Electrophilic substitutions of benzene (2023)

Electrophilic substitution is the replacement of an electrophile (proton) from an aromatic ring with another electrophile. The aromatic ring is preserved.


Important Notes


An electrophilic substitution is the replacement of an electrophile (aproton) from the aromatic ring with another electrophile. The aromatic ring is preserved.


The mechanism of electrophilic substitution consists of two steps. In the first step, the aromatic ring uses two of themPI NumberThe electrons bond with the electrophile, creating a positively charged intermediate. In step 2, the proton is removed from the ring and the electrons from the broken C-H bond are used for regenerationPI Numberbinds and restores the aroma.

intermediate stabilization

The electrophilic substitution is aided by the fact that the positively charged intermediate is stabilized by resonance, leading to delocalization of the positive charge. Because the intermediate is stabilized, the reaction proceeds more easily.


Benzene can be halogenated with chlorine and bromine. Lewis acid such as FeBr3Lubricate FeCl3is required to activate the halogen and make it more electrophilic.

Friedel-Crafts Alkylation and Acylation

The alkyl chains are attached to benzene by Friedel-Crafts alkylation using an alkyl chloride and a Lewis acid. The Lewis acid plays an important role in forming the acarbocation, which acts as the electrophile for the reaction. Primary alkyl chlorides are not ideal for Friedel-Crafts reactions because the primary carbocations produced can rearrange into more stable secondary or tertiary carbocations. The Friedel-Crafts alkylation can also be carried out with an alkene or an alcohol in the presence of a mineral acid. The Friedel-Crafts acylation is the reaction of benzene with an acid chloride and a Lewis acid. The acyl ion is formed as an electrophile and has the advantage over the carbocation that it is not rearranged. The product is an aromatic ketone. The keto group can be reduced to form alkyl chains that would be difficult to attach by Friedel-Crafts alkylation.

(Video) Electrophilic aromatic substitution | Aromatic Compounds | Organic chemistry | Khan Academy

sulfonation and nitration

Benzene sulfonated with concentrated sulfuric acid. The reaction produces sulfur trioxide, which acts as an electrophile. The nitration takes place with concentrated nitric acid and sulfuric acid. Sulfuric acid acts as an acid catalyst in the production of the electrophilic nitronion. Both electrophiles in these reactions are strong and no Lewis acid is required.


Aromatic rings undergo electrophilic substitution, e.g. B. the bromination of benzene (Fig. 1). An electrophile (Br+) replaces another electrophile (H+), leaving the aromatic ring intact. Hence, one electrophile replaces another and the reaction is called electrophilic substitution. (In this step we skip the formation of the bromine cation.)

Electrophilic substitutions of benzene (1)


In the mechanism (Grab. 2) The aromatic ring acts as a nucleophile, providing two of themPI NumberElectrons to form a bond with Br. The aromatic ring has now lost one of its formal double bonds, resulting in a positively charged carbon atom. This first step in the mechanism is the same as described for the electrophilic addition to alkenes, so the positively charged intermediate here corresponds to a non-electrophilic carbocation intermediate. However, in step 2, the mechanisms of electrophilic addition and electrophilic substitution differ. While the alkene carbocation intermediate reacts with the nucleophile to form an addition product, the aromatic ring intermediate loses a proton. CHPThe bond is broken and two electrons move to the ring to re-form the bondPI NumberBonding, regenerating the aromatic ring and neutralizing the positive charge on the carbon. This is the mechanism that occurs in all electrophilic substitutions. The only difference is in the nature of the electrophile (Grab. 3).

intermediate stabilization

The rate-limiting step in an electrophilic substitution is the formation of a positively charged intermediate. Therefore, the reaction rate depends on the energy level of the transition state leading to this intermediate. The transition state is similar in nature to an intermediate, so any agent that stabilizes the intermediate also stabilizes the transition state and lowers the activation energy required for the reaction. Therefore, electrophilic substitution is more likely if the positively charged intermediate can be stabilized. Stabilization is possible if the positive charge can be distributed between different atoms - a process called delocalization. The process by which this can happen is called resonance (Fig. 4).

