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__ .

Short answer

There are 4 meta directing groups: \(-\text{CHO}\), \(-\text{CN}\), \(-\text{NO}_2\), \(-\text{COOH}\).

Step-by-step solution

Understanding Meta Directing Groups

Meta directing groups are types of substituents on a benzene ring that direct incoming electrophiles to the meta position relative to themselves during electrophilic aromatic substitution. These groups are typically electron-withdrawing since they take electron density away from the ring.

Identifying Electron-Withdrawing Groups

To determine which groups are meta directing, we must identify the electron-withdrawing groups from the given options. The following are known to be electron-withdrawing: - -\(- ext{CHO}\), -\(- ext{CN}\), -\(- ext{NO}_2\),- -\(- ext{COOH}\).

Counting the Meta Directing Groups

Now we will count the number of groups identified as meta directing and electron-withdrawing. These include:1. \(- ext{CHO}\)2. \(- ext{CN}\)3. \(- ext{NO}_2\)4. \(- ext{COOH}\).Thus there are 4 meta directing groups in the list.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution, often abbreviated as EAS, is a key reaction in organic chemistry that explains how electrophiles attack aromatic rings like benzene. Aromatic compounds are special types of hydrocarbons that contain a benzene ring. The benzene ring is characterized by its stability and unique electron configuration, which makes it a prime target for electrophilic attacks.

During an EAS reaction, an electrophile, which is a positively charged or electron-deficient species, is attracted to the electron-rich benzene ring. This reaction results in a temporary disruption of the aromaticity, but the aromatic character of the benzene is restored by the removal of a hydrogen atom.

When a substituent is already present on a benzene ring, it can influence the position where the new electrophile will attach. Substituents that direct the electrophile to the meta position play a significant role in determining the outcome of these reactions. Understanding whether a group is electron-donating or electron-withdrawing helps predict these directing effects.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents that pull electron density away from the benzene ring. This characteristic typically affects both the reactivity and site selectivity of the benzene ring in electrophilic aromatic substitution reactions.

EWGs often contain electronegative atoms or are positively polarized, making them effective at reducing electron density. This electron deficiency is why EWGs tend to direct electrophiles to the meta position, away from their own electron-withdrawing effects.

Some common electron-withdrawing groups include:

• Carbonyl groups like \(-\mathrm{CHO}\) and \(-\mathrm{COOH}\)
• Cyanide group \(-\mathrm{CN}\)
• Nitro group \(-\mathrm{NO}_2\)

These groups withdraw electrons through resonance or induction, stabilizing the negative charge on the transition state when the electrophile attacks the meta position on the aromatic ring. Their withdrawal effect is why they are considered meta directing groups.

Benzene Ring Chemistry

In benzene ring chemistry, the structural and electronic characteristics of the benzene ring play a crucial role. Benzene, with its six carbon atoms forming a planar hexagon, is stabilized by resonance. The electrons in the bonds are delocalized, creating a ring of electron density above and below the plane of atoms. This creates a favorable environment for reactions that involve the attack by electrophiles.

A benzene ring is called aromatic due to its cyclic structure and resonance stability. Aromatic compounds resist addition reactions (which would break the resonance) and prefer substitution reactions like EAS to preserve their stability.

The presence of substituents on the benzene ring can further influence its reactivity and the position of incoming electrophiles. Understanding the nature of these substituents, whether they donate or withdraw electrons, is vital for predicting how an aromatic compound will behave in chemical reactions. These insights into benzene ring chemistry are fundamental for anyone looking to master the subject.