|
Carbocation
Carbocation is a general term for ions with a positively charged carbon atom. 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. Among the simplest carbocations are the methenium (a carbenium ion), methanium (a carbonium ion), acylium ions , and Vinyl cation, vinyl cations. Until the early 1970s, carbocations were called ''carbonium ions''. This nomenclature was proposed by George Andrew Olah, G. A. Olah. Carbonium ions, as originally defined by Olah, are characterized by a Three-center two-electron bond, 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. However ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carbenium Ion
The carbenium ion is a kind of cation, positive ion with the structure RR′R″C+, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. Carbenium ions are a major subset of carbocations, which is a general term for diamagnetic carbon-based cations. In parallel with carbenium ions is another subset of carbocations, the carbonium ions with the formula R5+. In carbenium ions charge is localized. They are isoelectronic with monoboranes such as B(CH3)3. Nomenclature Reactivity Carbenium ions are generally highly reactive due to having an incomplete octet rule, octet of electrons; however, certain carbenium ions, such as the tropylium cation, tropylium ion, are relatively stable due to the positive charge being delocalised between the carbon atoms.（It can even exist stably in aqueous solution.） Rearrangements Carbenium ions sometimes rearrangement reaction, rearrange readily. For example, when pentan-3-ol is heated with aq ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
2-Norbornyl Cation
In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo[2.2.1]heptyl cation) describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography confirmed the non-classical structure. Theory Hypovalency: the non-classical picture Advocates of the non-classical nature of the stable 2-norbornyl cation typically depict the species using either resonance structures or a single structure with partial bonds (''see Figure 2''). This hypovalent interaction can be imagined as the net effect of i) a partial sigma bond between carbons 1 and 6, ii) a partial sigma bond between carbons 2 and 6, and iii) a partial pi bond between carbons 1 and 2. Each partial bond is represented as a full bond in one of the three resonance structures or as a dashed partial bond if the cation is depicted through a single structure. There has been some debate over how much the pi-bonded resonance structure actually contribu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Norbornyl Cation
In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo .2.1eptyl cation) describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography confirmed the non-classical structure. Theory Hypovalency: the non-classical picture Advocates of the non-classical nature of the stable 2-norbornyl cation typically depict the species using either resonance structures or a single structure with partial bonds (''see Figure 2''). This hypovalent interaction can be imagined as the net effect of i) a partial sigma bond between carbons 1 and 6, ii) a partial sigma bond between carbons 2 and 6, and iii) a partial pi bond between carbons 1 and 2. Each partial bond is represented as a full bond in one of the three resonance structures or as a dashed partial bond if the cation is depicted through a single structure. There has been some debate over how much the pi-bonded resonance structure actually contributes ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Vinyl Cation
The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula of the parent ion is . Vinyl cation are invoked as reactive intermediates in solvolysis of vinyl halides, as well as electrophilic addition to alkynes and allenes. History Vinyl cations have long been poorly-understood and were initially thought to be too high energy to form as reactive intermediates. Vinyl cations were first proposed in 1944 as a reactive intermediate for the acid-catalyzed hydrolysis of alkoxyacetylenes to give alkyl acetate. In the first step of their facile hydration reaction, which was the rate limiting step, a vinyl cation reactive intermediate was proposed; the positive charge was believed to formally lie on a dicoordinate carbon. This is the first time such a transition state can be found in the literature. In 1959, Grob and Cseh detected vinyl cations during solvolysis reactions of alpha-vinyl halides. Indeed, for this contribution, Grob has been called ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
George Andrew Olah
George Andrew Olah (born Oláh András György; May 22, 1927 – March 8, 2017) was a Hungarian-American chemist. His research involved the generation and reactivity of carbocations via superacids. For this research, Olah was awarded a Nobel Prize in Chemistry in 1994 "for his contribution to carbocation chemistry." He was also awarded the Priestley Medal, the highest honor granted by the American Chemical Society and F.A. Cotton Medal for Excellence in Chemical Research of the American Chemical Society in 1996. After the Hungarian Revolution of 1956, he immigrated to the United Kingdom, which he left for Canada in 1964, finally resettling in the United States in 1965. According to György Marx, he was one of The Martians. Early life and education Olah was born in Budapest, Hungary, on May 22, 1927, into a Jewish couple, Magda (Krasznai) and Gyula Oláh, a lawyer. After the high school of Budapesti Piarist Gimnazium, he studied under organic chemist Géza Zemplén at the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carbonium Ion
