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Ch. 16 - Aromatic Compounds
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 16 - Aromatic CompoundsProblem 39b
Chapter 16, Problem 39b

Recall (Section 16-10) that two positions of anthracene sometimes react more like polyenes than like aromatic compounds.

b. The Diels–Alder reaction of anthracene with maleic anhydride is a common organic lab experiment. Predict the product of this Diels–Alder reaction.

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1
Step 1: Recall the Diels–Alder reaction mechanism. It is a [4+2] cycloaddition reaction where a conjugated diene reacts with a dienophile to form a cyclic product. Anthracene acts as the diene, and maleic anhydride serves as the dienophile.
Step 2: Identify the reactive positions in anthracene. Anthracene has three fused aromatic rings, but the central ring is less aromatic due to resonance effects. The 9,10 positions of anthracene behave more like a conjugated diene and are the reactive sites for the Diels–Alder reaction.
Step 3: Analyze the structure of maleic anhydride. Maleic anhydride contains a double bond that acts as the dienophile in the reaction. The electron-withdrawing groups (two carbonyls) enhance its reactivity by making the double bond electron-deficient.
Step 4: Predict the product formation. The 9,10 positions of anthracene will react with the double bond of maleic anhydride, forming a new six-membered ring. The product will have the anthracene framework fused with the maleic anhydride structure, resulting in a bicyclic compound.
Step 5: Consider stereochemistry. The Diels–Alder reaction is stereospecific, typically forming the endo product due to favorable secondary orbital interactions. The maleic anhydride will be positioned in the endo configuration relative to the newly formed six-membered ring.

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Key Concepts

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

Diels–Alder Reaction

The Diels–Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and a dienophile, resulting in the formation of a six-membered ring. This reaction is significant in organic chemistry for synthesizing cyclic compounds and is characterized by its stereospecificity and regioselectivity. In this case, anthracene acts as the diene, while maleic anhydride serves as the dienophile.
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Aromaticity

Aromaticity refers to the enhanced stability of certain cyclic compounds due to the delocalization of π-electrons across the ring structure, following Huckel's rule (4n + 2 π-electrons). While anthracene is an aromatic compound, its reactivity in the Diels–Alder reaction indicates that some positions can behave like non-aromatic polyenes, allowing for different reaction pathways and products.
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Reactivity of Anthracene

Anthracene is a polycyclic aromatic hydrocarbon with three fused benzene rings. Its reactivity in organic reactions, such as the Diels–Alder reaction, is influenced by the presence of electron-rich double bonds in its structure. The specific positions on anthracene that react can lead to different products, highlighting the importance of understanding the molecular structure and electronic effects in predicting reaction outcomes.
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Related Practice
Textbook Question

Biphenyl has the following structure.

c. The heat of hydrogenation for biphenyl is about 418 kJ/mol (100 kcal/mol). Calculate the resonance energy of biphenyl.

d. Compare the resonance energy of biphenyl with that of naphthalene and with that of two benzene rings. Explain the difference in the resonance energies of naphthalene and biphenyl.

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Textbook Question

For each NMR spectrum, propose a structure consistent with the spectrum and the additional information provided.

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a. Elemental analysis shows the molecular formula to be C8H7OCl. The IR spectrum shows a moderate absorption at 1602 cm–1 and a strong absorption at 1690 cm–1.

b. The mass spectrum shows a double molecular ion of ratio 1:1 at m/z 184 and 186.

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Textbook Question

Biphenyl has the following structure.

a. Is biphenyl a (fused) polynuclear aromatic hydrocarbon?

b. How many pi electrons are there in the two aromatic rings of biphenyl? How does this number compare with that for naphthalene?

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Textbook Question

Before spectroscopy was invented, Körner’s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner’s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed.

c. A turn-of-the-century chemist isolated an aromatic compound of molecular formula C6H4Br2. He carefully nitrated this compound and purified three isomers of formula C6H3Br2NO2. Propose structures for the original compound and the three nitrated derivatives

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Textbook Question

Before spectroscopy was invented, Körner’s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner’s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed.

b. How many isomers are formed by nitration of p-xylene?

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Textbook Question

Anions of hydrocarbons are rare, and dianions of hydrocarbons are extremely rare. The following hydrocarbon reacts with two equivalents of butyllithium to form a dianion of formula [C8H6]2–. Propose a structure for this dianion, and suggest why it forms so readily.

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