Symmetrization reactions

Let us now turn to the surfaces themselves to learn the kinds of kinetic information they contain. First observe that the potential energy surface of Fig. 5-2 is drawn to be symmetrical about the 45° diagonal. This is the type of surface to be expected for a symmetrical reaction like H -I- H2 = H2 -h H, in which the reactants and products are identical. The corresponding reaction coordinate diagram in Fig. 5-3, therefore, shows the reactants and products having the same stability (energy) and the transition state appearing at precisely the midpoint of the reaction coordinate. 

A similar substitution reaction occurs with other strong bases. Treatment of bromobenzene with potassium amide (KNH2) in liquid Nhh solvent, for instance, gives aniline. Curiously, though, when bromobenzene labeled with radioactive 14C at the Cl position is used, the substitution product has equal amounts of the label at both Cl and C2, implying the presence of a symmetrical reaction intermediate in which Cl and C2 are equivalent. 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Although the above work showed that for the symmetrization reaction (and its reverse reaction) the mechanism is SE2 with two steps of dissimilar rate, a symmetrical transition state (XLIII),... 

A simple, high-yield procedure for the conversion of ArTlXj into ArjTlX compounds has recently been described 90). This symmetrization reaction, the mechanism of which is not known, can be effected by treatment of the ArTlX2 compound either with triethyl phosphite or with hot aqueous acetone. As a wide variety of ArTlXj compounds can now be easily prepared by electrophilic thallation of aromatic substrates with thallium(III) trifluoroacetate (q. v.), symmetrization represents the method of choice for the preparation of the majority of ArjTlX compounds. Only about twenty mixed compounds, RR TIX, have been prepared so far, and the only general synthetic procedure available consists of a disproportionation reaction between an RTIX2 species and another organometallic reagent [e.g., Eqs. (5)-(7)]. 

In addition to enhancing surface reactions, water can also facilitate surface transport processes. First-principles ab initio molecular dynamics simulations of the aqueous/ metal interface for Rh(l 11) [Vassilev et al., 2002] and PtRu(OOOl) alloy [Desai et al., 2003b] surfaces showed that the aqueous interface enhanced the apparent transport or diffusion of OH intermediates across the metal surface. Adsorbed OH and H2O molecules engage in fast proton transfer, such that OH appears to diffuse across the surface. The oxygen atoms, however, remained fixed at the same positions, and it is only the proton that transfers. Transport occurs via the symmetric reaction... 

The transfer reaction utilizes a sacrificial alkene to remove the dihydrogen from the pincer or anthraphos complex first, before the oxidative addition of the target alkane. The elementary reaction steps are slightly different from the thermal reaction, which is discussed in the next section, both in their order and their direction. For simplicity, we describe the symmetric reaction where the sacrificial alkene is ethylene and the reactant is ethane (21b). The elementary reaction steps for the mechanism of this transfer reaction involve IVR, IIIR, VIR, VI, III and IV, where the superscript R stands for the reverse of the elementary steps listed in Section III. These reverse steps (IVR, IIIR, and VIR) involve the sacrificial alkene extracting dihydride from the metal to create the Ir(I) species 8, while steps VI, III and IV involve oxidative addition of target alkane, p-H transfer and olefin loss. 

Hase and co-workers have recently carried out extensive trajectory studies of the dynamics of a number of 8 2 reactions, most notably the symmetric reaction between chloride ion and methyl chloride. Equation (5). Experimental results... 

The Marcus equation allows AG for RX + Y —> RY + X to be calculated from the barriers of the two symmetrical reactions RX + X - RX + X and RY + Y — RY + Y. The results of such calculations are generally in agreement with the Hammond postulate. Marcus theory can be applied to any single-step process where something is transferred... 

R. A. Marcus Prof. Miller, has your insightful quantum mechanical flux-flux correlation expression for the rate been used to test some of the simplified quantum mechanical tunneling calculations for reactions I recall that Coltrin and I found a simple path that agreed to a factor of 2, over six or so orders of magnitude of tunneling, with the quantum mechanical results for the collinear symmetric reaction H + H2 — H2 + H [1]. Truhlar has proposed an extension for asymmetric reactions. 

