Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following groups, \(-\mathrm{CH}_{3},-\mathrm{Cl},-\mathrm{OCH}_{3}\), \(-\mathrm{CHO},-\mathrm{CN},-\mathrm{NO}_{2}\) and \(-\mathrm{COOH}\), number of meta directing groups is

Step-by-Step Solution

Answer

There are four meta-directing groups: -CHO, -CN, -NO2, -COOH.

Step 1: Understanding Meta-Directing Groups

In organic chemistry, substituents on an aromatic ring can influence the position at which new substituents are likely to attach to the ring during electrophilic aromatic substitution. Meta-directing groups usually possess an electron-withdrawing characteristic and often bear either a positive charge or a strong electronegative element. They make the meta position the most reactive site. Common examples are \(-NH_3^+, -NO_2, -CN, -CHO, -COOH\) and halogens like \(-Cl\).

Step 2: Analyzing Each Group

Let's analyze each group to determine if it is meta-directing:- \(-CH_3\): It is an electron-donating group, ortho/para directing.- \(-Cl\): Despite being electron-withdrawing due to its electronegativity, it is ortho/para directing because it has lone pairs that can be donated.- \(-OCH_3\): An electron-donating group, ortho/para directing.- \(-CHO\): A strong electron-withdrawing group, meta-directing.- \(-CN\): A strong electron-withdrawing group, meta-directing.- \(-NO_2\): A strong electron-withdrawing group, meta-directing.- \(-COOH\): A strong electron-withdrawing group, meta-directing.

Step 3: Counting the Meta-Directing Groups

Based on the analysis, we identify the meta-directing groups among \(-CHO, -CN, -NO_2, -COOH\). Count the number of groups that are identified as meta-directing.

Step 4: Conclusion

The meta-directing groups from the given list are \(-CHO\), \(-CN\), \(-NO_2\), and \(-COOH\), totaling four meta-directing groups.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a critical reaction in organic chemistry. It involves the replacement of a hydrogen atom on an aromatic ring with an electrophile. Aromatic rings like benzene are highly stable due to their delocalized π-electrons, forming a conjugated system.
This stability makes direct addition reactions unlikely; instead, the aromatic ring undergoes substitution, which maintains the aromaticity.

• The process begins with the formation of a resonating carbocation intermediate, also known as the "arenium ion," when an electrophile attacks the electron-rich aromatic ring.
• The next step involves deprotonation, which restores the aromaticity of the ring.

Electrophilic aromatic substitution reactions are highly influenced by substituents already present on the ring. These existing groups can either activate or deactivate the ring toward further substitution and direct incoming electrophiles to particular positions: ortho, meta, or para.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents that draw electron density away from the aromatic ring. Their presence alters the electronic environment, influencing reactivity and the distribution of electrons in the ring.
These groups often contain electronegative atoms or functional groups with electron-accepting characteristics like

• carbonyl groups (-C=O)
• cyano groups (-CN)
• nitro groups (-NO₂)

EWGs stabilize the positively charged intermediate formed during electrophilic aromatic substitution through resonance or inductive effects, leading to meta-directing behavior. For instance, the strongly electron-withdrawing -NO₂ group significantly enhances the likelihood of substitutions occurring at the meta position rather than ortho or para.

Aromatic Chemistry

Aromatic chemistry revolves around understanding the unique properties and reactions of aromatic compounds. These compounds, like benzene, are characterized by their stable, cyclic, and conjugated ring structures, which possess a high degree of resonance stabilization.
The concept of aromaticity requires that these compounds satisfy Huckel's rule, which states that aromatic rings must contain 4n+2 π-electrons (where n is an integer) to maintain stability and exhibit aromaticity.

• This rule explains why reactions that maintain or restore aromaticity are favored, as they preserve the ring's stability.
• Understanding structures like benzene sets a foundation for more complex structures with similar behaviors.

Aromatic compounds often participate in reactions such as electrophilic aromatic substitutions, driven by their ability to retain resonance and stability during chemical transformations. This makes them invaluable in countless applications, from synthetic dyes to pharmaceuticals.