Free carbonium ion

Olivier and Berger335, who measured the first-order rate coefficients for the aluminium chloride-catalysed reaction of 4-nitroben2yl chloride with excess aromatic (solvent) at 30 °C and obtained the rate coefficients (lO5/ ) PhCI, 1.40 PhH, 7.50 PhMe, 17.5. These results demonstrated the electrophilic nature of the reaction and also the unselective nature of the electrophile which has been confirmed many times since. That the electrophile in these reactions is not the simple and intuitively expected free carbonium ion was indicated by the observation by Calloway that the reactivity of alkyl halides was in the order RF > RC1 > RBr > RI, which is the reverse of that for acylation by acyl halides336. The low selectivity (and high steric hindrance) of the reaction was further demonstrated by Condon337 who measured the relative rates at 40 °C, by the competition method, of isopropylation of toluene and isopropylbenzene with propene catalyzed by boron trifluoride etherate (or aluminium chloride) these were as follows PhMe, 2.09 (1.10) PhEt, 1.73 (1.81) Ph-iPr, (1.69) Ph-tBu, 1.23 (1.40). The isomer distribution in the reactions337,338 yielded partial rate factors of 2.37 /mMe, 1.80 /pMe, 4.72 /, 0.35 / , 2.2 / Pr, 2.55337 339. 

Further evidence against the formation of a free carbonium ion in the alkylation reaction is obtained from the fact that in the presence of boron trifluoride-phosphoric acid catalyst, but-l-ene, but-2-ene, and i-butene react at different rates with alkylbenzenes, yet they would each give the same carbonium ion. In addition, only the latter alkene gave the usual activation order (in this case the hyper-... 

With regard to the composition of the electrical effect, examination of the p values reported in Table XVII shows that in six of the sets which gave significant correlation, the localized effect is predominant (in these sets, either Pr < 50 or / is not significant). Thus it would appear that in so far as substituent effects are concerned, there are two major classes of electrophilic addition to the carbon-carbon double bond predominance of the localized effect or predominance of the delocalized effect. This behavior may well be accounted for in terms of the reaction mechanism. The rate-determining step in the electrophilic addition reaction is believed to be the formation of an intermediate which may be either bridged or a free carbonium ion. 

Those sets for which the resonance effect is predominant are the sets which are most likely to give rise to the free carbonium ion 5, as the substituents in these sets (sets 15-14 and 15-17 and possibly 15-18) are all donors by resonance, as is shown by their Or values. Those sets for which the localized effect is predominant may be accounted for in terms of intermediates 3 or 4. Sets 15-5, 15-7B2, and 15-12 gave significant values of jS. It is difficult to account for this fact in terms of intermediate 4. The results can be accounted for in terms of intermediate 3, however, if this species resembles other three-membered rings, such as cyclopropane, in its behavior. Sets 15-6, 15-8, 15-9, 15-12, and 15-15 include both donor and acceptor substituents. The successful correlation of... 

Wiberg prefers mechanism A to the carbonium-ion mechanism with the proviso that the radical is oxidised before inversion occurs. The carbonium ion formed must rapidly acquire an oxygen atom to prevent inversion and the two processes may be synchronous. The minor role which free carbonium ions may play in the reaction has been discussed . 

A number of possible unimolecular mechanisms can be envisaged for decarboxylation ranging from one involving a free carbanion intermediate [Eq. (4)], to one involving a free carbonium ion intermediate [Eq. (5)]. In... 

In the second type of mechanism (dissociation), the bond to the leaving group is broken before the new bond is created, the reaction proceeding through an intermediate free carbonium ion. For example, in the hydrolysis of tertiary-butyl chloride ... 

Carbonium ions are well-defined transition states in many reactions. Indeed, one of the earliest applications of MM method to the reactivity problem was concerned with carbonium ions. At present, only the Schleyer force field (26b) and its predecessor (253) are capable of handhng carbonium ions, although the parameterization principle used earlier can be readily improved upon to the present standards. Schleyer s measure of steric strain in carbonium ions, Ajtrain (the difference in steric energies of free carbonium ion and its... 

THF can be polymerized only with cationic initiators, for example, boron trifluoride or antimony pentachloride. The initial step consists of the formation of a cyclic oxonium ion one of two activated methylene groups in the a-position to the oxonium ion is then attacked by a monomer molecule in an S 2-reaction, resulting in the opening of the ring. Further chain growth proceeds again via tertiary oxonium ions and not, as formerly assumed, via free carbonium ions ... 

