Question

Arrange the following compounds in the order of increasing tendency to undergo electrophilic substitution (1) Cyanobenzene (2) Benzene (3) Phenol (4) Toluene (5) Trimethyl phenyl ammonium ion (a) \(5<1<2<4<3\) (b) \(3>2>1>4>5\) (c) \(1>2>5>3>4\) (d) \(5<2<4<1<3\)

Short answer

The order is (a) \( 5<1<2<4<3 \).

Step-by-step solution

Identify Activating and Deactivating Groups

To determine the tendency of compounds to undergo electrophilic substitution, identify whether each substituent is activating or deactivating. - Cyanobenzene: has a cyano group (-CN), which is strongly deactivating. - Benzene: no substituents, neutral. - Phenol: the hydroxyl group (-OH) is a strong activating group due to resonance. - Toluene: the methyl group (-CH₃) is activating but less so than -OH. - Trimethyl phenyl ammonium ion: the positive charge deactivates the ring.

Rank Based on Activating/Deactivating Groups

Rank the compounds based on the influence of their substituents: - Activating groups increase the tendency for electrophilic substitution, while deactivating groups decrease it. - Phenol (3) is the most activating, followed by Toluene (4) due to the electron-donating CH3 group. - Benzene (2) has no groups, so it is moderately reactive. - Cyanobenzene (1) is strongly deactivated by the cyano group. - Trimethyl phenyl ammonium ion (5) is the most deactivated due to the positive charge.

Write the Order and Compare with Options

From most to least reactive based on the analysis: - Phenol (3) > Toluene (4) > Benzene (2) > Cyanobenzene (1) > Trimethyl phenyl ammonium ion (5). - Compare this order to the given options: - (a) 5 < 1 < 2 < 4 < 3 (b) 3 > 2 > 1 > 4 > 5 (c) 1 > 2 > 5 > 3 > 4 (d) 5 < 2 < 4 < 1 < 3 - Option (a) matches.

Key concepts

Activating Groups

Activating groups are an integral part of understanding electrophilic substitution in aromatic compounds. These are groups attached to a benzene ring that enhance its reactivity toward electrophilic attacks. How do they do this? Activating groups donate electrons into the benzene ring, making it more electron-rich.
As a result, the ring becomes more attractive to electrophiles—the positive entities that seek out electrons. A general way to spot these groups is by looking for atoms or groups that can donate electrons through resonance or induction.
Some general characteristics of activating groups include:

• Possession of lone pairs that can engage in resonance.
• Usually, contain an electronegative atom like oxygen or nitrogen.
• Typical examples include hydroxyl groups (-OH) and amino groups (-NH₂).

In the original exercise, phenol has an -OH group, which is a highly activating group. This explains why phenol is very reactive towards electrophilic substitution reactions.

Deactivating Groups

In contrast, deactivating groups decrease the reactivity of a benzene ring towards electrophilic substitution. These groups pull electron density away from the benzene ring, making it less electron-rich and, therefore, less attractive to electrophiles.
Deactivating groups often contain electronegative atoms or positive charges that draw electrons away from the aromatic ring through resonance or induction.
Key characteristics of deactivating groups include:

• Often contain double or triple bonds with electronegative atoms.
• May have a full or partial positive charge, which attracts ring electrons.
• Common examples include nitro groups (-NO₂) and cyano groups (-CN).

In the exercise, the cyano group in cyanobenzene and the positive charge in the trimethyl phenyl ammonium ion are examples of deactivating groups. These groups make the compounds less likely to undergo electrophilic substitution.

Aromatic Compounds Chemistry

Aromatic compounds, primarily benzene and its derivatives, constitute an essential part of organic chemistry. These compounds are known for their stability and unique reactivity patterns, attributed to the delocalized pi electrons in the benzene ring. The pi electrons create a cloud of electron density both above and below the plane of the carbon atoms, giving these compounds their characteristic stability.
Electrophilic substitution reactions are a hallmark of aromatic chemistry. These reactions allow the introduction and modification of various functional groups on the benzene ring. The key stages in these reactions generally include the formation of an electrophile, the attack of the benzene ring on the electrophile, and the re-formation of aromaticity by losing a hydrogen ion.
Understanding which substituents activate or deactivate a benzene ring is crucial in predicting and manipulating the outcome of these reactions. While phenol and toluene activate the benzene ring by adding electron density, cyanobenzene and trimethyl phenyl ammonium ion deactivate it by withdrawing electron density.
Knowing this helps chemists design synthesis pathways and predict the reactivity of different aromatic compounds, providing valuable insights into reaction mechanisms and product formations.