Inductive and coordination effects: electron-donating groups and electron-withdrawing groups ｜Hatsudy (2023)

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

Two particularly important effects of substituents are the induction effect (I effect) and the coordination effect (R effect). The acidity of the molecule varies depending on the substituents attached to it. It also depends on whether the substituent is attached to an alkyl chain or a benzene ring.

There are two types of substituents: electron donating and electron withdrawing. Electron-donating groups donate electrons and electron-withdrawing groups withdraw electrons. By identifying them, we can deduce the reactivity of the molecule.

So make sure you understand the principles of induction and resonance effects. Once you understand the nature of the substituents and learn to distinguish between electron-donating and electron-withdrawing groups, you can determine the acidity of the molecule.

Differences between induction effect (I effect) and resonance effect (R effect)

First: what is the inductive effect (I-effect)? And what is the resonance effect (R-effect)? Let's think of both as effects caused by the functional groups attached to the molecule. Depending on which substituents are attached to the molecule, there are many differences, such as different acidity.

The differences between the two are as follows.

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

Electron orbitals include the 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 substitution effect of the σ-bond is the induction effect.

On the other hand, some molecules form double or triple bonds. The part of the molecule that forms a double or triple bond is called a π bond. When a pi bond is present, one can write coordination structures.The coordination effect results from the coordination of the substituent, which changes the orientation (reactivity on the benzene ring) and the acidity of the molecule.

In general, understand that the induction 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 occur? Occurs when an atom (or molecule) with a high degree of electronegativity is bound.

Linkage with an alkyl chain is particularly important. An alkyl chain without double or triple bonds does not cause coordination. For this reason,For alkyl chains we only need to consider the inductive effect.

When an atom forms a bond with a high degree of electronegativity, it strongly attracts electrons. This causes the molecule to develop both a positive and a negative charge even though it is the same molecule. This is called polarization. Water, ammonia and hydrogen chloride are known to be polarized.

Polarization due to differences in electronegativity also occurs in alkyl chains.When a high electronegativity substituent is attached to an alkyl chain, it leads to a difference in acidity.

An example of an acidic substance is acetic acid. When a carboxylic acid is present, it exhibits acidic properties. However, even if it is the same carboxylic acid, the acidity will vary depending on the substituents present on the surrounding carbon.

For example, what happens when a chlorine atom, a halogen, bonds with acetic acid (CH)?₃COOH); In this case, the result is as follows.

When a chlorine atom is attached, the electrons are attracted to the chlorine atom. This means that chloroacetic acid has a stronger negative charge than carboxylic acid. This means that the acidity of chloroacetic acid is higher.

Electrons are withdrawn from the halogen, making chloroacetic acid more acidic than carboxylic acid.

The effect of the electron-withdrawing substituent is an inductive effect. This effect is responsible for acid strength. Obviously, the more halogens are bound, the greater the acidity.

As explained in the acid example, it also affects basicity in the same way. The inductive effect depends on the degree of basicity.

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

• Nitro Group (-NO₂)
• Aminogruppe (-NH₂)
• Cyan Group (-CN)
• carbonyl group (-CO)
• Carboxygruppe (-COOH)
• Sulfone group (-SO₃THE)
• Grupo Methoxy (-AND₃)
• Hydroxyl group (-OH)
• Halogen (-Cl, -Br, -I)

All attract electrons attached to the alkyl chain, causing the inductive effect.Functional groups containing oxygen or nitrogen atoms or halogens are all electron withdrawing groups.

Limited effect of electron withdrawing groups

What is the inductive effect of the electron withdrawing group? Let's be clear that this is limited.

Although electronegativity attracts electrons, the effect is small over long distances.For neighboring carbon atoms to which the electron-withdrawing group is attached, there is an effect due to the inductive effect. However, as the distance increases, the effect of electronegativity decreases.

Although the inductive effect reduces the electron density, the area of ​​influence 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, nitrogen and halogen atoms, carbon atoms repel electrons. This is the opposite of electron withdrawing.

So when many carbon atoms are bonded together, the electron density is greater.

Resonance effects are responsible for the displacement of electrons

The inductive effect on alkyl chains is simple. The inductive effect (Effect I) is the reduction of the electron density due to the binding of functional groups with high electronegativity. This leads to a higher degree of acidity (or basicity).

The resonance effect (R effect), on the other hand, is a bit more complicated. Unlike the induction effect, which only needs to consider the effects of single bonds, the resonance effect requires consideration of the effects of double and triple bonds. The result of the π bond is the resonance effect.

Between these double bondsThe resonance effect has a strong effect on the orientation and acidity of aromatic compounds (compounds with benzene rings).It is also responsible for the reaction rate of organic chemical reactions.

For compounds with conjugated structures, the coordination effect must be taken into account. In general, we can think of resonance effects as being 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. Thus, when there are hydroxy groups (-OH), methoxy groups (-OCH).₃) and amino groups (-NH₂) are attached to the alkyl chain, they become electron-withdrawing groups. Any of these substituents can be considered an electron withdrawing group.

