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In chemistry a carbene is a short-lived and highly reactive organic molecule with a divalent carbon atom with only six valence electrons and the general formula: R1R2C: [1]. The carbon atom is sp2 hybridised with an empty p-orbital extending above and below a plane containing R1 and R2 and the free electron pair. Multicolored chemicals are frequent hallmarks of chemistry. ... An organic compound is any of a large class of chemical compounds whose molecules contain carbon, with exception of carbides, carbonates and carbon oxides. ... It has been suggested that Valence number be merged into this article or section. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... It has been suggested that Valence number be merged into this article or section. ... four sp³ orbitals three sp² orbitals In chemistry, hybridisation is the mixing of atomic orbitals belonging to a same electron shell to form new orbitals suitable for the qualitative description of atomic bonding properties. ... A lone pair is an electron pair without bonding or sharing with other atoms. ...


The parent carbene is H2C: also called methylene. An often encountered carbene is Cl2C: or dichlorocarbene which can be generated in situ from chloroform and a strong base. In chemistry, methylene is a divalent functional group CH2 derived formally from methane. ... In chemistry a carbene is a short-lived and highly reactive organic molecule with a divalent carbon atom with only 6 valence electrons and the general formula: R1R2C:. The carbon atom is sp2 hybridised with a empty p-orbital extending above and below a plane containing R1 and R2 and... In situ (in place in Latin), a term used in: biology, where it means to examine the phenomenon exactly in place where it occurs (without removing it in some special medium etc. ... PEL-TWA (OSHA) 50 ppm (240 mg/m3) IDLH (NIOSH) 500 ppm Flash point non-flammable RTECS number FS9100000 Supplementary data page Structure & properties n, εr, etc. ... A base is: in mathematics: A number that is raised to a power, or base of an exponential function. ...

Contents


Structure

Singlet and triplet carbenes
Singlet and triplet carbenes

Generally there are two types of carbenes; singlet or triplet carbenes. Singlet carbenes have a pair of electrons and sp2 hybrid structure. Triplet carbenes have two unpaired electrons. They may be either sp2 hybrid or linear sp hybrid. Most carbenes have nonlinear triplet ground state with the exception of carbenes with nitrogen, oxygen, sulfur atoms, and dihalocarbenes. Image File history File links Carbenes. ... In theoretical physics, a singlet usually refers to a one-dimensional representation (e. ... A triplet is a set of three items, and includes in particular: one of three babies in a multiple birth a preparation of opal as a gemstone, with a thin layer of opal backed with a dark material and covered with cap of clear quartz in poetry, a tercet (three... four sp³ orbitals three sp² orbitals In chemistry, hybridisation is the mixing of atomic orbitals belonging to a same electron shell to form new orbitals suitable for the qualitative description of atomic bonding properties. ...


Singlet and triplet carbenes are named so because of the electronic spins they possess. Triplet carbenes are paramagnetic and may be observed by electron spin resonance spectroscopy if they can exist long enough without undergoing further reactions. The total spin of singlet carbenes is one while that of triplet carbenes is three (calculated as total number of unpaired electrons+1). For simple hydrocarbons, triplet carbenes usually have energies 8 kcal/mole (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. Substituents that can donate electron pairs may stabilize singlet state by delocalizing the pair into empty p-orbital. Bond angles are 125-140° for triplet methylene and 102° for singlet methylene (determined by EPR). The carbene 9-fluorenylidene has been shown to be a rapidly equilibrating mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference [2]. Triplet carbenes are generally stable in gaseous state while singlet carbenes are often found in aqueous media. The terms spin and SPIN have several meanings, including those primarily discussed as spinning: For spin in sub-atomic physics, see spin (physics) For the stalled aircraft maneuver or any of several forms of loss of control in aircraft, see spin (flight) For the periodical, see Spin Magazine For the... Paramagnetism is the tendency of the atomic magnetic dipoles, due to quantum-mechanical spin, in a material that is otherwise non-magnetic to align with an external magnetic field. ... Overview Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) is a spectroscopic technique which detects species that have unpaired electrons, generally meaning that it must be a free radical, if it is an organic molecule, or that it has transition metal ions if it is a inorganic complex. ... A calorie refers to a unit of energy. ... The mole and its simple conversions into different units of measurements. ... A kilojoule (abbreviation: kJ) is a unit of energy equal to 1000 joules. ... Hunds rule of maximum multiplicity, often simply referred to as Hunds rule, is a principle of atomic chemistry which states that a greater total spin state usually makes the resulting atom more stable, most commonly manifested in a lower energy state, because it forces the unpaired electrons to... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). ... In organic chemistry, a substituent is an atom or group of atoms subsituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... In chemistry, methylene is a divalent functional group CH2 derived formally from methane. ... EPR may refer to the following: Electron paramagnetic resonance, more commonly called Electron spin resonance, used especially in Analytical Chemistry EPR paradox by Einstein, Podolsky and Rosen (Quantum Mechanics) Extended producer responsibility Enlisted Performance Report, an evaluation form used by the United States Air Force Ejército Popular Revolucionario, a... Chemical equilibrium is the state in which a chemical reaction proceeds at the same rate as its reverse reaction; the rates of the forward and reverse reactions are equal, and the concentration of the reactants and products stop changing. ...


