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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 14a(vi)
Chapter 17, Problem 14a(vi)

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.
a. Use resonance forms of a sigma complex to show why a phenyl substituent should be ortho, para-directing.
(vi)

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1
Draw the structure of biphenyl, which consists of two benzene rings connected by a single bond. Label one of the benzene rings as the 'substituted ring' and the other as the 'phenyl substituent.'
Recall that a phenyl substituent is an electron-donating group due to resonance. This means it can donate electron density into the benzene ring it is attached to, activating the ring and making it more reactive toward electrophilic aromatic substitution.
To understand why the phenyl substituent is ortho, para-directing, draw the resonance structures of the sigma complex (also called the arenium ion) formed when an electrophile attacks the ortho position of the substituted ring. Show how the positive charge is delocalized over the ring and how the phenyl substituent stabilizes the intermediate through resonance.
Repeat the process for the para position. Draw the resonance structures of the sigma complex formed when an electrophile attacks the para position of the substituted ring. Again, show how the positive charge is delocalized and how the phenyl substituent stabilizes the intermediate.
Compare the resonance structures for the ortho and para positions to those for the meta position. For the meta position, draw the sigma complex and note that the phenyl substituent cannot stabilize the positive charge through resonance. Conclude that the phenyl substituent is ortho, para-directing because it provides additional resonance stabilization at these positions.

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Key Concepts

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

Resonance Structures

Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons. In the case of phenyl substituents, resonance allows for the distribution of positive charge across multiple atoms, stabilizing the sigma complex formed during electrophilic aromatic substitution. This delocalization is crucial for understanding why certain positions on the benzene ring are more reactive.
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Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The presence of substituents on the ring can influence the reaction's regioselectivity, determining whether the substitution occurs at the ortho, meta, or para positions. Understanding EAS is essential for predicting the outcomes of reactions involving biphenyl and its derivatives.
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Activating and Deactivating Groups

Substituents on an aromatic ring can be classified as activating or deactivating based on their electronic effects. Activating groups, such as phenyl, increase the electron density of the ring, making it more reactive towards electrophiles. They also direct incoming electrophiles to the ortho and para positions due to the stabilization provided by resonance, which is key to understanding the substitution patterns in biphenyl.
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Related Practice
Textbook Question

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.

b. Predict the mononitration products of the following compounds

(iv)

2
views
Textbook Question

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.

b. Predict the mononitration products of the following compounds

(vi)

1
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Textbook Question

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.

b. Predict the mononitration products of the following compounds

(v)

4
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Textbook Question

Predict the mononitration products of the following compounds.

f. o-hydroxyacetophenone

7
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Textbook Question

Predict the mononitration products of the following compounds.

d. p-methoxybenzoic acid

6
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Textbook Question

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing.

a. Use resonance forms of a sigma complex to show why a phenyl substituent should be ortho, para-directing.

(i)

2
views