The benzene ring is very important in organic chemistry. Compounds with a benzene ring are called aromatic compounds.
The benzene ring contains a very large number of electrons. This is the so-called electron-rich state, where the electrons in the benzene ring can react with other molecules to cause an organic chemical reaction. This is called electrophilic aromatic substitution reactions.
However, when an electrophilic substitution reaction occurs at the benzene ring, the position at which the chemical reaction occurs is fixed. This is called orientation. In more detail, the orientation of an aromatic compound changes depending on the substituent group.
In addition, the reactivity (how efficiently the reaction occurs) also differs depending on the functional group. Here, we will explain how orientation and reactivity in electrophilic aromatic substitution reactions differ depending on the substituents.
Table of Contents
- 1 Electrophilic Aromatic Substitution Reactions Vary in Location
- 2 The Orientation of Benzene Rings Varies with Electron-Donating and Electron-Withdrawing Groups
- 3 Halogens (Chlorine and Bromine) are ortho- and para-Oriented
- 4 Alkylbenzene (Toluene) Has ortho-para Orientation
- 5 Reactivity Varies Greatly with Electron-Donating and Electron-Withdrawing Groups
- 6 Understanding Electrophilic Aromatic Substitution Reactions
Electrophilic Aromatic Substitution Reactions Vary in Location
When an electrophilic substitution reaction occurs for a benzene ring, the substitution site differs depending on the type of aromatic compound.
In the case of the benzene ring, the positions can be divided into three, starting from the substituent.
- The ortho position
- The meta position
- The para position
For example, electrophilic aromatic substitution reactions include Friedel-crafts reactions. When Friedel-crafts is performed on methoxybenzene (anisole), the following compounds are formed.
Electrophilic aromatic substitution reactions create a new substituent at the ortho or para position. Why is this selective for certain positions? Doesn’t a substitution reaction occur at the meta-position? This is because orientation is involved.
Resonance Effect by Substituents (R Effect)
When an aromatic compound undergoes an electrophilic substitution reaction, there are different types. Consider the following two types of benzene ring orientations.
- Ortho-para orientation
So let’s understand that in the benzene ring, either the substitution reaction occurs at ortho-para, or the substitution reaction occurs at meta.
-Different People Have Different Ways of Explaining Orientation
There are different ways to explain the reasons for this orientation, which vary from person to person. For example
- You can write a lot of resonance structures of reaction intermediates.
- You can write resonance structures that satisfy the octet rule in the reaction intermediates.
- There is charge repulsion (or stability) in the reaction intermediate.
- Resonance effect (R effect) is present.
These are all correct answers. In science, it is common for results from experiments to come out and prepare theories later to make sense of it all. In short, any method of explanation is fine as long as you understand why.
However, the easiest way to understand the orientation of aromatic compounds is to explain them using the resonance effect (R effect). The effect of molecular resonance is called the resonance effect (R effect). For example, aniline resonates as follows.
The influence of the substituents present in the benzene ring causes it to resonance in this way. Once we understand these resonances, we can easily understand the orientation of the benzene ring.
The Orientation of Benzene Rings Varies with Electron-Donating and Electron-Withdrawing Groups
So how does the resonance effect affect the orientation? In this regard, the orientation depends on whether the substituent in the aromatic compound is an electron-donating or an electron-withdrawing group.
For the resonance effect, the orientation of the benzene ring would be as follows.
- Electron-donating group: ortho-para orientation
- Electron-withdrawing group: meta-orientation
The substituents attached to the benzene ring can be divided into two groups: electron donating and electron withdrawing. And depending on whether they are electron-donating or electron-withdrawing groups, the orientation of the benzene ring changes.
Why does this happen? The reason is that the electron density on the benzene ring varies from place to place as a result of resonance.
Phenol and Aniline Are Electron-Donating Groups: ortho-para Orientation
Typical functional groups that show electron-donating properties by binding to the benzene ring are as follows.
