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Aromatic Electrophilic Substitutions (EArS Reactions)

Written by tutor Erin D.


Recall: When alkenes undergo reactions with electrophiles, an addition reaction occurs.

However, aromatic systems do not reaction in the same fashion as alkene containing systems, in part due to their remarkable stability.

Therefore, when aromatic compounds undergo reactions with electrophiles, a substitution reaction occurs. These types of reactions are known as electrophilic aromatic substitution (EArS or EAS) reactions.

Many functional groups can be added to compounds via EArS reactions. The chart below lists the most common types of EArS reactions. Later on, we will discuss each reaction type individually.

Reaction Type      Reagents           Electrophile      Product                Notes
Nitration HNO3/H2SO4 NO2+ E+ formed by loss of water from nitric acid (HNO3)
Sulfonation SO3/H2SO4
(fuming sulfuring acid)
SO3 Reversible
Halogenation X2/Fe or
FeX3
Cl+ or Br+ E+ formed by Lewis acid (FeCl3) removing Cl- or Br-
Friedel Crafts Alkylation R-Cl/AlCl3
R-OH H+
C=C/H+
R+ E+ formed by Lewis acid (AlCl3) removing X-
E+ formed by loss of water from alcohol
E+ formed by protonation of alkene
Friedel Crafts Acylation RCOCl/AlCl3
RCO2COR/AlCl3
RCO+ E+ formed by Lewis acid (AlCl3) removing Cl-
E+ formed by Lewis acid (AlCl3) removing RCO2-

General Mechanism

The overall transformation that occurs is as follows:

There are three crucial components to an EArS reaction:

I) Formation of a new σ-bond from a C=C nucleophile
II) Removal of a substituent (usually a proton, H+) by breaking a σ-bond (typically C-H)
III) Reforming the C=C π-bond to restore aromaticity.

(Formation of the reactive
electrophile E+ from the reagents is not shown but will be discussed later.)                                   
Slow reaction of the arene C=C nucleophile with the E+ to give a resonance stabilized carbonation. Fast reaction whereby H+ is lost from the carbonation to restore the C=C π-bond and the aromaticity of the ring.

You will note that in the mechanism the carbocation does not capture a nucleophile; instead it loses a proton. This occurs because capturing a nucleophile would result in an addition reaction and, in turn, eliminate the positive charge, which would encumber the subsequent restoration of aromaticity.

The intermediate carbocation (aka cyclohexadienyl cation, arenium ion, or sigma complex) is resonance stabilized.

The reaction with the arene is the slow step since it results in loss of aromaticity even though the carbocation is resonance stabilized. Therefore, when we look at the reaction profile of an EArS reaction, the energy required for the first step is very large, making it the rate-determining step (RDS).

Nitration of Benzene

Stepwise Mechanism

Step 1: An acid/base reaction in which the hydroxyl group of the nitric acid becomes protonated, thereby providing a superior leaving group.

Step 2: Loss of the leaving group, a water molecule, to provide the nitronium ion, the reactive electrophile.

Step3: (RDS) The electrophilic nitronium ion reacts with the p-electrons of the nucleophilic C=C of the arene. This step destroys the aromaticity of the benzene ring and affords the cyclohexadienyl intermediate.

Step 4: Water functions as a base to remove the proton from the sp3 carbon bearing the nitro group and reforms the C=C and the aromatic system.

Sulfonation of Benzene

Stepwise Mechanism

Note: Fuming sulfuric acid typically is used in sulfonation EArS reactions. Fuming sulfuric acid is the common name for 7% SO3 in H2SO4. Sulfur trioxide is the anhydride of sulfuric acid, meaning that the addition of water to SO3 affords H2SO4. While SO3 has an overall neutral charge, it is a strong electrophile, containing three sulfonyl (S=O) bonds that draw electron density away from the sulfur atom.

Step 1: (RDS) The electrophilic sulfur trioxide reacts with the p-electrons of the nucleophilic C=C of the arene. This step destroys the aromaticity of the benzene ring and affords the cyclohexadienyl intermediate.

Step 2: Loss of a proton from the sp3 carbon bearing the sulfonyl group reforms the C=C bond and restores aromaticity.

Step 3: Protonation of the conjugate base of the benzenesulfonic acid by sulfuric acid produces the final compound.

All these steps are reversible, which means that the sulfonic acid group may be removed from the aromatic ring by heating in dilute sulfuric acid. In practice, steam often is used as both water and heat for desulfonation.

Halogenation of Benzene

Stepwise Mechanism

Step 1: Bromine reacts with the Lewis acid (FeBr3) to form a complex that makes the terminal bromine more electrophilic.

Step 2: (RDS) The electrophilic bromine complex reacts with the p-electrons of the nucleophilic C=C of the arene, displacing iron tetrabromide. This step destroys the aromaticity of the benzene ring and affords the cyclohexadienyl intermediate.

Step 3: Removal of the proton from the sp3 carbon bearing the bromo group and reforms the C=C and the aromatic system.

Friedel-Crafts Alkylation of Benzene

Stepwise Mechanism

Step 1: The alkyl chloride reacts with the Lewis acid (AlCl3) to form a complex that makes the alkyl group more electrophilic.

