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Directive Influence of Groups on Electrophilic Aromatic Substitution – Ortho-Para Directing Groups and Meta Directing Groups

Directive Influence of Groups on Electrophilic Aromatic Substitution – Ortho-Para Directing Groups and Meta Directing Groups

Assume you live in a 4BHK. You only utilise the first-floor bedroom of the four total bedrooms; the other three are for visitors. There are two rooms on each of the first and ground floors.

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Every time a visitor arrives at your home, you put them up in one of the three rooms. Depending on the type of visitor that has come, you decide in which room they stay. If the visitor is a friend or a relative, they can stay in the first-floor room that is close to yours so you may have fun with them. However, if the visitor is an elderly relative, they can stay in ground-floor accommodation because they will find it difficult to climb stairs frequently. But in the end, you get to make the call.

Similarly in chemistry, when monosubstituted benzene is exposed to substitution reaction, it accommodates the new functional group to either in ortho, para or meta position based on the type of group.

In this article, we will understand the directive influence of groups on electrophilic aromatic substitution reactions.

TABLE OF CONTENTS

  • Electrophilic Aromatic Substitution Reaction
  • Directive Influence of Groups on EAS
  • Ortho-Para Directing Groups
  • Meta Directing Groups
  • Directive Influence of Halogens
  • Practice Problems
  • Frequently Asked Questions – FAQ

Electrophilic Aromatic Substitution Reaction

Electrophilic aromatic substitution reactions occur when an electrophile replaces an atom attached to an aromatic ring. These reactions typically involve the replacement of a hydrogen atom from the benzene ring with an electrophile.

An electrophilic aromatic substitution reaction preserves the aromaticity of the aromatic system. For example, when bromobenzene is formed from the reaction of benzene and bromine, the aromatic ring's stability is not lost. This reaction is depicted in the diagram below.

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Because benzene is an electron-rich system with delocalized electrons, it undergoes electrophilic substitution reactions.

The general reaction of electrophilic aromatic substitution can be represented as

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Directive Influence of Groups on EAS

All the six hydrogen atoms of a benzene ring are equivalent. Therefore, the replacement of any one atom by any substituent always gives a single monosubstituted benzene derivative. However, when a monosubstituted benzene derivative is converted into a disubstituted benzene derivative, the substituent already present on the benzene ring determines the position of the incoming group. The ability of an existing group in the benzene ring to direct an incoming group to a specific position is referred to as the directive influence of groups.

The rate of the reaction and the site of the electrophilic attack on monosubstituted benzene depends on the functional group already attached to it. Activating groups are those that make the benzene ring more reactive while deactivating groups are those that make it less reactive. The new group may be primarily directed to the ortho, meta, or para position depending upon the nature of the group already present on the benzene ring, and the electrophilic substitution reaction on monosubstituted benzene may proceed more slowly or more quickly than it would with benzene alone.

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Depending on how these groups affect the direction of attack by the incoming electrophile, they are divided into two categories. The groups which increase the electron density at the ‘meta’ position are known as "meta directors," and the ones that increase the electron density at the ‘ortho’ and ‘para’ positions are known as "ortho-para" directors.

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Ortho-Para Directing Groups

Ortho-para directors typically place the incoming group in an ortho or para position to itself. All electron-donating groups are typically ortho, para-directing. Further, it may be pointed out here that although two o- (ortho) and one p- (para) positions are available for disubstitution, due to steric hindrance with the incoming group at ortho-position, it is usually the para-isomer which predominates in these substitution reactions. Generally, electron-releasing groups (+M, +I) are ortho-para directing groups and activating towards electrophilic reactions. The following are some of the activating groups that are ortho-para directing.

-NH2, -NHR, -NHCOCH3, -OCH3, -CH3, -C2H5, etc,.

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Explanation

With the exception of alkyl and phenyl groups, all other groups have at least one lone pair of electrons on the atom directly attached to the benzene. This lone pair of electrons is involved in resonance with the -electrons of the benzene ring. As a result of resonance, the electron density increases at all the nuclear positions of the benzene ring but the increase in electron density is much more at the ortho and para positions than at the meta position.

Consider the directive influence of -OH group on benzene.

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Evidently, the electrophile will attack the benzene ring at the position where the electron density is high. Since the electron density is high at the ortho and para-positions than at the meta positions in phenol, the electrophile will attack preferentially at the ortho and the para positions. Thus, -OH group is ortho-para directing. The reactivity of the benzene ring towards an electrophile increases by the +M effect of -OH group. The -OH group activates the benzene ring for attack by an electrophile and is therefore called an activating group.