In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom. They are a type of carbocation. In older literature, the name "carbonium ion" was used for what is today called carbenium. Carbonium ions charge is delocalized in three-center, two-electron bonds. The more stable members are often bi- or polycyclic. 2-Norbornyl cation The 2-norbornyl cation is one of the best-characterized carbonium ions. It is the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, it has a symmetric structure with an RCH2+ group bonded to an alkene group, stabilized by a bicyclic structure. Cyclopropylmethyl cation A non-classical structure for is supported by substantial experimental evidence from solvolysis experiments and NMR studies. One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity. The NMR spectr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Superacid
In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid. Commercially available superacids include trifluoromethanesulfonic acid (), also known as triflic acid, and fluorosulfuric acid (), both of which are about a thousand times stronger (i.e. have more negative ''H''0 values) than sulfuric acid. Most strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. A strong superacid of this kind is fluoroantimonic acid. Another group of superacids, the carborane acid group, contains some of the strongest known acids. Finally, when treated with anhydrous acid, zeolites (microporous aluminosilicate minerals) will contain superacidic sites within their pores. These m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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] |
Resonance (chemistry)
In chemistry, resonance, also called mesomerism, is a way of describing Chemical bond, 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. The resonance hybrid is the accurate structure for a molecule or ion; it is an average of the theoretical (or hypothetical) contributing structures. 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 suffi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The International Union of Pure and Applied Chemistry (IUPAC) Preferred IUPAC name, recommends using the name "alkene" only for Open-chain compound, acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloalkadiene, etc. for Cyclic compound, cyclic ones; and "olefin" for the general class – cyclic or acyclic, with one or more double bonds. Acyclic alkenes, with only one double bond and no other functional groups (also known as mono-enes) form a homologous series of hydrocarbons with the general formula with ''n'' being a >1 natural number (which is two hydrogens less than the corresponding alkane). When ''n'' is four or more, isomers are possible, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Methenium
In organic chemistry, methenium (also called methylium, carbenium, methyl cation, or protonated methylene) is a cation with the formula . It can be viewed as a methylene radical (:) with an added proton (), or as a methyl radical (•) with one electron removed. It is a carbocation and an enium ion, making it the simplest of the carbenium ions. Structure Experiments and calculations generally agree that the methenium ion is planar, with threefold symmetry. The carbon atom is a prototypical (and exact) example of sp2 hybridization. Preparation and reactions For mass spectrometry studies at low pressure, methenium can be obtained by ultraviolet photoionization of methyl radical, or by collisions of monatomic cations such as and with neutral methane. In such conditions, it will react with acetonitrile to form the ion . Upon capture of a low-energy electron (less than ), it will spontaneously dissociate. It is seldom encountered as an intermediate in the condensed phase. It i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Acylium Ions
In organic chemistry, acylium ions are cations with the formula RCO+, where R = alkyl or aryl. They are a kind of carbocation. Structure, bonding, synthesis In acylium ions, the C-C-O linkage is linear. The oxygen and the central carbon can be described as sp hybridized. A triple bond exists between C and O. Acylium ions can be viewed as C-alkylated derivatives of carbon monoxide (which also has a C-O triple bond). They are typically prepared by removal of chloride from acyl chlorides using strong Lewis acids such as antimony pentachloride. Several acylium salts have been characterized by X-ray crystallography, including the propionyl and mesityl derivatives. Because acylium cation is highly electrophilic, its salts can only be isolated with weakly coordinating anions. : Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples. The strength of the bond is indicated by the frequency of its vibration (νCO). These values are 2300 and 2200 cm−1, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