Step 2: (RDS) The electrophilic alkyl complex reacts with the p-electrons of the nucleophilic C=C of the arene, displacing aluminum tetrachloride. This step destroys the aromaticity of the benzene ring and affords the cyclohexadienyl intermediate.

Step 3: Removal of the proton from the sp3 carbon bearing the alkyl group and reforms the C=C and the aromatic system.

Limitations to Friedel Crafts Alkylation Reactions

1. The alkyl halide cannot be a vinyl or an aryl halide because their corresponding carbocations are too unstable.
2. Carbocation rearrangements are common with these types of alkylation reactions.
3. The benzene undergoing the reaction needs to as reactive or more reactive than a monohalobenzene.
4. Over alkylation is common since the product is more reactive than the starting material.
5. AlCl3 can complex with aryl amines, making them unreactive.

Friedel-Crafts Alkylation of Benzene

Stepwise Mechanism

Step 1: The acyl chloride reacts with the Lewis acid (AlCl3) to form a complex that makes the alkyl group more electrophilic.

Step 2: (RDS) The electrophilic acyl complex reacts with the p-electrons of the nucleophilic C=C of the arene, displacing aluminum tetrachloride. This step destroys the aromaticity of the benzene ring and affords the cyclohexadienyl intermediate.

Step 3: Removal of the proton from the sp3 carbon bearing the acyl group and reforms the C=C and the aromatic system.

Limits to Friedel Crafts Acylation Reactions

1. Acylation only can give ketones; formyl chloride decomposes too readily.
2. The benzene undergoing the reaction needs to as reactive or more reactive than a monohalobenzene.
3. AlCl3 can complex with aryl amines, making them unreactive.
4. Amines and alcohols can give competing N or O acylations.

Substituent Effects on EArS Reactions

Recall: Substitution patterns on di-substituted benzene rings are characterized by ortho-, meta-, and para.

The table below depicts the effect of substituents on both the rate and the orientation of EArS reactions.

Hydrogen is an arbitrary reference group and, as such, is considered to have no effect.

Activating groups increase the rate of reactions, while deactivating groups have the opposite effect and decrease the rate of reaction.

EDG add electron density to the aryl system, making it more electrophilic. EDGs are most easily identified by lone pairs of electrons on the atom adjacent to the aryl system. Alkyl, aryl, and vinyl groups also are EDGs due to hyperconjugation. (Recall: Hyperconjugation is the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital, anti-bonding π orbital, or filled π orbital, to give an extended molecular orbital that increases the stability of the system.) EDGs direct the incoming substituent to the ortho/para positions.

EWGs remove electron density from the aryl system, making it less nucleophilic. EWGs are most easily identified by either the atom adjacent to the aryl system have multiple bonds to electronegative atoms OR a formal +-charge. However, halogen substituents are deactivating, but ortho/para directing. EWGs direct the incoming substituent to the meta position.

Effects of More than One Subtituent on EArS Reactions

When an aromatic system possesses two or more substituents, they will exert a combined effect on the reactivity of aryl ring. If the groups serve to buttress each other, the outcome will be easy to predict; for example, if both groups are activating, EDGs, the aromatic ring will be highly reactive and the incoming substituent will be directed to the open ortho or para position.

When there is a conflict between an activating group and a deactivating group, the activating group usually directs the incoming substitution pattern. This is because EDGs stabilize the cyclohexadienyl cation through resonance. Nonetheless, the stronger substituent will predominate.

EArS Reactions of Heteroaromatic Compounds

Recall: Heteroaromatic compounds are aromatic compounds that contain a heteroatom (i.e., N, O, or S) as a part of the conjugated ring. Some common heteroaromatic systems are listed below.

The presence of a heteroatom can influence the reactivity of the system when compared with the reactivity of benzene. Many 5- and 6-membered heteroaromatics are more reactive than benzene because they can further delocalize the positive charge of the sigma-complex.

Pyridine, however is less reactive than benzene because the electronegativity of the N-atom, making it a π-electron deficient aromatic compound. The basic nature of the N-atom interferes and can interact with the incoming electrophile, which serves to further deactivate the ring.

EArS Reactions Quiz

Use the following reaction for questions 1 and 2

1. In the reaction below, toluene is the ______________ and Cl+ is the _______________.

A. electrophile/nucleophile
B. electrophile/electrophile
C. nucleophile/electrophile
D. nucleophile/nucleophile
The correct answer here would be C.

2. FeCl3 serves as Lewis ________________ to make the electrophile ______________ reactive.

A. base/less
B. acid/less
C. base/more
D. acid/more
The correct answer here would be D.

3. All of the following names describe the intermediate carbocation in an EArS reaction, EXCEPT:

A. sigma complex
B. cyclohexadienyl complex
C. arenium ion
D. none of the above
The correct answer here would be D.

4. A nitro group will direct an incoming electrophile to ____________ position.

A. Para
B. Ortho
C. Ortho and para
D. Meta
The correct answer here would be D.

5. In Friedel-Crafts acylation reactions, multiple acylations do not occur because the resulting compound __________________ the product.

A. deactivates
B. neutralizes
C. activates
D. sterilizes
The correct answer here would be A.

6. Inductive effects of substituent groups in aromatic compounds shuttle electrons via

A. delta bonds
B. sigma bonds
C. pi bonds
D. ionic bonds
The correct answer here would be B.
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