Problem 7 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Among the following compounds (I-III) the correct order of reaction with electrophile is: |CBSE 1997] COc1ccccc1 c1ccccc1 O=[N+]([O-])c1ccccc1 II 111 (a) \(11>11 I>I\) (b) III < \(I<11\) (c) \(1>11>111\) (d) \(\mathrm{I}=\| \geq \mathbb{I I}\)

Step-by-Step Solution

Answer

The reactivity order with an electrophile is I > II > III, matching option (c).

Step 1: Understanding Electrophilic Reactions

Electrophilic aromatic substitution (EAS) reactions involve an electrophile attacking an aromatic ring, and the reactivity of benzene derivatives in EAS depends on the substituents attached to the ring. Activating groups, such as alkoxy groups like methoxy (-OCH₃), increase reactivity by donating electrons through resonance, while deactivating groups, such as nitro groups (-NO₂), decrease reactivity by withdrawing electrons.

Step 2: Identify the Compounds' Structures

Assess the given compounds: 1. Compound I - Methoxybenzene or Anisole (\(\text{COc}_1\text{cccc}1\) ) has a methoxy group, known to activate the ring.2. Compound II - Benzene (\(\text{c}_1\text{ccccc}1\) ), which has no substituents and serves as a baseline reactivity.3. Compound III - Nitrobenzene (\(\text{O}=[N+]([O-])\text{c}_1\text{ccccc}1\) ), featuring a nitro group, which is strongly deactivating.

Step 3: Determine Reactivity with Electrophile

The order of reactivity in EAS follows: activating groups > benzene > deactivating groups. Therefore, based on the substituents: 1. Anisole (I) has an activating methoxy group, thus it reacts fastest. 2. Benzene (II) has no activators or deactivators, so it is neutral. 3. Nitrobenzene (III) has a deactivating nitro group, making it the least reactive.

Step 4: Conclude the Reaction Order

Arranging the compounds based on their functional group influence, we obtain the order: I > II > III. Methoxybenzene is most reactive due to the electron-donating methoxy group, followed by plain benzene, and finally, nitrobenzene is the least reactive due to the electron-withdrawing nitro group.

Key Concepts

Aromatic Compounds Reactivity

Aromatic compounds, like benzene, exhibit unique chemical behaviors primarily due to their stable ring structure. This stability is a result of resonance, where electrons are delocalized around the ring, creating a cloud of electron density. This electron-rich environment generally makes aromatic compounds reactive towards electrophiles, which are positively charged species that seek out electrons.
The type and nature of substituents attached to the aromatic ring can significantly affect the reactivity. The basic reactivity pattern in electrophilic aromatic substitution (EAS) reactions aligns as follows:

• Rings with electron-donating groups are more reactive compared to benzene itself.
• Rings with electron-withdrawing groups are less reactive than benzene.

This understanding of reactivity is crucial when predicting how different aromatic compounds will behave in EAS reactions.

Activating and Deactivating Groups

In the context of electrophilic aromatic substitution, substituents on the benzene ring can be classified into two categories: activating and deactivating groups.

Activating Groups:

• These are electron-donating groups that increase the electron density of the benzene ring through resonance or inductive effects.
• Common examples include -OH, -OCH₃ (methoxy), and -NH₂ (amino) groups.
• They make the ring more attractive to electrophiles because the added electron density can stabilize potential positive charges formed during reactions.

Deactivating Groups:

• These are electron-withdrawing groups that decrease the electron density of the benzene ring via resonance or inductive effects.
• Examples include -NO₂ (nitro), -CF₃ (trifluoromethyl), and -CN (cyano) groups.
• These groups make the ring less reactive toward electrophiles, as they draw electron density away, making it harder for the positively charged electrophile to attack.

Identifying if a substituent is an activator or deactivator is essential for predicting reaction outcomes.

Role of Substituents in Reactivity

The role of substituents in the reactivity of aromatic compounds, particularly during electrophilic aromatic substitution, is pivotal. Substituents not only influence the overall reactivity of the compound but also direct subsequent additions to preferred positions on the ring.

• Ortho/Para Directing Groups:
  • Mostly activating groups, which promote substitution at the ortho (adjacent) and para (opposite) positions relative to themselves. The methoxy group in anisole is a classical example, enhancing reactivity significantly.
• Meta Directing Groups:
  • Typically deactivating groups, which guide incoming substituents to the meta position (the third carbon away). The nitro group is a classic meta director, making the compound less reactive.

This directing effect arises because the substituents can stabilize or destabilize the transition states of the intermediates in the EAS reaction, depending upon their electron-donating or withdrawing nature. Understanding these roles helps chemists design and predict the outcomes of reactions more accurately.