The Mechanism of Contact Elimination, A Contribution to Understanding the Function of Polar Catalysts

Abstract

The current ideas of organic chemists based on the work of Ingold and his school are applied to heterogeneous catalytic eliminations (mostly from haloalkanes and aliphatic alcohols). It is deduced from the activity of the catalysts, the reactivity of the substrates (reactants), and the primary product distribution that these eliminations proceed by a heterolytic mechanism similar to that involved in the liquid phase. The activity of the catalysts (salts and oxides) increases with increasing charge and decreasing radius of the cations and with increasing basicity of the anions. The reactivity of the substrates behaves in much the same manner as in the liquid phase. In contrast with theliquid-phase reaction, the cis-olefins are frequently favored as primary products. The stereospecificity of the reaction is determined from the relative strengths of the interactions between the catalyst cation and the leavinggroup X -, and between the catalyst anion and the leaving proton. Only trans elimination has so far been found in the concerted mechanism.

Dehydrobromination of 1,1-diaryl-1,2-dibromoalkanes catalyzed by iron and iron bromides. An ion-pair-mediated reaction?

“…that a similar ion-pair-mediated mechanism may be involved in hydrogen bromide elimination from l a catalized by alkali lysts and those of dehydrobrominations carried out with transition metal catalysts could provide decisive evidence on the We report here the results of research into the possible participation of an ion-pair-initiated mechanism in these reactions, as had been proposed for metal-ion-catalyzed dehydrohalogenation of haloalkanes in the gas phase (3)(4)(5)(6).…”

“…strates over solid polar catalysts has been studied by Noller et al (3,4). Dehydrochlorination of chloroalkanes with lithium 1,2-Dibromo-l , l -diphenylethane, la, yields ~-bromo-l,l -diand sodium halides has been interpreted on the basis of the phenylethene, 2a, in the Presence of catalytic amounts of some Lewis acidity of the cations and coordination chemistry contransition and transition-like metals and their bromides.…”

Elimination and Addition Reactions

“…The conclusions from homogeneous reactions applicable to heterogeneously catalyzed eliminations have been detailed by Noller and Kladnig [6]. Several reviews centered on dehydration have been published, by Pines and Manassen [7], Notari [8], Knözinger [9,10], Noller, Andreu and Hunger [11], John and Scurrell [12], and Winterbottom [13]. A unified treatment of kinetics and mechanism of heterogeneously of catalyzed elimination and addition reactions has been presented by Beranek and Kraus [14].…”

“…This problem has been extensively investigated for dehydration and for dehydrohalogenation [11,14]. The ratio of the isomers obtained is governed by the nature of the catalyst and the structure of reactants through the chosen mechanism, and may vary considerably [14].…”

“…Experimental data indicate that the preferential elimination mode is anti [11,[28][29][30]. However, one would expect that with two-point adsorption on the surface of a catalyst the obvious mode would be syn.…”