Methyl cation, shape

The most effective single criterion of whether free base or conjugate acid nitrates is the comparison of a model compound in which the possibilities of prototropic and tautomeric equilibria have been eliminated. For example, the model compound for a substitued pyridinium cation is its N-methyl cation. If the nitration of the pyridine proceeds via the conjugate acid, the rate profile will have the same shape as for the N-methyl model compound. The individual rates of nitration for the model compound may or may not be exactly the same as for the pyridine itself, depending upon the effect of the methyl group upon the reactivity. If, on the other hand, nitration of the pyridine takes place on the free base, then the N-methyl cation will not be nitrated under the same conditions. 

Problem 2.3 Besides free radicals, we shall encounter two other kinds of reactive particles, carbonium ions (positive charge on carbon) and carbanions (negative charge on carbon). Suggest an electronic configuration, and from this predict the shape, of the methyl cation, CHj of the methyl anion, CH3 . 

The simplest, commonly encountered, charged hydrocarbon species is the methyl cation, CH3+. Draw a dot and cross structure of this ion, and suggest a shape for it. 

Earlier we mentioned the shape of the methyl cation suggest the shape of the /-butyl cation, (CH3)... 

We discussed the planar shape of the methyl cation in Chapter 4 (p. 103), and the terf-butyl cation is similar in structure the electron-deficient central carbon atom has only six electrons, which it uses to form three a bonds, and therefore also carries an empty p orbital. Any carbocation will have a planar carbon atom with an empty p orbital. Think of it this way only filled orbitals contribute to the energy of a molecule, so if you have to have an unfilled orbital (which a carbocation always does) it is best to make that unfilled orbital as high in energy as possible to keep the filled orbitals low in energy, p orbitals are higher in energy than s orbitals (or hybrid sp, sp2, or sp3 orbitals for that matter) so the carbocation always keeps the p orbital empty. 

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. 

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

Goto et al. (2004) measured the reaction kinetics of one-electron oxidation of A -methyl-p-anisidine in AN. In the electrode process, oxidation was performed at the platinum disk-shaped anode, in the chemical process, by means of the tris(p-bromophenyl)amine cation-radical. In both the cases, after one-electron oxidation, dimerization took place leading to the formation of the dye variamine blue. According to the kinetic data, the mechanism of this dye formation is different in the electrode and chemical processes (see Scheme 2.34). Namely, in the electrode oxidation, the cation-radical appears to be surrounded by a huge amount of the initial (nonoxidized) A-methyl-p-anisidine... 

Rates of detritiation of [2- H]-benzimidazole (10) and the corresponding 1-methyl compound have been measured over a pH range at 85° and the bell-shaped pH-rate profile (Fig. 1) accounted for in terms of rate-determining attack by OH on the benzimidazole cation with the formation of an ylide intermediate. Because of the ionization of the N-H group at high pH, the development of negative charge a to the... 

The reaction between A-chlorobenzotriazole and l-methyl-2-phenylindole involves formation of the indole radical cation and benzotriazole radical via an initial electron transfer <82JOC4895, 91JCS(P2)1779>. Chemical reactions of benzotriazole on a freshly etched surface of metallic copper are studied by surface-enhanced Raman scattering, x-ray photoelectron spectroscopy, and cyclic voltammetry. The surface product is (benzotriazolato)copper(-l-), which covers the surface in the shape of polymeric material and shows good anticorrosion effects for copper <91JPC7380>. 

If NaCl is replaced by the butyl-3-methyl imidazolium (BMIM) chloride IL, a 30% decrease in retention factor associated with a remarkable peak shape improvement is observed. In this case, the IL cation adsorbs on the C18 stationary phase more than Cl", thereby preventing detrimental attractive silanophilic interaction of the cationic additive. Charge-charge repulsion occurs, the retention factor is lower, and the peak shape is better. The analyte cation is largely retained by hydrophobic fast interactions. When BMIM BF4 IL replaces NaCl, both the cation and anion of the IL adsorb on the C18 surface and all the interactions cited above take place simultaneously and contradict each other. Global retention depends on the extent to which one interaction is stronger than the other [124],... 

Saunders and Rosenfeld (1969) extended their H-nmr investigation to temperatures above 100°C and discovered another, slower process which exchanges the two methylene protons with the nine methyl protons, resulting in coalescence of these bands above 130°C. The band shape analysis gave an activation energy of 18.8 1 kcal mol" for this new process. Since any mechanism involving primary alkyl cations is expected to have a barrier of ca 30 kcal mol" (the enthalpy difference between tertiary and primary carbo-cations), the formation of a methyl-bridged (corner protonated cyclopropane)... 

When pubhshed reports of the diffusivity of paraffins in ZSM-5 catalysts obtained from uptake rate measurements appeared grossly inconsistent with catalytic behavior. Werner participated in resolving the problem by determining diffusivities from catalytic behavior of catalysts of very different particle sizes. The analysis not only confirmed the many orders of magnitude higher true diffusivities but also allowed Werner to extend the technique to demonstrate that shape selectivity could occur due to lack of fit of a reactant (e.g., diffusion of a dimethyl paraffin) in the structure or lack of fit of a reaction complex (transition state) that must be created on the active site (e.g., the methyl paraffin/propyl cation complex). 

The rate of initial electron transfer from A,7V-dimethylaniline to [Fe(phen)3] + is diffusion-limited. This is followed by the rate-determining proton transfer from the radical cation to pyridine to give the deprotonated a-amino radical which is rapidly oxidized by a second equivalent of [Fe(phen)3] + to yield the product iminium ion. Kinetic isotope effects [kii/kjf) for the proton transfer were determined from the J3/tfo ratios of the products derived from p-substituted A-methyl-A-trideuteromethylanilines. The k /kx) value first increases and then decreases with increasing pAa of p-substituted A,A-dimethylaniline. Such a bell-shaped isotope effect profile is typical of proton-transfer reactions [82, 85]. The maximum kn/fco value is determined as 8.8 which is much larger than the corresponding value for the demethylation of the same substrate by cytochrome P-450 (2.6) [79]. 

The spectrum of the dvuenesemiquinone radical-cation exhibits a marked alternating line-width effect which is temperature-dependent it is ascribed to cis-trans isomerism of (68) and (69), the life-time of each isomer being comparable with the inverse frequency separation between the methyl proton-splittings (Bolton and Carrington, 1962). Carrington (1962) has discussed the theoretical line-shapes to be expected for different rates of isomerization and has estimated that the life-times of the isomers at room temperature are about 10 sec. 

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