- Methoxy group (-OCH3)
- Hydroxy group (-OH)
- Amino group (-NH2)
The oxygen and nitrogen atoms in phenols and anilines act as electron-donating groups. Writing the resonance structure, these aromatic compounds have a higher electron density in the ortho and para positions.
In the following, we again describe the resonance structure of aniline.
Thus, you can write a resonance where electrons exist in the ortho and para positions. There are many electrons in the ortho and para, not in the meta.
In an organic synthesis reaction, the electrons attack other molecules to cause the reaction. This resonance structure predicts that when an electrophilic aromatic substitution reaction takes place, the electrons in the ortho and para react chemically with other molecules. As a result, the electrophilic substitution reaction occurs at ortho and para.
Earlier we explained that in methoxybenzene (anisole), the substitution reaction occurs at ortho and para. This is because the electron-donating group binds to the benzene ring and becomes ortho-para oriented.
Orientation of ortho and para Involves Steric Hindrance
Which of the ortho-para orientations is produced more frequently, ortho or para? This is largely influenced by the substituents.
As you can see from the resonance structure, ortho can have two resonances written. Para, on the other hand, has a single resonance structure. So, statistically, the synthetic reaction should produce twice as many compounds with substituents in the ortho position.
However, in practice, this does not happen. In most cases, the substitution reaction occurs at the para position, not the ortho position. The probability that a compound with a substituent in the ortho position will be produced, but a compound with a substituent in the para position is more likely to be synthesized.
What is the reason for this? It is because of steric hindrance. In the ortho position, a substitution reaction occurs next to the already existing substituted group. On the other hand, in the para position, the substituent is far enough away from the substituent that no steric hindrance occurs.
When you sit on the couch, do you prefer to sit in a seat with a fat person in the middle? In that case, the seating is very uncomfortable. Even if there are a number of sofas around, if there is a fat person sitting on all of them you will consciously avoid them.
By contrast, what if you find a couch in the distance with no one sitting on it? Even if it’s far away, you will walk over to that couch and sit down.
People and substituents alike don’t like three-dimensional obstacles. When a substituent cannot physically enter a space, steric hindrance occurs. As a result, the substitution reaction occurs at the para instead of the ortho. Also, the larger the substituent, the more likely it is that steric hindrance will occur.
Electron-Withdrawing Groups Such As Nitro and Carbonyl Groups Are meta-Oriented
On the other hand, what about electron-withdrawing groups? Electron-withdrawing substituents include, for example, the following.
- Carbonyl groups (-CO)
- Carboxy group (-COOH)
- Sulfone group (-SO3H)
- Nitro group (-NO2)
- Cyano group (-CN)
They are all functional groups with double or triple bonds. For these π-bonded substituents, they will be electron-withdrawing.
For these electron-withdrawing groups, they are meta-oriented. When an electrophilic substitution reaction occurs in an aromatic compound, we can think of it as occurring in the meta position rather than in the ortho or para position. For example, nitrobenzene resonates as follows.
Considering the resonance effect, we can see that cations (positively charged carbons) are produced in the meta and para positions. Due to the low probability of electrons, electrophilic substitution reactions are unlikely to occur at this location. As a result, the substitution reaction occurs at the meta position where the electrons are present.
Sometimes Explained by the Stability of the Intermediate
Orientation by electron-withdrawing groups is often explained by the stability of the intermediate. For example, when a synthetic reaction occurs with nitrobenzene, the intermediates are as follows.
If a substitution reaction occurs at ortho or para, we can write the positive charges next to each other in the intermediate resonance, as noted in the diagram above. When the same charges are next to each other, they repel each other. It turns out that these resonance structures are unstable and do not contribute to stabilization by resonance.
On the other hand, what about when an electrophilic substitution reaction occurs in the meta position? The intermediate is not unstable, because there is no resonance structure with adjacent positive charges. The result is meta-orientation.