Equation (93) was derived for a pure or limiting SN1 reaction, i.e. one involving a free carbonium ion intermediate. For such reactions the plot of log k against a should not only be linear, it should also have a constant slope. However, it turned out29 that a similar linear relation was also obtained for the rates of the reactions of such chlorides with iodide ion in dry acetone ... 

This is a reaction in which a transfer of hydrogen takes place, either as a proton or as a hydride ion. In the literature the term carbonium ion is often used carelessly to indicate this kind of reaction, without proof of the existence of a reaction intermediate. A more accurate description would be a reaction in which a positive charge is developed at the reaction centre, irrespective of whether the reaction is concerted or involves a free carbonium ion. 

However, under somewhat different conditions (aqueous acetonitrile at 85°), no evidence for the free carbonium ion could be found (Bethell et al., 1965). Moreover, the invariance of the product proportions when water is replaced by deuterium oxide, coupled with the observation of a large tritium isotope effect on the formation of diphenylmethanol, is consistent only with the ylid mechanism (equation 21) (Bethell et al., 1969). For reaction of diarylmethylenes with alcohols, substantial hydrogen-isotope effects are observed, consistent with both equations 21 and 22. 

Also shown in Table II are the propagation rate constants for several other free carbonium ions and one free carbanion. 

The equilibrium between ions and ion pairs is not maintained in all polymerizing systems. For example, the cationic polymerization induced by ionizing radiation produces the positive and negative ions, the latter initiating a free carbonium ion propagation which is terminated by their collision with negative ions. Such a collision destroys the free ions, and hence their stationary, but not the equilibrium, concentration is determined by their rate of formation and destruction. 

Olah et al. (1979a) reinvestigated ion [241] prepared from [240 R = OH] to confirm its proposed free carbonium ion nature and obtained essentially the same results. It is interesting to note that the C-chemical shift sum of ion [241] actually is 48 ppm less than the corresponding hydrocarbon [240] (R = H) confirming its nonclassical structure according to the criterion of Schleyer et al. (1980). 

No satisfactory explanation has yet been offered for the marked change in product composition with change in acid medium. It seems possible that the polarity of the solvent is important, for aqueous media would presumably transfer a proton from H3O+ to the dienone to form a highly solvated free carbonium ion. In the less polar acetic anhydride the acidic catalyst would exist as an ion-pair , and so, probably, would the activated steroid species. Preferential association of the anion with the less-hindered j0-face of the mesomeric carbonium ion may be a factor favouring migration of C(9> from the a -side of C(io). 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

The next step in the reaction scheme—decomposition of the a-bonded alkylpalladium (XIV or XV)—has caused some controversy. To account for the results of several deuterium-labelling studies (15, 36, 54), a. palladium-assisted hydride transfer reaction (Reaction 4) has been proposed (36, 54). A number of inconsistencies in the studies using 2-deuteropro-pylene as substrate (54) have been discussed (i). In addition, the formation of a free carbonium ion such as VII [as proposed by Moiseev (36)], while accounting well for the formation of ethylidene diacetate, is much less satisfactory in accounting for the production of the unsaturated esters in an acetate-acetic acid medium. A simple elimination of -hydrogen (Reactions 13a and b) could also account for the products formed. While not necessary for the reaction, chloride assistance for proton removal is a possibility and has been postulated previously for a similar reaction (i, 37). 

In certain cases, there is no free carbonium ion involved. Instead, the alkyl group is transferred—without a pair of electrons—directly to the aromatic ring from the polar complex, I, between AICI3 and the alkyl halide ... 

The mercurinium ion is attacked by the nucleophilic solvent—water, in the present case— to yield the addition product. This attack is back-side (unless prevented by some structural feature) and the net result is anti addition, as in the addition of halogens (Sec. 7.12). Attack is thus of the Sn2 type yet the orientation of addition shows that the nucleophile becomes attached to the more highly substituted carbon— as though there were a free carbonium ion intermediate. As we shall see (Sec. 17.15), the transition state in reactions of such unstable three-membered rings has much SnI character. 

An explanation of these results would seem to follow from the interpretation already given for the solvent dependence of the stereochemistry of diazonium ion reactions. The highly polar and in some cases acidic reaction conditions, coupled with the relatively long life expectancy of a primary diazonium ion, are conducive to ion pair dissociation or destruction at the stage of the diazonium ion. Moreover, the carboxylate ion pairs that do survive to form carbonium ions might be expected to yield ester exclusively on account of the very high reactivity of the n-propyl cation. If this is the case, the bulk of the alcohol should be derived from free carbonium ions formed from already dissociated diazonium ions. 

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