Solvation of ions. Part X. Kinetic and thermodynamic properties of bimolecular reactions between anions and cations and between dipoles in protic and dipolar aprotic solvents

Abstract

Rate data are reported for two anion-cation reactions, the SN2 decompositions of trimethylsulphonium thiocyanate and trimethylsulphonium bromide, and for a dipole-dipole reaction between methyl bromide and dimethyl sulphide in a protic and a dipolar aprotic solvent of the same dielectric constant. The entropy and enthalpy of ion-pair formation are compared with the entropy and enthalpy of activation of the anion-cation reaction and with the overall entropy and enthalpy change for the decomposition of trimethylsulphonium bromide in dimethylacetamide. The anion-cation reaction has a high activation energy and a large positive entropy of activation, and the reverse dipole-dipole reaction has a low activation energy and a large negative entropy of activation. This situation is less pronounced in 88% methanol-water than in dimethylacetamide. Solvent and ionic strength influence the rate of the anion-cation reaction much more than that of the dipole-dipole reaction, although both reactions pass through the same transition state. Changes in the equilibrium and rate constants for anion-cation reactions, on transfer from dipolar aprotic to protic solvent, can be predicted from medium effects on anion-dipole reactions, on trihalide equilibria, and on the e.m.f. of halogen-halide cells. All are influenced by the much greater hydrogenbonding solvation of anions and the weaker solvation of dipolar molecules, transition states, or cations, by protic solvents relative to dipolar aprotic solvents.THE SN2 decomposition of trimethylsulphonium bromide [forward reaction (l)] is an excellent model for demonstrating some features of bimolecular anion-cation reactions in solution. Such reactions are more common in inorganic systems than with carbon compounds, but the latter provide a simpler, better understood model. Following the work of Hughes and Ingold2 and of Moel~yn-Hughes,~ perhaps more is known about the kinetics and mechanism of bimolecular substitution at a saturated carbon atom and, in particular, at a methyl carbon atom than about most reactions.We considered earlier4 the effect of ion association and of ionic strength on the rate of anion-cation reactions, using forward reaction (1) in ethanol a t 100" as a model. We now report rates for both the forward and the backward reactions (l), and thermodynamic data for the equilibrium (l), in dimethylacetamide, a dipolar aprotic solvent, and in 88% (w/w) methanolwater, a protic solvent of the same dielectric constant. ki CH,-S+(CH,), + Br-+ CH,Br + (CH,),S (I) kb Solvent, ionic strength, and temperature strongly influence the equilibrium constant, K = kf/kb, for reaction (1) because, contrary to classical ideas,2 the anion-cation reaction is much more susceptible to changes in these variables than is the dipole-dipole reaction. Much of the discussion is along conventional linesJe7 for solvent and salt effects on reaction rate. We feel, however, that the large effects which have been observed for two simple reactions of different charge type, but having the same transition state, ...

Solvent Influence on Rates and Mechanisms

“…For example, Pocker and Parker [195] and Mac et al [196] have studied the reaction CH 3 -S + (CH 3 ) 2 + Br" <± [Br · · CH 3 ··· S(CH 3 ) 2 ] <± CH 3 BR(CH 3 ) 2 S (5.323) and found the rate to decrease with solvent polarity in agreement with the Hughes-Ingold theory of solvent effect on reactions of this charge type. For example, Pocker and Parker [195] and Mac et al [196] have studied the reaction CH 3 -S + (CH 3 ) 2 + Br" <± [Br · · CH 3 ··· S(CH 3 ) 2 ] <± CH 3 BR(CH 3 ) 2 S (5.323) and found the rate to decrease with solvent polarity in agreement with the Hughes-Ingold theory of solvent effect on reactions of this charge type.…”

“…The reactions unlike S N 2 reactions at saturated carbon do show unusual salt effects in acetone [101,132]. They have activation entropies and enthalpies which are unlike S N 2 reactions when both are of the anioncation charge type [101,134], and show nucleophilic tendencies of halide ions toward platinum [101,132] which are at variance with those for S N 2 at carbon [135].…”

“…This reaction has been studied in 88% CH 3 OH-12% H 2 0 [134], in dimethylacetamide [134], in methanol [144], in ethanol [112,144], in dimethylformamide [112], dimethylacetamide (DMAC) [134], and in nitromethane [101]. At 100°C 88% MeOH-12% H 2 0 and DMAC have the same dielectric constant, but the forward reaction rate diminished by 10 4,68 (log/: changes from +1.24 to -3.44) on transfer from the dipolar aprotic solvent DMAC to the protic solvent 88% MeOH-12% H 2 0 .…”

“…(5.331). It was, therefore, estimated [196] that the difference (P c -P d ) is less than 100 atm and that the difference (Pl l2 -P d 12 ) is approximately 1. This ratio for a given solvent divided by the ratio for the gas phase gave (k s '/k g f ).…”