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# Resonance

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Resonance Structures & Hydbrid Orbitals
15:38 — by Dennis H.

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## Key Questions

• #### Answer:

Resonance refers to the existence of numerous forms of a compound, and is a component of valence bond theory.

#### Explanation:

Resonance refers to the existence of numerous forms of a compound, and is a component of valence bond theory. Shown below are the two resonance structures of benzene:

These appear to be simple mirror images of each other, but this in fact represents a process that occurs continuously throughout the lifetime of a benzene ring. Observe that benzene is composed of both $\text{C - C}$ single bonds, and $\text{C = C}$ double bonds.

Since a $\text{C = C}$ double bond is stronger than a $\text{C - C}$ single bond, due to the presence of a $\pi$ bond, it follows that the $\text{C = C}$ bonds must have a shorter bond length than the $\text{C - C}$ bonds. The bond length of a $\text{C - C}$ single bond is $154 \text{pm}$, whilst the bond length of a $\text{C = C}$ double bond is $133 \text{pm}$.

Yet the bond length shown in benzene is $139 \text{pm}$. Not only that, but all of the bonds have the same bond length. How can this be? Molecular orbital theory is able to explain this, because resonance is integral to its conception, but valence bond theory must take a different tact if it wishes to explain this.

Recall that $\pi$ bonds result from the extension of half-filled, unhybridised p atomic orbitals from the bonding plane, which can overlap side-on - perpendicular to the $\sigma$ bond. In doing this, the overlapping regions will result in a single $\pi$ bond that contains a pair of bonding electrons. When this happens in benzene, we see an alternating pattern of $\text{C - C}$ single bonds and $\text{C = C}$ double bonds, as shown in the first diagram.

But a p atomic orbital on, say, ${\text{C}}^{1}$, can overlap with, to form a $\pi$ bond, a p orbital on ${\text{C}}^{2}$ or ${\text{C}}^{6}$, can it not? Upon consideration, neither of the two local carbon atoms is any more favorable than the other for this purpose. Because of this, valence bond theory postulates that the structure resonates: it holds in one formation for an instant, before moving onto the next one, then back again, because neither one is more stable than the other.

This actually strengthens the structure, because what appear to be simple $\text{C - C}$ single bonds actually experience double bonded properties instantaneously. The reason for the intermediate bond length in benzene is due to the fact that these bonds are effectively intermediate in identity between single and double bonds.

Resonance plays an important structural role in covalent compounds in accordance with valence bond theory. It can be applied not only to benzene, but also to other compounds such as ozone and carbocations.

• Resonance is a molecule's way of spreading out its electron density, and that helps to minimize its ground-state energy.

We as chemists draw so-called resonance structures to depict each possible snapshot of the molecule that contribute to the overall observed resonance hybrid structure.

Consider acetate anion and the allyl carbocation:

The acetate anion has two resonance structures of the same energy;

• one in which the $\pi$ bond is on one oxygen,
• the other where it's on the other oxygen.

These snapshots "overlap" to give the resonance hybrid, with half-$\pi$-bonds across the $\text{C"-"O}$ internuclear distance.

The allyl carbocation is stabilized in a similar manner with two resonance structures of the same energy. The hybrid structure spreads out the negative charge to stabilize the positive charge.

Or, consider the following resonance structures of urea:

• $\text{I}$ is the most commonly observed form, and is the lowest-energy resonance structure. It contributes the most to the observed structure.
• $\text{II}$ is unstable because carbon only has six valence electrons.
• $\text{III}$ is unstable because oxygen is more electronegative than nitrogen, and can withstand more of the negative charge in a double bond than nitrogen can.
• $\text{IV}$ is the same energy as $\text{III}$ by symmetry.
• Each resonance structure only corresponds to one chemical formula... but a single resonance structure has a fixed structure by definition of it being a snapshot of a molecular electronic configuration.

The one chemical formula can have more than one resonance structure, and typically, a stable molecule will have more than one resonance structure.

Take nitrate anion for example... ${\text{NO}}_{3}^{-}$:

Each resonance structure corresponds to only one chemical structure, and that's the one you see on the page. In this case we just have three degenerate structures; if you rotate the first one ${120}^{\circ}$ clockwise you get the second one, and so on.

But that is no coincidence. This rotational symmetry is what then sensibly gives the resonance hybrid structure, which has 3-fold rotational symmetry axis through the page:

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