Electron-withdrawing power effect

Figure 5.1 Electron-withdrawing power effect of R group in RCOCl on the log ortho-C/O reactivity ratio.

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

This effect is observable within a series of very similar electrophiles. Zollinger27 found that in reactions of diazonium ions substituted with 4-C1, 3-C1, and 3-N02 substituents (i.e. the reactivity and electron-withdrawing power of the ion increased along the series) the respective kinetic isotope effects were 6.55, 5.48 and 4.78. 

Results for these CEBEs are presented in Table 1. As can be seen, for the carvone variants I-V the various substitutions have absolutely no effect at the carbonyl C=0 core, and are barely significant at the chiral center that lies between the carbonyl and substituent groups in these molecules. Only upon fluorine substitution at the tail (molecule VI) does the C=0 CEBE shift by one-half of an electronvolt the second F atom substitution adjacent to the C=0 in the difluoro derivative, VII contributes a further 0.6-eV shift. This effect can be rationalized due to the electron-withdrawing power of an F atom. Paradoxically, it is these fluorine-substituted derivatives, VI, VII, that arguably produce b curves most similar to the original carvone conformer, I, yet they are the only ones to produce a perturbation of the ground-state electron density at the C li core. This contributes further evidence to suggest that, at least for the C li... 

A group with a more powerful (electron-withdrawing) inductive effect, e.g. NOa, is found to have rather more influence. Electron-withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised it orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro-anilines are found to have related p a values ... 

It has been shown that it is favorable for surface carbon to go into the first subsurface layer of cobalt.71 Diffusion to octahedral sites of the first subsurface layer is thermodynamically preferred by 50 to 120 kJ/mol and the corresponding activation energy is low. Theoretical calculations on the conversion of surface carbidic to subsurface carbon on Co (0001) found that the electron withdrawing power, and therefore the poisoning effect on potential CO adsorption, is maximal for subsurface carbon.41 Metal dx/ orbitals are less likely to accept electrons from the CO 5a orbital, and thus metal-CO bonding will weaken. The dxz orbital will in turn be less able to back-donate into the CO 2n orbital, resulting in additional... 

As previously mentioned above, the chiral recognition abilities of the phenyl-carbamates of polysaccharides are greatly influenced by the substituents on the phenyl groups. In order to evaluate the effect of the substituents on the interaction between CSPs and solutes, the retention times of acetone and the first-eluted isomer of l-(9-anthryl)-2,2,2-trifluoroethanol (39) on 3- and 4-substituted CSPs are plotted against the Hammett parameter a of the substituents (Figure 3.33).130 The retention times of acetone tend to increase as the electron-withdrawing power of the substituents increases, whereas those of the first-eluted isomer of 39 tend to decrease. These results indicate that... 

A quantity called the radical stabilization energy (RSE) may be defined to relate the stabilities of substituted carbon radicals to the methyl radical. The effects of adjacent X , Z, and C substituents on the RSEs of carbon-centered radicals has been widely investigated [142,143]. The expectations based on simple orbital interaction theory as espoused above are widely supported by the experimental findings, except that when the the n donor or n acceptor ability of the group is weak and the inductive electron-withdrawing power is large, as in F3C and (Me N+CHj, the net effect is to destabilize the radical relative to the methyl radical [143]. The BDE of a C—H bond of a compound R—H is another measure of stability of the product radical, R. It is related to the RSE by... 

---