Carbocation-ion pair

Fragmentation of alkoxychlorocarbenes provides a new entry to carbocation-ion pair chemistry that complements the classical solvolytic method. Because of the high rate constants for fragmentation (lO -lO and the low activation ener-... 

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

B. Formation of carbocation ion pair from alkene and electrophile... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Attack by a nucleophile or the solvent can occur at either of the ion pairs. Nucleophilic attack on the intimate ion pair would be expected to occur with inversion of configuration, since the leaving group would still shield the fiont side of the caibocation. At the solvent-separated ion pair stage, the nucleophile might approach fiom either fece, particularly in the case where solvent is the nucleophile. Reactions through dissociated carbocations should occur with complete lacemization. According to this interpretation, the identity and stereochemistry of the reaction products will be determined by the extent to which reaction occurs on the un-ionized reactant, the intimate ion pair, the solvent-separated ion pair, or the dissociated caibocation. 

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

On the other hand, a freely rotating open carbocation would be expected to give both the syn and anti addition products. If the principal intermediate were an ion pair that collapsed faster than rotation about the C—C bond, syn addition could predominate. 

In summary, it appears friat bromination usually involves a charge-transfer complex which collapses to an ion-pair intermediate. The cation can be a carbocation, as in the case of styrenes, or a bromonium ioiL The complex can evidently also be captured by bromide ion when it is present in sufficiently high concentration. 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

Figure 11.11 Ion pairs in an S l reaction. The leaving group shields one side of the carbocation intermediate from reaction with the nucleophile, thereby leading to some inversion ol configuration rather than complete racemization.

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

It is unlikely that free carbanions exist in solution. Like carbocations, they usually exist as either ion pairs or they are solvated. " Among experiments that demonstrated this was the treatment of PhCOCHMe with ethyl iodide, where was Li ", Na", or K" . The half-lives of the reaction were for Li, 31 x 10 Na, 0.39 X 10 and K, 0.0045 x 10 , demonstrating that the species involved were not identical. Similar results were obtained with Li, Na, and Cs triphenylmethides (PhsC M Where ion pairs are unimportant, carbanions are solvated. Cram " demonstrated solvation of carbanions in many solvents. There may be a difference in the structure of a carbanion depending on whether it is free (e.g., in the gas phase) or in solution. The negative charge may be more localized in solution in order to maximize the electrostatic attraction to the counterion. ... 

Like the kinetic evidence, the stereochemical evidence for the SnI mechanism is less clear-cut than it is for the Sn2 mechanism. If there is a free carbocation, it is planar (p. 224), and the nucleophile should attack with equal facility from either side of the plane, resulting in complete racemization. Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5-20% inversion, though in a few cases, a small amount of retention of configuration has been found. These and other results have led to the conclusion that in many SnI reactions at least some of the products are not formed from free carbocations but rather from ion pairs. According to this concept," SnI reactions proceed in this manner ... 

As with other SnI reactions, there is clear evidence that SnI reactions can involve ion pairs. If the intermediate attacked by the nucleophile is a completely free carbocation, then, say, should give the same mixture of alcohols when reacting with... 

No matter how produced, RN2 are usually too unstable to be isolable, reacting presumably by the SnI or Sn2 mechanism. Actually, the exact mechanisms are in doubt because the rate laws, stereochemistry, and products have proved difficult to interpret. If there are free carbocations, they should give the same ratio of substitution to elimination to rearrangements, and so on, as carbocations generated in other SnI reactions, but they often do not. Hot carbocations (unsolvated and/or chemically activated) that can hold their configuration have been postulated, as have ion pairs, in which OH (or OAc , etc., depending on how the diazonium ion is generated) is the coun-... 

The lUPAC designation is Dn + De (or Dn+Dh). This mechanism normally operates without an added base. Just as the E2 mechanism is analogous to and competes with the Sn2, so is the El mechanism related to the SnE In fact, the first step of the El is exactly the same as that of the SnI mechanism. The second step differs in that the solvent pulls a proton from the P carbon of the carbocation rather than attacking it at the positively charged carbon, as in the SnI process. In a pure El reaction (i.e., without ion pairs), the product should be completely nonstereospecific, since the carbocation is free to adopt its most stable conformation before giving up the proton. 

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