No abstract

Rates and equilibrium constants have been determined for the proton-transfer reaction of 4nitrophenylnitromethane, NOzC6H4CH2N02, and its cta-deuterated analogue NO2C6H4CD2NO2, with the strong base tetramethylguanidine [HN=C(NMe2)2], at temperatures between -60°C and + 65°C in a range of aprotic solvents. Spectrophotometry and the stopped-flow technique were used.The reaction is a simple proton-transfer process leading to an ion-pair. The kinetic isotope effects are correlated with the polarity of the solvents, as measured by the dielectric constant or by the empirical parameter ET. In the less polar solvents they are exceptionally large. In toluene, for example, at 25°C the rate ratio kH/kD = 45 k 2, the activation energy difference E p -EF = 4.3 kO.3 kcal mol-l (16 kJ mol-l),$ and the ratio of the pre-exponential factors loglo(AD/AH) = 1.5k0.2 ; and even larger values of loglo(AD/AH) are found for mesitylene (1.94k 0.06) and cyclohexene (2.4rf: 0.2). Positive deviations from linear Arrhenius plots are found for these solvents. Tunnelling is the only interpretation that can account for these results. For the more polar solvents (dielectric constant 7 to 37), the isotope effects are closer to the range predicted by semi-classical theory.The isotope effects in all solvents have been fitted to Bell's equation for a parabolic barrier, and the barrier dimensions calculated for each solvent, in two ways. (a) It is first assumed that only the proton moves, so that the effective mass LVH = 1 a.m.u. ; the best values of the barrier height (EH> and width at the base (2b) are determined by trial and error. The tunnelling corrections so calculated depend on the solvent polarity ; for the less polar solvents they are large, and the values also explain quantitatively the deviations of the Arrhenius plots from linearity. The calculated barrier dimensions are also solvent-dependent ; EH is lower in solvents of higher polarity by a factor of about 2 compared with those of low polarity, and 2b is greater by 0.1 -0.2 A. (b) It is assumed, alternatively, that the effective mass may be increased above 1 by reason of solvent or other motions coupled to that of the proton ; the values of E H derived in (a) are assumed, and the best values of LVH and 2b' determined by trial and error. The calculated values of 2b' are then all about equal, those of the less polar solvents being unchanged from (a) ; the values of m& in the less polar solvents are 1.00 a.m.u. as before, but in the more polar solvents they are 1.17-1.27 a.m.u., increasing with polarity.The suggested interpretation is that the solvent-solute interactions affect the height of the barrier and that motions of solvent molecules are coupled with the motion of the proton in the more polar solvents but not in the less polar ones ; reorganization of solvent molecules accompanies the protontransfer in the more polar solvents, but only electron-polarization in the less polar. Tunnelling has large effects in the less polar solvents, where the proton is the only atom that moves in the...

OEt-+ H , O S E t O H + OH-in ethanol-water at 25" has been determined. In mixtures containing a few per cent. of water, and in pure ethanol, the equilibrium favours ethoxide ion. It is deduced that solutions made by dissolving alkali hydroxides in ethanol will contain mainly ethoxide ion rather than hydroxide. For instance, in a 0-lM-solution of sodium hydroxide in 99% ethanol, 96% of the total base is ethoxide. Solutions made by dissolving sodium in ethanol or aqueous ethanol are also considered. The relative acid dissociation constants of ethanol and water, in (a) ethanol, and (b) water, are estimated; in each solvent they differ by less than a power of 10.

Activation parameters of solvent exchange and ligand substitution reactions of metal ions are discussed in relation to structural properties. Solvent exchange is treated as a particular case of ligand substitution. A general relation between enthalpy and entropy of activation is shown to hold for both types of reaction a t divalent metal ions for a wide variety of solvents and ligands, after corrections have been made for ligand-field stabilisation. Water occupies a middle position in the range of solvents. The results are interpreted in terms of a structural model for a solvated ion in solution, based on that of Frank and Wen.

The rates and activation parameters for substitution of bipyridyl and terpyridyl at NiII and Corl ions have been measured in water and some non-aqueous solvents. A comparison is made with the corresponding rates of solvent exchange for these ions, and the applicability of the model usually suggested for aqueous solutions is examined. The behaviour of water is apparently not general. Approximate relationships are described linking the divergence of substitution rates from exchange rates with properties of the solvent which reflect its structure, such as its enthalpy of evaporation and its fluidity.MUCH evidence has accumulatedl in support of the hypothesis that, in the substitution of a ligand (L) into the first hydration sphere of a metal cation (M2+), the rate-determining step involves the loss of a solvent molecule (S) from that sphere. This is thought to be preceded by the rapid formation of an ' outer-sphere complex,' as in equation (1). This reaction schemepredicts the correct rate law, first-order in M2+ and in L.If k , is the first-order rate constant for exchange of a solvent molecule between the first hydration sphere and the bulk solvent, is the second-order rate constant for the overall forward reaction, and KO is the equilibrium constant for the formation of the outer-sphere complex, then the kinetic behaviour in the simplest case, with M2+ ion in large excess, should be given by equation (2). Even for aqueous solutions, this equationis obeyed much less accurately than is often supposed. For instance, the observed values for the reactions of nickel(I1) ion with uncharged ligands vary by a factor of ca. 20 (leaving aside the reactions of aliphatic diamines and related ligands for which a special mechanism is proposed).4 These variations in ki remain largely unexplained. They are sometimes rationalised by adjusting the value of K O ; for uncharged ligands this is not independently measurable ,5 but large variations would not be expected (cj. below) and the interpretation is unconvincing. Moreover, the lifetime of such a complex, if its formation is diff usion-controlled, would not be much longer than the time of an encounter.The outer-sphere-complex treatment leads to the same transition state as the interchange mechanism MS,