Kinetic Studies on the Elimination Reactions of 1-(p-Nitrophenyl) 2-(2 and 4-quinolyl)ethyl Acetates in Some Protic Solvents

Abstract

Eliminations of acetic acid from 1-(p-nitropheny1)-2-(2 and 4-quinolyl)ethyl acetates ([1 a]and [1b]) to give trans-2 and -4-(p-nitrostyryl)quinolines ([2 a] and [2 b]) have been studied kinetically in some alcohols at 60°C. The elimination was catalyzed by a base (KOH)and several acids (RCO₂H, HCl and BF₃ ). But no reaction was observed in the presence of sodium acetate alone or without any catalyst (Table 1). The catalytic ability of the acids decreased in the order: RCO₂H (R; Cl₃ C>Cl₂CH>CF₃>ClCH₂>Ph>CH₃) >HCl>BF₃ (Tables 1 and 3, and Fig.1). In the acid-catalyzed reaction, the rate of [1 a] was greater than that of [1 b], but [1 a] was less reactive than [ 1 b] in the hydroxide-catalyzed reaction (Tables 1and 4). The rate was expressed as v=k[RCO₂H]₃ [Substrate] for the carboxylic acid catalyzed elimination (Table 2). The rate ratio r= k([1 a])1k([1 b]) decreases from 5.7 to 1.2 with increasing acidity of carboxylic acids and dielectric constant of solvents (Fig.2). Salt effect on the acetic acid-catalyzed elimination was studied in ethanol-water (22: 1 v/v). No normal ionic strength and common anion effects were observed for both substrates. However, the rate increase of [1 b] was proportional to [AcOM]₀^1/2 (Fig.3). The kinetic isotope effect (k_h/k_D)was measured for the elimination of [1 a] in some protic solvents (Table 5). The value of k_H/k_D was larger than unity in all cases, suggesting a rate-determing elimination of β-hydrogen. The value of k_H/k_D increases with the increasing basicity of the attacking base on the β-hydrogen for the carboxylic acid-catalyzed elimination. This is consistent with a transition state from the nearly El to the symmetrical. Possible mechanisms are proposed and discussed.