Essential knowledge in organic chemistry is resonance. Molecules can write a resonance structure if there is a double or triple bond. Electrons don’t stay in one place; they move around in many places. As a result, many different structural formulas can be written for even a single molecule.

When you learn about resonance, some people may imagine that electrons are moving incessantly one after another. But that’s not the concept of resonance.

One of the first concepts in organic chemistry that all people learn about is resonance. However, many people don’t understand the concept of resonance and how to write it, and as a result, they dislike organic chemistry in college.

Once you learn how to take into account the resonance structure, you will be able to understand the concepts of molecular stabilization and acidity. In this section, we will explain the principles of resonance.

Ozone and Benzene Cannot Be Explained By the Lewis Structure

The Kekule and Lewis structures are frequently used to describe the structure of molecules. The Kekule structure is used to represent molecular bonds as lines. On the other hand, the Lewis structure is used to represent molecular bonds as dots.

The following are the Kekule and Lewis structures.

In actual organic chemistry, the Kekule structure and the Lewis structure are used in combination.

However, there are some situations that do not make sense when written in a Lewis structure. Commonly used examples include ozone (O3), benzene, and nitrate ion (NO3). The structural formula for each is as follows.

For example, let’s look at ozone. In ozone, there are single and double bonds in the molecule. If this structure is correct, then the bonds should be stronger at the double bonded part of the molecule. But in reality, the single and double bonded parts of the molecule in ozone have the same bond strength.

The same is true for benzene and nitrate ions (NO3). Why is this the case? This is because the following structures are mixed in ozone, benzene and nitrate ions.

For example, in ozone, there is a mixture of two structures. In nitrate ion, there are three structures. Despite the fact that there are multiple structural formulas, writing in a single structural formula leads to inconsistencies.

Due to Delocalization, Electrons Are Spread Throughout the Molecule

To resolve this inconsistency, we use resonance structures. By writing an expression in which the electrons move, we note that there are multiple structural formulas. For example, in ozone we have the following.

You can write a resonance structure by describing the movement of an unshared electron pair (lone pair). This allows us to understand how many structural formulas exist in a mixture.

-It Is Not Resonance that Causes Electrons to Move One After Another

It is important to note that in resonance, electrons are not moving one after another within the molecule. In writing a resonance structure, we utilize arrows to note the electrons as they move. Therefore, some people think that electrons are constantly moving within the molecule and that the structural formula is changing.

However, this is not true. It is important to understand that resonance is a mixture of multiple structural formulas. Of course, the movement of electrons can change the structural formulae. However, a mixture of several structures is resonance.

Therefore, in ozone, the following is the correct structural formula.

In ozone, the two structural formulas exist in a 1:1 ratio. Similarly, in benzene and nitrate ions, multiple structural formulas are mixed in a certain ratio.

-Resonance Causes Electrons to Delocalize

What would it look like if we could write a resonant structure? Electrons are not confined to one place but can be found in many places. The more you can write resonance structures, the higher the probability that electrons exist in all places.

This is called the delocalization of electrons. Electrons don’t have to stay in one place; they can be found everywhere in the molecule.

The More Resonance Structures You Can Write, the More Stable the Electrons Become

So what does it mean to write these resonance structures? When you can write a resonance structure, the electrons are stabilized.

It is known that the more electrons go inside a small box, the higher the energy. Electrons are waves, and in quantum mechanics, they are represented by a wave function. In short, you can understand that electrons are waves.

Even though electrons are waves, there are waves of various widths (frequencies). Suppose we confine these waves to a small area. Then waves of various frequencies (waves with various motive forces) have to be confined to a small area, resulting in high energy.

On the other hand, if a large number of structural formulas can be written in resonance structures, the range of electrons can be extended. Delocalization allows electrons to move freely within the molecule instead of being confined to a small area.

As a result of expanding the area in which electrons can exist, the electrons become more stable. This is the reason why electrons are more stable as they can write resonance structures.

  • Being able to write a resonance structure = stabilization of electron

We are taught to do this, but we rarely get a chance to think about the reasons for this. Normally, we are supposed to explain the reason for electron stabilization by using difficult formulas, but this makes the content very difficult. Therefore, only physics students who are interested in quantum mechanics should try to figure out the reasons while solving the equations.

The Presence of a Double Bond (pi Bond) Causes Resonance

When does resonance occur? Roughly, consider that you can write a resonance structure when there is a double bond.

In molecular bonding, the s and p orbitals are involved in the bond. In the case of a single bond (σ-bond), it is not possible to write a resonance structure. As a double or triple bond, a resonance structure can only be formed when a π bond is present.

The ozone, benzene, and nitric ion mentioned earlier all have one thing in common. They all have a double bond in their molecules. Let’s think of it as having a double bond or a triple bond that allows us to draw a resonance structure.

