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The densities of anthracene, tetracyanoethylene, maleic anhydride, N phenylmaleineimide, trans,trans 1,4 diphenylbuta 1,3 diene, and their Diels-Alder adducts were measured in the solid state and in solution at 25 °С. The reaction volumes in the solid state were calculated from the difference in molar volumes. They turned out to be low, close to each other (-4 to -11 cm 3 mol -1 ), and slightly different from the reaction volumes (-8±1 cm 3 mol -1 ) calcu lated from the van der Waals radii. The reaction volumes in solutions (-15 to -32 cm 3 mol -1 ) were found from the difference in partial molar volumes of the reactants in dioxane, acetoni trile, and 1,2 dichloroethane, The experimental Diels-Alder reaction volumes in solution are determined not only by the formation of new bonds in an adduct: a considerably higher contribution (to 75%) is made by a change in the volume of intermolecular empty spaces in solution on going from reactants to adducts.A change in the volume on going from the initial to transition or final state of a reaction corresponds to an activation volume (∆V ≠ ) or reaction volume (∆V ). The activation or reaction volumes can be calculated from the plot of the free activation energy (dG ≠ /∂p, Eq. (1)) or free reaction energy (∂G/∂p, Eq. (2)) vs. external pres sure (p).(dlnk/dp) Т = -1/RT (dG ≠ /dp) Т = -∆V ≠ /RT,(dlnK/dp) Т = -1/RT (dG/dp) Т = -∆V/RT.The reaction volumes determined independently from the difference in partial molar volumes of the products and reactants are usually close to the values calculated by Eqs (1) and (2). 1 The activation volumes can be deter mined only from the plot of the rate constant vs. external pressure. In common belief, 2-4 the sign and value of an activation volume provide additional information on the mechanism of the chemical reaction. In several cases, the pressure effect on the rate or equilibrium of reactions (Eqs (1) and (2)) can be caused by an energy contribution р∆V ≠ or р∆V and also by a change in the properties of the solvent due to a pressure increase. For instance, for reac tions with a high polar transition state, an elevated pres sure can affect the process rate due to the contribution р∆V ≠ and also due to an increase in the dielectric constant of the solvent. 1,5 Several examples are known for which an elevated pressure results in a significant increase in the viscosity of the medium, and the diffusion rate can be come determining even for comparatively slow pro cesses. 6,7 In catalytic reactions, an elevated pressure can change the concentration of active species and thus change the reaction rate. 2-4In the absence of similar complications under an el evated pressure, the volume parameters can reliably be determined from relations (1) and (2).Another question arises: what is an origin for the values of activation and reaction volumes? In the first approxi mation, it seems evident that, when a transition state and adducts are achieved, a decrease in the volume of a sys tem in the addition and cycloaddition reactions is caused by the formation...

For several non‐electrolytes and for lithium perchlorate the solvent effect on the heat of solution and partial molar volumes was studied. In the absence of the donor–acceptor and electrostatic interactions, clear proportionality can be seen only in the alkane solutions and these changes are defined by the non‐alkane component. For solutions of π‐acceptor (tetracyanoethylene) and ν‐acceptor (gallium chloride) in the presence of π,π‐, π,ν‐ and n,ν‐complexes, the relation between the changes of interaction energy and the values of the partial molar volumes can be seen. The maximum change in the value of partial molar volumes (up to 50 cm3 mol−1) was noted for lithium perchlorate in ten solvents. The volume change is proportional to the compressibility coefficients of the solvents and correlates less with the values of heat of solution and solvent permittivity. Copyright © 2006 John Wiley & Sons, Ltd.