EAS Reactions - Organic Chemistry 2

Polyalkylation is a significant problem in Friedel-Crafts alkylation but not in Friedel-Crafts acylation. What is the fundamental reason for this difference?
Answer: The alkyl group added during alkylation activates the ring, while the acyl group added during acylation deactivates the ring. The key difference lies in the electronic effect of the newly introduced substituent. An alkyl group is electron-donating and activates the aromatic ring, making the product (e.g., toluene) more reactive towards further electrophilic attack than the starting material (benzene). This leads to polyalkylation. In contrast, an acyl group (R-C=O) is strongly electron-withdrawing and deactivates the ring, making the product (an acylbenzene) much less reactive than the starting material. This effectively prevents further acylation.
In EAS, what is the key aromatic intermediate formed after electrophile addition to the ring?
Answer: Arenium ion ($ $-complex) This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the key intermediate in EAS after electrophile addition. The correct answer, B, reflects the accurate understanding that the arenium ion (σ-complex) is the key intermediate. A common distractor, A, might mislead students by suggesting benzyne, which is for elimination-addition mechanisms. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
In nitration of toluene (HNO $_3$ /H $_2$ SO $_4$ ), how does Me affect rate and o/p orientation?
Answer: Activates; directs ortho/para via hyperconjugation This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the effect of the methyl group in toluene on the nitration reaction using HNO₃/H₂SO₄. The correct answer, B, reflects the accurate understanding of the methyl group as an activator that directs ortho/para through hyperconjugation and inductive donation. A common distractor, A, might mislead students by suggesting that alkyl groups deactivate and direct meta, which is actually characteristic of electron-withdrawing groups like nitro. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
Which of the following substituted benzenes will fail to undergo a Friedel-Crafts acylation reaction with acetyl chloride and aluminum chloride?
Answer: Nitrobenzene Friedel-Crafts reactions, both alkylation and acylation, do not proceed on benzene rings that are moderately or strongly deactivated by electron-withdrawing groups. Anisole and toluene have activating groups (-OCH₃ and -CH₃, respectively). Chlorobenzene has a weakly deactivating group (-Cl), but it is not deactivated enough to prevent the reaction. Nitrobenzene contains a strongly deactivating nitro group (-NO₂), which makes the aromatic ring too electron-poor to react with the acylium ion electrophile.
Which set of reagents and conditions will produce p-bromotoluene as the major product when starting from toluene?
Answer: Br₂, FeBr₃ The goal is to brominate the aromatic ring, which is an electrophilic aromatic substitution (EAS) reaction. This requires a source of electrophilic bromine (Br⁺), which is generated by reacting molecular bromine (Br₂) with a Lewis acid catalyst like iron(III) bromide (FeBr₃). The methyl group of toluene is an ortho,para-director, so this reaction yields a mixture of o- and p-bromotoluene, with the para isomer often being the major product due to reduced steric hindrance. Br₂ with UV light (A) would cause free-radical substitution at the benzylic methyl group. HBr with peroxides (C) is for anti-Markovnikov addition to alkenes. NaBr in acetone (D) is for Sₙ2 reactions.
Which ring reacts faster toward chlorination with Cl $_2$ /FeCl $_3$ : benzene or anisole, and why?
Answer: Anisole; OMe donates by resonance, stabilizing $ $-complex This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes which ring reacts faster in chlorination: benzene or anisole. The correct answer, B, reflects the accurate understanding that anisole reacts faster due to resonance donation from OMe stabilizing the σ-complex. A common distractor, A, might mislead students by claiming deactivation, ignoring the activating effect. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
When benzene is treated with one equivalent of chloromethane and AlCl₃, a mixture of toluene, xylenes, and other polymethylated benzenes is obtained. What is the primary reason for this lack of selectivity for the monosubstituted product?
Answer: The methyl group of the toluene product activates the ring, making it more reactive than benzene. This phenomenon is known as polyalkylation. The methyl group introduced onto the benzene ring is an electron-donating group, which activates the ring towards further electrophilic aromatic substitution. As a result, the product, toluene, is more nucleophilic and more reactive than the starting material, benzene. Consequently, as soon as some toluene is formed, it competes with benzene for the electrophile, leading to the formation of disubstituted (xylenes) and higher substituted products. Carbocation rearrangements (B) are not an issue with a methyl group.
Which directing effect is expected for acylamino (NHAc) substituent in EAS on acetanilide?
