Ortho Meta Para Directors - Activating and Deactivating Groups

Activating Groups (Ortho-Para Directors)
📌 Strongly activating groups like $-\text{NH}_2$, $-\text{NHR}$, $-\text{OH}$ (e.g., phenol), and $-\text{OR}$ increase ring nucleophilicity via strong resonance electron donation.
📌 Moderately activating groups such as amides and esters (where the ring is attached to the oxygen) are also ortho-para directors but are less potent because the carbonyl group pulls electron density from the oxygen.
📌 Weakly activating groups like alkyl ($\text{CH}_3$) groups direct ortho/para via inductive effects; substitution at ortho/para positions leads to a more stable tertiary carbocation intermediate compared to the secondary carbocation formed by meta substitution.

Deactivating Groups (Meta Directors)
📌 Weakly deactivating groups (halogens like $\text{F}$, $\text{Cl}$, $\text{Br}$, $\text{I}$) withdraw electrons inductively but donate via resonance; the inductive withdrawal exceeds resonance donation, making them net deactivating.
📌 Despite being deactivating, halogens remain ortho-para directors because their resonance effect is sufficient to place a negative charge on the ortho and para carbons, increasing their local nucleophilicity.
📌 Moderately and strongly deactivating groups ($\text{CHO}$, $\text{C=O}$ attached to the ring, $\text{NO}_2$, $\text{CN}$, $\text{SO}_3\text{H}$) are meta directors because resonance withdrawal places positive charges on the ortho and para ring carbons, leaving the meta carbon as the most nucleophilic site for electrophilic attack.

Key Points & Insights
➡️ The directing effect (ortho/para vs. meta) is determined by whether the group's resonance stabilization places negative charge density at the ortho/para positions or if its electron-withdrawing nature leaves the ortho/para positions electron-deficient (positive charge).
➡️ Activating groups donate electrons via resonance, stabilizing the electrophilic attack intermediates at ortho/para positions.
➡️ Strongly deactivating groups (like $\text{NO}_2$) make the ring less reactive towards electrophilic aromatic substitution but more reactive towards nucleophilic aromatic substitution.
➡️ For alkyl groups, the stability of the arenium ion intermediate (tertiary vs. secondary carbocation) explains the ortho-para directionality.

📸 Video summarized with SummaryTube.com on Feb 27, 2026, 01:11 UTC