Activating/Deactivating Groups and Directing Effects - Organic Chemistry 2

1. Which substituent is meta-directing mainly due to a strong -I (inductive withdrawal) effect?
   Answer: Trifluoromethyl (-CF3) This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. The -CF3 group is meta-directing primarily through strong -I effect from electronegative fluorines, lacking resonance. This deactivates the ring and favors meta positions. The correct answer choice picks -CF3, differing from +R donors like -OCH3. A common distractor might be methoxy, confusing effects. To help students: Emphasize inductive withdrawal in halogenated alkyls. Compare to resonance-based meta directors.
2. For the electrophilic bromination of 3-methoxybenzoic acid, which position is the most likely site of substitution?
   Answer: Position 6 The molecule has two competing directing groups: a methoxy group (-OCH₃) at C1 and a carboxylic acid group (-COOH) at C3. The methoxy group is a strong activating, o,p-director. The carboxylic acid group is a strong deactivating, m-director. The powerful activating effect of the -OCH₃ group will control the position of substitution. It directs to its ortho (C2, C6) and para (C4) positions. The -COOH group directs to its meta position (C5). The positions most activated for substitution are C2, C4, and C6. However, positions C2 and C4 are ortho to the deactivating -COOH group, which disfavors substitution there. Position C6 is ortho to the activator and meta to the deactivator, making it the most favorable site for electrophilic attack.
3. A benzene ring with -CO2H is nitrated; what is the major directing outcome?
   Answer: Meta substitution dominates This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. The -CO2H group is deactivating and meta-directing due to its electron-withdrawing carbonyl, leading to meta nitration. This occurs because meta sigma complex is least destabilized. The correct answer choice indicates meta dominance, unlike ortho/para for activators. A common distractor might be para, confusing with donors. To help students: Examine resonance in carboxylic acids for withdrawal. Practice predicting products based on directors.
4. Which substituent is strongly deactivating and meta-directing in EAS?
   Answer: Sulfonic acid (-SO3H) This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Strongly deactivating meta directors like -SO3H withdraw electrons via resonance and induction, favoring meta substitution. This group significantly reduces ring reactivity compared to benzene. The correct answer choice selects -SO3H, contrasting with activators like -OCH3. A common distractor could be methoxy, from misclassifying effects. To help students: Classify substituents by strength and directing type. Analyze sigma complex energies for meta preference.
5. Which sequence of reactions is most likely to produce 2-chloro-4-nitrobenzoic acid as the major product, starting from toluene?
   Answer: 1. Nitration (HNO₃, H₂SO₄); 2. Chlorination (Cl₂, FeCl₃); 3. Oxidation (KMnO₄) The order of operations is crucial. Step 1: Nitrating toluene gives a mixture of o- and p-nitrotoluene; the para isomer is major and can be separated. Step 2: In p-nitrotoluene, the methyl group is an o,p-director and the nitro group is a m-director. Both groups direct substitution to the same position (C2, which is ortho to methyl and meta to nitro). This makes the chlorination highly regioselective. Step 3: The methyl group can be oxidized to a carboxylic acid using a strong oxidizing agent like KMnO₄. Other sequences fail. For example, in (A), oxidizing first gives benzoic acid, which is a meta-director, leading to the wrong isomer. In (B), chlorinating first gives p-chlorotoluene, and nitrating it would place the nitro group ortho to the methyl group, not the chlorine, also leading to the wrong final product.
6. In synthesis, directing effects mainly help chemists control what outcome in EAS?
   Answer: Regioselectivity on the ring This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Directing effects control regioselectivity, determining substitution position on the ring. This is crucial for targeted synthesis. The correct answer choice specifies regioselectivity, central to EAS planning. A common distractor could be stereochemistry, irrelevant to planar aromatics. To help students: Apply directing rules in multi-step syntheses. Predict products based on substituent effects.
7. Which EAS reaction is reversible and used to install a temporary blocking group on benzene?
   Answer: Sulfonation This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Sulfonation is reversible, allowing -SO3H as a temporary meta-directing blocker. This aids in synthesis control. The correct answer choice identifies sulfonation, distinct from irreversible like nitration. A common distractor might be nitration, overlooking reversibility. To help students: Explore synthetic applications of reversible EAS. Tie to directing strategies.
8. Among the halobenzenes (fluorobenzene, chlorobenzene, bromobenzene), which is the most deactivated towards electrophilic aromatic substitution and why?
