of a particular carbene depends on the substituent groups, preparation method, reaction conditions such as presence or absence of metals. In Organic chemistry, a substituent is an atom or group of atoms substituted in place of a Hydrogen atom on the Parent chain of a Hydrocarbon The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across Some of the reactions carbenes can do are insertions into C-H bonds, skeletal rearrangements, and additions to double bonds. Carbenes can be classified as nucleophilic, electrophilic, or ambiphilic. Reactivity is especially strongly influenced by substituents. For example, if a substituent is able to donate a pair of electrons, most likely carbene will not be electrophilic. Alkyl carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds. An alkyl is a Univalent radical consisting of Carbon and Hydrogen atoms arranged in a chain

Carbenes add to double bonds to form cyclopropanes. Cyclopropane is a Cycloalkane Molecule with the molecular formula C3H6 consisting of three Carbon Atoms linked to A concerted mechanism is available for singlet carbenes. Triplet carbenes do not retain stereochemistry in the product molecule. Stereochemistry, a subdiscipline of Chemistry, involves the study of the relative spatial arrangement of Atoms within Molecules An important branch Addition reactions are commonly very fast and exothermic. In Thermodynamics, the word exothermic "outside heating" describes a process or reaction that releases Energy usually in the form of Heat, but The slow step in most instances is generation of carbene. A well-known reagent employed for alkene-to-cyclopropane reactions is Simmons-Smith reagent. The Simmons-Smith reaction is an Organic reaction in which a Carbenoid reacts with an Alkene (or Alkyne) to form a Cyclopropane. This reagents is a system of copper, zinc, and iodine, where the active reagent is believed to be iodomethylzinc iodide. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 Iodine (ˈaɪədaɪn ˈaɪədɪn or /ˈaɪədiːn/ from ιώδης iodes "violet" is a Chemical element that has the symbol I and Atomic Reagent is complexed by hydroxy groups such that addition commonly happens syn to such group.

Insertions are another common type of carbene reactions. The carbene basically interposes itself into an existing bond. The order of preference is commonly: 1. X-H bonds where X is not carbon 2. C-H bond 3. C-C bond. Insertions may or may not occur in single step.

Intramolecular insertion reactions present new synthetic solutions. Intramolecular in Chemistry describes a process or characteristic limited within the structure of a single Molecule; a property or phenomenon limited to the extent Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no intermolecular insertions are seen. In Physics, Chemistry, and Biology, intermolecular forces are forces that act between stable Molecules or between functional groups of In flexible structures, five-membered ring formation is preferred to six-membered ring formation. Both inter- and intramolecular insertions are amendable to asymmetric induction by choosing chiral ligands on metal centers.

Alkylidene carbenes are alluring in that they offer formation of cyclopentene moieties. Cyclopentene is a Chemical compound with the formula 58 It is a colorless Liquid with a Petrol -like Odor. To generate an alkylidene carbene a ketone can be exposed to trimethylsilyl diazomethane. For tetramethylsilane which also abbreviated as TMS see Tetramethylsilane. Diazomethane is the chemical compound CH2N2 In the pure form at room temperature it is a Yellow Gas, but it is almost universally used

Carbenes and carbene ligands in organometallic chemistry

Carbenes can be stabilized as organometallic species. Organometallic chemistry is the study of Chemical compounds containing bonds between Carbon and a Metal. These transition metal carbene complexes fall into three categories, with the first two being the most clearly defined:

• Fischer carbenes, in which the carbene is tethered to a metal that bears an electron-withdrawing group (usually a carbonyl). A transition metal carbene complex in Organometallic chemistry is a compound bearing a formal carbon-metal bond. A transition metal carbene complex in Organometallic chemistry is a compound bearing a formal carbon-metal bond.
• Schrock carbenes, in which the carbene is tethered to a metal that bears an electron-donating group. A transition metal carbene complex in Organometallic chemistry is a compound bearing a formal carbon-metal bond.
• Persistent carbenes, also known as stable carbenes or Arduengo carbenes. A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive Anthony J Arduengo III, PhD (born in 1952 is a Chemist perhaps most famous for his work in the field of Stable carbene research These include the class of N-heterocyclic carbenes (NHCs) and are often are used as ancillary ligands in organometallic chemistry. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally
• Foiled carbenes derive their stability from proximity of a double bond (i. foiled carbene in Organic chemistry is a special type of stabilized Carbene due to the proximity of a Double bond. e their ability to form conjugated systems).

