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Hyperconjugation
In organic chemistry, hyperconjugation (σ-conjugation or no-bond resonance) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to give a pair of extended molecular orbitals. However, sometimes, low-lying antibonding σ* orbitals may also interact with filled orbitals of lone pair character (n) in what is termed ''negative hyperconjugation''. Increased electron delocalization associated with hyperconjugation increases the stability of the system. In particular, the new orbital with bonding character is stabilized, resulting in an overall stabilization of the molecule. Only electrons in bonds that are in the β position can have this sort of direct stabilizing effect — donating from a sigma bond on an atom to an orbital in another ato ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Anomeric Effect
In organic chemistry, the anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the ''axial'' orientation instead of the less hindered ''equatorial'' orientation that would be expected from steric considerations. This effect was originally observed in pyranose rings by J. T. Edward in 1955 when studying carbohydrate chemistry. The term ''anomeric effect'' was introduced in 1958. The name comes from the term used to designate the lowest-numbered ring carbon of a pyranose, the ''anomeric'' carbon. Isomers that differ only in the configuration at the anomeric carbon are called ''anomers''. The anomers of D-glucopyranose are diastereomers, with the ''beta'' anomer having an OH group pointing up equatorially, and the ''alpha'' anomer having that OH group pointing down axially. The anomeric effect can also be generalized to any cyclohexyl or linear syst ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Negative Hyperconjugation
In organic chemistry, negative hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ*-orbital. This phenomenon, a type of resonance, can stabilize the molecule or transition state. It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital. Negative hyperconjugation is seldom observed though its most commonly observed when the σ*-orbital is located on certain C–F or C–O bonds, In negative hyperconjugation, the electron density flows in the ''opposite'' direction (from π- or p-orbital to empty σ*-orbital) than it does in the more common hyperconjugation (from a lone pair of electrons to an empty p-orbital). See also *Conjugated system In theoretical chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented . ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotational Barrier
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation). While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations, conformations that correspond to local minima on the potential energy surface are specifically called conformational isomers or conformers. Conformations that correspond to local maxima on the energy surface are the transition states between the local-minimum conformational isomers. Rotations about single bonds involve overcoming a rotational energy barrier to interconvert one conformer to another. If the energy barrier is low, there is free rotation and a sample of the compound exists as a rapidly equilibrating mixture of multiple conformers; if the energy barrier is high enough then there is restricted rotation, a molecule may exist for a r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carbocation
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered (e.g., ethylene dication ). Until the early 1970s, all carbocations were called ''carbonium ions''. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further classified in two main categories according to the coordination number of the charged carbon: three in the carbenium ions and five in the carbonium ions. This nomenclature was proposed by G. A. Olah. Carbonium ions, as originally defined by Olah, are characterized by a three-center two-electron delocalized bonding scheme and are essentially synonymous with so-called 'non-classical carbocations', which are carbocations that contain bridging C–C or C–H σ-bonds. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Beta-silicon Effect
The beta-silicon effect also called silicon hyperconjugation in organosilicon chemistry is a special type of hyperconjugation that describes the stabilizing influence of a silicon atom on the development of positive charge at a carbon atom one position removed (β) from the silicon atom. The C-Si σ orbital is said to partially overlap with the σ* anti-bonding orbital of the C-leaving group, lowering the energy of the transition state leading to the formation of a carbocation. A prerequisite for the hyperconjugation to occur is an antiperiplanar relationship between the Si group and the leaving group.''Silicon in Organic Synthesis'' Colvin, E. Butterworth: London 1981 This allows for the maximum overlap between the C-Si σ orbital and the σ* anti-bonding orbital of the leaving group. Silicon hyperconjugation explains specific observations regarding chemical kinetics and stereochemistry of organic reactions with reactants containing silicon. The picture below shows the partial ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radical (chemistry)
In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes. A notable example of a radical is the hydroxyl radical (HO·), a molecule that has one unpaired electron on the oxygen atom. Two other examples are triplet oxygen and triplet carbene (꞉) which have two unpaired electrons. Radicals may be generated in a number of ways, but typical methods involve redox reactions. Ionizing radiation, heat, electrical discharges, and electrolysis are known to produce radicals. Radicals are intermediates in many chemical reactions, more so than is apparent from the balanced equations. Radicals are important in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes. A majority ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Resonance (chemistry)