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Directive Influence of Alkyl Group

The alkyl group (R) does not have a lone pair of electrons. Its directive influence can be explained on the basis of the hyperconjugation effect as shown below.

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The electron density increases at the ortho and para positions and hence, the directive influence of CH3 group is ortho para directing.

Effect of Ortho-Para Directing Groups on Reactivity

Since the ortho-para directing groups increase the electron density on the benzene ring, the ring gets activated and further electrophilic substitution on the ring becomes easier. It is because of this reason that all ortho-para directing groups except halogens are called activating groups. Further, because of the ability of these groups to donate electrons to the benzene ring, ortho-para directing groups are also called electron-repelling or electron-donating groups (EDG). In light of the above discussion, two cases arise.

  1. Activating groups facilitate further electrophilic substitution reactions.

For example, the nitration of toluene occurs faster than that of benzene because of the hyperconjugation effect of the CH3 group in toluene. Due to this effect, the electron density in toluene is higher than that in the benzene ring.

  1. Higher the electron-donating ability of the substituent, the more facile the reaction.

For example, the bromination of aniline occurs faster than that of phenol due to lower electronegativity of N than O. The electron-donating ability of -NH2 group is much higher than that of -OH group. As a result, the electron density in the aniline ring is much higher than that in phenol, and hence, the bromination of aniline occurs faster than that of phenol.

Order of activating effects of ortho-para directing groups

-NH2> -HNR > -NR2 > -OH> -OR > -NHCOR> -OCOR> -R

Meta Directing Groups

Meta-directing groups are substituents or groups that direct the incoming group to the meta position. For example, (CH3)3N+-, -NO2, -CN, -CF3, -CHO, -COR-COOH, -COOR, -SO3H, CCl3, CF3, etc, are meta directing groups. In general, all electron-withdrawing groups are m-directing. Generally, electron-withdrawing groups (–M, –I) are meta-directing groups and deactivating towards electrophilic reactions.

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Thus, the nitration of nitrobenzene mainly gives m-dinitrobenzene.

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Explanation

With the exception of trifluoromethyl and trichloroethyl (CCl3, CF3) groups, the atom directly attached to the benzene ring in all other groups has one more electronegative atom linked to it by a multiple bond. This more electronegative atom pulls the electrons of the multiple bond towards itself, which in turn withdraws electrons from all the nuclear positions. As a result, the electron density decreases at all the nuclear positions, but the decrease is much more at the ortho and para positions than at the meta positions of the benzene ring.

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In other words, the electron density is comparatively higher at meta positions than at ortho and para positions and hence, the electrophile will attack preferentially at the meta position in benzaldehyde. Thus, -CHO group is meta directing.

Similarly, we can explain the meta-directing influence of the -NO2, group. Due to electron-withdrawing resonance effect (i.e., -R-effect) of the - NO2 group, electron density decreases more at the ortho and the para positions than at the meta positions as shown below.

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In other words, the electron density is comparatively higher at meta positions than at ortho and para positions, and therefore, further substitution in nitrobenzene will occur at the meta positions.

Effect of Meta Directing Groups on Reactivity

Since meta directing groups decrease the electron density in the benzene ring, the ring gets deactivated. Hence, further electrophilic substitution becomes difficult. It is because of this reason that meta-directing groups are also called deactivating groups. Further, because of their ability to withdraw electrons from the benzene ring, meta-directing groups are also called electron-attracting or electron withdrawing groups (EWG). In the light of above discussion, the following two cases arise:

  1. Deactivating groups make further electrophilic substitution more difficult.

For example, the nitration of benzene occurs faster than that of nitrobenzene. This is due to the reason that the nitro group in nitrobenzene decreases the electron density on the benzene ring as it is an electron-withdrawing group. Benzene lacks electron-withdrawing groups, and therefore, there is no such effect operating in benzene. As a result, the electron density in benzene ring is higher than that in nitrobenzene ring, and hence, the electrophile (ie., NO2+) will attack benzene faster than nitrobenzene.

  1. Higher the electron-withdrawing ability of the substituent, more difficult is the reaction.

For example, the nitration of nitrobenzene occurs much slower than that of benzoic acid since -NO2 group is much more powerful electron-withdrawing group than carboxylic acid group -COOH. As a result, electron density in the benzoic acid ring is much higher than that in nitrobenzene. Hence, further electrophilic substitution will occur faster in benzoic acid than in nitrobenzene.