Reactivity

Carbene addition to alkenes
Carbene addition to alkenes

Singlet and triplet carbenes do not demonstrate the same reactivity. Singlet carbenes generally participate in cheletropic reactions as either electrophiles or nucleophiles. Singlet carbene with its unfilled p-orbital should be electrophilic. Triplet carbenes should be considered to be diradicals, and participate in stepwise radical additions. Triplet carbenes have to go through an intermediate with two unpaired electrons whereas singlet carbene can react in a single concerted step. Addition of singlet carbenes to olefinic double bonds is more stereoselective than that of triplet carbenes. Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved. Image File history File links Singletriplet. ... An electrocyclic reaction is a type of pericyclic rearrangement reaction where the net result is one pi bond being converted into one sigma bond. ... In chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. ... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... In chemistry free radicals are uncharged atomic or molecular species with unpaired electrons or an otherwise open shell configuration. ... In chemistry a reactive intermediate is a short-lived high energy highly reactive molecule. ... In chemistry, a concerted reaction is one that takes place in a single step. ... A synonym for the more widely accepted term, alkene. ...


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 [3]. Photolysis refers to any chemical reaction in which a compound is broken down by light. ... A diazo compund is any class of organic substances that have the characteristic atomic grouping: as a part of their molecular structure. ... In chemistry, a stereospecific chemical reaction yields different stereoisomeric reaction products from two stereoisomeric reactants depending on the reaction conditions. ...


Reactivity of a particular carbene depends on the substituent groups, preparation method, reaction conditions such as presence or absence of metals. 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. In organic chemistry, a substituent is an atom or group of atoms subsituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... Hot metal work from a blacksmith Look up Metal in Wiktionary, the free dictionary. ... An Alkyl is a univalent radical containing only carbon and hydrogen atoms arranged in a chain. ...

Carbene cyclopropanation
Carbene cyclopropanation

Carbenes add to double bonds to form cyclopropane. A concerted mechanism is available for singlet carbenes. Triplet carbenes do not retain stereochemistry in the product molecule. Addition reactions are commonly very fast and exothermic. The slow step in most instances is generation of carbene. A well-known reagent employed for alkene-to-cyclopropane reactions is Simmons-Smith reagent. This reagents is a system of copper, zinc, and iodine, where the active reagent is believed to be iodomethylzinc iodide. Reagent is complexed by hydroxy groups such that addition commonly happens syn to such group. Image File history File links Cyclopropanation. ... Molecule structure formula of cyclopropane Cyclopropane is a cycloalkane molecule with the molecular formula C3H6 consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms. ... The different types of isomers. ... Exothermic means to release energy in the form of heat. ... The Simmons-Smith reaction is an organic reaction in which a carbenoid reacts with an alkene (or alkyne) to form a cyclopropane. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Atomic mass 126. ... This prefix in chemical nomenclature indicates the presence of a hydroxyl functional group (-OH). ... SYN can stand for: The SYN (TCP) synchronize packet in transmission control protocol (TCP) Synchronous Idle — control character in the C0 set. ...


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.

Carbene insertion
Carbene insertion

Intramolecular insertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no intermolecular insertions are seen. 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. Image File history File links Insertion. ... Intramolecular describes a process or characteristic limited within the structure of a single, or each molecule; a property or phenomenon limited to the extent of a single, or each molecule. ... Intermolecular describes a process or characteristic that extends from one molecule to an adjacent one; a property or phenomenon that extends from one molecule to another. ...

Carbene intramolecular reaction
Carbene intramolecular reaction
Carbene intermolecular reaction
Carbene intermolecular reaction


Alkylidene carbenes are alluring in that they offer formation of cyclopentene moieties. To generate an alkylidene carbene a ketone can be exposed to trimethylsilyl diazomethane. Image File history File links Carbene_intra. ... Image File history File links Carbene_inter. ... Cyclopentene is a highly flammable cycloalkene with chemical formula C5H8 and CAS number 142-29-0. ... Chemical structure of Tetramethylsilane Chemical structure of a Trimethylsilyl group The name Tetramethylsilane stands for a chemical compound whose molecular structure is essentially like a silane (SiH4) molecule with four methyl groups substituted for the four hydrogen atoms in it. ... A diazo compund is any class of organic substances that have the characteristic atomic grouping: as a part of their molecular structure. ...

