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	Organic Chemistry 4e Carey
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Chapter 12: Reactions of Arenes. Electrophilic Aromatic Substitution

Overview

Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution

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

There are three fundamental components to an electrophilic substitution reaction:

formation of the new s bond from a C=C in the arene nucleophile
removal of the proton by breaking the C-H s bond
reform the C=C and restore aromaticity

curly arrows for electrophilic aromatic substitution

The mechanism is represented by the following series of events:

Formation of the reactive electrophile, E⁺
Slow reaction of the arene C=C with the E⁺ to give a resonance stabilized 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.

Why Substitution not Addition ?

Overall an electrophilic aromatic substitution can be represented as follows:

But we have previously seen the C=C generally react via an electrophilic addition pathway: electrophilic addition to C=C

So why don't arenes react in a similar fashion to alkenes and give overall addition ?

The first step is common to both, E⁺ adds a C=C to give a carbocation intermediate. For an addition pathway, the nucleophile then adds to the electrophilic carbocation giving a cyclohexadiene derivative.
For a substitution pathway, the "nucleophile" functions as a base and removes a proton from the sp³ C to recreate the C=C and restore the aromaticity.

Recall that the resonance energy of benzene is about 152 kJ / mol (36 kcal / mol) and a conjugated diene is 16 kJ / mol (4 kcal/mol). This extra stability of the aromatic system is responsible for favouring the substitution reaction.

Reactions and Reagents:

The following table contains a summary of the key reactions of aromatic systems covered by this chapter. More detailed information on each reaction can be accessed by following the link from the reaction column. Note that the reagent combination and reaction conditions for reactions of benzenes are usually more severe than reactions of alkenes.... this tends to make them more readily recognizable.

The following pointers may aid your understanding of these reactions:

Recognize the electrophile present in the reagent combination
The electrophile adds first to the arene
Substitution is preferred over addition in order to preserve the stable aromatic character

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
Halogenation	Cl₂ / Fe or FeCl₃	Cl⁺		E⁺ formed by Lewis acid removing Cl^-
	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^-
	RCO₂COR / AlCl₃	RCO⁺		E⁺ formed by Lewis acid removing RCO₂^-

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