Question

Arrange the following groups in order of decreasing 0 - and \(\mathrm{p}\) -directing strength. \(\mathrm{NII}_{2}\), OII, \(\mathrm{Cl}, \mathrm{R}\) (1) \(-\mathrm{Cl}>-\mathrm{R}>-\mathrm{OH}>-\mathrm{NH}_{2}\) (2) \(-\mathrm{NH}_{2}>-\mathrm{OH}>-\mathrm{R}>-\mathrm{Cl}\) (3) \(-\mathrm{OH}>-\mathrm{NH}_{2}>-\mathrm{R}>-\mathrm{Cl}\) (4) \(-\mathrm{R}>-\mathrm{CI}>-\mathrm{NH}_{2}>-\mathrm{OH}\)

Short answer

(2) \( -\text{NH}_{2} > -\text{OH} > -\text{R} > -\text{Cl} \)

Step-by-step solution

- Understand 0- and p-directing groups

0-directing (ortho-directing) and p-directing (para-directing) groups are substituents that influence electrophilic aromatic substitution reactions. Groups that donate electrons to the benzene ring, either through resonance or induction, typically activate the ring and direct incoming substituents to the ortho and para positions.

- Analyze the given options

Examine the groups provided in the options: - \( -\text{NH}_{2} \) is a strong electron-donating group through resonance and induction. - \( -\text{OH} \) is also a strong electron-donating group but less compared to \( -\text{NH}_{2} \). - \( -\text{R} \) (alkyl group) is an electron-donating group through induction. - \( -\text{Cl} \) is a weak electron-donating group through resonance, but it has a strong -I effect, making it less activating than the others.

- Rank groups based on electron-donating ability

Compare the electron-donating abilities of the groups: \( -\text{NH}_{2} > -\text{OH} > -\text{R} > -\text{Cl} \).

- Choose the correct option

Identify the option that matches the ranking determined in Step 3 is: \( -\text{NH}_{2} > -\text{OH} > -\text{R} > -\text{Cl} \). This corresponds to option (2).

Key concepts

ortho-directing groups

Ortho-directing groups influence electrophilic substitution reactions in benzene rings. These groups tend to activate the benzene ring and direct new substituents to the ortho position. The ortho position is adjacent to the substituent already attached to the benzene ring.
Groups that are typically ortho-directing have lone pairs available for donation or are strong electron donors. For example:

• Amino group (-NH₂)
• Hydroxy group (-OH)
• Alkyl groups (-R)

These groups increase the electron density at the ortho positions through either direct donation or via resonance, making those positions more reactive for further substitution.

para-directing groups

In benzene chemistry, para-directing groups are substituents that guide incoming electrophiles to the para position on the benzene ring. The para position is directly opposite to the original substituent. Similar to ortho-directing groups, para-directing groups usually activate the benzene ring.
These groups can be strong electron donors, enhancing electron density both at the ortho and para positions due to their ability to either donate electrons through resonance or inductive effects. Common para-directing groups include:

• Methoxy group (-OCH₃)
• Alkyl groups (-R)

since they stabilize the formation of intermediates during substitution.

electron-donating groups

Electron-donating groups (EDGs) play a crucial role in electrophilic aromatic substitution reactions. These groups increase the electron density in the benzene ring, making it more reactive towards electrophiles. They can donate electrons either through:

• Inductive effect: Through sigma bonds as seen in alkyl groups (-R).
• Resonance effect: Through pi bonds, such as -OH and -NH₂ groups.

Electron-donating groups are generally ortho/para-directing because they increase electron density in these positions, making them more favorable for substitution.

inductive effect

The inductive effect is a concept where atoms or groups of atoms transmit their electron-withdrawing or electron-donating effects through sigma bonds in a molecule.
Substituents like alkyl groups show a +I or positive inductive effect. They push electron density towards the benzene ring, making it more reactive.
While weaker than resonance, inductive effects are significant in determining the reactivity and directing effects of substituents. Strong +I groups like alkyls make the ortho/para positions in the benzene ring reactive to further electrophilic attack.

resonance effect

The resonance effect describes the delocalization of electrons within a molecule via pi bonds. In benzene rings, substituents can either donate or withdraw electron density through resonance.
For example, groups with lone pairs like -OH and -NH2 can donate electrons into the benzene ring. This increases electron density at the ortho and para positions.
Conversely, some groups may withdraw electron density, making the benzene ring less reactive. However, in the context of ortho/para-directing groups, we focus on electron-donating substituents which stabilize intermediates and enhance reactivity at specific positions.