An important element in organic chemistry is the influence of substituents. The properties of a molecule change depending on what functional group is attached to the alkyl chain or benzene ring.

Two particularly important effects of the substituents are the inductive effect (I effect) and the resonance effect (R effect). The acidity of the molecule differs as a result of the change in the substituents bound to it. It also depends on whether the substituents are alkyl chains or benzene rings.

Substituents can be either electron-donating or electron-withdrawing. Electron-donating groups provide electrons and electron-withdrawing groups pull electrons. By identifying these, we can infer the reactivity of the molecule.

So make sure you understand the principles of the inductive and resonance effects. Once you understand the nature of the substituents and learn how to distinguish between electron-donating and electron-withdrawing groups, you’ll be able to figure out what the acidity of the molecule is.

Differences Between the Inductive Effect (I Effect) and the Resonance Effect (R Effect)

First, what is the inductive effect (I effect)? And what is the resonance effect (R effect)? Let’s assume that both of these effects are due to the functional groups that are attached to the molecule. Depending on what substituents are attached to the molecule, many differences will occur, such as different levels of acidity.

The differences between the two are as follows.

  • Inductive effect: effect of σ-bond (single bond)
  • Resonance effect: Effect of π-bond (double and triple bonds)

Electronic orbitals include s and p orbitals, and these orbitals form bonds. Among the bonds formed by these s and p orbitals, the single bond is the σ-bond. The substituent effect of the σ-bond is the inductive effect.

On the other hand, some molecules may form double or triple bonds. The part of the molecule that forms a double or triple bond is called a π bond; if a pi bond is present, the molecule can write a resonance structure. The resonance effect is the result of resonance by the substituent, which changes the orientation (reactivity on the benzene ring) and the acidity of the molecule.

Roughly speaking, understand that the inductive effect (I effect) affects the alkyl chain and the resonance effect (R effect) affects the benzene ring.

The Acidity Varies with the Degree of Electronegativity Due to the Inductive Effect

The inductive effect affects the single bond (σ-bond). When does the inductive effect appear? It occurs when an atom (or molecule) with a high degree of electronegativity is attached.

Particularly important is when it is attached to an alkyl chain. There are no double or triple bonds in the alkyl chain, and therefore no resonance occurs. In an alkyl chain, only the inductive effect needs to be considered.

When an atom with a high degree of electronegativity bond together, they will strongly attract electrons. As a result, the molecule will produce a positive and negative charge, even if it is the same molecule. This is known as polarization. Water, ammonia and hydrogen chloride are known to be polarized.

Such polarization due to differences in electronegativity also occurs in alkyl chains. The addition of substituents with high electronegativity to the alkyl chain leads to differences in acidity.

For example, acetic acid is a known acidic substance. When a carboxylic acid is present, it is acidic. However, the acidity of the same carboxylic acid differs depending on what substituents are present in its surrounding carbons.

For example, what happens if one chlorine atom, a halogen, is bonded to acetic acid (CH3COOH)? In this case, the result is as follows

As the chlorine atoms bond, electrons are attracted to the chlorine atoms. As a result, chloroacetic acid is more negatively charged than a carboxylic acid. This means that the acidity of chloroacetic acid is higher.

The electrons are pulled by the halogen and, as a result, the chloroacetic acid is more acidic than the carboxylic acid.

The effect of the substituent pulling the electrons is an inductive effect. This effect causes the acidity to become more or less involved. Of course, the more halogens are bound, the greater the acidity.

As we explained in the example of acidity, the same thing affects basicity. The inductive effect is related to the degree of basicity.

Since it is a functional group with strong electronegativity, the inductive effect is involved in many cases. When an oxygen or nitrogen atom or a halogen is attached to an alkyl chain, it causes an inductive effect and lowers the electron density.

  • Nitro group (-NO2)
  • Amino group (-NH2)
  • Cyano group (-CN)
  • Carbonyl group (-CO)
  • Carboxy group (-COOH)
  • Sulfone group (-SO3H)
  • Methoxy group (-OCH3)
  • Hydroxy group (-OH)
  • Halogen (-Cl, -Br, -I)

All of these attract electrons by binding to the alkyl chain, causing the inductive effect. In functional groups with oxygen or nitrogen atoms or halogens, they are all electron‐withdrawing groups.

