Intermediaires aryl carbonium en bromation. Nivellement de la contribution d'un cycle aromatique par les substituants donneurs de l'ion carbonium ArC+(R′)R″

The bromination of trans α-methyl stilbenes, Ar(Me)CHCHPh, is studied as a model of reactions with carbonium ion intermediates. Direct measurement of ρ, the sensitivity of bromination to the effect of substituents in an aromatic ring over a wide range of reactivity, is possible. Despite the linearity of the ρσ⁺ relationship, the real value of ρ, ?4·87, is only obtained by a Yukawa-Tsuno treatment which is justified by the absence of coplanarity in the aromatic ring and the carbonium ion. The effect of substituents in the ring decreases on going from the stilbenes, ρ = ?5·05, to the α-methyl stilbenes, ρ = ?4·87. Similar decreases are observed in solvolysis, protonation and electrophilic aromatic substitution. We conclude that the decrease in ρ, which occurs when the electron-donating ability of R′ and R″ in the cation ArC⁺(R′)R″ is increased, is a general feature of reactions with carbonium ion intermediates. This is attributed to two types of effect; the increase in the stability of the intermediate and “saturation” of charge stabilization by the substituents R′ and R″. The variation of ρ with structure of substituents is analysed.