Methyl anion methane

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

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 ... 

An estimate of the standard enthalpy change for the formation of the methyl anion and a hydrogen cation from methane may be obtained by a calculation based on Hess s Law, as shown in Table 1. 

As shown in Appendix 1, the pAia value for methane is approximately 50-75, and the pAia value for acetonitrile is approximately 25. Thus, the methyl anion is more reactive than the acetonitrile anion and is therefore the better nucleophile. 

As an illustration, the results of the application of the ESYCAD program to the oxidative coupling of methane are explained. For this reaction, methane may by activated at strong basic sites of the catalyst by hcterolytic chemisorption, resulting in methyl anions which may be oxidized to radicals. In the selective reaction, these radicals dimerize to ethane as the primary product. Acid sites or /j-conductivity should be avoided because they lead to total oxidation. Under reaction conditions the catalyst should be stable, i.e. not be oxidized or reduced or volatize, which can be checked by thermodynamics. 

The thermal decomposition of a variety of tetramethylammonium salts of weak bases has been studied and the major product is generally the methylated anion 92>. The temperature required to decompose the bromide and chloride salts to trimethylamine and the methyl halide is near 360 °C and the products recombine on cooling. The fluoride salt decomposes at a much lower temperature (180 °C) and the products do not recombine 92>. Decomposition of the borohydride salt at 225 °C gives trimethylamine borane and methane 11h... 

A trispyrazolylborate platinum complex was found to activate C-H bonds via a Pt(ii)/Pt(iv) sequence. Removal of a methyl anion from [(77 -Tp )RhMe2] leads to an unsaturated species that then adds to the hydrocarbon C-H bond (Equation (12)). The related protonatcd species Tp PtMc2H loses methane at 110°G and then oxidatively adds benzene- 6. The new methyl-phenyl-deuteride derivative then reversibly exchanges deuterium into the methyl group, indicating reversible methane activation. In addition, the Pt-H bond can undergo reaction with oxygen to... 

To explain the high activity of cerium dioxide for SOF oxidation, recall that SOF consists mainly of high molecular weight paraffins. There is consensus that the important step in alkane oxidation is cleavage of a C-H bond. On oxide catalysts, most authors favor a heterolytic mechanism, with, for example, methane splitting to yield a proton and a methyl anion. An important observation in support of this comes from a comparison of methane and fluoromethane. The fluorine atom would be expected to stabilize a transition state of the form FH2C and one finds that fluoromethane is an order of... 

Fig. 2.7 Basicities of some molecules, calculated by the CBS-4 method. The proton affinities refer to the acidity of the conjugate acids. Methane is extremely weak as a base and the methyl anion is extremely strong...

Fig. 6.4 Basicities of some compounds, calculated by the CBS-4 M method. These are 0 K enthalpy differenees between the hydrocarbon and its conjugate base. CBS-4 calculations (CBS-4M in Gaussian 03). Methane is of extremely low basicity, the methyl anion is of extremely high basicity...

Fig. 12.20 Calculated (B3LYP/6-31G ) basicities (as OK gas-phase proton afiSnities) of some molecules. For reference points, methane is an extremely weak base and the methyl anion is extremely strong, while ammonia is moderately strong. Cf. the proton alEnities in Chapter 2, Fig. 2.7...

In principle, one might try to study the ionic dissociation of an acid (equation 7.3) directly in the gas phase, but AH for dissociation of a neutral species to a proton and an anion is usually quite large without solvent stabilization of the ions. For example, the AH for the gas phase dissociation of methane to methyl anion and a proton (AH° jj) was calculated to be - -417kcal/mol. This is much greater than the homolytic C—H bond dissociation energy of methane (-I-104 kcal/mol), so thermolysis of methane in the gas phase leads to radicals instead of ions. The pKg value of an acid can be determined indirectly, however, by measuring the equilibrium for proton transfer from the acid to a base with a known pKg. With a series of measurements, a scale of gas phase acidity values can be established by referencing one compound to another. 

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. 

It is generally most difficult for even a strong base (B ) to remove a proton from a hydrocarbon to give an anion. In other words, hydrocarbons are generally very weak acids. We have already had a brief discussion of the methyl anion in Chapter 2 (p. 62), but methane, the parent of this anion, is an extraordinarily weak acid (Fig. 3.63)."... 

If one would assume that heterolytic splitting of the weakly acidic methane occurs on the highly basic surface, Zn may act as an electron acceptor from the methyl anion forming Zn + as a transitional state. Neutral methyl species could then easily escape as radicals from the surface to the gas phase where they could recombine to ethane. Adsorbed oxygen could easily be reduced by Zn + hereby its transformation into lattice oxygen would be facilitated. This phenomenon might also contribute to a rapid removal of any adsorbed oxygen species from the surface which are assumed to be responsible for non-selective oxidation. 

During his frequent trips to the Computer Center, Pete Owens discovered computer graphics and, in particular, the ability to display perspective and contour diagrams of three-dimensional functions with the CalComp plotter. We recognized how useful this facility would be in representing orbitals and electron density functions and published some applications to the methane molecule and methyl anion. We subsequently published a book containing many such computer plots to help undergraduates understand orbitals. 

The reaction of [Re(Me)(NO)(PPh3)Cp] with trifluoromethanesulfonic acid (triflic acid) in dichloromethane at 0 °C results in cleavage of the Re-Me bond and formation of [Re(0S02CF3)(N0)(PPh3)Cp] by formal ion exchange of methyl anion for the triflate anion. 3,64 Methane was not detected in this reaction. However, in other protonation reactions of rhenium alkyls, [Re(R)(NO)(PPh3)Cp], formation of alkane, RH, has been confirmed.63... 

The strategy described above is applicable to fluorotris(trimethylsilyl)methane (157) it reacts with two molecules of benzaldehyde to give 2-fluoroallyl alcohol 158 (Scheme 5.40) [57]. The reaction involves five events in a single operation generation of a naked methyl anion, aldehyde addition, Peterson elimination, generation of a naked sp anion, and the second aldehyde addition. 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Considerable deuteriation occurs at the C-methyl group when the salt (107) is treated with methan[2H]ol-sodium methoxide, indicating that the resonance-stabilized anion (108) is formed as well as the ylide (109),... 

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