Electrophilic Aromatic Substitution: Enthalpies of Hydrogenation of the Ring Determine Reactivities of C6H5X. The Direction of the C6H5–X Bond Dipole Determines Orientation of the Substitution

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 (Cδ+–Xδ−) but in the opposite direction (Cδ−–Xδ+) for other deactivating groups. This reverses the directing effect. For all X, including the halogens, ipso-Cδ+–Xδ− results in ortho/para substitution. p-13C NMR shifts of C6H5X greater than that of benzene predict meta substitution. A linear relationship exists between p-13C NMR shift and ΔH hyd, except for X = halogen. With halobenzenes, the ortho/para ratios of the products are linearly related to the ipso/ortho ratios of the 13C shifts of C6H5X for chlorinations, brominations, nitrations, and protonations. The relative reactivities of the halobenzenes are linearly related to the p-13C NMR shifts. The electronegativities of X are linearly related to the 13C NMR shifts of the ipso carbon.