In electrophilic aromatic substitution reactions, a substituent already present on the benzene ring influences both the rate of the reaction and the position where the new incoming electrophile will attach. These substituents are known as directing groups.

Directing groups can be broadly classified into two types based on where they direct the incoming electrophile:

• Ortho, Para Directors: These groups direct the electrophile to the positions adjacent to the substituent (ortho) and opposite to the substituent (para). They are generally activating groups, meaning they increase the reactivity of the benzene ring towards electrophilic attack, except for halogens which are deactivating but ortho, para directors.
• Meta Directors: These groups direct the electrophile to the positions located two carbons away from the substituent (meta). They are generally deactivating groups, meaning they decrease the reactivity of the benzene ring towards electrophilic attack.

We are asked to find the group among the given options that is a meta directing group.

Understanding Directing Effects and Meta Directing Groups

The directing ability of a substituent depends on its electronic properties. Electron-donating groups typically activate the ring and are ortho, para directors. Electron-withdrawing groups typically deactivate the ring and are meta directors. Identifying meta directing groups involves understanding how different functional groups interact electronically with the benzene ring.

Analyzing the Given Functional Groups for Meta Directing Properties

Let's examine each option:

• -Cl (Chloro): This is a halogen. Halogens are unique; they are electron-withdrawing through the inductive effect (\(\sigma\)-withdrawal), which deactivates the ring. However, they have lone pairs that can be donated to the ring via resonance (\(\pi\)-donation), which is activating. The inductive effect primarily governs reactivity (deactivating), while the resonance effect primarily governs direction (ortho, para). So, -Cl is a deactivating ortho, para director.
• -NH₂ (Amino): The nitrogen atom has a lone pair of electrons that it can donate to the benzene ring via resonance. This resonance effect pushes electron density into the ring, particularly at the ortho and para positions, strongly activating the ring. Therefore, -NH₂ is a strongly activating ortho, para director.
• -CH₃ (Methyl): Alkyl groups like methyl are electron-donating through hyperconjugation and a weak inductive effect. This increases electron density in the ring, activating it. They direct the incoming electrophile to the ortho and para positions. So, -CH₃ is a weakly activating ortho, para director.
• -CN (Cyano): The cyano group (\( \text{-C} \equiv \text{N} \)) is strongly electron-withdrawing through both inductive and resonance effects. The nitrogen is highly electronegative, and the triple bond allows resonance structures that pull electron density out of the ring, especially from the ortho and para positions. This leaves the meta positions relatively more electron-rich and thus the preferred site for electrophilic attack. The -CN group strongly deactivates the ring and is a meta directing group.

Summary of Directing Effects

Here is a summary of common directing and activating/deactivating groups:

Identifying the Correct Meta Directing Group from Options

Comparing the options provided with the understanding of directing effects and common meta directing groups, the -CN (cyano) group fits the description of a meta director because it is a strong electron-withdrawing group that deactivates the ring and guides the electrophile to the meta position.