Question

Resonance energy is defined as (A) Energy difference between most stable resonating structure and least stable resonating structure (B) Energy difference between least stable resonating structure and resonance hybrid (C) Energy difference between least stable and most stable resonating structure (D) Energy difference between most stable resonating structure and resonance hybrid

Short answer

The correct definition of resonance energy is the energy difference between the most stable resonating structure and the resonance hybrid. Therefore, the correct answer is option (D).

Step-by-step solution

Define resonance energy

Resonance energy is usually defined as the energy difference between the actual energy of a molecule (its resonance hybrid) and the energy of the most stable contributing structure, that would be expected if resonance was not occurring. This means that resonance energy represents the stabilization that occurs due to the electrons being delocalized over several atoms in the molecule rather than being localized to a specific pair of atoms.

Analyze the given options

Now let's analyze each option to see which one matches the definition of resonance energy. (A) Energy difference between most stable resonating structure and least stable resonating structure: This option does not consider the resonance hybrid; it's focused only on the contributing structures. This is not the correct definition. (B) Energy difference between the least stable resonating structure and resonance hybrid: This option considers the resonance hybrid but focuses on the energy difference with the least stable structure instead of the most stable structure. This is not the correct definition. (C) Energy difference between the least stable and most stable resonating structure: This option is essentially the same as option (A), and it doesn't involve the resonance hybrid. This is not the correct definition. (D) Energy difference between the most stable resonating structure and resonance hybrid: This option matches the correct definition of resonance energy, as it considers the energy difference between the resonance hybrid (actual energy of the molecule) and the most stable contributing structure.

Choose the correct answer

Based on our analysis, we can now conclude that option (D) is the correct answer. The definition of resonance energy is the energy difference between the most stable resonating structure and the resonance hybrid.

Key concepts

Resonating Structures

In organic chemistry, resonating structures are different Lewis structures with the same placement of atoms but a different arrangement of electrons. These structures are used to represent molecules where the electron distribution can be depicted in two or more ways. Molecules with such characteristics often have electrons that can move within a certain framework of adjacent atoms, leading to multiple valid electron dot structures.

For instance, benzene, with its alternating double and single bonds, is commonly represented by two resonating structures. Each of these structures, however, does not fully describe the electron distribution within benzene. Rather, the actual molecule is a hybrid of these structures. This is where the concept of resonance plays a crucial role in understanding the behavior of electrons in such molecules.

Resonance Hybrid

The term resonance hybrid comes into play as we realize that no single resonance structure completely defines the position and behavior of electrons within a molecule. A resonance hybrid can be envisioned as a blend of all the possible resonating structures. It's important to understand that a molecule exists only as its resonance hybrid and not as any of its individual resonating structures.

The resonance hybrid represents the overall delocalization of electrons within the molecule. For instance, in the case of benzene mentioned earlier, the resonance hybrid would depict an equal distribution of electrons across all carbon-carbon bonds rather than alternating single and double bonds. This representation is more accurate and suggests that all bonds have partial double-bond character, contributing to its stability and unique properties.

Delocalized Electrons

Delocalized electrons are electrons in a molecule that are not associated with a single atom or a single bond. Instead, these electrons are spread out or 'delocalized' over several atoms. This concept is a key factor in stabilizing molecules with resonance, as it allows for the distribution of electrons over a larger volume of the molecule.

In the context of our earlier example, benzene's electrons are delocalized across the carbon atoms that form the ring. This delocalization lowers the energy of the molecule compared to what it would be if the electrons were localized. It is this energy stabilization realized through delocalization that is known as resonance stabilization, a critical concept for understanding the stability and reactivity in organic chemistry.

Organic Chemistry JEE

The Joint Entrance Examination (JEE) for engineering in India places a significant focus on organic chemistry, which includes resonance energy as one of its concepts. The understanding of resonating structures, resonance hybrids, and delocalized electrons is key for students preparing for the JEE. Clarity in these topics can mean the difference in tackling multiple-choice questions effectively, like the one provided in our exercise.

JEE aspirants should be well-versed in the visual representations and implications of resonance as it not only pertains to the stability of molecules but also influences their chemical behavior and reactions. This makes resonance a recurrent theme in problems involving mechanisms, acid-base chemistry, and the properties of aromatic compounds, all of which are core to organic chemistry on the JEE.