Question

Explain the following observations: (1) The ortho-para ratio of the products obtained by sulfonation of toluene is lower than that of nitration; (2) The ortho-para ratio of the products obtained by nitration of isopropylbenzene is lower than that of nitration of toluene.

Short answer

The ortho-para ratio differences in sulfonation versus nitration of toluene can be attributed to the larger size and steric hindrance of the sulfonating electrophile (\(HSO_3^{+}\)) compared to the nitrating electrophile (\(NO_2^{+}\)). This results in a lower o/p ratio for sulfonation. For isopropylbenzene and toluene nitration, the greater steric hindrance of the isopropyl group compared to the methyl group leads to a lower o/p ratio for the nitration of isopropylbenzene. These differences are due to the electrophiles' relative reactivity and steric hindrance effects.

Step-by-step solution

1. Ortho-para ratio in sulfonation of toluene vs. nitration of toluene

First, let's discuss the two reactions: sulfonation and nitration. Sulfonation of an aromatic compound involves the reaction of the compound with sulfuric acid (\(H_2SO_4\)), leading to the substitution of a hydrogen atom with a sulfonate group (-SO3H). Nitration involves the reaction of an aromatic compound with a mixture of concentrated nitric acid (\(HNO_3\)) and sulfuric acid (\(H_2SO_4\)) to substitute a hydrogen atom with a nitro group (-NO2). Both sulfonation and nitration reactions follow the mechanism of electrophilic aromatic substitution (EAS). However, the reaction rates of these two processes are different, with sulfonation being slower than nitration. This is because the sulfonating electrophile (\(HSO_3^{+}\)) is less powerful and more sterically demanding compared to the nitrating electrophile (\(NO_2^{+}\)). Now, let's consider the effect of the methyl group in toluene. The methyl group is an activating group in EAS reactions, making the ortho- and para- positions more reactive relative to the meta- position. Activating groups can stabilize the intermediate arenium ion via hyperconjugation. In the case of sulfonation, the large, bulky sulfonate group has a higher steric hindrance than the nitro group, which results in a lower o/p ratio compared to nitration. In essence, the greater steric hindrance prevents the sulfonate group from preferentially attaching to ortho- and para- positions.

2. Ortho-para ratio in nitration of isopropylbenzene vs. nitration of toluene

Now, let's compare the nitration of isopropylbenzene with that of toluene. Both toluene and isopropylbenzene have activating alkyl groups (methyl and isopropyl, respectively) that increase the reactivity of the ortho- and para- positions. However, the isopropyl group in isopropylbenzene is bulkier than the methyl group in toluene. The larger size of the isopropyl group can lead to greater steric hindrance in the ortho- and para- positions of isopropylbenzene, making it more difficult for the electrophile (\(NO_2^{+}\)) to react with these positions compared to the ortho- and para- positions in toluene. This results in a lower o/p ratio for the nitration of isopropylbenzene compared to that of toluene. In conclusion, the observed differences in the ortho-para ratios of the products obtained from the sulfonation and nitration of toluene and isopropylbenzene can be attributed to the steric hindrance and relative reactivity of the electrophiles involved in these reactions.

Key concepts

Sulfonation

Sulfonation is a key reaction in the world of chemistry, particularly within electrophilic aromatic substitution (EAS). This reaction involves the treatment of an aromatic compound with sulfuric acid ( H_2SO_4 ).
This results in a hydrogen atom on the aromatic ring being substituted with a sulfonate group (-SO3H).
One important factor to note about sulfonation is its reaction rate. It is generally slower than nitration due to the nature of the sulfonating electrophile. The electrophile here is the sulfonic acid ion ( HSO_3^{+} ), which is not as potent as the nitrating ion.
Additionally, since this group is bulkier, it comes with higher steric hindrance. Because of this bulkiness, the sulfonic group prefers positions on the ring with less steric strain, affecting the ortho-para product ratio.

• Uses sulfuric acid ( H_2SO_4 ) as the catalyst.
• Introduces a -SO3H group to the aromatic ring.
• The larger size gives it more steric hindrance, influencing the outcomes of the reaction.

Sulfonation, despite being slower and more sterically constrained, remains valuable for its ability to introduce sulfonic acid groups onto aromatic compounds.

Nitration

Nitration is another classic example of electrophilic aromatic substitution reactions. In this reaction, an aromatic compound is treated with a mixture of concentrated nitric acid ( HNO_3 ) and sulfuric acid ( H_2SO_4 ).
This results in the introduction of a nitro group (-NO2) onto the aromatic ring.
One significant note about nitration is the reactivity of the nitrating electrophile. This involves the nitronium ion ( NO_2^{+} ), which is quite powerful and less sterically demanding than the sulfonate ion.
Due to this, the reaction typically occurs faster compared to sulfonation.

• Utilizes a mix of HNO_3 and H_2SO_4 .
• Substitutes a hydrogen on the aromatic ring with a -NO2 group.
• The NO_2^{+} ion is relatively smaller, creating less steric hindrance.

As a result, nitration is usually more favorable when looking for high ortho-para ratios in aromatic substitution, especially against bulkier groups.

Ortho-para Directing Groups

In electrophilic aromatic substitution, an important aspect is understanding ortho-para directing groups.
These groups influence which positions – ortho, meta, or para – are most likely to be substituted on the aromatic ring during a chemical reaction.
Activating groups, such as methyl and isopropyl groups, significantly boost the reactivity of the ortho and para positions relative to the meta position.
This is because they help stabilize the reaction's positively charged intermediate through methods like hyperconjugation or inductive effects.

• Methyl groups are smaller, allowing easier access to ortho and para positions.
• Isopropyl groups are bulkier, potentially adding steric hindrance, yet they still direct ortho and para positions.
• Ortho-para directing groups can interact favorably in reactions such as sulfonation or nitration by influencing reaction pathways.

Therefore, the presence of such activating groups can greatly impact the final ortho-para ratio of a chemical reaction on an aromatic ring.