The Octet Rule Contributes to a Stable Structure

Does any molecule resonate if it has a π bond? Of course, this is not the case. Even if a resonance structure can be written, it is meaningless without a stable structural formula. To be more precise, it is common to write a resonance structure after satisfying the octet rule.

The stability of an atom with eight electrons is called the octet rule. In particular, carbon, nitrogen, oxygen, and fluorine atoms must satisfy the octet rule.

For example, if the electrons are free to move, we can write the following resonance structure.

But in practice, this resonance does not occur. This is because it produces a carbocation. In a carbocation, there are only six electrons in a carbon atom. If there are eight valence electrons, the octet rule is satisfied, and the carbocation is stable. The carbocation, however, does not satisfy the octet rule and is unstable.

In other words, such resonance does not occur in most cases. If there is a double bond (pi-bond), the molecule does not necessarily resonate freely.

On the other hand, if a resonance structure can be written to satisfy the octet rule, the electrons are delocalized. Such resonances can be said to make a significant contribution to structural stabilization.

The Presence of Oxygen and Nitrogen Atoms and Conjugated Structures Is Important for the Resonance Structure

When does a molecule resonate?  Is there a way to find it? We mentioned above that the presence of π-bonds (double or triple bonds) is a prerequisite for resonance. Another way to find the molecule is to add the following conditions to the molecule, which make it more likely to resonate

  • The presence of an oxygen or nitrogen atom.
  • There’s a conjugate structure.

In fact, there are a number of other conditions that can cause it to resonate. For example, atoms such as phosphorus (P) and sulfur (S) will resonate when they are present. Just to make it easier to understand, let’s focus on these conditions at first.

If you can write a resonance, in many cases, there is an oxygen atom (O) or a nitrogen atom (N) in the molecule. These atoms have unshared electron pairs and can even make double bonds. They can also be positively or negatively charged. Because they can change their properties, they are an easy starting point for resonance.

However, it is not possible to write a resonant structure even if only oxygen or nitrogen atoms are present; a double bond is required. In particular, if there is a conjugated structure, you can write a resonance. A structure with multiple double bonds linked together is conjugated. For example, a benzene ring is a type of conjugation.

The following is the resonance structure of aniline.

In this way, aniline can draw resonance structures. The conjugated structure of the aromatic ring (benzene ring), which originates from the electrons of the nitrogen atom, allows multiple resonances to be drawn.

Of course, even without the conjugated structure, there are cases where multiple resonances can be drawn, such as nitrate ions (NO3) and carbonate ions (CO32-). But in any case, you can write a resonance, starting from an oxygen or nitrogen atom and using double or triple bonds.

The Molecules Stabilize Because They Resonate and Are Involved in Acidity and Basicity

What can we learn from writing these resonances? As we explained earlier, many resonances cause delocalization of the molecule, which makes the molecule more stable. Should the octet rule be met, the wide dispersion of electrons allows the molecule to exist in a stable manner.

This plays a role in the acidity and basicity of the molecule.

For example, a hydroxy group (-OH) is a neutral molecule. It is neither acidic nor basic. Phenol, on the other hand, is known to be an acidic molecule.

Phenol has -OH attached to the benzene ring, but why is it not neutral like the hydroxy group attached to the alkyl chain? In phenol, you can write resonance structures by ionizing it. They are as follows.

In the case of propanol, it is not possible to write a resonance structure because it is all single bonds (σ-bond). On the other hand, in the case of phenol, after ionization, it is possible to write a number of resonance structures with the benzene ring. In other words, the structure stabilizes after ionization. This is the reason why phenol shows acidity.

The stabilization by resonance is related to the acidity of the structure. In high school chemistry, we only learn that phenol shows acidity, and we don’t understand why. So if you learn about resonance, you will understand why phenol is acidic while the hydroxy group (-OH) attached to the alkyl chain is neutral.

Resonance Structure Is the Foundation of Organic Chemistry

A concept that is basic but difficult to understand is resonance. When first learning about resonance, many people wonder why electrons move on their own. As a result, we lose our understanding of organic chemistry.

In some cases, such as ozone, there are several structures that exist in more than one. So by writing a resonance structure, we need to understand that there are multiple molecules mixed together. By writing a resonance, we can show that electrons are not in constant motion, but rather exist over a wide area.

In general, however, you need to write the resonances in such a way that they satisfy the octet rule. If you understand these rules and focus on the pi bonds (double and triple bonds) and the oxygen and nitrogen atoms, you will be able to distinguish whether or not you can draw a resonance structure.

Learning these resonance structures is also useful when understanding synthetic reactions in organic chemistry. Without understanding the concept of resonance, it is impossible to understand the reaction mechanisms of organic chemistry. Therefore, it is important to understand the concept of resonance and to be able to write resonance structures.