Why can the activation volume of the cycloadduct decomposition in isopolar retro‐Diels–Alder reactions be negative?

Abstract

Rate constants of the Diels-Alder cycloaddition reaction of anthracene with tetracyanoethylene, enthalpy of solution of reactants and adduct, enthalpy of the reaction in solution, enthalpy and entropy of activation of the forward and retro-Diels-Alder reactions were determined in 14 solvents. Temperature and pressure effects on the rate of the decomposition of the adduct formed from 9-chloroanthracene and tetracyanoethylene were studied. Since the electrostriction effect can be excluded from the consideration of the isopolar Diels-Alder reaction, negative values of the activation volume in the retro-Diels-Alder reactions can be caused by the different possibilities of penetration of the solvent molecules to large steric branched structures of the transition states and adducts.

Heats of Solution of Liquid Solutes in Various Solvents

“…From comparison of the independent values of Δ solv TS for the direct and back reactions, a good correlation was observed in line with the common nature of activated complex for the reversible reactions [13].…”

“…is the enthalpy transfer of reagents 1 and 2 from the reference solvent (S 0 ) to other one (S ), and Δ ≠ (S ) and ≠ (So) are the enthalpies of activation of reaction in these solvents [12,13].…”

Anomalies in the Change of Volumetric Parameters of the Diels–Alder Reaction in Solution

“…Only a strong donor-acceptor interaction, which is observed, for example, for π -acceptor tetracyanoethylene in the medium of π -donors (alkylbenzenes), significantly reduces the activity of dienophile and its rate in the direct reaction [4,[11][12][13][14]. However the reaction rates of the adduct 11 decomposition are insensitive to the properties of the solvent [11]. Therefore there is no reason to assume the existence of solvent electrostriction in the process of the AC solvation to explain the observed negative activation volumes of the retro-process.…”

“…Recently it was shown [11] that there is an absence of correlation (r = 0.56, n = 10) between the change of enthalpy of solvation of AC in several solvents for the DAR Scheme 1 Structures of the reactants and adducts considered between tetracyanoethylene 5 with anthracene 3 and the change in enthalpy of solvation of adduct 11 (Scheme 1). Based on these data, it has been proposed [11] that the different capability of the solvent molecules to penetrate into the large branched structures of the AC and adducts ("steric solvation effect") is involved. This corresponds to the idea that different capability of the solvent to penetrate into the reactants, AC and adduct may affect their PMVs.…”

“…This means that the molar volume of the AC should be less than that of the cycloadducts, and negative values of the volume of activation of retro DARs should be expected. It is well known [1,4,[9][10][11] that the rate of isopolar DAR is weakly dependent (if any) on the polarity of the medium. Only a strong donor-acceptor interaction, which is observed, for example, for π -acceptor tetracyanoethylene in the medium of π -donors (alkylbenzenes), significantly reduces the activity of dienophile and its rate in the direct reaction [4,[11][12][13][14].…”