Figure 2. Initially, in a thermal reaction, there are two electrons in each of the ethylene -orbitals, and it is apparent that if the reaction follows the symmetrical reaction path, the initial state correlates with a highly excited state of the product. Configuration interaction between the two Aig states leads to an avoided crossing, but there is still a considerable activation energy (Figure 3a). The thermal reaction is...

The principle here is general, but the absence of a break in a linear correlation does not exclude an intermediate. Values of A/9 or Ap maybe so small, or errors in determining the slopes of the components so large, that the break point is undetectable. Equally, prediction of the values of the parameters at which the break point occurs (i = k2) is not straightforward and it may lie outside the range of available reactants. The technique has been applied most often in the demonstration of non-accumulating intermediates in substitution processes, both at carbon and at other elements, in quasi-symmetrical reactions [50]. 

Fig. 11.8 The quasi-symmetrical reaction of phenolate ions with 4-nitrophenyl acetate.

In Fig. 7 we have taken a symmetrical reaction where, apart from the isotopic mixing, AG ° = 0. One of the first successes of the Marcus theory was the correlation of rates for such homogeneous reactions with the rates found for the same electron transfer taking place on an electrode (Marcus, 1963). The theory then went on to predict the rates of cross reactions between two different redox couples in terms of the kinetic and thermodynamic properties of the two redox couples. The free energy profile for an unsymmetrical cross reaction such as (17) is shown in Fig. 8. The free energy of activation depends... 

The free energies in (18) are illustrated in Fig. 10. It can be seen that GA is that part of AG ° available for driving the actual reaction. The importance of this relation is that it allows AGXX Y to be calculated from the properties of the X and Y systems. In thermodynamics, from a list of n standard electrode potentials for half cells, one can calculate j (m — 1) different equilibrium constants. Equation (18) allows one to do the same for the %n(n— 1) rate constants for the cross reactions, providing that the thermodynamics and the free energies of activation for the symmetrical reactions are known. Using the... 

Fig. 9 Test of the Marcus theory of electron transfer where fcca,c for the cross-reaction O, + R - R, + On is calculated from the thermodynamic free energies and the free energies of activation of the symmetrical reactions. The symbols are as follows O, Ce(IV), x IrCl -, + Mo(CN)j-, Fe(CN) ", R O, Fe(CN)J , A Mo(CN)f, W(CN)<-...

We now explore whether the pattern of reactivity predicted by the Marcus theory is found for methyl transfer reactions in water. We use equation (29) to calculate values of G from the experimental data where, from (27), G = j(JGlx + AG Y). The values of G should then be made up of a contribution from the symmetrical reaction for the nucleophile X and for the leaving group Y. We then examine whether the values of G 29) calculated for the cross reactions from (29) agree with the values of G(27) calculated from (27) using a set of values for the symmetrical reactions. The problem is similar to the proof of Kohlrausch s law of limiting ionic conductances. 

The values of Gx x for the symmetrical reactions are found by first calculating the difference between adjacent rows or columns of the league table. If the Marcus pattern is obeyed, the difference should be constant. The values, together with their standard deviations, are displayed as A and a in the table. To within a few kJ mol 1 the differences are indeed found to be constant. Now if is the difference between two rows then (39) holds. Thus for rt... 

Fig. 12 Test of the Marcus theory for methyl transfers in H,0. The graph compares the values of G(29) calculated by equation (29) from experimental free energies with C,7) calculated by equation (27) from the Cx x for the symmetrical reactions...

While the values of m xx for the symmetrical reactions are similar, the smaller harder X (e.g. Cl- and CN 0.7) have larger values than the... 

The Marcus pattern can be tested for three pairs of reactions as shown in Table 23. The difference for each pair is roughly constant and hence gives some support for the Marcus pattern given by (97). We can then use the results for the I-, I" symmetrical reaction to obtain values for the fractionation factors in the symmetrical reactions. The results are given in Table 24. 

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