Chapter 23, Problem 23.46P

Interpretation Introduction

(a)

Interpretation:

The isomer from the given pair that will undergo electrophilic aromatic substitution faster is to be determined.

Concept introduction:

The electron donating groups activate whereas the electron withdrawing groups deactivate the aromatic ring. The electron donating groups are ortho-para directing, which means electrophilic aromatic substitution preferably occurs at the ortho-para position.

Answer to Problem 23.46P

The $\text{m-Dimethylbenzene}$ undergoes electrophilic aromatic substitution faster than $\text{o-Dimethylbenzene}$.

Explanation of Solution

The given pair is $\text{o-Dimethylbenzene}$ and $\text{m-dimethylbenzene}$; structures for both are shown below:

In both structures, benzene has two methyl substituents, which are activating groups. They activate the ring at ortho-para direction. As the position of methyl groups is different in these structures, their activating position on the ring would be different. In both the structures, each methyl activates the ring at two positions, as shown below:

The activating positions of first methyl are indicated by letter $\text{(a)}$, and those of second methyl are indicated by letter $\text{(b)}$. In case of $\text{o-Dimethylbenzene}$, both methyl activate different positions, i.e., represented as $\text{a}\ne \text{b}$, and there are four possible positions. In case of $\text{m-Dimethylbenzene}$, both methyl activate the same positions, i.e., represented as $\text{a}=\text{b}$, and there are three possible positions. As the methyl groups in $\text{m-Dimethylbenzene}$ activate the same carbons in the ring, it must undergo electrophilic aromatic substitution faster than $\text{o-Dimethylbenzene}$.

Conclusion

It is determined that $\text{m-Dimethylbenzene}$ undergoes electrophilic aromatic substitution faster than $\text{o-Dimethylbenzene}$ based on the activating positions in the ring.

Interpretation Introduction

(b)

Interpretation:

The isomer from the given pair that will undergo electrophilic aromatic substitution faster is to be determined.

Concept introduction:

The electron donating groups activate whereas the electron withdrawing groups deactivate the aromatic ring. The electron donating groups are ortho-para directing, which means electrophilic aromatic substitution preferably occurs at the ortho-para position.

Answer to Problem 23.46P

The $\text{m-Dimethylbenzene}$ undergoes electrophilic aromatic substitution faster than $\text{p-Dimethylbenzene}$.

Explanation of Solution

The given pair is $\text{m-dimethylbenzene}$ and $\text{p-dimethylbenzene}$; structures for both are shown below:

In both structures, the benzene has two methyl substituents, which are activating groups. They activate the ring at ortho-para direction. As the position of methyl groups is different in these structures, their activating position on ring would be different. In both the structures, each methyl activates the ring at two positions, as shown below:

The activating positions of first methyl are indicated by letter $\text{(a)}$, and those of second methyl are indicated by letter $\text{(b)}$. In case of $\text{p-Dimethylbenzene}$, both methyl activate different positions, i.e., represented as $\text{a}\ne \text{b}$, and there are four possible positions. In case of $\text{m-Dimethylbenzene}$, both methyl activate the same positions, i.e., represented as $\text{a}=\text{b}$, and there are three possible positions. As the methyl groups in $\text{m-Dimethylbenzene}$ activate the same carbons in the ring, it must undergo electrophilic aromatic substitution faster than $\text{p-Dimethylbenzene}$.

Conclusion

It is determined that $\text{m-Dimethylbenzene}$ undergoes electrophilic aromatic substitution faster than $\text{p-Dimethylbenzene}$ based on the activating positions in the ring.

Interpretation Introduction

(c)

Interpretation:

The isomer from the given pair that will undergo electrophilic aromatic substitution faster is to be determined.

Concept introduction:

The electron donating groups activate whereas the electron withdrawing groups deactivate the aromatic ring. The electron donating groups are ortho-para directing, which means electrophilic aromatic substitution preferably occurs at the ortho-para position.

Answer to Problem 23.46P

The $\text{1, 2, 3, 5-tetramethylbenzene}$ undergoes electrophilic aromatic substitution faster than $\text{1, 2, 3, 4-tetramethylbenzene}$.

Explanation of Solution

The given pair is $\text{1, 2, 3, 4-tetramethylbenzene}$ and $\text{1, 2, 3, 5-tetramethylbenzene}$; structures for both are shown below:

In both structures, the benzene has four methyl substituents, which are activating groups. They activate the ring at ortho-para direction. As the position of methyl groups is different in these structures, their activating position on ring would be different. In both the structures, each methyl activates the ring at two positions, as shown below:

The activating positions of first methyl are indicated by letter $\text{(a)}$, second by letter $\text{(b)}$, third by letter $\text{(c)}$, and fourth by letter $\text{(d)}$.

In case of $\text{1, 2, 3, 4-tetramethylbenzene}$, first and third methyl activate the same positions, i.e., represented as $\text{a}=\text{c}$; second and fourth methyl activate the same position, i.e., represented as $\text{b}=\text{d}$. Thus each position is activated by two methyl groups. In case of $\text{1, 2, 3, 5-tetramethylbenzene}$, all three methyl groups – first, second, and fourth – activate the same positions, i.e., represented as $\text{a}=\text{b}=\text{c}$, and there are three possible positions. The ortho-para position of the third methyl is blocked.

The methyl groups in $\text{1, 2, 3, 5-tetramethylbenzene}$ activate the same carbons in the ring; thus it must undergo electrophilic aromatic substitution faster than $\text{1, 2, 3, 4-tetramethylbenzene}$.

Conclusion

It is determined that $\text{1, 2, 3, 5-tetramethylbenzene}$ undergoes electrophilic aromatic substitution faster than $\text{1, 2, 3, 4-tetramethylbenzene}$ based the activating positions in the ring.