Chapter 22 — Key Takeaways

What you should leave Chapter 22 with

1. Substituents on a benzene ring affect both the rate of EAS (activating vs deactivating) and the regiochemistry (o/p- vs meta-directing).

2. Two questions define every EAS: (1) Where does the electrophile attack? (regiochemistry, controlled by directing effects); (2) How fast? (rate, controlled by activation/deactivation).

3. Activating groups (electron donors, EDG) speed up EAS by stabilizing the arenium ion. Strong activators: -O⁻, -NH₂, -OH, -OR (methoxy). Weak: alkyl groups (-CH₃, etc.).

4. Deactivating groups (electron acceptors, EWG) slow EAS by destabilizing the arenium ion. Strong: -NO₂, -CN, -CF₃, -SO₃H. Moderate: -COOH, -COOR, -CHO. Weakly deactivating: -F, -Cl, -Br, -I (the halogens; exceptional).

5. Ortho/para vs meta directing depends on which positions the substituent stabilizes (or destabilizes) in the arenium ion.

6. General rules: - Activating groups → ortho/para-directing (donors stabilize the arenium more at o/p than at meta). - Deactivating groups → meta-directing (acceptors destabilize all arenium ions; the destabilization is greatest at o/p, so meta is the "least bad" option). - Exception: halogens are deactivating BUT ortho/para-directing.

7. Halogen exception explained: halogens are σ-acceptors (electronegativity → inductive deactivation) AND π-donors (lone pairs → resonance into the o/p arenium). Inductive effect (deactivation) wins on rate; resonance effect (o/p direction) wins on regiochemistry.

8. π-donors (lone pair on the atom attached to the ring) stabilize the o/p arenium by an extra resonance structure: the lone pair donates into the cation. Examples: -OH, -NH₂, -OCH₃, -NHR, -O⁻.

9. π-acceptors (π* orbital extending from the atom; usually involves a multiple bond) destabilize the arenium ion by withdrawing electron density. Examples: -NO₂, -CN, -CHO, -COR, -COOR, -COOH.

10. σ-effects: -CF₃ has no lone pair (so no resonance donation) and is strongly inductively electron-withdrawing → meta-directing. -NH₃⁺ similarly: protonated amines have no available lone pair → become meta-directors!

11. Multi-substituted rings: predict by considering both substituents' directing effects:

    • Reinforcing: both groups direct to the same position(s) → predict confidently.
    • Conflicting: groups direct to different positions → strong activator usually wins.

12. Hammett equation: $\log(k/k_0) = \rho\sigma$. The reaction constant ρ measures sensitivity to substituent effects; σ measures the substituent's electronic character. Negative ρ = cation-stabilizing reaction (favored by donors). Positive ρ = anion-stabilizing reaction (favored by acceptors). EAS has ρ ≈ -12 (very negative; very sensitive).

13. Multi-step synthesis requires planning the order of installation. Examples:

    • 3-bromobenzoic acid: install -COOH first (meta-director) → bromination goes meta to -COOH → product.
    • 4-bromobenzoic acid: install Br first (o/p-director) → install methyl by FC alk → oxidize methyl to -COOH → product.

14. Substituent modification allows exploiting reactivity at one stage and then changing the group:

    • -NO₂ → -NH₂ by reduction (Sn/HCl, Fe/HCl, H₂/Pd).
    • -NH₂ → -N₂⁺ (diazonium) at 0 °C with HNO₂; diazonium is a versatile platform.
    • Sandmeyer reactions: Ar-N₂⁺ + CuX → Ar-X (X = Cl, Br, CN).
    • -N₂⁺ → -F (Schiemann reaction, with BF₄⁻).
    • -N₂⁺ → -OH (aqueous, hot).
    • -N₂⁺ → -H (H₃PO₂; for removing the amine after using it for direction).

15. Acetaminophen synthesis (Case Study 1): phenol → para-nitrophenol → 4-aminophenol → acetaminophen. -OH directs ortho/para; the 3-step synthesis is governed by Chapter 22.

16. Azo dyes (Case Study 2): diazonium electrophile + activated arene → azo bond formed at ortho/para position. The whole synthetic dye industry rests on Chapter 22.

17. Industrial relevance: TNT, picric acid, aspirin, acetaminophen, ibuprofen, sulfa drugs, dyes — all built using Chapter 22 directive effects.

18. The two key ideas to remember:

    • Donors o/p-direct AND activate.
    • Acceptors meta-direct AND deactivate.
    • Halogens are the deactivating o/p-director exception.

19. Typical product distribution (for mono-substituted nitration):

    • Anisole (-OCH₃): >90% para.
    • Toluene (-CH₃): ~60% para, 40% ortho.
    • Bromobenzene (-Br): ~70% para, 30% ortho.
    • Nitrobenzene (-NO₂): ~93% meta.
    • Methyl benzoate (-COOR): ~80% meta.

20. Mastery of Chapter 22 is essential for: drug synthesis (regioselective installation), dye chemistry, industrial aromatics, and understanding why aspirin, acetaminophen, and ibuprofen all have specific substitution patterns.

Cross-references

• Chapter 6 — Acid-base chemistry (substituent effects on phenol pKa parallel Hammett).
• Chapter 19 — Conjugated dienes (resonance principles).
• Chapter 20 — Aromaticity (foundation for arene chemistry).
• Chapter 21 — EAS reactions (Ch 22 governs where on a ring they happen).
• Chapter 23 — SNAr (substituent effects also govern leaving group activation here).
• Chapter 25 — Carbonyl additions (similar resonance considerations).
• Chapter 30 — Amines (aniline; diazonium chemistry).
• Chapter 35 — Drug design (drugs are rich in functional groups; Ch 22 predicts where substituents place for binding).
• Appendix B — pKa table (Hammett σ values for benzoic acids).
• Appendix C — Reaction summary (Sandmeyer, diazonium, dye synthesis).

Study tip

For each EAS problem on a substituted ring, work in three steps:

1. Classify the substituent: activating or deactivating? o/p- or meta-directing?
2. Apply the directing rule: where does the new electrophile go?
3. Apply the rate rule: are conditions adjusted for the activation/deactivation?

If you can answer (1)-(3) correctly for any single substituent, you've mastered Chapter 22.

For multi-substituted rings: 1. Apply (1)-(3) for each substituent. 2. If reinforcing → predict confidently. 3. If conflicting → strong activator typically wins; halogens "lose" to activators but "win" against pure acceptors.

For multi-step synthesis: 1. Retrosynthetically, work backward from the product. 2. Each step: which substituent directs the next step? Plan the order. 3. Use modification chemistry (-NO₂ → -NH₂ → -N₂⁺) to maneuver groups around.

If you can plan a 3-step EAS synthesis, you've internalized Chapter 22.