Effects of Para-Substituents on the Mechanisms of Solvolysis of Styrene Oxides

Article abstract of DOI:10.1021/jo00056a027

Rate and product studies of the hydronium ion-catalyzed, hydroxide ion-catalyzed, and spontaneous reactions of styrene oxide and its p-CH3O, p-CH3, p-Cl, and p-O2N-derivatives in water solutions have been carried out.A Hammett correlation of log k for the acid-catalyzed reactions vs ?⁺ gives a slope ρ⁺ of -4.2 and only diol products are formed.An intermediate in the hydronium ion-catalyzed hydrolysis of p-methoxystyrene oxide is trapped, subsequent to its rate-limiting formation, by azide ion.The spontaneous reactions of p-methylstyrene oxide and styrene oxide yield only diol products, and their reactions in 18O-water indicate that >/= 98percent and ca. 95percent, respectively, of 18O is incorporated into the benzyl positions.Nucleophilic addition of water to the benzyl carbon of neutral epoxide is proposed as the mechanism of the primary component of the spontaneous reaction for the p-CH3-, p-H- and p-Cl-substrates on the basis of a Hammett ρ of -2.0.The spontaneous rate constant for reaction of p-methoxystyrene oxide is much greater than that expected from a Hammett correlation for the p-CH3, p-H-, and p-Cl-styrene oxides.This increased rate is attributed to the incursion of a reaction that yields mainly (>80percent) p-methoxyphenylacetaldehyde.The regiochemistries of addition of hydroxide ion in 18O-water to p-CH3-, p-H-, and p-Cl-, and p-O2N-substituted styrene oxides were also determined and found to vary as functions of the para substituent.Addition of hydroxide and methoxide ions to the α-carbon is favored by electron-donating groups in the phenyl ring, and addition of these nucleophiles to the β-carbon is favored by electron-withdrawing groups in the phenyl ring.