Electrons donated, number

Fig. 22. Alkyne carbon l3C chemical shift (ppm) versus formal alkyne electron donation number, N.

When hydroxypteridines are considered, it must be borne in mind that these compounds exist principally in the pteridinone forms, containing thermodynamically stable amide functions, and consequently have low reactivity. Their stability towards acid and alkali correlates well with the number of electron-donating groups which apparently redress the deficit of ir-electrons located at the ring nitrogen atoms. Quantitative correlations can be seen in the decomposition studies of various pteridinones (Table 7). These results are consistent with the number of the oxy functions and their site at the pteridine nucleus. The... 

Electron donation from pyrrole-like nitrogen, or to a lesser extent from analogous sulft or oxygen atoms, helps electrophilic attack at azole carbon atoms, but as the number c heteroatoms in the ring increases, the tendency toward electrophilic attack at both C an N decreases rapidly. 

A number of Lewis acids have been utilized in the Pomeranz-Fritsch reaction, including polyphosphoric acid and boron trifluoride-trifluoroacetic anhydride. Under the latter conditions yields were best when electron-donating groups were present in the 3- or 3, 4- position of imine 20, whereas unactivated aldehydes failed to cyclise at all. ... 

Toluene (methylbenzene) is similar to benzene as a mononuclear aromatic, but it is more active due to presence of tbe electron-donating metbyl group. However, toluene is much less useful than benzene because it produces more polysubstituted products. Most of tbe toluene extracted for cbemical use is converted to benzene via dealkylation or disproportionation. Tbe rest is used to produce a limited number of petro-cbemicals. Tbe main reactions related to tbe cbemical use of toluene (other than conversion to benzene) are the oxidation of the methyl substituent and the hydrogenation of the phenyl group. Electrophilic substitution is limited to the nitration of toluene for producing mono-nitrotoluene and dinitrotoluenes. These compounds are important synthetic intermediates. 

The Fricdel-Crafts type polyalkylation of alkyl-substituted benzenes with 3 becomes easier and faster as the number of electron-donating methyl groups on the phenyl group increases. This is consistent with the fact that the alkylation occurs in the fashion of electrophilic substitution. The tendency of starting incthylben-zenes to form reoriented products also increases in the same order from toluene to mesitylene. 

A very important criterion for electron structure is the percent d-character, which characterizes the number of unpaired electrons in the rf-orbitals of the individual metal atom. Because of the vacancies existing in these orbitals, metals will interact with electron-donating species forming electron pairs. It is this interaction that determines the special features of adsorption of these species and, as a consequence, the catalytic activity of a given metal. 

Mo6 octahedron) the cluster is electron-precise, the valence band is fully occupied and the compounds are semiconductors, as, for example, (Mo4Ru2)Se8 (it has two Mo atoms substituted by Ru atoms in the cluster). In PbMo6Sg there are only 22 electrons per cluster the electron holes facilitate a better electrical conductivity below 14 K it becomes a superconductor. By incorporating other elements in the cluster and by the choice of the electron-donating element A, the number of electrons in the cluster can be varied within certain limits (19 to 24 electrons for the octahedral skeleton). With the lower electron numbers the weakened cluster bonds show up in trigonally elongated octahedra. 

Abramovitch, Roy, and Uma 51> disagreed with this, pointing out a number of inconsistencies with that conclusion. Thus, while the total rate ratios are not much different from unity, as expected for a homolytic substitution, the values of °h A = 1.0, °HeA = 0.96, and °hA = 0.80 do not support this mechanism since such electron-donating substituents should facilitate attack by an electrophilic free radical 59-60> and lead to total rate ratios greater than unity. Also, the partial rate factor calculated for attack at the meta position of toluene was unusually low, and it is not clear why this position should be deactivated towards attack either by a free radical or by an electrophilic species. 

On this basis an electron-donating substituent, Y, should be somewhat better at promoting attack by H p-(57a, R = H), rather than o-(51b, R = H), to Y because of the slightly more effective delocalisation of +ve charge that thereby results. The figures quoted in (57) would point to an expected value for the log partial rate ratio, log /o-/log fp.y of =087, and values very close to this have indeed been observed for protonation of a number of different C6H5Y species. 

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