Diels-Alder reactions activation entropy

Enthalpy of reaction, maleimide, 266 Enthalpy values, MA polymerization, 244 Entropy of activation, Diels-Alder reactions, 138 Entropy values, MA polymerization, 244 Environmental Protection Agency, regulations, in MA production, 17... 

Unfortunately, the number of mechanistic studies in this field stands in no proportion to its versatility" . Thermodynamic analysis revealed that the beneficial effect of Lewis-acids on the rate of the Diels-Alder reaction can be primarily ascribed to a reduction of the enthalpy of activation ( AAH = 30-50 kJ/mole) leaving the activation entropy essentially unchanged (TAAS = 0-10 kJ/mol)" . Solvent effects on Lewis-acid catalysed Diels-Alder reactions have received very little attention. A change in solvent affects mainly the coordination step rather than the actual Diels-Alder reaction. Donating solvents severely impede catalysis . This observation justifies the widespread use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis-acid catalysed Diels-Alder reactions. 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

With application of reasonable values for trapping parameters and AS2, it was possible to bracket the enthalpy and entropy of activation for isomerization of cyclobutadiene. Hence, A/Zj was estimated to fall between 1.6 and lOkcal/mol, where the upper limit was consistent with theoretical predictions for square-planar cyclobutadiene. Most surprising, though, was the conclusion that AS for automeriza-tion must lie between -17 and -32cal/(molK), based on the AS values normally observed for Diels-Alder reactions as a model for AS2. ... 

The antibody-catalyzed Diels-Alder reaction developed by Schultz utilized a Diel-Alderase enzyme-like catalyst evolved from an antibody-combining site (Eq. 12.13). The idea is that the generation of antibodies to a structure that mimics the transition state for the Diels-Alder reaction should result in an antibody-combining site that lowers the entropy of activation by binding both the diene and dienophile in a reactive conformation. 

The retro-Diels-Alder reaction has been reviewed.A fully concerted cyclic transition state has been proposed for conrotatory opening of cyclobutenes, in order to account for the low activation entropy and unexpected activation volume of ca —2 to —3cm mol . ... 

Equation 12.35) a low activation enthalpy and highly negative activation entropy (AH = 15.3 kcal mole-1, AS = —35 cal mole-1 °K-1) suggest a concerted mechanism similar to that of the Diels-Alder reaction.70... 

By measuring activation parameters (Eq. (2) and Table 2), it has been shown that the cceleration arises from a favorable change of activation entropy, which is an indication of the implication of the hydrophobic effect [32], Concentrated aqueous carbohydrate solutions (glucose and saccharose for instance) have been shown to accelerate the Diels-Alder reaction. The acceleration is even greater than that observed with saturated fi-cyclodextrin solution (Eq. (3) and Table 3) [32],... 

The computed activation energy of 19.6 kcal/mol in vacuum at 298 K is in excellent agreement with the experimental value of 19.7 kcal/mol [70]. In toluene, the experimental activation enthalpy is 15.8 1.4 kcal/mol, with an activation entropy of —38 4 cal/mol K [71]. Therefore the computed 19.1 kcal/mol gas phase activation enthalpy of acrolein and butadiene should be lowered by 3.3 kcal/mol in toluene. In a related Diels-Alder reaction of cyclopentadiene and methyl acrylate, the experimental activation enthalpy is 15.1 kcal/mol in toluene, which is ca. 0.7 kcal/mol lower than that reported for acrolein and butadiene [81],... 

The author has tried to collect in Table 9 a representative sample of Diels-Alder reactions with different types of reactants, for which the usual activation parameters could be determined with care (in most cases from kinetic runs at four or more different temperatures, in a range of at least 20°C). Other data (E and log A values) have been given in Tables 2 and 3 (pp. 93, 94) the relevant values of the activation entropy can be calculated from log A by the conversion formulae ... 

ENERGY AND ENTROPY OF ACTIVATION OF REVERSE DIELS-ALDER REACTIONS... 

Several types of 1,3-cycloaddition are represented in Table 15. Reaction velocities at room temperature are expressed by rate coefficients (l.mole. sec ) ranging from the order of 10 , cases (f) and (i) to about 10", (c) and (e). This variation is due (Table 15) to values of the activation energy ranging from 8.6 to 18.0 kcal.mole", while entropies of activation span from —40 to —24 eu, a range similar to that typical of the Diels-Alder reaction. Very few published A5 values of 1,3-cycloadditions are outside the range indicated. 

By applying transition-state theory, we can calculate the activation entropy AS of this Diels-Alder reaction from Eq. (4), where R is the molar gas constant, A is the preexponential factor in the Arrhenius equation, T is the absolute temperature, kt is the Boltzmann constant, and h is Planck s constant. The values of and AS are presented in Table 19.2. 

The progress of the Diels-Alder reaction was assessed by contact angle measurements performed at room temperature (Fig. 19.7). Again, the reaction was studied systematically at six different temperatures. We observed that the Diels-Alder reaction could be described as a pseudo-second-order reaction (Fig. 19.8). Similarly to the Diels-Alder reaction on monolayers, the third-order rate constants koA. calculated from the least-squares fits shown in Fig. 19.8 for the Diels-Alder reaction in the polymer thin film, obey the Arrhenius equation (Fig. 19.9). The activation energy 3 = 48.1 3.7 kj moh and the activation entropy AS = -538.2 16.4 J mol at 298 K (Table 19.2) are determined at the polymer surface in the same way as for the monolayers. 

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