Chapter 18, Problem 18.47P

For each of the following substituted benzenes: [1] ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$; [2] ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$; [3] ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_{\text{3}}$:

a. Does the substituent donate or withdraw electron density by an inductive effect?

b. Does the substituent donate or withdraw electron density by a resonance effect?

c. On balance, does the substituent make a benzene ring more or less electron rich than benzene itself?

d. Does the substituent activate or deactivate the benzene ring in electrophilic aromatic substitution?

Interpretation Introduction

(a)

Interpretation: The substituents present on the given benzene donate or withdraw electron density by an inductive effect is to be predicted.

Concept introduction: The replacement or substitution of one functional group with another different functional group in any chemical reaction is termed as substitution reaction. The electron deficient chemical species that contains positive charge are known as electrophile. In electrophilic aromatic substitution reaction, electrophile takes the position of hydrogen atom by attacking the electron rich carbon atom of benzene.

Answer to Problem 18.47P

The substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$, ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$ ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ and withdraws electron density by an inductive effect.

Explanation of Solution

The inductive effect appears in a molecule due to the difference in electron negativity of atoms. The groups attached to benzene ring in the given compound are $\text{Br}$, $\text{CN}$ and ${\text{OCOCH}}_{\text{3}}$. Each group is bonded to benzene ring by making bond with carbon atom.

The electron negativity of bromine is more than carbon atom. Hence, the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$ withdraws electron density by an inductive effect.

The electron negativity of nitrogen is more than carbon atom. Hence, the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$ withdraws electron density by an inductive effect.

The electron negativity of oxygen is more than carbon atom. Hence, the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ withdraws electron density by an inductive effect.

Conclusion

The substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$, ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$ ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ and withdraws electron density by an inductive effect.

Interpretation Introduction

(b)

Interpretation: The substituents present on the given benzene donate or withdraw electron density by a resonance effect is to be predicted.

Concept introduction: The replacement or substitution of one functional group with another different functional group in any chemical reaction is termed as substitution reaction. The electron deficient chemical species that contains positive charge are known as electrophile. In electrophilic aromatic substitution reaction, electrophile takes the position of hydrogen atom by attacking the electron rich carbon atom of benzene.

Answer to Problem 18.47P

The substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$ and ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ donate electron density by a resonance effect, whereas the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$ withdraw electron density by a resonance effect.

Explanation of Solution

The positive resonance effect is observed when the substituent present on a benzene ring donates its lone pair of electrons. Whereas, the negative resonance effect is observed, when the substituents present on a benzene ring pull its electrons towards itself.

The bromine atom attached to a benzene ring in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$ donates its lone pair of electrons and hence, increases the electron density of benzene ring. Therefore, the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$ donate electron density by a resonance effect.

The carbon atom of $\text{CN}$ group does not have any lone pair of electrons. However, it shows negative resonance effect due to presence of partial positive and partial negative charge on carbon and nitrogen atom, respectively.

Figure 1

Therefore, the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$ withdraw electron density by a resonance effect.

The oxygen atom attached to a benzene ring in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ donates its lone pair of electrons and hence, increases the electron density of benzene ring. Therefore, the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ donate electron density by a resonance effect.

Conclusion

The substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{Br}$ and ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ donate electron density by a resonance effect, whereas the substituent in ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{CN}$ withdraw electron density by a resonance effect.

Interpretation Introduction

(c)

Interpretation: The substituents make benzene ring more or less electron than benzene itself is to be predicted.

Concept introduction: The replacement or substitution of one functional group with another different functional group in any chemical reaction is termed as substitution reaction. The electron deficient chemical species that contains positive charge are known as electrophile. In electrophilic aromatic substitution reaction, electrophile takes the position of hydrogen atom by attacking the electron rich carbon atom of benzene.

Answer to Problem 18.47P

Substituents $\text{Br}$ and $\text{CN}$ make benzene ring less electron rich than benzene itself whereas substituent ${\text{OCOCH}}_{\text{3}}$ makes benzene ring more electron rich than benzene itself.

Explanation of Solution

The electron donating groups or activating groups make benzene ring more electron rich than benzene itself, whereas the electron withdrawing groups or deactivating groups make benzene ring less electron rich than benzene itself.

The $\text{Br}$ and $\text{CN}$ are deactivating groups whereas ${\text{OCOCH}}_{\text{3}}$ is an activating group. Therefore, the benzene ring of ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{OCOCH}}_3$ is more electron rich than benzene itself.

Conclusion

Substituents $\text{Br}$ and $\text{CN}$ make benzene ring less electron rich than benzene itself whereas substituent ${\text{OCOCH}}_{\text{3}}$ makes benzene ring more electron rich than benzene itself.

Interpretation Introduction

(d)

Interpretation: The substituents activate or deactivate the benzene ring in electrophilic aromatic substitution is to be predicted.

Concept introduction: The replacement or substitution of one functional group with another different functional group in any chemical reaction is termed as substitution reaction. The electron deficient chemical species that contains positive charge are known as electrophile. In electrophilic aromatic substitution reaction, electrophile takes the position of hydrogen atom by attacking the electron rich carbon atom of benzene.

Answer to Problem 18.47P

Substituents $\text{Br}$ and $\text{CN}$ activate the benzene ring in electrophilic aromatic substitution whereas substituent ${\text{OCOCH}}_{\text{3}}$ deactivate the benzene ring in electrophilic aromatic substitution.

Explanation of Solution

The electron donating groups activate the benzene ring in electrophilic aromatic substitution, whereas the electron withdrawing groups deactivate the benzene ring in electrophilic aromatic substitution.

The $\text{Br}$ and $\text{CN}$ are deactivating groups whereas ${\text{OCOCH}}_{\text{3}}$ is an activating group. Therefore, substituents $\text{Br}$ and $\text{CN}$ activate the benzene ring in electrophilic aromatic substitution whereas substituent ${\text{OCOCH}}_{\text{3}}$ deactivate the benzene ring in electrophilic aromatic substitution.

Conclusion

Substituents $\text{Br}$ and $\text{CN}$ activate the benzene ring in electrophilic aromatic substitution whereas substituent ${\text{OCOCH}}_{\text{3}}$ deactivate the benzene ring in electrophilic aromatic substitution.