Electrophilic substitutions of benzene (2)

The resonance process involves twoPI NumberThe electrons shift their position in the ring to give the "top" carbon a fourth bond, thus neutralizing its positive charge. The next carbon in the ring remains unbonded and acquires a positive charge. This process can be repeated so that the positive charge is distributed to the third carbon. withdrawn structuresGrab. 4are called resonance structures.


A stable aromatic ring means that aromatic compounds are less reactive than alkenes toward electrophiles. For example, analken reacts with Br2while the aromatic ring does not. Therefore, we need to activate the aromatic ring (i.e. make it a better nucleophile) or activate Br2(i.e. make it a better electrophile) if we want a reaction to take place. We will later explain how electron-donating substituents in an aromatic ring increase the nucleophilicity of an aromatic ring. Here we will see how Father2The molecule can be activated to make it more electrophilic. This can be done by adding a Lewis acid such as FeCl3, February3AlCl3to the reaction environment. All of these compounds contain a central atom (iron or aluminum) that is highly electrophilic and does not have a complete valence electron shell. This allows the central atom to accept a lone pair of electrons, even from a weakly nucleophilic atom such as a halogen. In the example shown (Grab. 5) Bromine uses a lone pair of electrons to bond with the Fe atom in FeBr3and is positively charged. The bromine is now activated to behave as an electrophile and will more readily react with the nucleophile (aromatic ring) due to the normal electrophilic substitution mechanism.

(Video) Electrophilic Aromatic Substitution

The aromatic ring can be chlorinated with Cl in a similar manner2in the presence of FeCl3.

Electrophilic substitutions of benzene (3)

Friedel-Crafts Alkylation and Acylation

Friedel-Crafts alkylation and acylation (Pussy. 6) are two other examples of electrophilic substitutions that require the presence of a Lewis acid and are particularly important as they allow the construction of larger organic molecules by addition of alkyl (R) or acyl (RCO) side chains to the aromatic ring.

Electrophilic substitutions of benzene (4)

An example of a Friedel-Crafts alkylation is the reaction of benzene with 2-chloropropane (Abb.7). Lewis-Syre (AlCl3) promotes the formation of the carbocation required for the reaction by accepting a lone pair of electrons from chlorine and forming an unstable intermediate that fragments to form the carbocation and AlCl4-(Grab. 8). Once the carbocation is formed, it reacts as an electrophile with the aromatic ring via the electrophilic substitution mechanism previously described (Grab. 9).

Electrophilic substitutions of benzene (5)

There are limitations to Friedel-Crafts alkylation. For example, the reaction of 1-chlorobutane with benzene gives two products containing only 34% of the desired product (Grab. 10). This is because the resulting primary carbocation can rearrange into a more stable secondary carbocation in which the hydrogen (and the two sigma electrons that form the C-H bond) "slide" over the adjacent carbon atom (Grab. 11This is the so-calledhydride shiftand that is because the secondary carbocation is more stable than the primary carbocation. Such rearrangements limit the type of alkylation that can be performed in the Friedel-Crafts reaction.

Electrophilic substitutions of benzene (6)

(Video) Electrophilic Aromatic Substitution Reactions Made Easy!

Against this background, how can structures such as 1-butylbenzene be prepared in good yield? The answer to this problem can be found hereFriedel-Crafts Acylierung(Abb.12). By reacting benzene with butanoyl chloride instead of 1-chlorobutane, the required 4-C backbone is attached to the aromatic ring and no rearrangement occurs. The carbonyl group can then be removed by reduction with hydrogen over a palladium catalyst to give the desired product.

Electrophilic substitutions of benzene (7)

The mechanism of Friedel-Crafts acylation is the same as Friedel-Crafts alkylationAcylliumion instead of a carbocation. Similar to the Friedel–Crafts alkylation, a Lewis acid is required to generate the acyl ion (R–C=O).+, but unlike the carbocation, the acyl ion does not rearrange since the oxygen stabilizes the resonance (Abb.13).

