Carbenium ions triphenylmethyl

We now examine the theory recently put forward by Penczek [6]. Strictly, we would not need to concern ourselves with it, as the system to which it is said to apply does not involve initiation by perchloric acid, but by triphenylmethyl salts. None the less, it is useful to consider it briefly. Penczek believes that the propagating species in his systems is an oxy-carbenium ion stabilised by co-ordination of two oxygens from a polymer molecule, as shown in structure 6 ... 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

Allyl halides heterolyze just as easily as benzyl halides because they also produce a resonance-stabilized carbenium ion. Even faster heterolyses are possible when the charge of the resulting carbenium ion can be delocalized by more than one unsaturated substituent and can thereby be stabilized especially well. This explains the remarkably high SN1 reactivities of the benzhydryl halides (via the benzhydryl cation) and especially of the triphenylmethyl halides (via the trityl cation) ... 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Carbenium ions have three bonds to the central carbon and are planar, with the bonds directed toward the corners of a triangle (sp hybridization). They have six electrons in outer shell of carbon and a vacant p orbital. Carbenium ions are important intermediates in a number of organic reactions, notably the S l mechanism of NUCLEOPHILIC SUBSTITUTION. It is possible tO produce stable carbenium ions in salts of the type (C H5)3C C1", which are orange-red solids. In these the triphenylmethyl cation is stabilized by delocalization over the three phenyl groups. It is also possible to produce carbenium ions using SUPERACIDS. 

Carbon atoms are the most frequently found radical centers with their electron septet occupying an intermediate position between the carbenium ions and the carbanions. Radicals are often called free radicals, a term that arose from early nomenclature systems in which a radical was a substituent group that was preserved as a unit through a chemical transformation. Thus, the CH3 group as a substituent was known as the methyl radical, so a neutral CHj group became a free radical. The terms radical and free radical are now used interchangeably. Some common examples of radicals include the methyl radical (1), vinyl radical (2), phenyl radical (3), triphenylmethyl radical (4), allyl radical (5), and benzyl radical (6) (Figure 4.1). 

Table 11.7 shows a series of Hammett sensitivity parameters for various SnI reactions using o substituent constants. The p value is indicative of the amount of positive charge present in the transition state. The larger the magnitude of p, the more positive the charge. Some trends are noteworthy. The smallest negative number (entry 1) is for triphenylmethyl cation, the most stable carbenium ion. This is as expected, because the reaction would be the least endothermic, and by the Hammond postulate the transition state would be most reactant-like, thereby having the least cationic character. 

An acetal group readily participates in hydride transfer reactions because the resulting carboxonium ion is stabilized by electron pairs of the two neighboring oxygen atoms. Thus if compounds capable of acting as hydride ion acceptors, such as (triphenylmethyl)carbenium ions, are used as initiators, initiation of DXL polymerization proceeds through intermediate l,3-dioxolan-2-ylium ion, which is a real initiator, as shown in Scheme... 

In addition to the triphenylmethyl salts, other sources of carbenium ions are availaHe for use in one-electron oxidation reactions. Thus, the complex, [W(NO)2(P(OMe)3K n5-Cp)l is oxidized to the corresponding... 

Comparison of the IR and Raman spectra of the trimethylcarbenium ion with those of its isoelectric analogue — trimethylboron — has conflrmed the planar structure of alkylcarbenium ions. The planar structure of the carbenium fragment —C < was also concluded from the IR spectra of salts of the triphenylmethyl cation (CgH5)3C 449-451) analogues having different substituents at the para positions of the... 

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