Carbocations methyl cation

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. 

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)... 

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... 

Table 14 contains the reaction enthalpies relative to the methyl cation from MINDO/3 calculations. The gas phase values demonstrate the following graduation of stability of the carbocations ... 

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

The pKR+ value for the parent tri(l-azulenyl)methyl cation (2a+) is 11.3. Hydrocarbon-based carbocations, which comprise only of carbon and hydrogen, are generally very reactive species. Some extremely stable hydrocarbon carbocations, which exist even under basic conditions, were reported in the literature (5). However, most of these examples are cyclic cations, such as cyclopropenylium or tropylium ions (Figure 8). The tropylium ion 8+ annelated to three bicyclo[2.2.2]octane units is one of the most stable hydrocarbon-based carbocation ever reported (9). 

Tripheny(methyl cation (10 ) is effectively stabilized by electron donating substituents. B. W. Laursen et al. reported the highly stable carbocation 11+ with a pKr+ value of 19.7 (10). Recently, they reported a similar carbocation with a much higher p R+ value (23.7) (11). In our continuing efforts to prepare extremely stable carbocations, we have investigated the effect of introduction of electron donating substituents into each azulenyl group. 

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

Alkyl groups are able to decrease the concentration of positive charge on the carbocation by donating electrons inductively, thus increasing the stability of the carbocation. The greater the number of alkyl groups bonded to the positively charged carbon, the more stable is the carbocation. Therefore, a 3° carbocation is more stable than a 2° carbocation, and a 2° carbocation is more stable than a 1° carbocation, which in turn is more stable than a methyl cation. 

Other mechanisms with the involvement of an incipient methyl cation were also proposed.418,460,462 An attack of methyl cation released from extensively polarized methoxy groups on the carbon-hydrogen bond forms pentacoordinated carbocation intermediates that, in turn, yields ethyl methyl ether after the loss of a proton ... 

It will be obvious that replacement of hydrogen atoms in one or other of the phenyl rings of triphenyl methyl cation (Ph3C+) by electron releasing conjugating substituents such as RO-, R2N-, RS-, aryl, etc. will increase still further the carbocation stability. Hetero atom effects (e.g. from O, N, S) are even more manifest in carbocations having such substituents attached directly to the electron deficient site as in... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

The kinetics of the reaction of various silyl hydrides with tris(2,6-dimethoxyphenyl)methyl cation and some other stable carbocations in acetic acid have been systematically investigated by Carey and Wang-Hsu (92). The results were interpreted in terms of a four-center transition state involving a trigonal bipyramid at silicon with nucleophilic participation of the solvent. 

A carbocation (also called a carbonium ion or a carbenium ion) is a species that contains a carbon atom bearing a positive charge. The positively charged carbon atom is bonded to three other atoms, and it has no nonbonding electrons, so it has only six electrons in its valence shell. It is sp2 hybridized, with a planar structure and bond angles of about 120°. For example, the methyl cation (+CH3) is planar, with bond angles of exactly 120°. The unhybridized p orbital is vacant and lies perpendicular to the plane of the C—H bonds (Figure 4-13). The structure of +CH3 is similar to the structure of BH3, discussed in Chapter 2. 

Borane is isoelectronic with the methyl cation, CH3. All the arguments we have just applied to borane also apply to Me+ so it too is sp2 hybridized with a vacant p orbital. This will be very important when we discuss the reactions of carbocations in Chapter 17. 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

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. 

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