In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or ''canonical structures'') into a resonance hybrid (or ''hybrid structure'') in valence bond theory. It has particular value for analyzing delocalized electrons where the bonding cannot be expressed by one single Lewis structure. Overview Under the framework of valence bond theory, resonance is an extension of the idea that the bonding in a chemical species can be described by a Lewis structure. For many chemical species, a single Lewis structure, consisting of atoms obeying the octet rule, possibly bearing formal charges, and connected by bonds of positive integer order, is sufficient for describing the chemical bonding and rationalizing experimentally determined molecular properties like bond lengths, angles, and dipole moment. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ethane
Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petrochemical by-product of petroleum refining. Its chief use is as feedstock for ethylene production. Related compounds may be formed by replacing a hydrogen atom with another functional group; the ethane moiety is called an ethyl group. For example, an ethyl group linked to a hydroxyl group yields ethanol, the alcohol in beverages. History Ethane was first synthesised in 1834 by Michael Faraday, applying electrolysis of a potassium acetate solution. He mistook the hydrocarbon product of this reaction for methane and did not investigate it further. During the period 1847–1849, in an effort to vindicate the radical theory of organic chemistry, Hermann Kolbe and Edward Frankland produced ethane by the reductions of propionitrile ( ethy ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Baker–Nathan Effect
In organic chemistry, the Baker–Nathan effect is observed with reaction rates for certain chemical reactions with certain substrates where the order in reactivity cannot be explained solely by an inductive effect of substituents. This effect was described in 1935 by John W. Baker and W. S. Nathan. They examined the chemical kinetics for the reaction of pyridine with benzyl bromide and with a range of benzyl bromides with different para alkyl substituents (reaction product a pyridinium salt). : The reaction is facilitated by electron-releasing substituents (the inductive effect) and in general the observed order (with decreasing reactivity) is tert-butyl > isopropyl > ethyl > methyl. The observed order in this particular reaction however was methyl > ethyl> isopropyl > tert-butyl. In 1935 Baker and Nathan explained the observed difference in terms of a conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb f ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Zaitsev's Rule
In organic chemistry, Zaitsev's rule (or Saytzeff's rule, Saytzev's rule) is an empirical rule for predicting the favored alkene product(s) in elimination reactions. While at the University of Kazan, Russian chemist Alexander Zaitsev studied a variety of different elimination reactions and observed a general trend in the resulting alkenes. Based on this trend, Zaitsev proposed that the alkene formed in greatest amount is that which corresponded to removal of the hydrogen from the alpha-carbon having the fewest hydrogen substituents. For example, when 2-iodobutane is treated with alcoholic potassium hydroxide (KOH), 2-butene is the major product and 1-butene is the minor product. : More generally, Zaitsev's rule predicts that in an elimination reaction the most substituted product will be the most stable, and therefore the most favored. The rule makes no generalizations about the stereochemistry of the newly formed alkene, but only the regiochemistry of the elimination reaction ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chem , a place in Finland
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Chem may refer to: * Chemistry practical waali mam * Chemistry * Chemical * ''Chem'' (journal), a scientific journal published by Cell Press *Post apocalyptic slang for "drugs", medicinal or otherwise in the Fallout video game series. In Ancient Egyptian usage: * ''Khem'' (also spelt ''Chem''), the Egyptian word for "black" * Min (god), in the past erroneously named ''Khem'' CHEM may refer to : *A metabolic panel: for instance, CHEM-7, which is the basic metabolic panel *CHEM-DT, a Canadian television channel See also *Chemo (other) *Kemi Kemi (; sme, Giepma ; smn, Kiemâ; sms, Ǩeeʹmm; Swedish (historically): ''Kiemi'') is a town and municipality of Finland. It is located very near the city of Tornio and the Swedish border. The distance to Oulu is to the south and to Rovani ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nature (journal)
''Nature'' is a British weekly scientific journal founded and based in London, England. As a multidisciplinary publication, ''Nature'' features peer-reviewed research from a variety of academic disciplines, mainly in science and technology. It has core editorial offices across the United States, continental Europe, and Asia under the international scientific publishing company Springer Nature. ''Nature'' was one of the world's most cited scientific journals by the Science Edition of the 2019 '' Journal Citation Reports'' (with an ascribed impact factor of 42.778), making it one of the world's most-read and most prestigious academic journals. , it claimed an online readership of about three million unique readers per month. Founded in autumn 1869, ''Nature'' was first circulated by Norman Lockyer and Alexander Macmillan as a public forum for scientific innovations. The mid-20th century facilitated an editorial expansion for the journal; ''Nature'' redoubled its efforts in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
