In the search for a quantitative correlation between reactivity and electronic configuration of aromatic hydrocarbons, the electron density, at each carbon atom, of the highest occupied π-orbital in the ground state of the molecule is calculated by means of the LCAO method. Comparing the result of such a calculation on fifteen condensed aromatic hydrocarbons with their chemical reactivities, we find that the position at which the electron density is largest is most readily attacked by electrophilic or oxidizing reagents. It is, therefore, concluded that distinct from other π-electrons the pair of π-electrons occupying the highest orbital, which is referred to as frontier electrons, plays a decisive role in chemical activation of these hydrocarbon molecules. The theoretical significance of this discrimination of the frontier electrons in relation to the chemical activation is discussed.

provided comparisons are made only between closely related compounds. This observation suggests that the central atom is strongly involved in the transition concerned. Although the data presented here are insufficient to make a decision on this point, it is not unlikely that the end absorption corresponds to a dissociation process, as Milazzo has proposed. 7 EXPERIMENTAL All compounds were either obtained from commercial sources or prepared in this laboratory by known THE JOURNAL OF CHEMICAL PHYSICS methods. Commercial samples were purified by recrystallization or distillation. Unless otherwise indicated, the spectra were determined in solution in commercial 95 percent ethanol which was transparent above 210 m,u. A Beckman DU spectrophotometer and 1 em quartz cells were used throughout. The concentrations of the solutions were so chosen as to obtain optical densities between 0.3 and 0.7 at the absorption maxima, and no optical density data below 0.15 were used.

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The kinetics of the reactions of trimethylamine, dimethylamine, methylamine, and ammonia with methyl iodide in aqueous solutions have been investigated at temperatures ranging from 0 to 40°C. Kinetic analyses of the consecutive methylations of dimethylamine and methylamine have been conducted according to the method of French. In the case of ammonia a standard second-order kinetic treatment can be successfully used because of the relative sluggishness of the first stage as compared with the succeeding stages. The observed rates are in the sequence: Me3N>Me2NH>MeNH2>NH3. The factors which affect this rate sequence are discussed.

Rates in the SN2 and E2 reactions of a series of β-substituted ethyl chloride (RCH2CH2Cl, R=Et, MeO, PhO, Gl, and H) in aqueous solutions have been measured at temperatures ranging from 90 to 150°C, using sodium acetate as a nucleophile. The mild rate-retarding effect of β-subtituents in the SN2 reactions, observed in the rate order, H>Et>MeO>Cl>PhO, may be attributed to the steric effects. In a similar fashion, the retardation of E2 reaction rates by the β-substituents, which is in the rate sequence H>MeO>Et>PhO>Cl, may be ascribed to the steric effects, which outweigh the electronic effects. The β-substituent effects of several other substituents, which are known in the literature, have also been discussed.