Kinetics and mechanism of acid-catalysed addition of methanol to α-methoxystyrenes

Abstract

A kinetic study of the complex behaviour of ring-substituted α-methoxystyrenes in acidic (HBr) methanol is reported. The reaction yields the corresponding acetals as the only kinetic products; these are subsequently partly hydrolysed into acetophenones with rates only a little lower than those of their formation. The rate-limiting step for methanol addition corresponds to proton attachment to the double bond, yielding an oxocarbenium ion which is trapped by methanol with a rate constant 10⁴ higher than that of ion deprotonation. The rate constants for proton transfer from CH₃OH₂⁺ are in the same range of magnitude as those for proton transfer from H₃O⁺ in water. The lyonium ion catalytic constants fit a good Young–Jencks equation with ρⁿ–2.86 and ρ^r–1.28.