Limited Influence of Electron‐Withdrawing Groups

So what is the extent of the inductive effect of the electron‐withdrawing group? Let’s understand that this is limited.

Even though electrons are attracted by the electronegativity, the effect is less pronounced at greater distances. For the neighboring carbon atoms to which the electron‐withdrawing group is bound, the effect of the inductive effect is strong. However, the effect of the electronegativity becomes less pronounced as the distance increases.

Although the inductive effect reduces the electron density, the scope of the effect is small.

-The More Carbon Atoms There Are, the Higher the Electron Density

For reference, the presence of carbon atoms increases the electron density. Unlike oxygen and nitrogen atoms and halogens, carbon atoms push out electrons. This is the opposite of electron‐withdrawing.

Therefore, if there are many carbon atoms bonded together, the electron density will be higher.

Resonance Effects Take Into Account the Delocalization of Electrons

The inductive effect in alkyl chains is simple. The inductive effect (I effect) is the lowering of the electron density due to the bonding of functional groups with a high degree of electronegativity. This results in a higher degree of acidity (or basicity).

The resonance effect (R effect), on the other hand, is a bit more complicated. Unlike the inductive effect, which only had to consider the effects of single bonds, the resonance effect has to consider the effects of double and triple bonds. The effect of the π-bond is the resonance effect.

Among these double bonds, the resonance effect has a particularly profound effect on the orientation and acidity of aromatic ring compounds (compounds involving benzene rings). It is also involved in the reaction rates of organic chemical reactions.

For compounds with conjugated structures, the resonance effect should be taken into account. Roughly speaking, however, we can think of resonance effects as those related to the reactivity of the benzene ring.

The Resonance Effect Is Stronger than the Inductive Effect

Oxygen and nitrogen atoms are typical examples of electron-withdrawing groups. So when hydroxy groups (-OH), methoxy groups (-OCH3) and amino groups (-NH2) are attached to the alkyl chain, they become electron-withdrawing groups. You can think of these substituents as being all electron-withdrawing groups when they are bonded together.

In aromatic compounds, on the other hand, the situation is different. If a substituent can be an electron-donating group, it can be an electron-withdrawing group. This is because they resonate.

The more you can write resonance structures, the more stable the compound is. If there is resonance, the electrons can move to many places with it. This is called the delocalization of electrons. The greater the degree of delocalization, the more stable the electron state becomes.

In order to draw a resonance structure, the compound must have a double bond. Since the benzene ring has a double bond, any aromatic ring compound can write a resonant structure. For example, the following is the resonance of aniline.

If we focus on the nitrogen atom of aniline, we can see that it is pushing electrons out of the nitrogen atom towards the benzene ring. In other words, the nitrogen of the aniline acts as an electron-donating group.

Because it is a nitrogen atom, there is a force that attracts electrons through the inductive effect. However, compared to the inductive effect, the effect of the delocalization of electrons due to resonance is much stronger. As a result, the amino group on the benzene ring becomes an electron-donating group.

How to Distinguish Between Electron-Donating and Electron-Withdrawing Groups in Aromatic Rings

So how do we distinguish between the electron-donating and the electron-withdrawing group on the benzene ring? To do this, look for double (or triple) bonds in the substituents.

For example, the following substituents are involved as electron-withdrawing groups on the benzene ring

  • Methoxy group (-OCH3)
  • Hydroxy group (-OH)
  • Amino group (-NH2)

If we focus on the atoms directly bonded to the benzene ring (oxygen and nitrogen atoms), we find that they can all be single bonded. As a result, these substituents are electron-donating groups on the benzene ring.

On the other hand, what if there is a double (or triple) bond within the substituent? In this case, they act as electron-withdrawing groups.

