Electrophilic Substitution in Methyl-substituted Naphthalenes III. Correlation between Experimental Results and Molecular Orbital Calculations of Reactivity Indices

Abstract

Common reactivity indices (electron density qr,self-polarizability πrr, frontier electron density fr, superdelocalizability Sr, and localization energy Lr) are calculated for electrophilic substitution in 25 methyl-naphthalenes. An elementary s.c.f. method in the form of a modified ω-technique is used, using the hyperconjugative-heteroatom model for the methyl groups, with ω = 1.4, hx = 2.0, kc–x = 0.8. This choice gives reasonably good ionization potentials and very good correlation for singlet transitions (p-band) in u.v. spectra of α-methylnaphthalenes. Purely static indices qr, fr, and πrr are found to be unsuitable for predicting reactive positions for chloromethylation, while Sr and Lr are very satisfactory. On the theory that the polarizing effect of the approaching reagent is important, the index qr′ = qr + πrr δαr may be obtained, which is also found to be very satisfactory for δαr = β. If the interaction is viewed as an interaction between a hard acid (chloromethyl) and soft base (methylnaphthalenes), the index ΔEr = aqr + bfr is obtained, which is likewise found to be satisfactory with a = 1, b = 0.15. These results show clearly that it is insufficient to base reactivity considerations in methylnaphthalenes entirely on the properties of the isolated substrate molecule, but that even a very simple description of the substrate–reagent interaction is sufficient since the four indices Sr, Lr, qr′ and ΔEr all have the same predictive value.