Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

The order of effectiveness of o- and p- directing groups is (a) \(\mathrm{OH}>\mathrm{NR}_{2}>\mathrm{NH} \cdot \mathrm{COCH}_{3}>\mathrm{Cl}>\mathrm{CH}_{3}\) (b) \(\mathrm{OH}>\mathrm{Cl}>\mathrm{CH}_{3}>\mathrm{NR}_{2}>\mathrm{NH} . \mathrm{COCH}_{3}\) (c) \(\mathrm{OH}>\mathrm{NH} . \mathrm{COCH}_{3}>\mathrm{Cl}>\mathrm{NR}_{2}>\mathrm{CH}_{3}\) (d) \(\mathrm{OH}>\mathrm{CH}_{3}>\mathrm{NH} . \mathrm{COCH}_{3}>\mathrm{Cl}>\mathrm{NR}_{2}\)

Step-by-Step Solution

Answer

Option (a) - \( \mathrm{OH} > \mathrm{NR}_2 > \mathrm{NH} \cdot \mathrm{COCH}_3 > \mathrm{Cl} > \mathrm{CH}_3 \) is correct, reflecting strong to weak o/p-directing effects.

Step 1: Understand o- and p- Directing Effects

O- and p- directing groups are known for activating or deactivating the benzene ring towards electrophilic substitutions, depending on their electronic nature. Activating groups increase the electron density on the ring and often enhance reactivity, while deactivating groups do the opposite. Generally, groups with lone pairs like OH and NR2 are stronger activators compared to groups like CH3 or Cl.

Step 2: Analyze the Activation Strength of the Groups

Groups that are strong activators (like OH and NR2) enhance reactivity due to their ability to donate electrons via resonance or inductive effects. Conversely, groups like Cl are weakly deactivating (due to its electronegativity), while CH3 is slightly activating. 1. OH is a strong activator due to lone pair donation. 2. NR2 is a strong activator due to resonance donation. 3. NH.COCH3 is moderately activating due to resonance, but less than free amines. 4. Cl is weakly deactivating due to its electron-withdrawing nature through resonance (mesomeric effect), but it's still o/p-directing. 5. CH3 is weakly activating due to hyperconjugation.

Step 3: Compare Given Options with Correct Order

The order of effectiveness depends on their ability to donate or withdraw electrons. Based on electron donation: 1. OH and NR2 are stronger than Cl and CH3. 2. NH.COCH3 will be less strong than free NR2 due to resonance donation being reduced by the acetyl group. Compare each option: (a) OH > NR2 > NH.COCH3 > Cl > CH3 suggests correct order based on strength of activation. (b), (c), and (d) do not show the correct decreasing activation order considering the explanations from Step 2.

Key Concepts

Electrophilic Substitution

Electrophilic substitution is a key reaction in organic chemistry, particularly involving aromatic compounds like benzene. In these reactions, an electrophile effectively replaces a hydrogen atom on the benzene ring. Electrophiles are species that seek electrons due to their positive charge or partial positive charge. The overall process involves three main steps:

• Formation of the Electrophile: Usually, a catalyst is used to generate a strong electrophile that can attack an electron-rich aromatic ring.
• Formation of the Carbocation Intermediate: The electrophile attacks the benzene, temporarily forming a less stable carbocation intermediate and breaking the aromaticity of the ring.
• Restoration of Aromaticity: A hydrogen atom is removed, restoring the aromatic nature of benzene.

Each step is crucial for the successful completion of electrophilic substitution and greatly depends on the nature of the substituents on the benzene ring.

Activation Strength

Activation strength refers to how different substituents on an aromatic ring affect its reactivity towards electrophilic attacks. Activating groups increase reactivity, whereas deactivating groups reduce it. How strongly a group activates the ring is determined by its ability to donate electrons:

• Strong Activators: Groups like hydroxyl (-OH) and amino (-NR₂) enhance electron density significantly through lone pair donation and are among the most potent activators.
• Moderate Activators: Substituents such as -NHCOCH₃ have reduced activating power compared to free amines due to restricted resonance.
• Weak Activators: Alkyl groups like -CH₃ weakly activate through hyperconjugation, subtly increasing electron density.
• Deactivators: Halogens such as -Cl are slightly deactivating due to their electron-withdrawing nature, despite being ortho- and para-directing.

Electron Donation

Electron donation is a mechanism by which substituents on a benzene ring influence its reactivity in electrophilic substitution reactions. The ability of a group to donate electrons is crucial for understanding their behavior as directing groups:

• Substituents with lone pairs, like -OH or -NR₂, donate electrons through resonance, increasing electron density on the ring and making it more reactive towards electrophiles.
• Alkyl groups, such as -CH₃, donate electrons through hyperconjugation, which slightly boosts reactivity.
• Halogens, although electron-withdrawing through inductive effects, can also donate electrons through resonance. This dual character explains their unique position as ortho/para-directors yet weak deactivators.

The effectiveness of electron donation plays a critical role in determining the overall activation or deactivation of the aromatic ring.

Resonance Effects

Resonance effects are a critical factor in dictating the behavior of substituents on an aromatic ring during electrophilic substitution. Resonance involves the delocalization of electrons across a molecule, which can stabilize or destabilize intermediates during reactions:

• Groups like hydroxyl (-OH) and amine (-NR₂) transfer electron density through resonance, significantly activating the ring by stabilizing the carbocation intermediate.
• For amide groups like -NHCOCH₃, the resonance donation is limited due to the presence of the acyl group, making them moderate activators.
• Chlorine (-Cl) exhibits a mixed resonance and inductive impact. Its resonance can compensate for some of its inductive electron-withdrawing character, allowing it to still direct electrophiles to ortho and para positions.

Understanding how resonance influences substituent effects helps predict the outcomes of electrophilic substitution reactions, including the most likely positions for new substituents to appear on the aromatic ring.