8.7.6. Resonance Structure

There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties.

Some organic molecules do not adequately correspond to just one structure as predicted by the Lewis model. Taking benzene as an example, singular Lewis structures suggest that it would have bonds of differing lengths because of the alternating single and double bonds. However, experimental measurements show that all carbon-carbon bonds in benzene are of equal length.

Imagine if you were trying to describe a person using only one photo. Just like how a single photo might not capture the full personality or the different sides of that person effectively, a single Lewis structure cannot portray all the nuances of a molecule's characteristics.

However, as determined experimentally benzene has a uniform C–C bond distances of 139 pm, a value intermediate between the C–C single(154 pm) and C=C double (134 pm) bonds.

Scientific studies quantitatively reveal that benzene's bond lengths are consistently measured at 139 pm. This length is notably between the typical single bond length (154 pm) and the double bond length (134 pm), indicating that no single bond representation is sufficient.

Think about a rubber band that’s stretched just right between two fingertips; it doesn’t exactly resemble any single length perfectly but is a functional balance of two states. Similar to benzene, it exists in a hybrid state rather than in a fixed single structure.

Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.

The concept of resonance comes into play as benzene can be drawn in two different yet equivalent structures, both indicating double bonds in different placements. While one might be a true depiction at any one moment, neither can solely represent the full behavior and characteristics of benzene as both contribute to its overall resonance hybrid.

Imagine a well-composed piece of music. Just listening to a single instrument can give you a perspective, but it’s the combination of all instruments harmonizing that creates a beautiful symphony, just as the mixture of benzene's resonating structures conveys its precise characteristics.

Another example of resonance is provided by nitromethane (CH3NO2) which can be represented by two Lewis structures, (I and II).

Nitromethane also features resonance. It can be depicted with two different Lewis structures presenting different placements of the double bonds between nitrogen and oxygen. This aligns with how resonance operates, indicating that neither structure precisely captures the molecule's reality, depicting a hybrid of the two instead.

Consider a chameleon that can portray different colors; if you just looked at it in one color, you'd miss its true complexity that involves it seamlessly blending multiple hues. Resonance captures the intricate behavior of molecules like nitromethane that 'switch colors' between forms.

The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures.

The resonance hybrid of a compound is more stable than any of the individual structures (called canonical structures). The energy difference between the actual structure and the lowest energy resonance structure is termed resonance stabilization energy. This stability indicates that delocalization of electrons among multiple bonds lowers the overall energy.

Think of it like a group of friends sharing responsibilities; when they combine their strengths and work collaboratively, the outcome is more stable and reliable than if only one person performed a task alone, leading to potentially more errors.

The following rules are applied while writing resonance structures: The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons.

When drawing resonance structures, the key principles to uphold include maintaining the positions of the atomic nuclei and ensuring the unpaired electrons are consistently depicted. The goal is to create hypothetical structures that are still capable of leading towards the physical characteristics observed in the real molecule.

Imagine casting a group of actors in a movie; they maintain their roles (nuclei) but might wear different clothes (bonds) in various scenes. Each version is part of the overarching story, even if each shot looks slightly different.

Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others.

In resonance structures, the most stable form is determined by the presence of more covalent bonds, proper electron configurations (meaning atoms should satisfy the octet rule), minimal charge separation, and optimal distribution of charges overall. The goal is to create a structure that resembles the actual molecular behavior as closely as possible.

Think of a well-balanced diet. Foods that offer a variety of nutrients and maintain the right proportions are healthier (more stable) than those that are overly dominated by a single food group or nutrient.

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