  • Carbonyl group (-CO)
  • Carboxy group (-COOH)
  • Sulfone group (-SO3H)
  • Nitro group (-NO2)
  • Cyano group (-CN)

The presence of a double bond (or triple bond) allows electrons to be drawn into the substituent group by resonance. A result is an electron-withdrawing group.

When an oxygen or nitrogen atom is attached, a substitution with only a single bond is an electron-donating group. If, on the other hand, a substituent with a double (or triple) bond is attached, it is an electron‐withdrawing group. There are some exceptions, but let’s roughly understand it this way.

The Acidity of the Benzene Ring Depends on the Substituents

Even if it is an electron-withdrawing group in an alkyl chain, it can be an electron-donating group in an aromatic ring compound. This fact must be understood first. In the resonance effect, it can be more complicated than in the inductive effect.

So why is it important to understand the resonance effect (R effect) in aromatic compounds? This is because it relates to acidity. Although the orientation (what part of the benzene ring undergoes a chemical reaction) also varies, we will focus on the acidity.

Even for the same aromatic compound, the acidity changes depending on the substituents. For example, phenolic compounds are listed in the following order of acidity.

Why does this difference show up?

Halogens attract electrons through an inductive effect. Therefore, it is understandable that they would be more acidic than phenol. Carbon atoms also push out electrons. Because they are electron-donating groups, they are less acidic than phenol.

On the other hand, what should we think of nitro and methoxy groups? These two functional groups should not be considered in terms of their inductive effects alone. Since the influence of resonance effects is very strong, we consider the acidity of the resonance.

The Stability of Electrons Varies with the Resonance Structure

For a phenol to be acidic, the more stable it is after becoming an ion, the more acidic it can be expected to be. In this case, the nitro group (p-nitrophenol) resonates as follows.

Thus, after the phenol is turned into an acid, many resonance structures can be written. The electrons are also delocalized up to the nitro group. The acidity of p-nitrophenol is high due to its high stability in becoming an acid. The presence of electron-withdrawing groups increases its acidity.

What about the methoxy groups on the other hand? The methoxy group in the aromatic ring acts as an electron-donating group. The methoxy group in the aromatic ring acts as an electron donor, and as a result, the acidity is reduced, as opposed to the previous example.

If the methoxy group resonates by donating electrons, we get the following

The methoxy group pushes electrons into the benzene ring, resulting in a negatively charged carbon and oxygen atom next to each other. When they become ions in this state, the negative charges exist next to each other.

The negative and negative charges are repelled. In other words, given the acidity of phenol as it becomes ionized, it is a disadvantage resonance structure. For this reason, the presence of an electron-donating group in the benzene ring reduces the acidity.

-The orientation of ortho-meta-para Changes Due to Resonance Effects

Note that it is not only the acidity that is involved in the resonance effect, but also the orientation. Orientation is a tool for predicting where in the benzene ring a synthetic reaction of organic chemistry will occur.

In the benzene ring, there are ortho, meta, and para positions starting from the substituents. Where the substituents bind in an aromatic compound depends on the electron-donating and electron-withdrawing groups. Different orientations also change the reactivity of aromatic compounds.

These are also affected by the resonance effect (R effect). More information on the orientation of the benzene ring is given below.

Electronegativity and Resonance Contribute to the Stability of the Compound

Various substituents are attached to the molecule. These types of functional groups lead to different properties of the molecule.

The most obvious is the inductive effect (I effect). The higher the electronegativity, the more electrons the substituent attracts. As a result, the acidity of the molecule will differ. The difference in acidity is very important not only in the degree of acidity and basicity, but also in the degree of reactivity.

However, even substitutes involved in the inductive effect as electron-withdrawing groups can function as electron-donating groups in aromatic compounds. So, it is important to be able to distinguish the difference between electron-donating and electron-withdrawing groups.

The resonance effect is stronger than the inductive effect. So, depending on what substituents are on the benzene ring, the acidity and orientation of the molecule can vary greatly.

Substituents play a major role in the properties of the molecule. The properties of the substituents change depending on whether it is an alkyl chain or a benzene ring that is bonded to the molecule. Make sure you understand this fact.