Carbocations carbenium

PHYSICAL ORGANIC CHEMISTRY NO-MENCATURE CARBOCATION CARBENIUM ION CARBONIUM ION BRIDGED CARBOCATION CARBOHYDRATE MODIFICATION REACTIONS... 

The fundamental step in acid-catalyzed hydrocarbon conversion processes is the formation of the intermediate carbocations. Whereas all studies involving isomerization, cracking, and alkylation reactions under acidic conditions (Scheme 5.1) agree that a trivalent carbocation (carbenium ion) is the key intermediate, the mode of their formation of this reactive species from the neutral hydrocarbon remained controversial for many years. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Table 17.1 Carbocations carbenium ions and carbonium ions ...

GENERAL CONCEPT OE CARBOCATIONS CARBENIUM VERSUS CARBONIUM IONS... 

The major carbon centered reaction intermediates in multistep reactions are carbocations (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [CH3], [CHi]", [CHi] , and [ CH2]. 

A carbon radical is a trivalent species containing a single electron in a p orbital. A carbanion is viewed as a tetrahedral species containing a pair of electrons in an orbital (1). We have viewed a carbocation (carbenium ion) as an sp hybridized, trigonal planar carbon with an empty p orbital (2). A radical, which contains one electron in an orbital, can be tetrahedral, planar, or in between, with properties of both a carbanion and a carbocation. As shown in 3, a reasonable in between structure is a flattened tetrahedron (the actual structure of radicals will be discussed below). In terms of its reactivity, radical 3 could be considered electron rich or electron poor. In most of its reactions, the electron-deficient characterization is the most useful for predicting products. 

G. Olah proposed to name the classical carbocations carbenium ions (cf. and those with penta-coordinated carbon atoms carbonium ions . In keeping with his recommendations the authors d ing with the above ion groups in their English-language publications now use the term arenium ions more often than the one used earlier ( arenonium ions ). 

One of my basic goals is to answer questions on stereoselectivity, catalysis, stability and reactivity of reactive intermediates, kinetic and thermodynamic aspects of chemical transformation, and so on. Many of the reactive intermediates of organic chemistry are charged species, such as carbocations (carbenium and carbonium ions) and carbanions, but there is an important subgroup of formally neutral... 

During the activation of the initiator consisting of a heterocyclic, aryl substituted, or aryl-ring fused sulfonium salt, a carbon-sulfur bond is broken via a ring-opening reaction, leading to formation of a sulfide and a carbocation (carbenium ion) within the same molecule. The functionality of... 

A carbocation is a structure with a positive charge that is associated primarily with a carbon center or, in the case of delocalized systems, a collection of carbons. Heteroatom substitution is allowed, and it will inevitably lead to some positive charge being on the heteroatoms. There are two types of carbocations—carbenium ions and carbonium ions. Carbenium ions are trivalent species with a formula of RaC. Carbonium ions encompass two related types of structures. First are pentavalent species of the general formula R5C. While not common in solution phase chemistry, such pentavalent carbons are common in the gas phase. Also referred to as carbonium ions are carbocations that have an important contribution from three center—two electron bonding. The nomenclature will become more clear as we present examples below. 

Carbocations (carbenium ions) are important intermediates in organic chemistry, and have been intensely studied since as long ago as 1900. The analogy between carbon and silicon is often stressed, but the isolation of a silylium ion, RaSi, took much longer than for carbon chemistry, although as silicon is more electropositive than carbon it should form cations more easily. But it was not until 1986 that Lambert first reported the synthesis of a free silylium cation, by the following reaction [52] ... 

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