Secondary Deuterium Isotope Effect in an Electrophilic Aromatic Substitution ‐ Protodesilylation of Trimethylphenylsilane

Reaction rates for the protodesilylation of trimethylphenylsilane and of [2,4,6‐2H3]‐trimethylphenylsilane by HCIO4, were measured in aqueous methanol (2:5, v/v) and the secondary deuterium isotope effect for the reaction was found to be kH/kD3 = 0.79. The magnitude of the observed isotope effect supports a mechanism in which the rate‐determining step is the proton transfer from the hydronium ion to the silane to form a σ‐intermediate.