Cations methyl

Calculate E(, G2] for the methyl cation CHj. Check your result against the value used in the section on energies of atomization and ionization in this chapter. 

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

FIGURE 4 14 Electro static potential maps of methyl cation and ethyl cation The region of high est positive charge is more concentrated in CH3 and more spread out in CH3CH2 (The electrostatic potentials were mapped on the same scale in order to allow direct comparison )... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Further examination of Table 1.10 reveals that the lowest unoccupied molecular orbital, i/ 4, is a pure p orbital, localized on carbon, since the coefficients are zero for all but the 2p orbital. The MO picture is in agreement with the usual qualitative hybridization picture for the methyl cation. 

Table 1.10. Coefficients of Wave Functions Calculated for Methyl Cation by the CNDO/2...

Substituent effects on intermediates can also be analyzed by MO methods. Take, for example, methyl cations where adjacent substituents with lone pairs of electrons can form 71 bonds, as can be expressed in either valence bond or MO terminology. 

Fig. 1.9. Total energy as a function of distortion from planarity for methyl cation, methyl radical, and methyl anion. [Reproduced from J. Am. Chem. Soc. 98 6483 (1976) by permission of the American Chemical Society.]...

Table 1.13. Calculated Stabilization Resulting from Substituents on the Methyl Cation...

The substituent stabilization effects calculated for the methyl cation and the methyl anion refer to the gas phase, where no solvation effects are present, and therefore are substantially larger, in terms of eneigy, than would be the case in solution, where solvation contributes to stabilization and attenuates the substituent effects. 

Fig 1.34. The carbon atoms in the methyl cation (a), radical (b), and anion (c), respectively (Reproduced from Angew. Chem. Int. Ed. Engt. 33 620 (1994) by permission of Wiiey-VCH.)... 

TIricooRlinate caibocations are fiequendy called carbonium ions. The terms methyl cation, butyl cation, etc., are used to describe the c >rTesixiiulir.ji tricoordinate cations. Chemical Abstracts uses as specific names methylium, ethyUum, propylium. We will use carbocation as a generic term for trivalent carbon cations. 

Adjacent atoms with one or more lone pairs of electrons strongly stabilize a carbocation. Table 1.13 (p. 30) indicates the stabilization of flie methyl cation by such... 

In Eq. (3), the unstable methyl diazonium ion decomposes by S l reaction into nitrogen and a methyl cation w hich combines with the anion Z to give CH3—Z. In Eq. (4) an Sk2 reaction occurs. The loss of the nitrogen from CH3—here takes place only wdth the participation of the anion as nucleophile. 

The photolysis of arenediazonium salts has been widely used for intramolecular cyclizations in the synthesis of 1-phenylethylisoquinoline alkaloids by Kametani and Fukumoto (review 1972). An example is the photolysis of the diazonium ion 10.73, which resulted in the formation of O-benzylandrocymbine (10.74) (Kametani et al., 1971). The mechanism of this cyclization is obviously quite complex, since the carbon (as cation or radical ) to which the diazonio group is attached in 10.73 does not react with the aromatic CH group, but with the tertiary carbon (dot in 10.73), forming a quinone-like ring (10.74). In our opinion the methyl cation released is likely to react with the counter-ion X- or the solvent. 

The formation of a further single bond between sulfur and carbon, as in the trimethylsulfonium cation, may be pictured as involving a 3sp3 unshared pair orbital on sulfur and an empty 2sp3 orbital on carbon in a methyl cation. Thus the three a bonds and the remaining unshared pair (in a 3sp3 orbital) in a trialkylsulfonium ion are distributed approximately tetrahedrally, i.e. the ion is pyramidal, with the sulfur atom at the apex (2). 

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