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Electrophilic substitution on the aromatic nucleus (benzene and derivatives)

Mechanism

Hückel Rule: The aromatic systems with 2n + 2 π electrons are particularly stable. With 2 · 1 + 2 = 6 π electrons, benzene falls under the rule. - X+ is a reagent that can perform an electrophile attack . - The attack removes 2 electrons from the aromatic system, Hückel's rule is no longer verified: stability is compromised. By splitting off H+, 2 electrons are added to the aromatic system: the substituted derivative has regained its stability!

Frequent electrophiles

X+ = Cl+Chloronium Ion

Formation: Chlorine and Aluminium Chloride

X+ = Br+ Bromonium Ion

Formation: Bromine und Iron(III) Bromide

X+ = NO2+ Nitronium Ion

Formation: Concentrated nitric acid and concentrated sulfuric acid

X+ = R+ Alkylium Ion or Alkanoylium Ion(R = Alkyl oder Acyl=Alkanoyl)

Formation: Chloroalcane (or Organic acid chloride) and Aluminium Chloride

Here R+ = CH3CH2(Ethylium)

X = SO3 Sulfur Trioxide( no cation, but electron gap and 2+ charge on the sulfur atom !)

Formation: In the fuming sulfuric acid (oleum)