No abstract

There are still some secrets left to this well-studied reaction. Previously unreported relationships discovered are as follows. The ordering of reactivities of C6H5X is the same as that of enthalpies of hydrogenation of the ring to the correspondingly substituted cyclohexane. The orientation of substitution (meta or ortho/para) is controlled by the dipole direction of the ipso-C-X bond, like an ON/OFF switch. The difference between the halogens and other deactivating groups is that the bond between the atom bonded to the ipso carbon has the positive end of the dipole on the ipso carbon for the halogens (C(δ+)-X(δ-)) but in the opposite direction (C(δ-)-X(δ+)) for other deactivating groups. This reverses the directing effect. For all X, including the halogens, ipso-C(δ+)-X(δ-) results in ortho/para substitution. p-(13)C NMR shifts of C6H5X greater than that of benzene predict meta substitution. A linear relationship exists between p-(13)C NMR shift and ΔHhyd, except for X = halogen. With halobenzenes, the ortho/para ratios of the products are linearly related to the ipso/ortho ratios of the (13)C shifts of C6H5X for chlorinations, brominations, nitrations, and protonations. The relative reactivities of the halobenzenes are linearly related to the p-(13)C NMR shifts. The electronegativities of X are linearly related to the (13)C NMR shifts of the ipso carbon.

The concept of establishing relative stabilities of alkenes by their enthalpies of hydrogenation is extended to the enthalpies of hydrogenation of the ring of substituted benzenes, C6H5X. Enthalpies of hydrogenation of the rings predict the ordering of their reactivities. p-13C NMR chemical shifts predict the directing effect of X, meta or ortho/para, as does the direction of the dipole of the C–X bond. Inclusion of these facts in organic chemistry curricula integrates and reinforces concepts known prior to reaching the topic on electrophilic aromatic substitutions.

No abstract

Although electrophilic addition to alkenes has been well studied, some secrets still remain. Halogenations, hydrohalogenations, halohydrin formations, hydrations, epoxidations, other oxidations, carbene additions, and ozonolyses are investigated to elucidate the relation of alkene reactivities with their enthalpies of hydrogenation (ΔHhyd ). For addition of electrophiles to unconjugated hydrocarbon alkenes, ln(k) is a linear function of ΔHhyd , where k is the rate constant. Linear correlation coefficients are about 0.98 or greater. None of the many previously proposed correlations of ln(k) with the properties of alkenes or with linear free-energy relationships match the generality and accuracy of the simple linear relationship found herein. A notable exception is acid-catalyzed hydration in water or in solvents stabilizing relatively stable carbocation intermediates (e.g., tertiary, benzylic, or allylic). (13) C NMR chemical shifts of the two alkene carbons also predict regioselectivity. These effects have not been noted previously and are operative in general, including addition to heteroatom-substituted alkenes.