Answer: Moderately activating, ortho/para-directing This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the directing effect of the NHAc group in acetanilide. The correct answer, B, reflects the accurate understanding that NHAc is moderately activating and ortho/para-directing due to resonance donation. A common distractor, A, might mislead students by assigning strong deactivation and meta direction. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
Which statement best describes sulfonation of benzene with fuming H $_2$ SO $_4$ and heat?
Answer: Reversible; higher temperature favors desulfonation This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the sulfonation of benzene with fuming H₂SO₄ and heat. The correct answer, B, reflects the accurate understanding that sulfonation is reversible, with higher temperatures favoring desulfonation. A common distractor, A, might mislead students by claiming irreversibility, ignoring the equilibrium nature. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
In a Friedel-Crafts acylation reaction, a stoichiometric amount (or slight excess) of the AlCl₃ catalyst is required. Why is a catalytic amount, which is sufficient for Friedel-Crafts alkylation, not effective for acylation?
Answer: The ketone product of the reaction is a Lewis base that forms a stable complex with AlCl₃, deactivating the catalyst. The carbonyl oxygen of the ketone product has lone pairs and acts as a Lewis base. It coordinates strongly with the Lewis acid catalyst, AlCl₃. This complex formation effectively removes the AlCl₃ from the reaction mixture, preventing it from activating more acyl chloride molecules. Therefore, at least one equivalent of AlCl₃ is needed for each equivalent of the carbonyl compound formed. In contrast, the alkylbenzene product of alkylation is a much weaker Lewis base and does not deactivate the catalyst, allowing it to be used in catalytic amounts.
In chlorination of benzene, why is FeCl $_3$ used with Cl $_2$ under standard EAS conditions?
Answer: Generates Cl $^+$ (or polarized Cl $_2$ ) by Lewis acid activation This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the role of FeCl₃ in benzene chlorination with Cl₂. The correct answer, A, reflects the accurate understanding that FeCl₃ generates Cl⁺ or polarized Cl₂ as the electrophile. A common distractor, B, might mislead students by suggesting a radical mechanism, which requires light or peroxides. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
In Friedel–Crafts alkylation using tert-butyl chloride and AlCl $_3$ , what electrophile attacks benzene?
Answer: tert-Butyl carbocation (or equivalent complexed cation) This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the electrophile in Friedel–Crafts alkylation with tert-butyl chloride and AlCl₃. The correct answer, A, reflects the accurate understanding that the tert-butyl carbocation (or complex) attacks the ring. A common distractor, B, might mislead students by suggesting an anion, which is not electrophilic. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
The sulfonation of naphthalene at 80°C yields naphthalene-1-sulfonic acid, but heating the reaction mixture to 160°C changes the major product to naphthalene-2-sulfonic acid. This observation is best explained by:
Answer: The reaction being under kinetic control at 80°C to form the faster, less stable product, and under thermodynamic control at 160°C to form the slower, more stable product. When you encounter a temperature-dependent product distribution in organic reactions, think about kinetic versus thermodynamic control. This fundamental concept explains how reaction conditions determine which product predominates. At 80°C, the sulfonation favors naphthalene-1-sulfonic acid because the 1-position is more reactive toward electrophilic aromatic substitution. The 1-position has greater electron density and forms the electrophile-naphthalene intermediate faster. However, this kinetic product is less thermodynamically stable due to steric crowding and electronic factors. At 160°C, the higher energy allows the reaction to reach equilibrium. Sulfonation becomes reversible - both products can form and decompose. Under these thermodynamic conditions, naphthalene-2-sulfonic acid predominates because it's more stable. The 2-position has less steric hindrance and better orbital overlap, making this isomer thermodynamically favored despite forming more slowly initially. Choice A is wrong because both temperatures involve the same EAS mechanism - only the control regime changes. Choice B incorrectly suggests the 2-isomer is always stable; actually, both isomers can undergo desulfonation at high temperatures, but equilibrium favors the 2-isomer. Choice C misunderstands the cause - steric hindrance doesn't increase with temperature; rather, higher temperature enables equilibration to the more stable product. Remember this pattern: when you see temperature affecting product ratios in reversible reactions, consider kinetic control (lower temperature, faster-forming product) versus thermodynamic control (higher temperature, more stable product). This principle applies broadly across organic chemistry.
