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)