Solution thermodynamics and kinetic solvent effects on a Diels-Alder reaction

Abstract

The kinetics of the Diels-Alder condensation of maleic anhydride with 1,3-butadiene has been measured in seven solvents at 25.00"C using a vapour-pressure technique. The rate constants are used in conjunction with activity coefficient data to find the relative activity in solution of the transition state from the Brransted-Bjerrum equation. These results show that the transition state resembles electronically the reactants rather than the product, and permit comment on the reaction mechanism and on the prediction of solvent effects for Diels-Alder reactions.Transition state theory provides a powerful tool for visualizing the molecular details of a chemical reaction in terms of thermodynamic functions. For example, the rate constant k (relative to its value in an ideal solution ko) for a bimolecular reaction of A and B, through a transition state M, is given in terms of thermodynamic activity coefficientsIn (klko) = In (YAYBlYM).(1) Although the properties of the transition state are not directly available experimentally, they can be obtained from the application of eqn (1). Such information can then prove useful for many purposes, such as for the interpretation of reaction mechanisms or for solvent Arnett and McKelvey have applied the transition state theory in a detailed study of the activation parameters for some solvolysis reactions in aqueous mixtures.In this work is reported the kinetics of the simple, bimolecular Diels-Alder condensation of maleic anhydride (A) and 1,3-butadiene (B) to form the product (P), cis-4-cyclohexene-1 ,Zdicarboxylic anhydride.This reaction has been carried out in seven solvents at 25°C using a vapour-pressure technique. The results are compared with measurements of activity of A, B and P in solutions as obtained from vapour pressure, solubility, and differential boiling point *Then the information about the solvent-solute interactions of the transition state as found from eqn (1) permits examination of the reaction mechanism as well as indicating an approach for solvent design for Diels-Alder react ions.

Structure and solvation of confined water and water–ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis

“…In contrast, the (C 2 H 5 OH)(H 3 O + )(H 2 O) 4-5 MARI contains only one non-polar ethyl group, which disrupts surrounding H 2 O networks to a lesser extent than the two ethyl groups of the transition state. This analysis is conceptually similar to that of the activation volume [114][115][116][117] in liquid-phase reactions or the activation area on two-dimensional surfaces, 118 where increasing pressures and concomitant increasing densication of the solvent phase destabilizes transition states with positive activation volume, according to the empirical equation: 115 v lnðkÞ vP…”

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

“…131 The surface pressure is analogous to the three-dimensional pressure used to account for similar forces on reaction volumes for homogeneous systems. [27][28][29][30][31][32][33]131 Such surface pressures must increase monotonically with increasing CO* coverages and thus with CO pressure (…”

“…The term in square brackets in eq 18 represents the activation area at a specific surface pressure, and its sign determines whether η increases or decreases as adlayers densify, in the same manner as activation volumes determine how hydrostatic pressure influences rates for liquid-phase reactions. [27][28][29][30][31][32][33]131 A DFT-derived activation area can be estimated from CO* and [*HCO−H*] ⧧ structures optimized on close-packed terraces of Ru 218 clusters at low coverages (0 spectating CO*, 0.02 ML) and high coverages (106 and 110 spectating CO* per 106 Ru surface atoms, 1.00 and 1.04 ML). Surface areas of each adsorbed species are taken as the area of the Ru surface occluded by their atoms, with atomic sizes determined by their van der Waals radii 132 (Table 1).…”

“…Eqs 17 and 18 are analogous to those derived using similar formalisms to account for the effects of (hydrostatic) pressure in homogeneous reactions occurring in the liquid or gas phase and mediated by intermediates present in their thermodynamically nonideal states. [27][28][29][30][31][32][33]131 These formalisms are translated here into concepts of adlayer compression and two-dimensional lateral pressure to describe chemical reactions occurring on densely covered surfaces. 131 Next, we estimate the magnitudes of the enhancement factors (η) and the k app values (eq 15) caused by CO* adlayer densification as consequence of the negative activation area for these CO hydrogenation reactions.…”

“…Coverage effects of this type have been previously considered through modified microkinetic models − and by kinetic Monte Carlo methods − that implicitly or explicitly account for co-adsorbate interactions. Here, we will instead account for these co-adsorbate interactions by treating dense adlayers of adsorbates as thermodynamically nonideal systems for which the rates of surface reactions depend on their “areas of activation”, through an analogy with the activation volumes often used for homogeneous reactions in dense and nonideal liquid and gaseous media. − Specifically, we address the effects of dense CO* adlayers and assess the adequacy of Langmuirian models for CO hydrogenation on Ru-based catalysts, ,− widely used along with Ni, − Co, ,− and Fe − catalysts, in methanation and Fischer–Tropsch synthesis (FTS). High CO pressures (>100 kPa) shift reaction selectivities from CH 4 products to larger hydrocarbons and lead to very dense saturated CO adlayers at all relevant conditions (450–550 K, 0.1–5 MPa).…”