Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of these is not compatible with arenes? (a) greater stability (b) resonance (c) delocalization of \(\pi\)-electrons (d) electrophillic addition

Step-by-Step Solution

Answer

Electrophilic addition (d) is not compatible with arenes.

Step 1: Understanding Arenes

Arenes, or aromatic hydrocarbons, are compounds that contain a benzene ring or similar structure. The benzene ring is known for its special stability due to the delocalization of    electrons (delocalization), which contributes to resonance.

Step 2: Analyzing Stability

Arenes are highly stable compared to other unsaturated compounds. This stability is because of the delocalized    electrons over the benzene ring structure, which leads to a lower energy state.

Step 3: Understanding Resonance

Resonance in arenes involves the delocalization of    electrons, which allows multiple contributing structures that enhance the overall stability of the compound. This is a key feature of arenes.

Step 4: Delocalization of    Electrons

The delocalization of    electrons in the benzene ring provides stability and contributes to its special chemical properties. This delocalization is a definitive characteristic of arenes.

Step 5: Electrophilic Addition Reactions

Unlike alkenes, arenes do not typically undergo electrophilic addition reactions. Instead, they undergo electrophilic substitution reactions, as adding directly to the benzene ring would disrupt its delocalized    electron system and stability.

Step 6: Identifying the Odd One Out

Based on the characteristics of arenes, which include stability, resonance, and    electron delocalization, electrophilic addition is the process that is not compatible with arenes because it would compromise their stability.

Key Concepts

Benzene Ring

The benzene ring is the most iconic example of aromatic hydrocarbons. It consists of six carbon atoms arranged in a hexagonal ring, each bonded to one hydrogen atom. Despite appearances, the bonds in a benzene ring are not isolated single and double bonds. Instead, all the bonds are of equal length. This uniformity results from the unique arrangement of electrons within the benzene structure.

The special stability of benzene arises from this structure. The electrons in the bonds are not confined to a pair of carbon atoms; instead, they are shared across the entire ring. This accounts for benzene's stability and unique chemical behavior. Such a structure is often represented by a hexagon with a circle inside, symbolizing the even distribution of electrons.

Delocalization of Electrons

Electrons in a benzene ring are not localized between individual carbon atoms, as they are in more typical carbon-carbon bonds. Instead, these electrons are spread or "delocalized" over the entire aromatic ring. This means that they are shared by all the carbons, creating a stronger, more stable, and low-energy structure.

This delocalization is significant for benzene's stability. By having electrons distributed throughout the ring, benzene compounds resist reactions that would disrupt this electron sharing. It is because of this phenomenon that benzene and similar aromatic structures do not easily undergo addition reactions, which would break the delocalized system and compromise its stability.

Electrophilic Substitution

A key chemical property of aromatic compounds like benzene is their preference for electrophilic substitution reactions. Unlike electrophilic addition, which would involve adding atoms to the benzene and disrupting its electron system, substitution preserves the aromatic integrity of the benzene ring.

Electrophilic substitution involves an electrophile substituting for one of the hydrogen atoms in the ring while maintaining the delocalized electron structure. This type of reaction is favored because it keeps the aromatic stability intact.

• Examples include reactions like nitration and sulfonation.
• These reactions are guided by the stability conferred by the delocalized electrons, preserving the aromatic nature.

Resonance

Resonance in chemistry refers to the concept that a molecule can be represented by two or more valid Lewis structures. For benzene, resonance involves different arrangements of the double bonds, contributing to the same overall chemical structure.

In benzene, resonance is the reason for its extra stability. The resonance forms are not distinct, isolated structures; rather, they illustrate the concept of delocalized electrons. The electrons can be visualized as moving around the ring, but they exist as a "cloud" that spreads evenly over the entire structure.

Resonance is crucial because it accounts for benzene's equal bond lengths and unexpected chemical stability. It also explains why benzene does not react like typical unsaturated hydrocarbons, as disrupting the resonance would require additional energy, which is not favored.