Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

Resonance energy per benzene ring is maximum for (a) naphthalene (b) anthracene (c) benzene (d) phenanthrene

Step-by-Step Solution

Answer

Benzene has the maximum resonance energy per benzene ring.

Step 1: Understanding Resonance Energy

Resonance energy is the extra stability a compound has due to the delocalization of electrons across multiple structures, specifically when molecules have multiple resonance forms. It's a key aspect of aromaticity, which provides stabilization to molecules like benzene, naphthalene, anthracene, and phenanthrene.

Step 2: Identifying Resonance Energy in Benzene

Benzene, a simple aromatic compound, consists of a single ring with 6 π-electrons delocalized over its 6 carbon atoms. Its structure provides a high degree of resonance stability per each ring due to complete conjugation and formation of identical Kekulé structures. This maximized electron delocalization in benzene contributes to its notable resonance energy.

Step 3: Calculating Resonance Energy per Ring

To find the resonance energy per benzene ring, consider the molecule's total resonance energy and divide it by the number of benzene rings. Benzene has a high resonance energy because it has only one ring - all resonance energy is per one ring.

Step 4: Comparing Resonance Energies of Given Compounds

Comparing benzene with polycyclic aromatic hydrocarbons like naphthalene (2 rings), anthracene (3 rings), and phenanthrene (3 rings), benzene has the highest resonance energy per its single ring. This occurs because while additional rings in naphthalene, anthracene, and phenanthrene provide some resonance stability, it does not exceed the per ring resonance energy of benzene.

Key Concepts

Benzene

Benzene is one of the most fundamental aromatic compounds, known for its remarkable stability due to electron delocalization. It consists of a single hexagonal ring with six carbon atoms. Each carbon atom in the benzene ring contributes one electron to a shared pool of electrons, known as π-electrons. These electrons are free to move around the ring, creating a stable structure with equal bond lengths.

• This unique arrangement leads to the formation of what are known as Kekulé structures.
• The electrons being spread over the entire ring confer extra stability to benzene.
• As a result, benzene exhibits famously high resonance energy.

The stability seen in benzene makes it a benchmark for comparing resonance energy and stability in other aromatic compounds.

Aromatic Compounds

Aromatic compounds, or arenes, are a class of compounds characterized by their rings of bonded carbon atoms with unique electronic configurations leading to enhanced stability. This stability is often described by the term "aromaticity."

• Aromatic compounds must have a planar ring structure.
• They should follow Hückel's rule, meaning they have a ($4n + 2$) π-electron count where n is a non-negative integer.
• Their stability surpasses that of non-aromatic systems due to their electron configuration.

Benzene is the simplest and most studied member of this group, and its structure is used as the defining feature of aromatic stability. In comparing multi-ring aromatic compounds, benzene's single ring demonstrates the maximum resonance energy per ring.

Delocalization of Electrons

The delocalization of electrons is a key feature that gives aromatic compounds their unique properties. In benzene and similar aromatic systems, π-electrons are not confined between just two atoms. Instead, they are spread out, or "delocalized," over a larger structure comprising multiple atoms.

• This delocalization contributes to lower reactivity compared to alkenes, because the stabilized energy makes breaking bonds more difficult.
• In benzene, all carbon-carbon bond lengths are equal, which is atypical of alternating single and double bonds.
• Electron delocalization is a central concept in understanding the nature of aromatic compounds and assessing their resonance energy.

In the specific case of benzene, the delocalization of its π-electrons is maximized, providing it with one of the highest resonance energy values per ring among aromatic compounds.