Structure and Reactivity of α,β-Unsaturated Ethers. IV. The Effects of Ring Substituents on the Acid-Catalyzed Hydrolysis Rate of Phenyl Vinyl Ether

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Abstract
The rates of hydrolysis of various ring-substituted phenyl vinyl ethers, CH2=CHOC6H4X, have been measured in 80 : 20 dioxane-water containing hydrochloric acid as catalyst. The ring substituents investigated include H, p-CH3O, m-CH3O, p-CH3, m-CH3, p-Cl and m-Cl. The reaction was first order with respect to both the acid and the ethers. It has been found that the effect of substituents on the hydrolysis rates obeys the Hammett ρσ relationship with the reaction constant ρ=−2.21 at 35°C. These results are compatible with the previous conclusion that the acid-catalyzed hydrolysis of alkenyl ethers involves the rate-determining protonation of the ethers at their β-carbon atom, to form intermediate oxycarbonium ions. It has been shown that the observed substituent effect can be rationalized from the π-electronic energy changes of the ethers on protonation, which have been calculated by an empirical molecular orbital method.