Ion-molecule pairs

The low efficiency of exchange in water can be explained by postulating that the ion-molecule pair (8.13 in Scheme 8-10) is almost completely trapped by water molecules, i.e., the first intermediate reacts so easily with this more strongly nucleophilic species that the reaction of the second intermediate (8.14) with N2 is not detectable. Therefore, the reaction coordinate diagrams for the dediazoniation in TFE and in water may be visualized as shown in Figure 8-4. 

Addition of hexafluorophosphate salts reduces the dediazoniation rate of 4-me-thylbenzenediazonium tetrafluoroborate in TFE/H20 (1 1) (Maskill and McCrud-den, 1992). However, as the concentration of these salts (0.12 — 0.23 M) does not affect the rate, it is evident that these salts are intercepting one of the intermediates, i.e., either the ion-molecule pair or the aryl cation. 

All the experimental data on the ion-molecule pair 8.13 discussed above are also compatible with a spirodiazirine cation 8.17 as intermediate. Differentiation between... 

If the deuterium isotope effect on the rearrangement rate ( H/ D3)r is larger than unity and is approximately equal to that on the rate of dediazoniation ( H/ D3)S, it can be concluded that the ion-molecule pair 8.13 is the more likely intermediate for the rearrangement reaction. On the other hand, an isotope effect on the rearrangement rate that is smaller than or equal to unity would indicate the involvement of the benzenespirodiazirine cation 8.17 as an intermediate. 

We will conclude this section on theory with such a case. In Section 8.3 it was shown that the influence of substituents on the rate of dediazoniation of arenediazonium ions can be treated by dual substituent parameter (DSP) methods, and that kinetic evidence is consistent with a side-on addition of N2. We will now discuss these experimental conclusion with the help of schematic orbital correlation diagrams for the diazonium ion, the aryl cation, and the side-on ion-molecule pair (Fig. 8-5, from Zollinger, 1990). We use the same orbital classification as Vincent and Radom (1978) (C2v symmetry). 

The 15N content was indeed lower when the experiment was performed This result justified the publication of a preliminary communication (Bergstrom et al., 1974). Later work (Hashida et al., 1978 Szele and Zollinger, 1978a Maurer et al., 1979) involving sophisticated statistical treatments suggested that, in a weakly nucleophilic solvent such as trifluoroethanol, the phenyl cation is formed in two steps and not in one, as in mechanism B (see Scheme 8-4 in Sec. 8.3), the first intermediate being a tight ion-molecule pair. 

For ion-molecule pairs in other solvolysis reactions, see Thibblin, A. J. Chem. Soc., Perkin Trans. 2, 1987, 1629. 

Compound 4, which is probably some kind of a tight ion-molecule pair, has been trapped with carbon monoxide. ... 

Extended kinetic measurements of dediazoniations in trifluoroethanol, water and other solvents107,108, and a statistical treatment109, demonstrated that a mechanism with two steady-state intermediates, namely initial formation of a tight ion-molecule pair (35) followed by the free (solvated) aryl cation (36), fits the experimental results significantly better than a mechanism with 36 only (Scheme 8). 

We defer (Section IV) a discussion of the mechanisms of process (2). Suffice to say that we have usually deduced the point of attack from the nature of the products, i.e. C adduct, Q adduct and terminal alkyne. The latter arises from an ion-molecule pair as in equation (54) where the coproduct (NuX) may undergo subsequent reaction... 

The competition for the nucleophile between sp carbon and halogen can also depend upon the solvent in a major way. The late development of process (2) may in part be attributed to the unfavourable solvents that were used. In a protic medium halogen abstraction not only becomes visible but appears to be promoted, e.g. for PhC=CBr-f(EtO)3P - and other examples of Table 6. In qualitative terms, the proton-solvent should favour ion-ion and ion-molecule pairs (14 and 16) over the larger species 15 and 17 in which the charge is more dispersed. Theoretical calculations (EHMO) tend to support this rationalization . ... 

