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Ch. 17 - Reactions of Aromatic Compounds
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 17 - Reactions of Aromatic CompoundsProblem 23a,b
Chapter 17, Problem 23a,b

Propose mechanisms and show the expected products of the following reactions.
(a) 2,4-dinitrochlorobenzene + sodium methoxide (NaOCH3)
(b) 2,4-dimethylchlorobenzene + sodium hydroxide, 350 °C

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1
Step 1: Analyze the reaction conditions for part (a). The substrate is 2,4-dinitrochlorobenzene, which contains electron-withdrawing nitro groups at the 2 and 4 positions. These groups increase the electrophilicity of the carbon attached to the chlorine atom, making it susceptible to nucleophilic aromatic substitution (SNAr). Sodium methoxide (NaOCH3) acts as a nucleophile.
Step 2: For part (a), propose the mechanism for nucleophilic aromatic substitution (SNAr). The methoxide ion (CH3O⁻) attacks the carbon attached to the chlorine atom, forming a Meisenheimer complex (an intermediate with resonance stabilization due to the nitro groups). The chlorine atom is then eliminated, resulting in the substitution of chlorine with the methoxy group (-OCH3).
Step 3: Analyze the reaction conditions for part (b). The substrate is 2,4-dimethylchlorobenzene, which contains electron-donating methyl groups at the 2 and 4 positions. These groups reduce the electrophilicity of the carbon attached to the chlorine atom, making nucleophilic aromatic substitution less favorable. However, under high-temperature conditions (350 °C) with sodium hydroxide (NaOH), elimination-addition (benzyne mechanism) can occur.
Step 4: For part (b), propose the mechanism for the benzyne pathway. Sodium hydroxide abstracts a proton adjacent to the chlorine atom, forming a benzyne intermediate (a highly reactive species with a triple bond in the benzene ring). The hydroxide ion then attacks one of the carbons in the benzyne intermediate, leading to the formation of a substituted product where the chlorine atom is replaced by a hydroxyl group (-OH).
Step 5: Summarize the expected products. For part (a), the product is 2,4-dinitroanisole (2,4-dinitrobenzene with a methoxy group at the position where chlorine was). For part (b), the product is 2,4-dimethylphenol (2,4-dimethylbenzene with a hydroxyl group at the position where chlorine was).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution (NAS) is a reaction where a nucleophile replaces a leaving group on an aromatic ring. This process typically occurs in aromatic compounds that have electron-withdrawing groups, which stabilize the negative charge that forms during the reaction. In the given reactions, sodium methoxide and sodium hydroxide act as nucleophiles, attacking the electrophilic carbon bonded to the chlorine atom.
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Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. This mechanism involves the formation of a sigma complex, where the aromaticity is temporarily lost, followed by deprotonation to restore aromaticity. Understanding EAS is crucial for predicting the products of reactions involving substituted aromatic compounds, such as 2,4-dimethylchlorobenzene.
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Reaction Conditions and Mechanism Pathways

The reaction conditions, such as temperature and solvent, significantly influence the mechanism and products of organic reactions. For instance, higher temperatures can favor elimination reactions or promote nucleophilic substitutions over electrophilic ones. In the case of 2,4-dimethylchlorobenzene with sodium hydroxide at 350 °C, the elevated temperature may facilitate a more favorable pathway for nucleophilic substitution, leading to different products compared to reactions at lower temperatures.
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