   Answer: Fluorobenzene, because fluorine's high electronegativity creates the strongest inductive withdrawal, which outweighs its resonance donation. All halogens are deactivating because their inductive electron-withdrawal outweighs their resonance electron-donation. The trend in reactivity depends on the balance of these two effects. Electronegativity (and thus the inductive effect) decreases down the group: F > Cl > Br. Resonance donation is actually most effective for fluorine due to better orbital size overlap with carbon's 2p orbital. However, the inductive effect is dominant. Since fluorine is the most electronegative, it has the strongest inductive withdrawal, making fluorobenzene the most deactivated (least reactive) of the halobenzenes.
9. A chemist plans a synthesis of p-ethylaniline starting from aniline. A proposed final step is the Friedel-Crafts ethylation of aniline using ethyl chloride and AlCl₃. Why is this a flawed synthetic step?
   Answer: The amino group of aniline reacts as a Lewis base with the AlCl₃ catalyst, deactivating the ring and causing meta-substitution. Friedel-Crafts reactions have a major limitation: they fail with rings containing an amino (-NH₂) group. The reason is that the nitrogen's lone pair is basic and reacts with the Lewis acid catalyst (AlCl₃). This acid-base reaction forms a complex, [C₆H₅NH₂-AlCl₃]⁻, which places a positive charge next to the ring. This complexed group is now a very strong deactivator and a meta-director, preventing the desired o,p-alkylation and shutting down the reaction. The correct way to achieve this transformation is to first acylate the amine (protecting it as an amide), perform the Friedel-Crafts reaction, and then hydrolyze the amide back to the amine.
10. Which statement best explains why chlorobenzene undergoes electrophilic substitution more slowly than benzene, yet directs incoming electrophiles primarily to the ortho and para positions?
    Answer: The chlorine atom's inductive effect withdraws electron density, deactivating the ring, while its resonance effect donates electron density, stabilizing the arenium ion for ortho/para attack. This question addresses the classic halogen anomaly. Chlorine is highly electronegative, so it withdraws electron density from the ring inductively, making the ring less nucleophilic and thus less reactive (slower rate) than benzene. This is the deactivating effect. However, chlorine also has lone pairs that can be donated into the ring via resonance. This resonance donation stabilizes the positively charged arenium ion intermediate, and this stabilization is most effective when the electrophile adds to the ortho or para positions. This preferential stabilization directs the substitution to the o/p positions.
11. Which substituent most strongly activates benzene toward EAS by resonance donation?
    Answer: Amino (-NH2) This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Strong activators like -NH2 enhance benzene reactivity through resonance electron donation, directing to ortho/para positions. The -NH2 group's lone pair participates in resonance, making it more activating than hyperconjugative groups like alkyl. The correct answer choice emphasizes -NH2's strong +R activation, contrasting with deactivators like -COCH3. A common distractor might be acetyl, from overlooking resonance donation strength. To help students: Compare activation strengths based on +R versus +I effects. Encourage drawing arenium ion structures to see stabilization by donors.
12. To synthesize m-bromoaniline from benzene, a common route involves nitration, bromination, and reduction. What is the correct sequence for these three steps?
    Answer: 1. Nitration; 2. Bromination; 3. Reduction The final product has two substituents, -Br and -NH₂, in a meta relationship. We must install the groups in an order that utilizes a meta-director. Step 1: Nitration of benzene gives nitrobenzene. The nitro group (-NO₂) is a meta-director. Step 2: Bromination of nitrobenzene places the bromine atom at the meta position, yielding m-bromonitrobenzene. Step 3: Reduction of the nitro group (e.g., with H₂/Pd, or Sn/HCl) converts it to an amino group (-NH₂), giving the final product, m-bromoaniline. Sequence (B) would produce the o/p isomers because the bromo group is an o,p-director.
13. Which EAS reaction introduces an acyl group using $\mathrm{RCOCl/AlCl_3}$?
    Answer: Friedel–Crafts acylation This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Friedel-Crafts acylation uses RCOCl/AlCl3 to introduce acyl groups via acylium ion. This reaction is influenced by ring activation state. The correct answer choice specifies acylation, separate from alkylation. A common distractor might be alkylation, confusing Lewis acids. To help students: Recall electrophile formation in FC reactions. Consider how deactivators prevent them.
14. Which substituent is most likely to prevent Friedel-Crafts alkylation due to strong deactivation?
    Answer: Nitro (-NO2) This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Strong deactivators like -NO2 make the ring too unreactive for Friedel-Crafts alkylation. This prevents carbocation electrophile attack. The correct answer choice identifies -NO2, contrasting with activators like -CH3. A common distractor might be methyl, overlooking deactivation. To help students: Note limitations of FC on deactivated rings. Relate to overall EAS reactivity.
15. A student performs a reaction on thioanisole (C₆H₅SCH₃) and finds the product is a strong meta-director in subsequent electrophilic nitration. What reaction was most likely performed on the thioanisole?
