Teaching Electrophilic Aromatic Substitution: Enthalpies of Hydrogenation of the Rings of C6H5X Predict Relative Reactivities; 13C NMR Shifts Predict Directing Effects of X

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, C 6 H 5 X. Enthalpies of hydrogenation of the rings predict the ordering of their reactivities. p - 13 C 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.