Some professors use this method to explain the meta-oriented of electron-withdrawing groups. It just makes it more difficult to understand. With writing the ortho-meta-para and all the resonance structures, the stable structure of the intermediate products can be determined. Therefore, the easiest way to understand meta-orientation is by using the resonance effect (R effect).
Halogens (Chlorine and Bromine) are ortho- and para-Oriented
So what is the case with halogens? A large number of aromatic compounds also have halogens. The important halogens in organic chemistry are as follows.
- Fluorine (F)
- Chlorine (Cl)
- Bromo (Br)
These halogens are ortho-para-oriented. Halogens have an unshared electron pair (lone pair), which causes them to resonance. For example, chlorobenzene resonates as follows.
Just as we explained in the case of electron-donating groups, chlorobenzene is ortho-para-orientated. Halogen substituents, including fluorine, chlorine, bromine, and iodine, can be understood as ortho-para-orientated.
Inductive Effect (I Effect) Reduces Reactivity
However, the properties of common electron-donating groups and halogens are quite different. Halogens are known to have a remarkably high degree of electronegativity, so the electron density on the benzene ring is low. This is because the halogen attracts electrons to the ring. This is called the inductive effect (I effect).
It is the electron-donating group that provides electrons by resonance and increases the electron density on the benzene ring. Halogens, on the other hand, attract electrons.
We don’t understand what it means to attract electrons while giving them away. However, this is because there are two completely different effects in the halogen.
As mentioned above, the halogen substituents are ortho-para-oriented because of the resonance effect. The effect of resonance is usually stronger than the electronegativity. Therefore, hydroxy and methoxy (-OH) and methoxy (-OCH3) and amino (-NH2) groups are electron-donating groups and have a higher electron density in the benzene ring.
Halogens, however, are known for their high electronegativity. The electron attraction is a phenomenon in which the electron is strongly attracted by the electronegativity, which is called the inductive effect. In the case of halogens, the inductive effect is stronger, resulting in a lower electron density on the benzene ring and a weaker reactivity.
- Resonance effect: higher electron density on aromatic rings (ortho-meta-orientation).
- Induction effect: Lowering the electron density of aromatic rings
In halogens, they work in two completely different ways. As a result, although it is ortho-meta oriented, it is less reactive in electrophilic substitution reactions by attracting electrons on the benzene ring.
Alkylbenzene (Toluene) Has ortho-para Orientation
So far, we’ve seen the orientation of various substitutions. However, there are also alkyl chains among the substituents. If alkyl chains are present in aromatic rings, what is the orientation?
In alkylbenzene, the orientation is ortho-para orientation. Alkyl chains are known to provide electrons. In other words, they are electron-donating groups. Although they do not have an unshared electron pair (lone pair) like oxygen or nitrogen atoms, and they do not actively resonate with each other, carbon atoms are known to be electron-donating groups that push out electrons.
Understand that any electron-donating substituent will show ortho-para orientation.
Tertiary Carbocation Is Easily Stabilized
So what makes alkylbenzene show ortho-meta orientation? Carbon atoms do not resonate in the same way as oxygen and nitrogen atoms, although they do provide electrons. Therefore, it is necessary to explain ortho-para orientation in a different way from the resonance effect.
This has to do with the stability of the carbocation. Although carbocation is an unstable substance, its stability varies with its structure. Among the carbocations, the carbocation with more carbon atoms bound to it is more stable.
This difference in properties leads to differences in orientation in alkylbenzenes.
Let’s consider toluene as an example. A methyl group is attached to the benzene ring to form toluene. When an electrophilic aromatic substitution reaction occurs at toluene, the resonance of the intermediate is as follows.
In the resonance structure of these intermediates, let’s focus on the carbocation state. Only when there are substituents in the ortho and para positions can we write the resonance structure of the tertiary carbocation. As a result, it is more stable than the substitution reaction in the meta.
As mentioned earlier, the tertiary carbocation is the more stable structure of the carbocation. This is the reason why alkylbenzenes such as toluene show ortho-para orientation. If you have trouble remembering it, you can say that the alkyl chain is electron-donating and therefore shows ortho-para-orientation.