Alkylidene carbene
Alkylidene carbene


Carbenes can be stabilized as organometallic species. These transition metal carbene complexes fall into two categories: Image File history File links Alkylidene_carbene. ... Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. ... A transition metal carbene complex is a compound bearing a formal carbon-metal double bond. ...

  • Fischer carbenes in which carbenes are tethered to a metal and an electron-withdrawing group (usually a carbonyl),
  • Schrock carbenes; in which carbenes are tethered to a metal and an electron-donating group. The reactions that such carbenes participate in are very different from those in which organic carbenes participate.

The following people whose family name is Fischer have articles: Andrea Fischer (born 1960), German politician Bobby Fischer (born 1943), Icelandic chess player David Hackett Fischer, professor of history Edmond Henri Fischer (born 1920) Edwin Fischer (1886-1960), Swiss pianist and conductor Ernst Otto Fischer (born 1918, German chemist, Nobel... Richard R. Schrock, professor at MIT and winner of the Nobel Prize in Chemistry in 2005 Ed Schrock, American politician This is a disambiguation page — a list of articles associated with the same title. ...

Generation of Carbenes

  • Most commonly, photolytic, thermal, or transition metal catalyzed decomposition of diazoalkanes is used to create carbene molecules. A variation on catalyzed decomposition of diazoalkanes is the Bamford-Stevens reaction, which gives carbenes in aprotic solvents and carbenium ions in protic solvents.
  • Another method is induced elimination of halogen from gem-dihalides or HX from CHX3 moiety, employing organolithium reagents (or another strong base). 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
Carbene preparation
  • Photolysis of diazarines and epoxides can also be employed. Diazarines contain 3-membered rings and are cyclic forms or diazoalkanes. The strain of the small ring makes photoexcitation easy. Photolysis of epoxides gives carbonyl compounds as side products. With asymmetric epoxides, two different carbonyl compounds can potentially form. 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.
  • Thermolysis of alpha-halomercury compounds is another method to generate carbenes.
  • Rhodium and copper complexes promote carbene formation.
  • Carbenes are intermediates in the Wolff rearrangement

Photolysis refers to any chemical reaction in which a compound is broken down by light. ... In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings: It commonly refers to any element in the d-block of the periodic table, including zinc and scandium. ... The Bamford-Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. ... In chemistry any solvent that carries hydrogen attached to oxygen as in a hydroxyl group or nitrogen as in a amine group is called a protic solvent. ... The halogens are a chemical series. ... An organolithium reagent is an organometallic compound with a direct bond between a carbon and a lithium atom. ... Image File history File links Carbene_made. ... Photolysis refers to any chemical reaction in which a compound is broken down by light. ... An epoxide is a cyclic ether with only three ring atoms. ... Photoexcitation is the mechanism of electron excitation by photon absorption, when the energy of the photon is too low to cause photoionization. ... Carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom. ... The two optical isomers of bromochlorofluoromethane Chiral synthesis (also called asymmetric synthesis) is organic synthesis which preserves or introduces a desired chirality. ... An Alkyl is a univalent radical containing only carbon and hydrogen atoms arranged in a chain. ... In the context of organic molecules, aryl refers to any member of the set of functional groups or substituents that are derived from a simple aromatic ring. ... Thermolysis (from thermo- meaning heat and -lysis meaning break down) is a chemical process by which a substance is decomposed into other substances by use of heat. ... Rh redirects here. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ... The Wolff rearrangement is a rearrangement reaction converting a α-diazo-ketone into a ketene in a photochemical reaction, by application of heat, by microwave chemistry or with a transition metal catalyst such as silver oxide. ...

Persistent carbenes

A carbene is a reactive intermediate but certain organic carbenes, so called persistent carbenes, discovered in the 1990s are also quite stable. Singlet carbenes derived from imidazolium or thiazolium salts are known as Arduengo Carbenes and can be crystallized and isolated as free carbenes and stored without chemical decomposition, if kept in an oxygen- and moisture-free atmosphere [4]. In chemistry a reactive intermediate is a short-lived high energy highly reactive molecule. ... Imidazole is a heterocyclic aromatic organic compound. ... chemical decomposition is the gradual fragmentation of a chemical compound into smaller molecules. ...

