Reactivity of the C-H bonds of methyl hexanoate toward the tert-butylperoxy radical

Abstract

155Carboxylic acid esters resulting from the liquid phase oxidation of hydrocarbons and their oxygen derivatives [1,2] can enter into free radical chain pro cesses involving both the acyl and alkoxyl moieties [1,3,4]. The electron withdrawing functional group affects the mechanisms of formation and transforma tion of the intermediates (peroxyl radicals and hydro peroxides) and also the reactivity of the C-H bonds nearest to this functional group [1,3,4]. It was shown for the reactions of tert butylperoxy radicals with cyclohexyl acetate at 333 K [5] that the ester group activates the α C-H bonds and substantially deacti vates the β and γ C-H bonds of the alkoxyl moiety with respect to the peroxyl radicals. Although there have been many works devoted to ester oxidations (see monograph [1] and review [3]), the influence of the functional group on the reactivity of various types of C-H bonds in the acyl moiety remains unclear. The discovery of a new mechanism and new products of the oxidation of carbonyl containing compounds at the β C-H bonds [6-8] makes disputable the results of an earlier study [9] in which the relative reactivity of the C-H bonds of methyl hexanoate were determined from the composition of the alcohols that resulted from the reduction of the oxidation products of this ester with lithium aluminum hydride at 413 K.The purpose of the present work is to determine, using methyl hexanoate as an example, the reactivity of the main types of C-H bonds in the acyl moiety of the ester toward the tert butylperoxy radical at 373 K. EXPERIMENTALMethyl hexanoate (analytical grade) was purified by vacuum distillation in flowing argon. According to GLC data, the product contained 99.5 ± 0.1% methyl hexanoate.tert Butyl hydroperoxide (TBHP) was synthesized and purified by a standard procedure [10]. As deter mined by iodometry, the synthesized compound con tained at least 99.3 ± 0.2% hydroperoxide. Peroxide compounds were quantified by a colorimetric method using a reagent containing Fe 2+ ions and N,N dime thyl para phenylenediamine [11].The reference substances necessary for gas chro matographic identification of methyl hexanoate oxi dation products were synthesized using standard methods. 2 Hydroxyhexanoic acid was obtained by the hydrolysis of 2 bromohexanoic acid [12]. Accord ing to GLC data, the 2 hydroxyhexanoic acid content of the product was 99.3 ± 0.2%. Methyl 6 hydroxy hexanoate was obtained by heating ε caprolactone with water in a sealed tube followed by the esterifica tion of the carboxyl groups with diazomethane. 2 Hexenic acid was synthesized by the Knoevenagel condensation of butanal and propanedioic acid in the presence of pyridine [13] and was purified by consec utive recrystallization from water and an ethanolwater (1 : 1) mixture. The melting point of the product was 32°С (reference value: 32-33°С [13]). 2,3 Epoxy hexanoic acid was obtained by treating 2 hexenic acid with a 30% aqueous solution of hydrogen peroxide in the presence of Na 2 WO 4 [14]. The purity of the result ing acid, according ...

The specific features of the liquid-phase oxidation of saturated esters. Kinetics, reactivity and mechanisms of formation of destruction products

“…37 It is found that recombination of peroxyl radicals without chain termination also takes place during oxidation of aliphatic esters. 40 In studies of the recombination of peroxyl radicals, including radicals formed during the oxidation of esters, an assumption was made that the Russell mechanism (Scheme 5) with six-membered transition state 12 is too simplifi ed, but it rather adequately refl ects the nature of the transformations of peroxyl radicals. 12,13,16,64…”

“…Now we can consider the data on the kinetics of accumulation of primary products of methyl hexanoate oxidation initiated with cumene hydroperoxide at 373 К (Fig. 1) 40 and cyclohexyl acetate oxidation initiated with azobisisobutyronitrile at 333 К (Fig. 2) 42 together with the data on the composition of products of neopentyl heptano- (7), and hexanoic acid (8).…”

“…16,[35][36][37][38][39] The eff ect of the electron withdrawing group on the reactivity of the α-, β-, and γ-СН bonds of acyl and alkoxyl moieties of esters in their reactions with electrophilic peroxyl radicals can be explained by its dual (activating and deactivating) eff ect on this reaction, 16 which allows one to highlight the specifi c features of the oxidation of esters, which distinguish it from the oxidation of saturated hydrocarbons. [16][17][18] Over the past two decades, new experimental data on the reactivity of the main types of CH bonds of acyl and alkoxyl moieties of esters have been obtained, [40][41][42][43] new mechanisms of oxidation of the acyl groups involving their β-СН bonds have been established, 40,41,44 a new channel for recombination of peroxyl radicals during the ester oxidation, which occurs without the chain termination step, has been revealed. 40,41,45 Taking the liquid-phase oxidation of tert-butylphenylacetate as an example, a ki-netic model was developed.…”

“…[16][17][18] Over the past two decades, new experimental data on the reactivity of the main types of CH bonds of acyl and alkoxyl moieties of esters have been obtained, [40][41][42][43] new mechanisms of oxidation of the acyl groups involving their β-СН bonds have been established, 40,41,44 a new channel for recombination of peroxyl radicals during the ester oxidation, which occurs without the chain termination step, has been revealed. 40,41,45 Taking the liquid-phase oxidation of tert-butylphenylacetate as an example, a ki-netic model was developed. 38 It well reproduces the experimentally observed concentrations of peroxide and non-peroxide products and makes it possible to estimate the contributions of channels and mechanisms of their formation and transformations.…”

"TRIPHENYLPHOSPHINE OXIDE INTERACTION WITH ZINC CHLORIDE IN 2-BUTANOL"

“…Опыт ГЖХ-анализа гидропероксидных продуктов окисления спиртов [7], кетонов [8], сложных эфиров [9,10] и карбоновых кислот [11] с использованием насадочных хроматографических колонок показывает, что присутствие ТФФ и ТФФО в пробах приводит лишь к необходимости регулярного проведения процедуры стабилизации хроматографической колонки при температурах выше рабочей температуры колонки для удаления из нее абсорбированных ТФФ и ТФФО. В ГЖХанализе образцов, содержащих ТФФ и ТФФО с использованием капиллярных хроматографических колонок, кроме необходимости регулярной стабилизации колонки возникает еще ряд проблем.…”