Reactivity Varies Greatly with Electron-Donating and Electron-Withdrawing Groups
We have explained that the orientation varies greatly depending on whether it is an electron-donating or an electron-withdrawing group. The electron-donating group results in ortho-para orientation, while the electron-withdrawing group results in meta-orientation.
Furthermore, the electronic state also has a significant effect on the reactivity of aromatic compounds. It is as follows.
- Electron-donating groups: higher reactivity.
- Electron-withdrawing groups: less reactive
There are many electrons in the Benzene ring. This is why electrophilic substitution reactions occur. As the term electrophilic substitution reaction suggests, the more electrons there are on the benzene ring, the more likely it is that a synthetic reaction will occur.
If an electron-donating group is present, electrons are actively pushed out into the benzene ring. Therefore, when an electron-donating group is present at the benzene ring, only a small amount of energy is needed to cause the reaction to occur.
On the other hand, the presence of an electron-withdrawing group makes the reaction less reactive. The electrons in the benzene ring are attracted to the functional group, so the benzene ring is no longer electron-rich. In the presence of electron-withdrawing groups, the activation energy required for a synthetic reaction is greater.
Consider that the state of the electrophilic substitution reaction depends on what the electronic state of the benzene ring is.
Summary of Orientation and Reactivity (Reaction Rate)
So what does this sum up to date look like? The key points are as follows.
- Orientation changes with electron-donating groups and electron-withdrawing.
- Reactivity (rate of reaction) varies depending on whether there are more or fewer electrons in the benzene ring.
- Halogens are ortho-para-oriented, but they attract electrons.
In an electrophilic aromatic substitution reaction, the following occurs.
|-NH2, -NHR, -NR2||ortho and para||Very fast|
|-OH||ortho and para||Very fast|
|-OR (R=CH3 etc)||ortho and para||Very fast|
|-R (Alkyl chain)||ortho and para||Fast|
|-F, -Cl, -Br, -I (halogen)||ortho and para||Slow|
|-COR (R=OH, OR, NH2 etc)||meta||Slow|
Why does it have this orientation? And why does the reactivity (reaction rate) differ in this way? This can all be explained by electron-donating and electron-withdrawing groups.
For halogens, it’s special: they are ortho-para-oriented but exceptionally less reactive. This is because of the high degree of electronegativity, causing an inductive effect.
Predicting Orientation from Substituents
By looking at the substituents, we can determine whether they are electron-donating or electron-withdrawing groups, and we can distinguish the orientation of the aromatic ring. For example, acetanilide has an electron-donating substituent.
At the nitrogen atom in the acetanilide, an unshared electron pair pushes electrons out to the benzene ring. The result is ortho-para-orientation by acting as an electron donor. They are also more reactive. Depending on the nature of the substituents, the orientation and reactivity of aromatic compounds can be predicted.
In the case of acetanilide, there is a substituent (-NH-CO-CH3). Since it is a large substituent, the steric hindrance is greater. For this reason, the possibility of electrophilic substitution reactions at the para position is higher than with ortho.
Understanding Electrophilic Aromatic Substitution Reactions
Since the benzene ring contains a large number of electrons, it undergoes an electrophilic substitution reaction. This is called an electrophilic aromatic substitution reaction.
However, when a benzene ring undergoes an electrophilic substitution reaction, there is a law. It is not a free substitution reaction. There are certain points in the ortho, meta, and para where the substitution reaction occurs. The reason for this has been explained so far.
Some of the examples are special, like halogens. Also, in ortho-para-orientation, steric hindrance due to substituents has to be taken into account. By taking these into account, we can see what kind of compounds can be synthesized. Reactivity and reaction energy can also be estimated.
Reactions at the benzene ring are very important in organic chemistry. Understand these orientations and reactivity, and consider the synthetic reactions so that you get the desired compounds.