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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 40a,b
Chapter 16, Problem 40a,b

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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Step 1: Analyze the structure of biphenyl. Biphenyl consists of two benzene rings connected by a single bond. Each benzene ring is aromatic, containing alternating double and single bonds in a cyclic structure.
Step 2: Determine if biphenyl is a fused polynuclear aromatic hydrocarbon. A fused polynuclear aromatic hydrocarbon has two or more aromatic rings that share one or more carbon atoms. In biphenyl, the two benzene rings are connected by a single bond but do not share any carbon atoms, so it is not a fused polynuclear aromatic hydrocarbon.
Step 3: Count the pi electrons in each aromatic ring of biphenyl. Each benzene ring has 6 pi electrons from the alternating double bonds. Since biphenyl has two benzene rings, the total number of pi electrons is 6 + 6 = 12.
Step 4: Compare the number of pi electrons in biphenyl to naphthalene. Naphthalene is a fused polynuclear aromatic hydrocarbon with two benzene rings sharing two carbon atoms. It has a total of 10 pi electrons (6 from one ring and 4 shared between the two rings). Biphenyl has 12 pi electrons, which is greater than the 10 pi electrons in naphthalene.
Step 5: Summarize the findings. Biphenyl is not a fused polynuclear aromatic hydrocarbon, and it has 12 pi electrons in its two aromatic rings, which is more than the 10 pi electrons in naphthalene.

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Key Concepts

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

Polynuclear Aromatic Hydrocarbons (PAHs)

Polynuclear aromatic hydrocarbons are organic compounds composed of two or more fused aromatic rings. They are characterized by their stability and unique electronic properties due to the delocalization of pi electrons across the rings. Biphenyl, consisting of two connected benzene rings, is an example of a PAH, but it is not fused since the rings are connected by a single bond rather than sharing carbon atoms.
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Aromaticity of Hydrocarbons

Aromaticity and Pi Electrons

Aromatic compounds are characterized by their cyclic structure, planarity, and the presence of a specific number of pi electrons that follow Hückel's rule (4n + 2, where n is a non-negative integer). In biphenyl, each benzene ring contributes six pi electrons, totaling twelve pi electrons. This contrasts with naphthalene, which has ten pi electrons from its two fused rings, demonstrating how the structure influences the electron count.
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Counting pi electrons

Comparison of Biphenyl and Naphthalene

Biphenyl and naphthalene are both aromatic hydrocarbons, but they differ in structure and electron count. Biphenyl has two separate aromatic rings, leading to a total of twelve pi electrons, while naphthalene has two fused rings, resulting in ten pi electrons. This difference in connectivity affects their chemical properties, stability, and reactivity, making it essential to understand these distinctions when studying aromatic compounds.
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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

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

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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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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Textbook Question

How would you convert the following compounds to aromatic compounds?

(a)

(b)

(c)

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