Full details of a study of leaving group-promoted solvolytic elimination reactions of 1-(1-methyl-l-arylethyl)pyridinium cations in 25 vol.% acetonitrile (aqueous) have been reported. Reactions of (34) and (35) are found to proceed via a common carbocation intermediate of ion-molecule pair type to give the suhstitution product (36) and elimination product (37) (Scheme 4). The total rate of reaction of (35) exceeds that for (34) by 1100-fold, corresponding to a Bronsted parameter of )S g = —0.93, and the fraction of (37) obtained is governed by = 0.12 for the dehydronation (kg) of the ion-molecule pair by the leaving group the product ratio is hardly affected hy the presence of substituted pyridines. For (34) and (35), = 1.85 0.10 (60 °C) and... 

The ion-pair concept thus predicts that S l reactions can display either complete racemization or partial inversion. The fact that this behavior is generally found is evidence that ion pairs are involved in many S l reactions. There is much other evidence for the intervention of ion pairs, includng ion-molecule pairs. ... 

The estimated rate at which a simple tertiary carbocation reacts with water solvent ( 1012s-1)28 is very close to the rate at which solvent reorganizes (10n-1012s-1).29 It has been suggested that simple tertiary carbocations that are formed as part of ion pairs or ion-molecule pairs react with a solvent molecule that is already present within the solvent shell that is present at the time of formation of the carbocation.30... 

The study of glycoside formation and breaking in the gas phase is not as esoteric as it might appear. The very short lifetimes of glycosyl oxocarbenium ions in solution require that their study be via indirect competitive kinetic experiments. Knowledge of the barrier for capture of these species, whether as free oxocarbenium ions or as ion molecule pairs, is difficult to obtain by direct methods. On the other... 

In addition to these expected solvolysis products, (4- -butyl-l-cyclohexenyl)io-dobenzenes (33) were obtained in yields of 13-15% in methanol and aqueous solutions and of 30-40% in TFE. The ortho derivative o-33 is the predominant isomer of these products, and evidently is formed via the internal return of the contact ion-molecule pair of cyclohexenyl cation (31) and iodobenzene. The rates... 

In the thermolysis the E isomers of fluoro- and chlorostyrene ( -84-F and -84-Cl) are formed exclusively, which indicates the intermediacy of the vinylenebenze-nium ion 29 (see Scheme 29) as the species abstracting a fluoride from the counterion BF4, or a chloride from the solvent. Photochemically both E- and Z-84 are formed, indicating the intermediacy of the primary styryl cation 82 (see Scheme 52). Preferentially the Z-isomer of 84-F is formed. The fluoride-abstracting species must not be 82 itself, but a tight ion-molecule pair of 82 and iodobenzene (see Scheme 53), which hampers the approach of the fluoride donor to the side of 82 where this group leaves. 

The quantity c, y is an estimate of the maximum possible cross section for an ion-molecule pair with the parameters a and fi. 

Ion-molecule Pair Electronic Polarizability (A3) Dipole Moment (D.U.) Reduced Mass m (kg(x 102 )) Moment of Inertia I (kg-m2 (X 10 ))... 

Fig. 16. Superposition of 15 movie frames for single reflection CHgCN parent-ion collision. Ion-molecule pair are designated in pre-reflection positions motion is traced during and after reflection,...

Theoretical calculations support the intermediacy of a singlet aryl cation 6 preceded by an ion-molecule pair (5).13... 

In the Langevin orbiting theory, only the ion-induced dipole interaction was considered as a long-range force operative between the ion—molecule pair. Thus the theory applies only to the reactions of ions with non-polar molecules. In fact, it has been pointed out that some ion—molecule reactions in which the neutral molecule has a permanent dipole have reaction cross-sections greater than those predicted by the Langevin theory [56—63]. Such ion—polar molecule reactions have also been treated classical mechanically by several authors [57, 58, 61, 64—68]. 

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