Directive Influence of Halogens

Halogens, which are deactivating, are ortho-para directing in nature. The reactivity of halobenzene towards electrophile decreases due to –I effect of halogens. Ortho-para directing nature is decided by +M effect of halogens.

Because of their strong -I-effect, halogens are little deactivating in the case of aryl halides. As a result, the total electron density on the benzene ring decreases. In other words, halogens are deactivating due the -I effect. However, because of the +R-effect, i.e., the participation of lone pairs of electrons on the halogen atom in resonance with the -electrons of the benzene ring, the electron density increases more at the ortho and para positions as compared to the meta positions.

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Practice Problems

1. Which of the following positions is activated by an activating substituent?

a. Para position
b. Ortho position
c. Both A and B
d. Meta Position

Answer: C

Solution: When a substituent that is electron-donating in nature, or in other words, activating, the ortho and the para positions are activated, and the meta positions are not activated.

For example, in phenol, the ortho and the para positions are activated due to the +M effect of -OH group.

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So, option C is the correct answer.

2. Which of the following groups is ortho-para directing?

a. NHCOCH3
b. –NO2
c. –CN
d. –CHO

Answer: A

Solution: NHCOCH3 is an ortho-para directing group. This is because, the nitrogen atom attached to the benzene ring contains a lone pair of electrons, and this lone pair of electrons is involved in resonance with the -electrons of the benzene ring. As a result of resonance, the electron density increases at all the nuclear positions of the benzene ring but the increase in electron density is much more at the ortho and para positions than at the meta position.

-NO2, -CN and -CHO are meta-directing groups as they are electron-withdrawing in nature.

So, option A is the correct answer..

3. Which of the following groups best activates the benzene ring?

a. – NHCOCH3
b. –OH
c. –NH2
d. –C6H5

Answer: C

Solution: Among the groups given in the options, –NH2 group best activates the benzene ring due to the presence of a lone pair of electrons on the nitrogen atom attached to the benzene ring. This lone pair of electrons is involved in resonance with the -electrons of the benzene ring. As a result of resonance, the electron density increases at all the nuclear positions of the benzene ring.

–OH group also contains a lone pair of electrons on the oxygen atom attached to the benzene ring that is in resonance with the -electrons of the benzene ring. But due to the high electronegativity of the oxygen atom, –OH group is less activating than the –NH2 group.

Since esters and amides generate resonance structures that drive the electron density away from the ring, they are less activating than other substituents. The – NHCOCH3 group has two resonance structures in which the lone pair of electrons on nitrogen interacts with the C=O group. Therefore, the lone pair of electrons on the nitrogen atom is less available for donation.

–C6H5 is an electron-withdrawing group (-I) and therefore is not activating in nature.

So, option C is the correct answer.

4. Which of the following is a meta directing group?

a. – NHCOCH3
b. –COOH
c. –OH
d. –OCH3

Answer: B

Solution: –COOH is a meta directing as it an electron-withdrawing group due to its -M effect. – NHCOCH3, –OH and –OCH3 are ortho-para directing groups due to the presence of lone pair of electrons on nitrogen in NHCOCH3 and on oxygen in –OH and –OCH3.

So, option B is the correct answer.

Frequently Asked Questions – FAQ

1. Why are ortho-para directing groups referred to as activating groups?
Answer
Generally, ortho-para directing groups are electron-donating in nature (+M, +I). Due to their ability to donate electrons to the benzene ring, they activate the benzene ring for further reaction by increasing the electron density of the ring. So, ortho-para directing groups are referred to as activating groups.

2. Why does benzene undergo electrophilic substitution reactions easily but not nucleophilic substitution reactions?
Answer
Due to the presence of an electron cloud containing 6 π-electrons above and below the plane of the ring, benzene is a rich source of electrons. As a result it draws electrophiles (electron-loving) and repels nucleophiles (electron-repelling). As a result, benzene easily undergoes electrophilic substitution reactions while finding it challenging to undergo nucleophilic substitution reactions.

3. What does electrophilic aromatic substitution serve?
Answer
One of the most significant reactions in synthetic organic chemistry is the electrophilic aromatic substitution. Important intermediates that can be employed as precursors for the manufacturing of medicinal, agrochemical, and industrial products are created by these processes.

4. Why is electrophilic substitution tedious in pyridine?
Answer
Pyridine enters electrophilic aromatic substitution processes less readily than benzene derivatives because of the electronegative nitrogen in the pyridine ring.

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