At first glance, this one looks tricky! Look at what is directly attached to each ring and identify it as electron withdrawing (ring deactivating) or electron donating (ring activatin) as the clue...Consider the molecule below, select which of the following site/s will be the most reactive toward electrophilic aromatic substitution.

Question

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To approach this question, we first identify the guiding principle: in electrophilic aromatic substitution (EAS), the site reactivity is governed by the local electron density, which is influenced by substituents attached to the ring. Electron-donating groups (EDGs) increase electron density via resonance or inductive effects and activate positions ortho and para to the substituent; electron-withdrawing groups (EWGs) decrease electron density and deactivate those positions.

Option a: Sites A, C, D and F. If this option claimed all four positions A, C, D, and F are activated, we would need to assess each site individually. Typically, only those positions that lie ortho/para to an EDG will be activated relative to competing positions, while positions that are adjacent to EWGs or involved in steric congestion may be less reactive. If D or F are adjacent to a withdrawing group (like a carbonyl or ester linkage) or in a non-resonance-stabilized context, their inclusion would overstate activation. In many diaryl-ester systems, some rings are deactivated at certain positions due to the strong withdrawing effect of the carbonyl/ester linkage.

Option b: Site E. Selecting only E as the reactive site would require E to be the sole position that is sufficiently activated. If E is ortho/para to an EDG and not overly deactivated by adjacent EWGs, this could be plausible; however, in a system with multiple potential activation sites, it is uncommon for only one position to be reactive unless that site is uniquely favored by resonance donation and free from deactivating influences. Without detailed structural context, this narrow choice is uncertain.

Option c: Sites D and F. If both D and F lie next to an electron-donating environment or are positioned where resonance structures can stabilize the arenium ion, they could be reactive. Yet, if either D or F is positioned next to a carbonyl or ester function that withdraws electron density, those sites would be less activated. Therefore, this choice could be plausible only if both D and F are indeed activated; otherwise it overestimates the number of active sites.

Option d: Site B. B would be reactive only if it is ortho/para to an effective EDG and not heavily deactivated by neighboring EWGs. If B sits next to an electron-withdrawing substituent (for example, the carbonyl-containing linker) or is in a region of decreased electron density, B would be a poor EAS site. Isolating B as the sole reactive site is typically unlikely in such a poly-substituted system unless B is uniquely activated.

Option e: Sites A and C. This option posits that A and C are both activated by electron donation and sufficiently activated relative to other positions, while others are less favorable due to deactivation or steric hindrance. In many diaryl systems with an ester linkage that can donate or withdraw via resonance, the rings at A and C can indeed be more activated if the substituents connected to those positions donate electron density into the ring and avoid strong deactivation. This would yield a consistent pattern where A and C are the most reactive toward electrophiles.

Option f: Sites B and E. If B and E were both activated, they would need to be the positions that best benefit from EDG effects while avoiding EWGs. However, in a typical arrangement with an ester linkage and multiple ring substituents, it is less common that both B and E are the sole activated sites unless both are ortho/para to strongly donating groups and not adjacent to withdrawing groups. This makes this choice less likely unless the structural details specifically support it.

Taking into account that the clue emphasizes distinguishing electron withdrawing (ring deactivating) versus electron donating (ring activating) substituents directly attached to each ring, the most consistent pattern is that the sites that are truly activated are those directly influenced by EDGs and not overwhelmed by EWGs. In this context, Sites A and C are the ones that best fit the activating pattern, while the other combinations involve at least one site associated with deactivation or reduced activation due to withdrawing effects or unfavorable positioning.

In summary, the reasoning supports that the most reactive sites for EAS in this molecule are A and C, aligning with the idea that these positions experience electron donation that increases the ring’s reactivity toward electrophilic attack. The other options typically include sites that are deactivated by withdrawing groups or fail to capture the combined activation pattern across both rings.

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