Halogens are ortho and para-directing in electrophilic aromatic substitution despite being ring deactivators due to resonance effects
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Retrieved literature challenges the traditional textbook explanation that halogen directing and deactivating effects are governed by resonance, pointing instead to alternative explanations like bond dipole direction or finding that halogens may actually enhance reactivity at the para position.
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.
ABSTRACT Electrophilic aromatic substitution (EAS) is a fundamental reaction introduced early in organic chemistry courses, highlighting the influence of substituents on reactivity and regioselectivity in benzene rings. This study revisits and challenges established concepts about the thermodynamics of nitration EAS, particularly the formation of the σ ‐complex intermediate, and reexamines the role of halogens traditionally classified as deactivating, ortho‐para ‐directing groups. Our findings reveal that the initial step of the reaction is exothermic and that F, Cl, and Br substituents enhance EAS reactivity at the para position compared to hydrogen. These insights suggest a need for updating organic chemistry textbooks to provide a more accurate and nuanced description of substituent effects, especially those of halogens, on the characteristics of EAS reactions.
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