Chlorination of chlorobenzene (Cl $_2$ /FeCl $_3$ ) gives mainly which regioisomers, despite deactivation?
Answer: Ortho/para, from resonance donation by halogen lone pairs This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the regioisomers from chlorination of chlorobenzene. The correct answer, B, reflects the accurate understanding that it gives mainly ortho/para products due to resonance donation from halogen lone pairs. A common distractor, A, might mislead students by suggesting meta direction, confusing it with strong deactivators. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
In Friedel-Crafts alkylation, which substrate typically fails because it strongly deactivates the ring?
Answer: Nitrobenzene This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel-Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes which substrate fails in Friedel-Crafts alkylation due to deactivation. The correct answer, A, reflects the accurate understanding that nitrobenzene is strongly deactivated by the nitro group. A common distractor, C, might mislead students by selecting an activator like anisole. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
Which statement best explains why meta directors favor meta substitution in EAS?
Answer: Ortho/para $ $-complex places cation adjacent to EWG, destabilized This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes why meta directors favor meta substitution. The correct answer, A, reflects the accurate understanding that ortho/para σ-complexes are destabilized by placing the cation adjacent to the EWG. A common distractor, B, might mislead students by claiming no arenium ion in meta attack, which is incorrect. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
A chemist wants to synthesize 4-bromo-2-nitrotoluene from toluene. Direct bromination of toluene produces a mixture of ortho and para isomers, making separation difficult. Which of the following sequences best utilizes a blocking group to favor the desired product?
Answer: 1) Toluene + fuming H₂SO₄; 2) Br₂, FeBr₃; 3) dilute H₂SO₄, heat; 4) HNO₃, H₂SO₄ This synthesis uses the reversible nature of sulfonation. 1) Toluene is sulfonated, and the bulky -SO₃H group preferentially blocks the less hindered para position, forming 4-methylbenzenesulfonic acid. 2) The remaining ortho position is then brominated. 3) The sulfonyl group is removed by treatment with dilute acid and heat. 4) The resulting o-bromotoluene is nitrated. The methyl group directs para (to the nitro group's final position), and the bromo group also directs para (to the same position), yielding the target 4-bromo-2-nitrotoluene. The other sequences either produce the wrong isomer or involve unrealistic steps.
Which EAS reaction is characteristically reversible under aqueous acid and can be used as a blocking group strategy?
Answer: Sulfonation This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes which EAS reaction is reversible and useful as a blocking strategy. The correct answer, B, reflects the accurate understanding that sulfonation is reversible under aqueous acid conditions. A common distractor, D, might mislead students by selecting Friedel–Crafts, which is irreversible. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.
Which of the following reaction sequences is the most effective method for synthesizing n-propylbenzene from benzene?
Answer: Benzene treated with propanoyl chloride and AlCl₃, followed by reduction with Zn(Hg), HCl Direct Friedel-Crafts alkylation with 1-chloropropane (A) or an acid-catalyzed reaction with propene (C) will generate a primary carbocation that rapidly rearranges to a more stable secondary carbocation, yielding isopropylbenzene as the major product. To synthesize the straight-chain n-propylbenzene, a Friedel-Crafts acylation must be performed first using propanoyl chloride, which introduces the propyl group as a ketone (propiophenone) and is not subject to rearrangement. This is then followed by a Clemmensen reduction (Zn(Hg), HCl) to reduce the carbonyl to a methylene group. Simple aqueous workup (D) after acylation would only isolate the ketone product.
How does a nitro group affect both rate and orientation of a second EAS on nitrobenzene?
Answer: Deactivates; directs meta via resonance withdrawal This question tests the understanding of electrophilic aromatic substitution reactions, focusing on nitration, sulfonation, halogenation, and Friedel–Crafts (CCSS.OC.2.EAS). These reactions involve the substitution of an electrophile onto an aromatic ring, where the substituents affect both rate and regioselectivity due to their activating or deactivating nature. In this specific case, the context describes the effect of a nitro group on rate and orientation in nitrobenzene. The correct answer, B, reflects the accurate understanding that nitro deactivates and directs meta via resonance withdrawal. A common distractor, A, might mislead students by incorrectly assigning activation. To assist students, instructors can emphasize the importance of understanding the role of substituents in determining regioselectivity and how conditions influence the reversibility of reactions like sulfonation. Practice identifying the role of catalysts and recognizing common reaction pitfalls.

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