    Answer: Oxidation of the sulfide group with an agent like H₂O₂ or KMnO₄. Thioanisole (-SCH₃) is a sulfide, which is an activating ortho,para-director due to resonance donation from sulfur's lone pairs. To convert it into a meta-director, the sulfur atom must be made electron-withdrawing. This is achieved by oxidation. Strong oxidation converts the sulfide (-S-) to a sulfone (-SO₂-). The methylsulfonyl group (-SO₂CH₃) is strongly electron-withdrawing due to the electronegative oxygen atoms and is a meta-directing deactivator. Alkylation (B) would form a sulfonium salt, which is also a meta-director, but oxidation is a more common transformation in this context. Reduction (C) or deprotonation (A) would not result in a meta-director.
16. For chlorobenzene, what is the directing effect of -Cl in EAS?
    Answer: Ortho/para-directing and deactivating This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Halogens like -Cl are deactivating via -I but ortho/para-directing by +R. This leads to slower but regioselective EAS. The correct answer choice describes -Cl accurately, as deactivating ortho/para. A common distractor could be meta-deactivating, treating like -NO2. To help students: Remember halogens' unique profile. Balance inductive and resonance effects.
17. Why does the Friedel-Crafts acylation of chlorobenzene require more vigorous conditions (e.g., higher temperature) than the acylation of toluene?
    Answer: The chloro group is a deactivating group due to its inductive effect, making the ring less nucleophilic than the activated toluene ring. Reaction rate in EAS is determined by the nucleophilicity of the aromatic ring. Toluene has an electron-donating methyl group, which activates the ring and increases its nucleophilicity, leading to a faster reaction. Chlorobenzene has an electron-withdrawing chloro group. While it has resonance donation that makes it o,p-directing, its strong inductive effect deactivates the ring overall, making it less nucleophilic and slower to react than toluene. Steric hindrance (C) and directing effects (A) influence regioselectivity but not the overall rate as much as the activation/deactivation effect.
18. When nitration of aniline is attempted using a standard mixture of concentrated HNO₃ and H₂SO₄, a substantial amount of m-nitroaniline is unexpectedly formed, despite the -NH₂ group being a strong ortho,para-director. What is the primary reason for this outcome?
    Answer: In the strongly acidic medium, the amino group is protonated to form the anilinium ion (-NH₃⁺), which is a strong meta-directing deactivator. Aniline is a base. In the presence of a strong acid like H₂SO₄, the amino group (-NH₂) is protonated to form the anilinium ion (-NH₃⁺). The positive charge on the nitrogen atom makes this group strongly electron-withdrawing via the inductive effect. It has no lone pair to donate via resonance. Consequently, the anilinium ion is a strong deactivator and a meta-director. The electrophilic substitution then occurs on this protonated form, leading to the meta product.
19. In EAS, which substituent on benzene is an activating group that directs ortho/para?
    Answer: Methoxy (-OCH3) This question tests intermediate-level organic chemistry skills: understanding activating/deactivating groups and directing effects in electrophilic aromatic substitution. Activating groups like -OCH3 increase the reactivity of benzene rings by donating electrons through resonance, directing electrophiles to ortho/para positions. In this case, the methoxy group (-OCH3) is a classic example of a strong activator due to its +R effect from the oxygen lone pairs. The correct answer choice reflects the established activating and ortho/para-directing effects of -OCH3, distinguishing it from deactivating groups like -NO2 or -CF3. A common distractor might be nitro, a misconception from confusing electron-withdrawing with donating substituents. To help students: Focus on classifying substituents based on their resonance and inductive effects. Encourage memorizing key examples of activators versus deactivators to predict EAS outcomes accurately.
20. The relative rates of electrophilic bromination for three compounds were measured against benzene (relative rate = 1). Compound X had a relative rate of 3 × 10⁻⁵, and Compound Y had a relative rate of 40. Which pair of structures most plausibly corresponds to X and Y?
    Answer: X = Benzoic acid; Y = Toluene A relative rate much less than 1 indicates a deactivated ring, while a rate greater than 1 indicates an activated ring. Benzoic acid (-COOH) is a strongly deactivating group, consistent with a very low relative rate (3 × 10⁻⁵). Toluene (-CH₃) is a moderately activating group, consistent with a relative rate of 40. While nitrobenzene and anisole (C) are deactivating and activating respectively, the values fit better with the moderate effects of -COOH and -CH₃ compared to the very strong deactivating -NO₂ group and very strong activating -OH group (A) or -OCH₃ group (C). The values provided are typical for benzoic acid and toluene.

Start this quiz