Primary cations

Methyl cation Ethyl cation (primary) Isopropyl cation (secondary) tert Butyl cation (tertiary)... 

Substituent effects Carbocations are formed in the S l reactions. The more stable the carbocation, the faster it is formed. Thus, the rate depends on carbocation stability, since alkyl groups are known to stabilize carbocations through inductive effects and hyperconjugation (see Section 5.2.1). The reactivities of SnI reachons decrease in the order of 3° carbocation > 2° carbocation > 1° carbocation > methyl cation. Primary carbocation and methyl cation are so unstable that primary alkyl halide and methyl halide do not undergo SnI reachons. This is the opposite of Sn2 reactivity. 

In the thiazolium cation the proton in the 2-position is acidic and its removal gives rise to the ylide/carbene 227. This nucleophilic carbene 227 can add, e.g., to an aldehyde to produce the cationic primary addition product 228. The latter, again via C-deprotonation, affords the enamine-like structure 229. Nucleophilic addition of 229 to either an aldehyde or a Michael-acceptor affords compound(s) 230. The catalytic cycle is completed by deprotonation and elimination of the carbene 227. Strictly speaking, the thiazolium salts (and the 1,2,4-triazolium salts discussed below) are thus not the actual catalysts but pre-catalysts that provide the catalytically active nucleophilic carbenes under the reaction conditions used. This mechanism of action of thiamine was first formulated by Breslow [234] and applies to the benzoin and Stetter-reactions catalyzed by thiazolium salts [235-237] and to those... 

The rather selective structure-template relationship of AlPOi,-20 and AlP0i,-17 contrasts with the large number of templates that make AlP0i,-5 (Figure 12). Among these templates are quaternary ammonium cations, primary amines, secondary amines, tertiary amines, cyclic amines, diamines, and alkanolamines. Individual templates contain as many as 13 C+N atoms to as few as 5. There seems to be no common thread connecting these templates, not size, not shape, not solubility, and not pKjj. 

Depending on the desired species to be analyzed, a variety of primary ion beams may be used such as Cs" ", Oj, O, Ar" ", and Ga" ". Whereas a cationic primary beam such as Cs+ is used to ionize electronegative elements e.g., O, C, N, chalcogens. 

The preceding reaction steps present another difficulty, namely, attack by the methyl cation. Primary carbocations that lack stabilizing groups are highly unstable, and the methyl cation is the least stable of the carbocations. In fact, even in superacid (FS03H-SbF5), no primary carbocation is stable enough to be detected. Consequently, a mechanism that invokes such a species should be looked upon with suspicion. 

Methyl cation Primary cation Secondary cation Tei tiary cation... 

Almaula et al. [72] investigated the binding site pocket of the cloned human 5-HT2A receptor expressed in COS-1 cells using mutation studies and computational dynamic simulations. It was shown that the full agonist 5-HT binds with its cationic primary amine group to Asp-155 (helix 3) and by a hydrogen-bond type interaction with Ser-159 (helix 3) whereby serine acts as a H-acceptor. 

FIGURE 3.77 Relative stability of the / -/-butyl cation (tertiary) and the isobutyl cation (primary). 

Depending on the desired species to be analyzed, a variety of primary ion beams may be used such as Cs+, Oj, O, Ar, and Ga. Whereas a cationic primary beam such as Cs is used to ionize electronegative elements (e.g., O, C, N, chalco-gens, halogens, etc.), oxygen atoms are used to ionize electropositive elements (e.g., Li, Mg, Na, B, Al, etc.). Liquid metal ion beams (e.g., Ga, Au/Ge) are used for high-resolution studies, since the beam may be focused to a smaller diameter (<50 nm) for micron and nanoscale analyses. 

Based on these examples, the detection of some of the fluorine-containing radicals reported in Tables II and IV comes as no surprise. For instance, a number of the radicals are formed by C-C bond breakage. Thus a radical formation mechanism similar to the decay of the cation primary in irradiated alkali acetate salts would... 

Experiments on other electrolyte solutions have so far revealed that the Ca and Na ions are surrounded by hydration shells closely resembling that of Ni, the main difference being that the peaks are not as sharp. A surprising result, and one that conflicts with other data, is that the g nature of the cation Ca about cation-cation distributions, but it is claimed that the experimental results at reasonable concentrations clearly show that the Cl does not penetrate the cation primary hydration shell. 

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