In the case of aromatic compounds, however, the situation is different. Sometimes the substituent becomes an electron donating group and sometimes an electron withdrawing group. And that's because they resonate.

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

To be able to design coordination structures, it is important that the compound has a double bond. Because the benzene ring has double bonds, we can draw coordination structures for any aromatic ring compound. For example, here is the aniline coordination.

If we focus on the nitrogen atom of aniline, we can see that the electrons are pushed away from the nitrogen atom toward the benzene ring. In other words,The nitrogen of aniline acts as an electron donating group.

Since it is a nitrogen atom, there is a force that attracts the electrons through induction. However,Compared to the inductive effect, the effect of electron delocalization due to resonance is much stronger.This makes the amino group on the benzene ring an electron-donating group.

How do you distinguish electron-donating and electron-withdrawing groups in aromatic rings?

How do we distinguish electron-donating and electron-withdrawing groups in the benzene ring? To do this, look for double (or triple) bonds in the substituents.

The following substituents, for example, are involved as electron donating groups on the benzene ring.

• Grupo Methoxy (-AND₃)
• Hydroxyl group (-OH)
• Aminogruppe (-NH₂)

If we focus on the atoms directly attached to the benzene ring (oxygen and nitrogen atoms), we find that they are all single bonds. Therefore, these substituents are electron-donating groups on the benzene ring.

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

• carbonyl group (-CO)
• Carboxygruppe (-COOH)
• Sulfone group (-SO₃THE)
• Nitro Group (-NO₂)
• Cyan Group (-CN)

The presence of a double bond (or triple bond) allows electrons to be attracted by coordination to the substituent group. This makes it an electron withdrawing group.

When an oxygen or nitrogen atom is attached to the substituent, it becomes an electron-donating group if the substituent has only single bonds.Secondly,When a substituent is attached to a double bond (or triple bond), it becomes an electron withdrawing group.Although there are exceptions, this is a rough understanding.

The acidity of the benzene ring depends on the substituents

Although 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 first be understood. It can be more complicated with the resonance effect than with the inductive effect.

So why is it important to understand the resonance effect (R-effect) in aromatic compounds? This is because it is acidity. Although orientation (which part of the benzene ring triggers the chemical reaction) also changes, let's focus on acidity.

Even with the same aromatic compound, the acid content changes depending on the substituent.For example, phenolic compounds are listed in the following order of acidity.

Why does this difference arise?

Halogens attract electrons by induction. Therefore, it is understandable that they are more acidic than phenol. Carbon atoms also eject electrons. Because the carbon atom is an electron-donating group, its acidity is lower than that of phenol.

On the other hand, what about nitro and methoxy groups? These two functional groups should not be considered only for their inductive effects. Since the resonance effect is very strong, we must consider the acuity while considering the resonance.

Coordination structures change the stability of electrons

For phenol to be acidic,We can assume that the more stable it is after it is converted into an ion, the stronger the acidity.In this case, the nitro group (p-nitrophenol) resonates as follows.

After phenol becomes an acid, various coordination structures can be written. Electrons are also shifted to the nitro group. The acidity of p-nitrophenol is high because of its stability when it becomes acid.The presence of an electron-withdrawing group increases acidity.

On the other hand, what about the presence of a methoxy group? The methoxy group on the aromatic ring acts as an electron donating group. This will reduce the sharpness compared to the previous example.

When coordinated to the electron-donating methoxy group, the result is as follows.

The methoxy group donates electrons to the benzene ring,This creates a negatively charged carbon and an oxygen atom next to each other.When they become ions in this state, negative charges coexist.

Negative and negative charges repel each other. This means that the coordination structure is unfavorable since phenol is acidic when ionized. Because of this, the presence of an electron donating group on the benzene ring reduces its acidity.

-The tuning effect changes the alignment of Ortho, Meta and Para

Not only is sharpness affected by the resonance effect, but so is orientation. Orientation is a tool for predicting which part of the benzene ring will undergo an organic chemical reaction.

In the benzene ring there are ortho, meta and para positions of the substituents. The position at which substituents are attached in an aromatic compound depends on the electron-donating and electron-withdrawing groups. Furthermore, the reactivity of the aromatic compound changes with a different orientation.

These are also affected by the resonance effect (R-effect).

electronegativityand coordination contribute to the stability of the connection

Various substituents are attached to molecules. Depending on the type of functional group, the molecule exhibits different properties.

The most obvious is the inductive effect (I-effect). The higher the electronegativity, the more electrons the substituent will attract. As a result, the acidity of the molecule will be different. The difference in acidity is very important because it causes not only a difference in resistance to acidity and alkalinity, but also a difference in reactivity.

However,A substituent that participates in the induction effect as an electron-withdrawing group can act as an electron-donating group in aromatic compounds.Therefore, it is important to be able to tell the difference between an electron-donating group and an electron-withdrawing group.

The resonance effect is stronger than the inductive effect. Therefore, depending on which substituents are on the benzene ring, the acidity and orientation of the molecule can vary widely.

The nature of the molecule depends largely on the substituents. Depending on whether the substituent is attached to an alkyl chain or a benzene ring, the properties of the substituent change. It is important to understand this fact.