Ad stands for the adamantane substituent

One stable N-heterocyclic carbene [5] has a structure analogous to borazine with one boron atom replaced by methylene. This results in a planar 6 electron compound. stable imidazolium carbene File links The following pages link to this file: Carbenes ... Adamantane (Tricyclo[3. ... In organic chemistry, a substituent is an atom or group of atoms subsituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... Borazine is an inorganic compound composed of the elements boron, nitrogen and hydrogen. ... General Name, Symbol, Number boron, B, 5 Chemical series metalloids Group, Period, Block 13, 2, p Appearance black/brown Atomic mass 10. ...

In the second step of this reaction sequence the proton is abstracted by LiTMP, two cyclohexyl groups shield the carbene.

Another mode of stabilization is present in Bertrand's arbene, which is bis(diisopropylamino)phosphino[trimethylsilylcarbene]. The compound is alternatively formulated as a phosphaalkyne. Image File history File links Borazine carbene File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Lithium tetramethylpiperidide or LiTMP (CAS 38227-87-1) is an organic base and a harpoon base. ... Cyclohexane is a molecule with the molecular formula C6H12 (molar mass = 84. ...


In 2001, Hideo Tomioka and his associates were able to produce a comparatively stable triplet carbene, taking advantage of resonance. Triplet bis(9-anthryl)carbene [6] has a half-life of 19 minutes. Resonance structures of Benzene Resonance structures are diagrammatic tools used predominately in organic chemistry to symbolize resonant bonds between atoms in molecules. ...


In 2006 the same group reported a triplet carbene with a half-life of 40 minutes [7]. This carbene is prepared by a photochemical decomposition of a diazomethane with expulsion of nitrogen gas at a wavelength of 300 nanometers in benzene. As with the other carbenes this one contains large bulky substituents, in this molecule bromine and the trifluoromethyl groups, that shield the carbene and prevent or slow down the process of dimerization to a 1,1,2,2-tetra(phenyl)alkene. In silico experiments show that the distance of the divalent carbon atom to its neighbours is 138 picometers with a bond angle of 158.8°. The dihedral angle is 85.7° which makes the phenyl groups almost at right angles to each other. Exposure to oxygen (diradical) converts the carbene to the corresponding benzophenone and the diphenylmethane compound is formed when it is trapped by 1,4-cyclohexadiene. Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... Photochemistry is the study of the interaction of light and chemicals. ... chemical decomposition is the gradual fragmentation of a chemical compound into smaller molecules. ... A diazo compund is any class of organic substances that have the characteristic atomic grouping: as a part of their molecular structure. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... The wavelength is the distance between repeating units of a wave pattern. ... A nanometre (American spelling: nanometer) is 1. ... General Name, Symbol, Number bromine, Br, 35 Chemical series halogens Group, Period, Block 17, 4, p Appearance gas/liquid: red-brown solid: metallic luster Atomic mass 79. ... in silico is an expression used to mean performed on computer or via computer simulation. ... Bond length or bond distance in molecular geometry is the distance between two bonded atoms in a molecule. ... Picometre (American spelling: picometer) is an SI measure of length that is equal to 10−12 of a metre. ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... In Aerospace engineering, the dihedral is the angle that the two wings make with each other. ... Benzophenone, also known as diphenylmethanone, phenyl ketone, diphenyl ketone, or benzoylbenzene. ...


Another example of a carbene is methanesulfinyl carbene, molecular formula CHS(O)CH3, having a hydrogen and a sulfinyl (S(O)CH3) group bound to the sp2 carbon. Using the Density Functional B3LYP method with a 6-31G+(d) basis set, calculations indicate that the triplet state of this molecule is more stable than its singlet counterpart. Further studies are being conducted in order to test the effects of added substituents onto the carbene. These studies are being conducted at the University of California, Irvine School of Physical sciences. Image File history File links Persistent_triplet_carbene. ...


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.
  3.   Structure of Carbene CH2 Philip S. Skell, Robert C. Woodworth; J. Am. Chem. Soc.; 1956; 78(17); 4496-4497. Abstract
  4.   A stable crystalline carbene Anthony J. Arduengo, , III Richard L. Harlow, Michael Kline; J. Am. Chem. Soc.; 1991; 113(1); 361-363. Abstract
  5.   Stable Planar Six--Electron Six-Membered N-Heterocyclic Carbenes with Tunable Electronic Properties Carsten Präsang, Bruno Donnadieu, and Guy Bertrand J. Am. Chem. Soc.; 2005; 127(29) pp 10182 - 10183 Abstract
  6.   Nature, 412, 626 (2001), also see [8]
  7.   Triplet Diphenylcarbenes Protected by Trifluoromethyl and Bromine Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature Tetsuji Itoh, Yoshimaru Nakata, Katsuyuki Hirai, and Hideo Tomioka J. Am. Chem. Soc.; 2006; 128(3) pp 957 - 967; (Article) Abstract

 
 

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