Carbenium ions trivalent, stabilization

These differences were explained by solvation effects in the liquid phase. The carbenium ions are more efficiently stabilized by solvation than carbonium ions, because the former have unsaturated trivalent carbon atoms. In this way, the energy barrier between the (solvated) carbenium ion and the carbonium ion transition state increases. 

The most significant conclusion that can be drawn from the data summarized in Table III is that substituent effects do not exert the same overwhelming importance for the thermodynamic stability of the higher homologues of carbenium ions, thus they do not play the dominant role as in carbocation chemistry. This can be traced back on (i) the inherent higher stability of the trivalent cations of the elements Si Pb and (ii) the weakness of the stabilizing interaction (in many cases of ii-type) of the most common substituents with the central element atom. 

Some of the most important reaction intermediates in organic chemistry are the carbocations. Neglecting some heteroatom-stabilized cations, most carbocations are divided into two groups trivalent carbenium ions and five-coordinate or higher coordinate carbonium ions. The parent carbenium ion is CHJ, and the parent carbonium ion is CHJ. Carbonium ions have been proposed as reactive intermediates in superacid-catalyzed reactions however, they have never been directly observed in condensed media. In contrast, a variety of carbenium ions have already been prepared in superacidic media and been characterized by various physical methods, mainly 13C NMR spectroscopy (5). 

The measured stabilization energies are consistent with the theoretically predicted large hyperconjugative interaction between the Si—C cr-bond and the formally empty 2p(C) orbital. The results for the trivalent carbenium ions show a consistent decrease of 10 kcal mol-1 with each successive methyl group substitution on the carbenium carbon (Table 1, entries 1,4,6). Even the very stable /-butyl cation Me3C+ is stabilized by an additional 24 kcalmol-1 by a /l-MejSi substituent (Table 1, entry 6). The stabilization of vinyl cations due to the presence of a /l-McsSi group is found to be smaller than for... 

Neighboring group interactions with the vacant p orbital of the carbenium ion center can contribute to ion stabilization via charge delocalization. Such phenomena can involve atoms with unshared electron pairs (w-donors), C—H and C—C hyperconjugation, bent o-bonds (as in cyclopropylcarbenium ions), and 7t-electron systems (direct conjugative or allylic stabilization). Thus, trivalent carbenium ions can show... 

Tab. 2.2 Stabilization of a Trivalent Carbenium Ion Center by Conjugating Substituents Experimental Findings and Their Explanation by Means of Resonance Theory...

Fig. 2.17. MO interactions responsible for the stabilization of trivalent carbenium ion centers by suitably oriented unsaturated substituents ("conjugation").

Vinyl cations [1], Oie dicoordinated unsaturated analogs of trivalent carbenium ions, were first detected by Grob and coworkers in the early 1960s in solvolysis reactions of a-aryl vinyl halides [2]. The direct NMR detection of vinyl cations in superacidic media was achieved in 1992 at temperatures below -100 °C [3]. We recently reported a convenient synthesis of unusually stable vinyl cations at room temperature [4,5]. One reason for the unusual high thermodynamic stabilization of these vinyl cations is the presence of two 3-silyl substituents. [4]. We report here details of the X-ray structure of the vinyl cation 1 and discuss the structural and spectroscopic consequences of P-SiC hyperconjugation [6]. 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

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