Electrophilic aromatic substitution or EAS

Electrophilic aromatic substitution or EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile. The most important reactions of this type that take place are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating Friedel-Crafts reactions.
aromatic substitution (EArS):

What does the term "electrophilic aromatic substitution" actually imply ?

An electrophile, E+, is an electron poor species that will react with an electron rich species (the arene)
Aromatic because the reaction is characteristic of aromatic systems.
A substitution implies that a group is replaced (usually H).

Electrophilic Aromatic Substitution
Overall an electrophilic aromatic susbtitution (EArS) can be represented as follows:

There are three fundamental components to an electrophilic aromatic substitution mechanism:

formation of the new σ bond from a C=C in the arene nucleophile
removal of the proton by breaking the C-H σ bond
reforming the C=C to restore the aromaticity

The mechanism is represented by the following series of events:

Formation of the reactive electrophile, E+ (not shown here) from the reagents
Slow reaction of the arene C=C with the E+ to give a resonance stabilised carbocation (see below)
Loss of H+ from the carbocation to restore the C=C and the aromatic system

The reaction of the electrophile E+ with the arene is the slow step since it results in the loss of aromaticity even though the resulting cation is still resonance stablised. This carbocation is also described as the cyclohexdienyl cation or arenium ion or as a sigma-complex.

Reactions and Reagents
The following table contains a summary of the key reactions to introduce a substituent onto an aromatic system.
More detailed information on each reaction can be accessed by following the link from the reaction column.
Reactions that can be used to convert existing aromatic substituents are listed separately.

Study Tips:

The reagent combinations and reaction conditions for reactions of benzenes are usually more severe than reactions of alkenes.... this tends to make them more readily recognisable.
Recognise the electrophile present in the reagent combination.

Reaction	Reagents	Electrophile	Product	Comments
Nitration	HNO3 / H2SO4	NO2+		E+ formed by loss of water from nitric acid
Sulfonation	H2SO4 or SO3 / H2SO4	SO3		Reversible (heat with aq. acid)
Halogenation	Cl2 / Fe or FeCl3	Cl+		E+ formed by Lewis acid removing Cl-
Halogenation	Br2 / Fe or FeBr3	Br+		E+ formed by Lewis acid removing Br-
Alkylation	R-Cl / AlCl3	R+		E+ formed by Lewis acid removing Cl-
	R-OH / H+	R+		E+ formed by loss of water from alcohol
	C=C / H+	R+		E+ formed by protonation of alkene
Acylation	RCOCl / AlCl3	RCO+		E+ formed by Lewis acid removing Cl-
Acylation	RCO2COR / AlCl3	RCO+		E+ formed by Lewis acid removing RCO2-

Nitration of Benzene

Reaction type: Electrophilic Aromatic Substitution
Summary

Overall transformation : Ar-H to Ar-NO2
Reagent : for benzene, HNO3 in H2SO4 / heat
Electrophilic species : the nitronium ion (i.e. NO2+) formed by the loss of water from the nitric acid

MECHANISM FOR NITRATION OF BENZENE
Step 1: An acid / base reaction. Protonation of the hydroxy group of the nitric acid. This provides a better leaving group.....
Step 2: Loss of the leaving group, a water molecule provides the nitronium ion, the reactive electrophile.
Step 3: The electrophilic nitronium ion reacts with the nucleophilic C=C of the arene. This is the rate determining step as it destroys the aromaticity of the arene.
Step 4: Water functions as a base to remove the proton from the sp3 C bearing the nitro- group and reforms the C=C and the aromatic system.

Sulfonation of Benzene

Reaction type: Electrophilic Aromatic Substitution
Summary

Overall transformation : Ar-H to Ar-SO3H, a sulfonic acid.
Reagent : for benzene, H2SO4 / heat or SO3 / H2SO4 / heat (= fuming sulfuric acid)
Electrophilic species : SO3 which can be formed by the loss of water from the sulfuric acid
Unlike the other electrophilic aromatic substitution reactions, sulfonation is reversible.
Removal of water from the system favours the formation of the sulfonation product.
Heating a sulfonic acid with aqueous sulfuric acid can result be the reverse reaction, desulfonation.
Sulfonation with fuming sulfuric acid strongly favours formation of the product the sulfonic acid.

MECHANISM FOR SULFONATION OF BENZENE
Step 1: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic S, pushing charge out onto an electronegative O atom. This destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 2: Loss of the proton from the sp3 C bearing the sulfonyl- group reforms the C=C and the aromatic system.
Step 3: Protonation of the conjugate base of the sulfonic acid by sulfuric acid produces the sulfonic acid.

Halogenation of Benzene

Reaction type: Electrophilic Aromatic Substitution
Summary

Overall transformation : Ar-H to Ar-X
Reagent : normally the halogen (e.g. Br2) with a Lewis acid catalyst
The active catalyst is not Fe (0) but the FeX3 formed by reaction of Fe with X2
Electrophilic species : the halonium ion (i.e. X +) formed by the removal of a halide ion by the Lewis acid catalyst
Restricted to Cl2 and Br2. I- or F- are usually introduced using alternative methods

MECHANISM FOR HALOGENATION OF BENZENE
Step 1: The bromine reacts with the Lewis acid to form a complex that makes the bromine more electrophilic.
Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic Br, and displacing iron tetrabromide. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3: Removal of the proton from the sp3 C bearing the bromo- group reforms the C=C and the aromatic system, generating HBr and regenerating the active catalyst.

Friedel-Crafts Alkylation of Benzene

Reaction type: Electrophilic Aromatic Substitution
Summary

Overall transformation : Ar-H to Ar-R
Named after Friedel and Crafts who discovered the reaction in 1877.
Reagent : normally the alkyl halide (e.g. R-Br or R-Cl) with aluminum trichloride, AlCl3, a Lewis acid catalyst.
The AlCl3 enhances the electrophilicity of the alkyl halide by complexing with the halide.
Electrophilic species : the carbocation (i.e. R +) formed by the "removal" of the halide by the Lewis acid catalyst
The reactive electrophile, the carbocation is prone to rearrangement to a more stable carbocation which will then undergo the alkylation reaction.

Friedel-Crafts reactions are limited to arenes as or more reactive than mono-halobenzenes
Other Lewis acids such as BF3, FeCl3 or ZnCl2 can also be used
Other sources of carbocations can also be used:
- from loss of water from alcohols treated with acid such as H2SO4
- from the protonation of alkenes by acid such as H2SO4
Alkylation products can also be obtained by the reduction of Friedel-Crafts acylation products (more details)

MECHANISM FOR THE FRIEDEL-CRAFTS ALKYLATION OF BENZENE
Step 1: The alkyl halide reacts with the Lewis acid to form a a more electrophilic C, a carbocation
Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3: Removal of the proton from the sp3 C bearing the alkyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.

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