Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compound(s) has/have delocalized \(\pi\) - electrons? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHCH}=\mathrm{CH}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{OCH}_{3}\) (d) C1=CCC=C1

Step-by-Step Solution

Answer

Compound (d) has delocalized \( \pi \) -electrons.

Step 1: Understanding Delocalized Electrons

Delocalized \( \pi \) -electrons occur in conjugated systems where alternating single and double bonds increase stability through resonance. This means electrons are shared across several atoms, not localized between two.

Step 2: Analyze Compound (a)

The compound \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHCH} = \mathrm{CH} - \mathrm{CH}_{3} \) has only one double bond. No consecutive alternating double bonds or conjugation are present as the amine (\(-NH\)) interrupts potential conjugation.

Step 3: Analyze Compound (b)

The compound \( \mathrm{CH}_{3}-\mathrm{C} \left(\mathrm{CH}_{3}\right) - \mathrm{CH}_{2} - \mathrm{CH} = \mathrm{CH}_{2} \) has an isolated double bond, without alternating single-double bonds, thus lacks delocalization.

Step 4: Analyze Compound (c)

\( \mathrm{CH}_{3} \mathrm{CH} = \mathrm{CH} - \mathrm{CH}_{2} - \mathrm{OCH}_{3} \) has one double bond between \( \mathrm{CH} \) and \( \mathrm{CH}_{2} \), not adjacent to another double bond or group enabling conjugation. Thus, no delocalization is present.

Step 5: Analyze Compound (d)

The compound depicted as \( \text{C}_1=\text{CCC}=\text{C}_1 \) or cyclopentadiene has alternating double bonds forming a conjugated system, allowing \( \pi \) -electrons to be shared across the entire ring.

Step 6: Conclusion

Only compound (d) has alternating double bonds in a cyclic arrangement allowing \( \pi \) -electron delocalization.

Key Concepts

Conjugated Systems

Conjugated systems are arrangements of alternating single and double bonds within a molecule. This configuration allows for a special interaction among the  electrons, called delocalization. In simple terms, delocalized electrons are not confined to a single bond or atom; instead, they "flow" over the entire system. This behavior contributes to the stability and unique chemical properties of the compound.
Conjugation can occur in a linear chain of atoms or within a cyclic (ring-shaped) structure. These systems are notable for their ability to conduct electricity, absorb light, and exhibit interesting reactivity patterns. A well-known example of a conjugated system is benzene, which has alternating single and double bonds forming a stable ring structure. Conjugated systems are often associated with aromatic compounds, which display additional stability due to delocalized electrons throughout their ring system.

Resonance

Resonance is a concept used to describe molecules that can't be represented by a single Lewis structure. Instead, they have multiple valid structures, known as resonance forms, which contribute to the true structure. These forms depict the different possible locations of  electrons across the molecule.
Resonance leads to electron delocalization, meaning that electrons are shared across a greater range of atoms than they would be in simpler structures. This sharing results in increased stability. Signs of resonance include the presence of alternating single and double bonds, and sometimes, lone pairs that can join the resonance system.

• Resonance increases a molecule's overall stability due to the delocalization of charge.
• Molecules exhibiting resonance usually have lower energy states compared to those described by a single structure.
• Compounds with resonance are better able to distribute positive and negative charges around the structure efficiently.

Alternating Double Bonds

Alternating double bonds are a key feature in the formation of conjugated systems. This pattern refers to sequences of bonds within a molecule that switch alternately between single bonds and double bonds. The significance of alternating double bonds lies in their ability to facilitate electron delocalization and thus increase molecular stability.
When a compound has alternating double bonds, particularly in a ring or chain, it is capable of supporting the movement of electron density seamlessly from one end of the molecule to another. This contributes to the compound's overall chemical stability and often results in unique optical and electrical properties. By having a system of alternating single and double bonds, the energy levels of the electrons can lower, giving rise to more stable structures.

Cyclic Compounds

Cyclic compounds consist of atoms connected in a ring-like structure. These rings can be saturated, with only single bonds, or unsaturated, containing double or triple bonds. When these rings have alternating double bonds, they can exhibit interesting properties due to conjugation.
When a molecule forms a closed ring structure with alternating double bonds, it enables further electron sharing across the entire ring. This delocalization is particularly pronounced in aromatic compounds, where the stability is significantly heightened. The classic example of a cyclic compound with such properties is benzene, known for its planar ring and equal bond lengths due to resonance.

• Cyclic compounds can be aromatic or non-aromatic, depending on their electronic configuration.
• In aromatic compounds, like benzene, the delocalization of  electrons is a defining feature.
• Aromaticity, a property of some cyclic compounds, provides additional stability beyond what resonance alone could provide.