Polycyclic Aromatic Hydrocarbons

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Questions and Answers

Why does naphthalene undergo electrophilic substitution at the 1-position preferentially over the 2-position?

Electrophilic attack at C-1 is favored because the positive charge in the intermediate is delocalized over a greater number of resonance structures, making it more stable. The C-2 substitution yields fewer resonance stabilized structures.

Explain the role of succinic anhydride in the Haworth synthesis of naphthalene. Be specific about its reactivity and the type of reaction it undergoes.

Succinic anhydride acts as a four-carbon building block, undergoing Friedel-Crafts acylation with an aromatic substrate to form a key intermediate acylbenzene derivative. This intermediate is then transformed through a series of reactions, including reduction, cyclization, and aromatization, to create the naphthalene ring system.

Describe how the presence of a methyl group on a naphthalene ring affects its oxidation, and explain why.

A methyl group activates the ring towards oxidation due to its +I (inductive) effect, which increases electron density, thus making the ring more susceptible to electrophilic attack by oxidizing agents. The ring bearing the methyl group is more easily oxidized compared to the other ring.

In the context of naphthalene, what is the significance of the terms 'α' and 'β' when used in nomenclature, such as in α-Naphthol and β-Naphthol?

α refers to the 1-position on the naphthalene ring, while β refers to the 2-position. α-Naphthol indicates a hydroxyl group at the 1-position, while β-Naphthol signifies a hydroxyl group at the 2-position. Signup and view all the answers

Discuss the rationale behind using different reducing agents (e.g., $Na/EtOH$ vs. $H_2/Ni$) in the reduction of naphthalene, and how the choice of reagent influences the product formed.

Different reducing agents yield different products due to their varying reduction potentials and mechanisms. $Na/EtOH$ promotes dissolving metal reduction, leading to the formation of 1,4-dihydronaphthalene. $H_2/Ni$ facilitates catalytic hydrogenation, resulting in complete saturation to form decahydronaphthalene (decalin). Signup and view all the answers

How does the presence of electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) on a naphthalene ring influence the regioselectivity of subsequent electrophilic substitution reactions? Provide specific examples.

EDGs direct electrophilic attack primarily to the ortho and para positions relative to the EDG, with the para position often favored due to steric hindrance at the ortho position. EWGs, being deactivating groups, direct electrophilic attack to the meta position. Signup and view all the answers

Outline the key steps involved in converting phthalic anhydride to anthracene using the Haworth synthesis.

First, phthalic anhydride undergoes Friedel-Crafts acylation with benzene to form o-benzoylbenzoic acid. Then, intramolecular cyclization occurs in the presence of sulfuric acid. Finally, reduction with zinc and distillation yields anthracene. Signup and view all the answers

Explain why electrophilic substitution in anthracene preferentially occurs at the C-9 and C-10 positions.

Electrophilic attack at C-9 (or C-10, due to symmetry) leads to a more stable intermediate carbocation that retains aromaticity in two benzene rings, while losing aromaticity only in one ring. Attack at other positions would disrupt aromaticity in both neighboring rings. Signup and view all the answers

How does the reaction of 1-naphthol with benzene diazonium chloride proceed, and what type of product is formed?

1-Naphthol undergoes electrophilic aromatic substitution with benzene diazonium chloride to form a reddish-brown azo dye. The diazonium ion acts as the electrophile and attacks the electron-rich aromatic ring of 1-naphthol. Signup and view all the answers

Discuss the key differences between the oxidation of anthracene and phenanthrene, including reagents and products.

Anthracene oxidation typically uses reagents like $H_2SO_4/NaClO_3$ or $V_2O_5/AcOH$ yielding 9,10-anthraquinone. Phenanthrene oxidation, often with $CrO_3/H_2SO_4$, gives 9,10-phenanthraquinone. The positions of oxidation reflect the most reactive sites on each molecule, influenced by their respective structures. Signup and view all the answers

Flashcards

Polycyclic Aromatic Hydrocarbons (PAHs)

Hydrocarbons containing multiple fused aromatic rings.

Naphthalene

A PAH composed of two fused benzene rings.