Methyl groups stabilization

From the solubilities of aromatic hydrocarbons in aqueous silver nitrate it is clear that there, is an interaction with the silver ion to give monoargentated and, to a lesser extent, diargentated hydrocarbons. The bond to silver ion is probably of the rc-type, since the differences in the association constants for the complex with the three isomeric xylenes are small and since a single methyl group stabilizes the complex almost as much as two.279... 

Ionic polymerization may also occur with cationic initiations such as protonic acids like HF and H2SO4 or Lewis acids like BF3, AICI3, and SnC. The polymerization of isobutylene is a common example, shown in Fig. 14.5. Note that the two inductively donating methyl groups stabilize the carbocation intermediate. Chain termination, if it does occur, usually proceeds by loss of a proton to form a terminal double bond. This regenerates the catalyst. 

We pointed out" that the lowest unoccupied (LU)MO of 4 must have the same cylindrical symmetry (oi in the Csv point group) at the y carbon that it has at the ipso carbon. Since methyl groups stabilize carbocations principally through hyper-conjugative donation of electrons from 7i-like combinations of C—H bonds, and since the C H orbitals of a methyl group at the y carbon with n symmetry (e in Csv) are orthogonal to the LUMO, a methyl substituent at the y carbon of 4 should provide much less stabilization than a methyl at an a carbon. 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

It was shown that the major difference between carbenium and silylium ions in solution arises from the fact that internal stabilization of R3O ions can be much larger than for I 3Si+ ions. [6] For example, three methyl groups stabilize a carbenium ion by 72 kcal/mol (9, Table 3) because of hyperconjugative interactions. In this way, the LUMO is raised in energy and, accordingly, it is less available for a nucleophilic interaction partner. Hence, increasing internal stabilization of a carbenium ion makes specific external solvation more difficult. 

Schreiner argues that a methyl group stabilizes an sp carbon over an sp carbon, thereby lowering the energy of the reactant. This is inconsistent with the cd criteria. 

The interaction of the Jt-type orbitals of a methyl group29 (see p. 9) with an empty p orbital (Fig. 3-26) causes a lowering in energy. For this reason a methyl group stabilizes a carbonium ion, and the effect is known as hyperconjugation. (Because the two Jt-type orbitals, nz and ny, are of the same energy, the interactions in the two conformations shown in Fig. 3-26 are, to a first approxima-... 

Methyl groups stabilize carbocations whereas nitro groups, -N02, stabilize carbanions. Provide an explanation, using the idea of delocalization, why both these substituents stabilize a radical centre conjugated with them,... 

Fig. 8.7 Chemical reaction dynamics of methanol oxidation to formaldehyde as calculated from ab initio Molecular Dynamics. One of the C-H bonds of the methyl group becomes elongated (top left) and eventually breaks (top right). The adsorbed hydrox-methyl group stabilized by forming a hydrogen bonded complex to a water molecule (bottom left) and dissociates rapidly into adsorbed formaldehyde and a hydronium ion (bottom right) (Source [105] with permission)...

An explanation for this behavior may relate to the location of the methyl group on the allylic radical. The methyl group stabilizes the di-radical... 

Predicted rate (13.3f = 177 Both methyl groups stabilize the cation at the same time... 

At the temperatures lower than Tg several secondary low-scale physical transitions were registered in cellulose. For example, in [36, 41] the transition at 180—200 K is related to the methyl group stability and polarization of the primary hydroxyl in the electric fieldAs far as the transition at 273-310 K in cellulose and its derivatives are concerned, the researchers [32, 38, 42] attribute them to libration of pyranose rings around a glucoside bond or to conformational alteration of an armchair-bath type at the glycopyranose group level. 

Stabilized relative to the neutral bases (actually the former show even larger saturation effects than those discussed in section 2.3.2 for the latter). On the other hand, Staley and Beauchamp find that, relative to phosphonium ions, phosphorus cation ion radicals are stabilized by methyl groups by roughly only 35 percent as much as the phosphonium ions are stabilized relative to their neutral bases (series 6). It is also to be noted that, relative to the neutral bases, methyl groups stabilize phosphonium ions by about 1.7 times as much as ammonium ions [50]. Finally, as noted previously, methyl, ethyl, i-Pr, and t-butyl groups stabilize ammonium ions relative to free base by successive increments of 2.7,2.3, and 2.0 kcal mol There is, however, very little difference between these substituents in their stabilizing effects on nitrogen cation radicals values of... 

Since this type of delocalization is hindered by poor orbital overlap of phosphorus, methyl groups stabilize phosphorus cation radicals... 

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