Ethyl stabilization energies

Unfortunately, good calculational as well as thermochemical data are not often published for isopropyl derivatives. They are more commonly available for methyl and ethyl derivatives. Thus, equations 7 and 8 define methyl and ethyl stabilization energies respectively for cyclopropanes... 

Some stabilization energies are collected in Table 3. The presentation provides an easy overview of the stabilization in singly and doubly substituted methyl radicals. Values in parentheses for doubly substituted radicals represent the sum of the stabilization energies derived from mono-substituted radicals. By comparing the values calculated directly for the doubly substituted radical, information is obtained on antagonistic, additive or synergetic substituent effects. Apart from methyl and ethyl radicals, all other radicals are stabilized. Some points merit comment. 

The gain in stabilization by substituents is compared for radicals and cations in Table 6.8. For radicals the change from methyl to ethyl or even tertiary butyl is not linked to a large gain in stabilization energy. In cations, however, alkyl and alkoxyl substituents have a dramatic effect. Thus, the stabilizing effect of substituents on radical cations is mainly due to the stabilization of the cation and only to a small extent to that of the radical. 

In Table 1 we list calculated (4-3IG) and experimental stabilization energies for a variety of monosubstituted cyclopropanes. Some interesting trends occur. First, there is a good correspondence between experimental and calcualted stabilization energies. One notable exception is the 6.1 kcal mol discrepancy for vinylcyclopropane and here we feel that the experiment should be redone. The Pierson linear regression coefficient (r) between experimental stabilization enthalpies and calculated stabilization energies is 0.96 (n = 6, standard error = 0.81 kcal mol" ) for the methyl comparison, 0.99 (n = 6, standard error = 0.34 kcal mol" ) for the ethyl comparison and 0.91 (n = 4, standard error = 0.98 kcal mol" ) for the isopropyl comparison. ... 

Stabilization energy model X Methyl Ethyl Isopropyl ... 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

Neurock and coworkers [M. Neurock, V. Pallassana and R.A. van Santen, J. Am. Chem. Soc. 122 (2000) 1150] performed density functional calculations for this reaction scheme up to the formation of the ethyl fragment, for a palladium(lll) surface. Figure 6.38(a) shows the potential energy diagram, starting from point at which H atoms are already at the surface. As the diagram shows, ethylene adsorbs in the Jt-bonded mode with a heat adsorption of 30 kj mol and conversion of the latter into the di-a bonded mode stabilizes the molecule by a further 32 kJ mol . ... 

The kinetics of formation of phosphonates by reaction of o-dinitrobenzene with phosphites have been examined. The energy of activation for the reaction increases as the nucleophilicity of the phosphite decreases, e.g. ethyl diphenylphosphinite 14kcalmol"S diethyl phenylphosphonite 16 kcalmol S and triethyl phosphite 21kcalmol . An intermediate of the type (61), formed by nucleophilic attack of the phosphite, was proposed. In (61) there is a particularly favourable electrostatic interaction. That p-dinitrobenzene is unreactive, is thought to stem from the fact that this compound cannot form an intermediate with such a stabilizing factor. 

Continuing to assume that Gibbs energies and enthalpies are essentially equal, the above reaction is endothermic by 14 kJ moP for R = vinyl. However, it is exothermic for the other R groups cyclopropyl, —6 ethyl, —20 propyl, —22 isobutyl, —26 neopentyl, —31 cyclobutyl, —35 and cyclopentyl, —39 kJmoP. The values are in the order expected for carbanion stability. 

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