Ion Intermediates

Fig. 5.5. Potential energy diagrams for substitution mechanisms. A is the S l mechanism. B is the Sjy2 mechanism with intermediate ion-pair or pentacooidi-nate species. C is the classical S).(2 mechanism. [Reproduced from T. W. Bentley and P. v. R. Schleyer, Adv.

Predict the struaure of the transition state and the two intermediate ion-moleculc complexes for the S). 2 reaction ... 

Draw Lewis structures (or a series of Lewis structures) for the intermediate ions formed by addition of N02 to naphthalene at the 1 and 2 positions (nitro-naphthalenium ions ). On this basis, are you able to anticipate which intermediate is likely to be the more stable Examine the energies of 1-nitronaphthalenium and 2-nitronaphthalenium ions to see which ion is actually more stable. Which substitution product should be favored Is this the same product anticipated by inspection of naphthalene s HOMO Is it the observed product ... 

Another way to assess thiophene s reactivity is to compare the intermediate ions formed by addition of N02. Examine the structures, charge distributions and electrostatic potential maps of thiophene+nitronium at C2 and thiophene+nitronium at C3. Draw all of the resonance contributors needed to describe these structures. Which, if either, better delocalizes the positive charge Compare the energies of the two intermediates. Which product should form preferentially if the reaction is under kinetic control Are these results consistent with FMO theory ... 

Several explanations for this seeming inconsistency can be offered. By far the most attractive is based on unreactivity of certain intermediate ions, and this interpretation is supported by the observation that the reaction of butene ion with ethylene (Reaction 8) appears to occur with a collision efficiency of only about 0.015. The mass spectrometric observation of Reaction 17c (16) indicates that there may be similar low efficiency reactions in sequences initiated by other ions as well. 

The rate constants for unimolecular dissociation of the intermediate ions suggested earlier indicate that all ions containing seven or more carbon atoms arise from reactions of the dissociation products of Steps 9, 13, and 17 when pressures are of the order of a few torr and of Step 20 and its analogues at pressures in excess of a few hundred torr. The product ions are generally quite complex, and the simple exothermicity rule given earlier will not apply. Thus, we may well expect that there will be inefficient ion-molecule reactions in the sequences originating with these ions as well. 

Intermediate ions, unreactivity of 260 Intramolecular isotope effects... 95... 

All these data could be obtained by means of two techniques, namely n.m.r. spectroscopy and the use of superacid solvent systems (such as HF—BF3, HF—SbFj, FHSO3—SbFs, SbFs—SOj). As will become evident in this article, this is equally true for the data of the carbonyl-ation and decarbonylation reactions (3). With less acidic systems the overall kinetics can, of course, be obtained but lack of knowledge concerning the concentrations of the intermediate ions prevents the determination of the rate constants of the individual steps. ... 

It would seem that all bonds must break in one of the two ways previously noted. But there is a third type of mechanism in which electrons (usually six, but sometimes some other number) move in a closed ring. There are no intermediates, ions, or free radicals, and it is impossible to say whether the electrons are paired or unpaired. Reactions with this type of mechanism are called pericyclic. ... 

Sneen et al. formulated an intermediate-mechanism theory. The formulation is in fact very broad and applies not only to borderline behavior but to all nucleophilic substitutions at a saturated carbon. According to Sneen, all SnI and Sn2 reactions can be accommodated by one basic mechanism (the ion-pair mechanism). The substrate first ionizes to an intermediate ion pair that is then converted to products ... 

Pyridine and other heterocyclic nitrogen compounds can be aminated with alkali metal amides in a process called the Chichibabin reaction The attack is always in the 2 position unless both such positions are filled, in which case the 4 position is attacked. Substituted alkali metal amides (e.g., RNH and R2N ) have also been used. The mechanism is probably similar to that of 13-15. The existence of intermediate ions such as 15... 

As noted earlier, the kinetics of electrochemical processes are inflnenced by the microstractnre of the electrolyte in the electrode boundary layer. This zone is populated by a large number of species, including the solvent, reactants, intermediates, ions, inhibitors, promoters, and imparities. The way in which these species interact with each other is poorly understood. Major improvements in the performance of batteries, electrodeposition systems, and electroorganic synthesis cells, as well as other electrochemical processes, conld be achieved through a detailed understanding of boundaiy layer stracture. 

In contrast to the above behavior, in the presence of. 1 M LiBr phenylacetylene yields the trans dibromide, C6HsCBr=CHBr, in greater than 99% yield upon the addition of Brj in acetic acid (35). This difference in behavior between the two systems has been accounted for by the formation of a different intermediate ion, 13, in the latter case. 

Solvolysis of the deuterium-labeled precursor, 54, indicates that the intermediate ion 51 arises from a symmetrical precursor, as the product ketone 55 has an equal distribution of deuterium in the 3 and 4 positions. 

Besides differences in reactant stability as well as differences in the stability of the intermediate ions, there may also be other reasons, such as differences in solvation and differences in backside nucleophilic assistance, for the large discrepancy in reactivity between alkyl halides and vinyl halides. 

Extended Hiickel calculations have been carried out on the 1-cyclopropylvinyl cation 156 (122). These results show that the most favorable conformation for this ion is the linear bisected structure 156a. However, Hanack et al. (166b), by means of a modified CNDO technique, calculate the most stable geometry of the intermediate ion resulting from homopropargyl participation to be a bridged cyclobutenyl cation rather than 156a. 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

It is well known that allylic substrates are more reactive under solvolytic conditions than their saturated counterparts because of the delocalization of the positive charge in the developing carbonium ion over the tt system and the overlap of the empty p orbital with the double bond in the intermediate ion. 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Another method for evaluating carbocation stability involves the measurement of solvolysis rates (14,45). Typically, the transition state of the rate-determining step in SN1 reactions is assumed to closely resemble the intermediate ion pair, on the basis of the Hammond postulate (46). Thus, the free energy of activation for this reaction, AG, reflects the relative thermodynamic stabilities of the intermediate carbocations. 

In the stepwise case, the intermediate ion radical cleaves in a second step. Adaptation of the Morse curve model to the dynamics of ion radical cleavages, viewed as intramolecular dissociative electron transfers. Besides the prediction of the cleavage rate constants, this adaptation opens the possibility of predicting the rate constants for the reverse reaction (i.e., the reaction of radicals with nucleophiles). The latter is the key step of SrnI chemistry, in which electrons (e.g., electrons from an electrode) may be used as catalysts of a chemical reaction. A final section of the chapter deals... 

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