Electrophilic substitutions of benzene (8)

The Friedel-Crafts alkylation can also be carried out with alkenes instead of alkyl halides. Lewis acid is not required, but amino acid is. Treatment of an alkene with an acid leads to a carbocation, which can then react with the aromatic ring via the same electrophilic substitution mechanism previously described (Abb.14). The alkene is another example of electrophilic addition, where a proton is added to one end of the double bond and a phenyl group is added to the other end.

Electrophilic substitutions of benzene (9)

Friedel-Crafts reactions can also be performed with alcohols in the presence of a mineral acid. The acid results in the removal of water from the alcohol, producing an alkene, which can then be converted to a carbocation as previously described (Grab. 15).

Electrophilic substitutions of benzene (10)

(Video) Nitration of Benzene Mechanism - Electrophilic Aromatic Substitution Reactions

sulfonation and nitration

Sulfonation and nitration are electrophilic substitutions that involve strong electrophiles and do not require the presence of a Lewis acid (Grab. 16).

Electrophilic substitutions of benzene (11)

In sulfonation, the electrophile is sulfur trioxide (SO).3), which is formed under acidic reaction conditions (Grab. 17). The protonated intermediate product (I) is formed by protonation of the OH group. When oxygen acquires a positive charge, it becomes a good leaving group and water is lost from the intermediate, forming sulfur trioxide. Although sulfur trioxide does not have a positive charge, it is a strong electrophile. This is because the sulfur atom is bonded to three electronegative oxygen atoms, all of which "draw" electrons from the sulfur, making it electronless (i.e., electrophilic). Under electrophilic substitution (Grab. 18), the aromatic ring forms a bond with sulfur and a medPI NumberThe bonds between sulfur and oxygen are broken. Both electrons move to the more electronegative oxygen, forming a third lone pair of electrons and creating a negative charge on that oxygen. This is eventually neutralized when the third lone pair of electrons is used to form a bond with the proton.

Electrophilic substitutions of benzene (12)

In nitration, sulfuric acid serves as an acid catalyst to form nitronium ions (NO).2), which is produced from nitric acid in a very similar way to how sulfur trioxide is produced from sulfuric acid (Grab. 19).

The mechanism of benzene nitration is very similar to that of sulfonation (Grab. 20Since the aromatic ring bonds to the electrophilic nitrogen atom, aPI NumberThe bond between N and O is broken and both electrons go to the oxygen atom. Unlike sulfonation, this oxygen retains a negative charge and does not capture a proton. This is because it acts as a counter ion to the neighboring nitrogen positive charge.

  • previous page
  • Next page

Learning Materials, Lecture Notes, Homework, Reference, Wiki Description, Explanation, Brief Details

Organic Chemistry: Aromatic Chemistry: Electrophilic Substitutions of Benzene |

(Video) Adding Cl to Benzene: Electrophilic Substitution


1. Reaction Mechanism 05 | Electrophilic Substitution 01 : Chlorination , Nitration in BENZENE JEE/NEET
(Physics Wallah - Alakh Pandey)
2. Aromatic Halogenation Mechanism - Chlorination, Iodination & Bromination of Benzene
(The Organic Chemistry Tutor)
3. Electrophilic Substitution of Benzene Mechanism (A-Level Chemistry)
(Chemistry Student)
4. Electrophilic Substitution Of Benzene
5. Electrophilic Substitution of Benzene | A-level Chemistry | OCR, AQA, Edexcel
6. 44h: Electrophilic aromatic substitution on benzene with two or more substituents
(Roxi Hulet)


Top Articles
Latest Posts
Article information

Author: Mrs. Angelic Larkin

Last Updated: 23/07/2023

Views: 5950

Rating: 4.7 / 5 (67 voted)

Reviews: 82% of readers found this page helpful

Author information

Name: Mrs. Angelic Larkin

Birthday: 1992-06-28

Address: Apt. 413 8275 Mueller Overpass, South Magnolia, IA 99527-6023

Phone: +6824704719725

Job: District Real-Estate Facilitator

Hobby: Letterboxing, Vacation, Poi, Homebrewing, Mountain biking, Slacklining, Cabaret

Introduction: My name is Mrs. Angelic Larkin, I am a cute, charming, funny, determined, inexpensive, joyous, cheerful person who loves writing and wants to share my knowledge and understanding with you.