Methyl cations methane

Fisher, J.J. Koyanagi, G.K. McMahon, T.B. The C2H7 Potential Eneigy Surface a Fourier Transform Ion Cyclotron Resonance Investigation of the Reaction of Methyl Cation with Methane. Int. J. Mass Spectrom. 2000, 795/796,491-505. 

Many mechanistic implications have been discussed, but we will concentrate here only on the most important structures in the context of dihydrogen-cation complexes. Deuterium-labeled methane and methyl cations were employed to examine the scrambling and dissociation mechanisms. The protonated ethane decomposition yields the ethyl cation and dihydrogen. Under the assumption that the extra proton is associated with one carbon only, a kinetic model was devised to explain the experimental findings, such as H/D scrambling. ... 

SDTQ)/6-311(dp)//MP2(fu)/6-31G(d) + ZPVE, corrected to 298 K], probably arising through a 1,1-elimination from 19, agrees very well with experiment (10.5 2 and 13.0 kcal moP Whereas the reaction of methane with the methyl cation... 

Even methane, the least reactive alkane, was shown to undergo carboxylation under superacidic conditions.115,176 The formation of carboxylated products (acetic acid and methyl acetate) from methane was first observed by Hogeveen and coworkers by trapping methyl cation formed in SbF5 (60°C, 50 atm CO pressure) followed by quenching with H20 or MeOH. The intermediate methylcarboxonium ion (CH3CO+) and CH3CH2CO+ formed in a similar reaction of ethane were identified by NMR spectroscopy.176,177... 

Methane, with its relatively nonpolar bonds, is inert to almost all reagents that could remove hydrogen as H or H e except under anything but extreme conditions. As would be expected, methyl cations CH3 and methyl anions CH3 e are very difficult to generate and are extremely reactive. For this reason, the following reactions are not observed ... 

It is not necessary to assume a complete cleavage of methonium ion [CH5]+38 to a free, energetically unfavorable methyl cation. The carbon-carbon bond formation can indeed be visualized as the C—H bond of methane reacting with the developing methyl cation [Eq. (5.68)]. 

Mechanistically, two pathways are logical (Scheme 3). The ethyl cation can directly alkylate methane via a pentacoordinated carbonium ion (Olah) (path a), or alternatively, although a less favorable pothway (b), the ethyl cation could abstract a hydride ion from methane. The methyl cation thus formed, which is less stable by 39 kcal/mole (26), could then react directly with ethylene. In the latter case, propylene and/or polymeric material would probably be formed since the hydrogen required for a catalytic reaction has been consumed by the formation of ethane. 

The first oxidative carbonylation reactions (cf. Section 2.1.2.5) with methane used superacid catalysts to perform the carbonylation in a Koch-type reaction which involved protolytic oxidation of methane to the methyl cation (eq. (29) [137]) ... 

H-D exchange in HF-SbFs via hypercoordinate isotopic methonium ions [Eq. (6.5)] without any detectable side reactions (see Chapter 5, Section 5.4.1.1). Exchange involving protolytic ionization via CH + HD is improbable in the case of methane, because of the unfavorable, highly energetic nature of the primary methyl cation. However, in higher homologous alkanes protolytic ionization takes place with ease. 

Pertritiated methane (produced by the reaction of aluminum carbide with T2O) yields tritiated methyl cations (symbolized as CTj"), whose gas phase chemistry has been extensively studied by Nefedov s group in Russia. One of their first major contributions was to provide evidence for methyl migration around protonated benzene rings, as summarized in Table In mixtures of xylenes (at their vapor pressure) and pertritiated methane,... 

The gas-phase reactions of PH/ (x = 0,1,2) ions with methane have been studied by a number of groups " These ions are reactive and undergo a series of insertion reactions to produce ions containing phosphorus carbon bonds (equation 67-69). The reaction between the methyl cation and phosphine also produces ions containing phosphorus-carbon bonds (equation 70) ". ... 

In 1993 Bergman discovered that an iridium (iii) methyl cation was capable of undergoing an exchange of the methyl group with other alkanes in a process that looked similar to the electrophilic activation of alkanes by Shilov s Pt(n) complex (Equation (17)). Theoretical treatment of this system provided evidence that the actual pathway involved oxidative addition of the alkane to give an Ir(v) dialkylhydride that then underwent reductive elimination of methane. ... 

Since methoxymethyl cation is not formed in the t-butyl chemical ionization of methoxymethyl formate and acetate, it is necessary to use a stronger gaseous acid to investigate the relative ease of formation of methoxymethyl cation from these two compounds. Consequently, methane chemical ionization spectra of the compounds have been obtained at several temperatures. We need not give the spectra, for it suffices to say that copious amounts of methoxymethyl methyl cation are formed from both esters by means of the reaction... 

Note that we have never invoked the exceedingly unstable methyl cation (CH3+), which is known only to occur in the gas phase. Once the ethyl cation is produced, it can react with additional methane to ultimately produce the relatively stable t-butyl cation. 

The geometries for acetylene, methyl cation, and methane correspond to the bond angles for the different hybridization states sp, sp, and sp respectively. Again, most organic molecules display measurable deviations from these ideals, but we still loosely refer to the atoms as sp, sp, or sp hybridized, even though we don t expect angles of exactly 180°, 120° or 109.5°. 

We call CH3 a methyl cation. To make this molecule, one could imagine removing a hydride ( H) from methane to produce CH3. It is the simplest example of a carbocation, a molecule containing a positively charged carbon (Fig. 2.13a). 

Methane can be substituted in many ways through replacement of one or more hydrogens with another atom or groups of atoms. In principle, removal of a hydrogen from methane can lead to the methyl anion ( CH3), the methyl radical ( CH3), or the methyl cation ( CH3) depending on the nature of the hydrogen removed ( "H, H or H). In this chapter, we have discussed only the shapes of these intermediates—reactions are coming later. It will be important to remember that carbocations are flat and s/> hybridized and that simple carbanions are pyramidal and approximately s/> hybridized. 

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