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) \(\mathrm{OH}>\mathrm{NH} \cdot \mathrm{COCH}_{3}>\mathrm{Cl}>\mathrm{NR}_{2}>\mathrm{CH}_{3}\) (d) \(\mathrm{OH}>\mathrm{CH}_{3}>\mathrm{NH} . \mathrm{COCH}_{3}>\mathrm{Cl}>\mathrm{NR}_{2}\)

Step-by-Step Solution

Answer

(a) is correct: OH > NR2 > NHCOCH3 > Cl > CH3.

Step 1: Understanding o-/p- directing groups

Ortho-para directing groups are substituents on an aromatic compound that direct incoming electrophiles to ortho or para positions relative to themselves during electrophilic aromatic substitution. These groups typically have lone pairs or are capable of donation of electron density into the aromatic ring, making ortho and para positions more reactive.

Step 2: Analyze Group Effects

Evaluate each of the given substituents based on their ability to donate or withdraw electron density to the benzene ring. Generally, electron-donating groups like -OH and -NR2 are strong ortho-para directors, with -OH often being the strongest due to its lone pair donation via resonance.

Step 3: Compare Substituent Strengths

Assess the list of groups based on their typical directing power: 1. Hydroxyl group (-OH) is usually highly activating. 2. Amines (-NR2) are highly activating as well. 3. Acetamido group (e.g., -NHCOCH3) provides moderate activation. 4. Halogens (e.g., -Cl) are deactivating but still ortho-para directing. 5. Alkyl groups (e.g., -CH3) slightly activate.

Step 4: Determine Correct Order

The correct order of strength based on typical reactivity is: -OH > -NR2 > -NHCOCH3 > -Cl > -CH3 This matches option (a) in the given choices.

Key Concepts

Ortho-Para Directing Groups

Ortho-para directing groups play a crucial role in electrophilic aromatic substitution reactions. When an electrophile attacks an aromatic ring, these groups determine the position (ortho, para, or meta) where the substitution occurs. Ortho and para are specific positions on the benzene ring relative to the substituent already present.

• Ortho position refers to the adjacent positions (1,2-positions) on the benzene ring.
• Para position refers to the opposite position (1,4-position) on the ring.

These groups typically enhance the electron density of the ring, which makes it more reactive and facilitates the electrophilic attack at these positions. For example, in a phenolic compound, the hydroxyl group (-OH) acts as an ortho-para director, making these positions more favorable for substitution.

Electron-Donating Groups

Electron-donating groups (EDGs) are essential players in enhancing the reactivity of aromatic compounds. They are characterized by their ability to donate electron density into the aromatic ring, enhancing its reactivity toward electrophilic aromatic substitution. EDGs usually possess lone pairs of electrons or participate in hyperconjugation.
Lone Pair Donation:

• Groups like -OH or -NH₂ donate electrons through a resonance effect, where their lone pairs overlap with the pi system of the benzene ring.

Hyperconjugation:

• This involves the donation of electron density from C-H sigma bonds adjacent to the aromatic system, as seen with alkyl groups like -CH₃.

With increased electron density, the aromatic ring becomes more nucleophilic, readily interacting with electrophiles. Thus, EDGs rank among the most potent activators in substitution reactions.

Resonance Effect

The resonance effect is a pivotal factor in understanding the influence of substituents on the reactivity of aromatic rings. It involves the delocalization of electrons within a molecule, enhancing stability and reactivity. When applied to EDGs, it significantly influences where electrophilic aromatic substitutions occur.
Mechanism of Resonance:

• Lone pairs from EDGs can overlap with the pi electrons of the aromatic ring, creating resonance structures that distribute electron density across the ring.
• Example: The hydroxyl group (-OH) can donate a lone pair via resonance, reinforcing electron density particularly at the ortho and para positions.

Resonance enhances the nucleophilic character of the aromatic ring making it more susceptible to electrophiles, thereby ensuring that reactions are more likely to occur at the ortho and para positions in the presence of these groups.

Substituent Effects

Substituent effects are crucial in dictating the behavior and reactivity of aromatic compounds in electrophilic substitution reactions. The nature of these substituents can be broadly categorized based on whether they are electron-donating or electron-withdrawing, affecting not only the rate but also the position of substitution.
Types of Substituents:

• Activating Groups: These increase the rate of reaction and include electron-donating groups like -OH, -NR₂, and -CH₃. They are mainly ortho-para directors.
• Deactivating Groups: These decrease reaction rates and include electron-withdrawing groups like -NO₂. Most are meta directors, except halogens.

The interplay of these effects decides the overall reactivity pattern of the ring. Understanding these substituent effects allows chemists to predict product outcomes more accurately in complex synthesis processes.