Generation of Carbenes

• Most commonly, photolytic, thermal, or transition metal catalyzed decomposition of diazoalkanes is used to create carbene molecules. Photodissociation, photolysis, or photodecomposition is a Chemical reaction in which a Chemical compound is broken down by Photons In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in A variation on catalyzed decomposition of diazoalkanes is the Bamford-Stevens reaction, which gives carbenes in aprotic solvents and carbenium ions in protic solvents. The Bamford-Stevens reaction is a Chemical reaction whereby treatment of Tosylhydrazones with strong base gives Alkenes It is named for the British chemist In Chemistry a protic solvent is a Solvent that has a Hydrogen atom bound to an Oxygen as in a Hydroxyl group or a Nitrogen
• Another method is induced elimination of halogen from gem-dihalides or HX from CHX₃ moiety, employing organolithium reagents (or another strong base). Abundance Owing to their high Reactivity, the halogens are found in the environment only in compounds or as Ions Halide ions and oxoanions An organolithium reagent is an Organometallic compound with a direct bond between a Carbon and a Lithium atom It is not certain that in these reactions actual free carbenes are formed. In some cases there is evidence that completely free carbene is never present. It is likely that instead a metal-carbene complex forms. Nevertheless, these metallocarbenes (or carbenoids) give the expected products.

Carbene preparation

• Photolysis of diazarines and epoxides can also be employed. Photodissociation, photolysis, or photodecomposition is a Chemical reaction in which a Chemical compound is broken down by Photons An epoxide is a cyclic Ether with only three ring atoms This ring approximately is an Equilateral triangle, i Diazarines contain 3-membered rings and are cyclic forms or diazoalkanes. The strain of the small ring makes photoexcitation easy. Photoexcitation is the mechanism of Electron excitation by Photon absorption when the energy of the photon is too low to cause Photoionization. Photolysis of epoxides gives carbonyl compounds as side products. In Organic chemistry, a carbonyl group is a Functional group composed of a Carbon Atom double-bonded to an Oxygen With asymmetric epoxides, two different carbonyl compounds can potentially form. Asymmetric synthesis, also called chiral synthesis, enantioselective synthesis or stereoselective synthesis, is Organic synthesis which The nature of substituents usually favors formation of one over the other. One of the C-O bonds will have a greater double bond character and thus will be stronger and less likely to break. Resonance structures can be drawn to determine which part will contribute more to the formation of carbonyl. When one substituent is alkyl and another aryl, the aryl-substituted carbon is usually released as a carbene fragment. An alkyl is a Univalent radical consisting of Carbon and Hydrogen atoms arranged in a chain In the context of organic molecules aryl refers to any Functional group or Substituent derived from a Simple aromatic ring, may it be Phenyl
• Thermolysis of alpha-halomercury compounds is another method to generate carbenes. For the biological process see Decomposition. For chemical decomposition in general see Chemical decomposition.
• Rhodium and copper complexes promote carbene formation. Rh redirects here For other uses see Rh (disambiguation Rhodium (ˈroʊdiəm is a Chemical element with the symbol Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29
• Carbenes are intermediates in the Wolff rearrangement

See also

• Transition metal carbene complex also known as carbenoids
• Atomic carbon a single carbon atom with chemical formula :C:, in effect a dicarbene. A transition metal carbene complex in Organometallic chemistry is a compound bearing a formal carbon-metal bond. In Chemistry a carbenoid is a Reactive intermediate that shares reaction characteristics with a Carbene. Atomic carbon in Chemistry is single carbon Atom with chemical formula:C - in effect a di Carbene. Also has been used to make "true carbenes" in situ.

References

1. ^ Organic Chemistry R. T Morrison, R. N Boyd pp 473-478
2. ^ Chemical and Physical Properties of Fluorenylidene: Equilibrium of the Singlet and Triplet Carbenes Peter B. Grasse, Beth-Ellen Brauer, Joseph J. Zupancic, Kenneth J. Kaufmann, Gary B. Schuster; J. Am. Chem. Soc.; 1983; 105; 6833-6845. The Journal of the American Chemical Society (usually abbreviated as J
3. ^ Electronic Stabilization of Ground State Triplet Carbenes Adelina Nemirowski and Peter R. SchreinerJ. Org. Chem. 2007, 72, 9533-9540 9533 doi:10.1021/jo701615x
4. ^ Structure of Carbene CH2 Philip S. The Journal of Organic Chemistry (abbreviated as J Org Chem or JOC) is a Scientific journal for original contributions of fundamental A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. Skell, Robert C. Woodworth; J. Am. Chem. Soc.; 1956; 78(17); 4496-4497. The Journal of the American Chemical Society (usually abbreviated as J Abstract

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