The Relative Favorability Of Placing Substituents Ortho Or Para In The Cationic Intermediate For Electrophilic Aromatic Substitution

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Journal Of Physical Organic Chemistry


Regioselectivity in electrophilic aromatic substitution (EAS) is partly governed by how well a preexisting substituent stabilizes or destabilizes the cationic intermediate formed by addition of an electrophile. Such intermediates were modeled computationally (G4) by adding a proton ortho, meta, or para to an existing substituent on a benzene ring. The effect of a meta substituent depends on the electronic character, as measured by the Hammett substituent constant σₘ or, better yet, σ⁺ₘ. Similarly, the effect of a para or ortho substituent correlates with σ⁺ₚ (= σ⁺). However, the influence of a substituent at the para position is 10% stronger than at the ortho position. Frontier molecular orbital arguments rationalize this observation. Consequently, the ortho:para product ratio in EAS is expected to depend, at least in part, on σ⁺ₚ, with greater electron-donating character associated with a greater preference for para over ortho. The ortho-substituted cationic intermediate can even be favored over both meta and para if the electron-withdrawing character is sufficient to favor ortho over para but insufficient to favor meta over either. This phenomenon is observed to occur in at least some cases when the quantity σ⁺ₚ–σₘ falls between –0.5 and 0.

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