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Alkyne Trimerization
An alkyne trimerisation is a +2+2nbsp; cycloaddition reaction in which three alkyne units () react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. Being a cycloaddition reaction, it has high atom economy. Many variations have been developed, including cyclisation of mixtures of alkynes and alkenes as well as alkynes and nitriles. Mechanism and stereochemistry Trimerisation of acetylene to benzene is highly exergonic, proceeding with a free energy change of 142 kcal/mol at room temperature. Kinetic barriers however prevent the reaction from proceeding smoothly. The breakthrough came in 1948, when Walter Reppe and W. J. Schweckendiek reported their wartime results showing that nickel compounds are effective catalysts: : Since this discovery, many other cyclotrimerisations have been reported. Mechanism In terms of mechanism, the reactions begin with the formation of metal- ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cycloaddition Reaction
In organic chemistry, a cycloaddition is a chemical reaction in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity". The resulting reaction is a cyclization reaction. Many but not all cycloadditions are concerted and thus pericyclic. Nonconcerted cycloadditions are not pericyclic. As a class of addition reaction In organic chemistry, an addition reaction is an organic reaction in which two or more molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, ..., cycloadditions permit carbon–carbon bond formation without the use of a nucleophile or electrophile. Cycloadditions can be described using two systems of notation. An older but still common notation is based on the size of linear arrangements of atoms in the reactants. It uses parentheses: whe ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chirality (chemistry)
In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotation (geometry), rotations, translation (geometry), translations, and some Conformational isomerism, conformational changes. This geometric property is called chirality (). The terms are derived from Ancient Greek (''cheir'') 'hand'; which is the canonical example of an object with this property. A chiral molecule or ion exists in two stereoisomers that are mirror images of each other, called enantiomers; they are often distinguished as either "right-handed" or "left-handed" by their absolute configuration or some other criterion. The two enantiomers have the same chemical properties, except when reacting with other chiral compounds. They also have the same physics, physical properties, except that they often have opposite optical activity, optical activities. A homogeneous mixture of the two enantiomers in equal parts is said to be racemic mixture, racem ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Strain (chemistry)
In chemistry, a molecule experiences strain when its chemical structure undergoes some Stress (mechanics), stress which raises its internal energy in comparison to a strain-free reference Chemical compound, compound. The internal energy of a molecule consists of all the energy stored within it. A strained molecule has an additional amount of internal energy which an unstrained molecule does not. This extra internal energy, or strain energy, can be likened to a compression (physics), compressed spring (device), spring.Anslyn and Dougherty, ''Modern Physical Organic Chemistry'', University Science Books, 2006, Much like a compressed spring must be held in place to prevent release of its potential energy, a molecule can be held in an energetically unfavorable conformation by the Chemical bond, bonds within that molecule. Without the bonds holding the conformation in place, the strain energy would be released. Summary Thermodynamics The Chemical equilibrium, equilibrium of two c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Benzocyclobutene
Benzocyclobutene (BCB) is a benzene ring fused to a cyclobutane ring. It has chemical formula . BCB is frequently used to create photosensitive polymers. BCB-based polymer dielectrics may be spun on or applied to various substrates for use in Micro Electro-Mechanical Systems (MEMS) and microelectronics processing. Applications include wafer bonding, optical interconnects, low-κ dielectrics, or even intracortical neural implants. Reactions Benzocyclobutene is a strained system which, upon heating to approximately 180 °C, causes the cyclobutene to undergo a conrotatory ring-opening reaction, forming ''o''-xylylene. Since this process destroys the aromaticity of the benzene ring, the reverse reaction is highly favored. ''o''-Xylylenes generated in this way have been used prolifically in cycloaddition reactions, which restore the aromaticity to the benzene ring, while forming a new annulated species. Uses The benzocyclobutene moiety has also appeared in a number of chemi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Molecular Tether
In chemistry, intramolecular describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule. Relative rates In intramolecular organic reactions, two reaction sites are contained within a single molecule. This configuration elevates the effective concentration of the reacting partners resulting in high reaction rates. Many intramolecular reactions are observed where the intermolecular version does not take place. Intramolecular reactions, especially ones leading to the formation of 5- and 6-membered rings, are rapid compared to an analogous intermolecular process. This is largely a consequence of the reduced entropic cost for reaching the transition state of ring formation and the absence of significant strain associated with formation of rings of these sizes. For the formation of different ring sizes via cyclization of substrates of varying tether length, the order of reaction rates (r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Organocobalt Compound
Organocobalt chemistry is the chemistry of organometallic compounds containing a carbon to cobalt chemical bond. Organocobalt compounds are involved in several organic reactions and the important biomolecule vitamin B12, vitamin B12 has a cobalt-carbon bond. Many organocobalt compounds exhibit useful catalytic properties, the preeminent example being dicobalt octacarbonyl. Alkyl complexes Most fundamental are the cobalt complexes with only alkyl ligands. Examples include Co(4-norbornyl)4 and its cation. Alkylcobalt is represented by vitamin B12, vitamin B12 and related enzymes. In methylcobalamin the ligand is a methyl group, which is electrophilic. in vitamin B12, the alkyl ligand is an adenosyl group. Related to vitamin B12 are cobalt porphyrins, dimethylglyoxime, dimethylglyoximates, and related complexes of Schiff base ligands. These synthetic compounds also form alkyl derivatives that undergo diverse reactions reminiscent of the biological processes. The weak cobalt(III) ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of its stoichiometric relationship to benzene, COT has been the subject of much research and some controversy. Unlike benzene, C6H6, cyclooctatetraene, C8H8, is not aromatic, although its dianion, ( cyclooctatetraenide), is. Its reactivity is characteristic of an ordinary polyene, i.e. it undergoes addition reactions. Benzene, by contrast, characteristically undergoes substitution reactions, not additions. History 1,3,5,7-Cyclooctatetraene was initially synthesized by Richard Willstätter in Munich in 1905 using pseudopelletierine as the starting material and the Hofmann elimination as the key transformation: : Willstätter noted that the compound did not exhibit the expected aromaticity. Between 1939 and 1943, chemists throughout ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Enyne
An enyne is an organic compound containing a double bond (alkene) and a triple bond (alkyne). It is called a conjugated enyne when the double and triple bonds are conjugated. The term is a contraction of the terms alkene and alkyne. The simplest enyne is vinylacetylene. Some examples of enynes found in nature are isanolic acid and exocarpic acid. See also *Enyne metathesis * Enediyne *Polyyne A polyyne is any organic compound with alternating Single bond, single and triple bonds; that is, a series of consecutive alkynes, with ''n'' greater than 1. These compounds are also called polyacetylenes, especially in the natural products and ... References {{organic-chemistry-stub Chemical nomenclature Alkenes Alkynes Conjugated hydrocarbons ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aromaticity
In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The earliest use of the term was in an article by August Wilhelm Hofmann in 1855. There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds. Aromaticity can also be considered a manifestation of cyclic delocalization and of resonance. This is usually considered to be because electrons are free to cycle around circular arrangements of atoms that are alternately single- and double- bonded to one another. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé (see History section below). Each bond may be seen as a hybrid of a single bo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
