Ethyl cation rearrangement

The pulsed electron beam MS technique was also used by Hiraoka and Kebarle842 to study the C4H + cations. In the ion-molecule reaction of ethane and the ethyl cation, two species were observed and identified as the 2-//-n-butoniu m cation 469 and the 2-C-w-butonium cation 470. C—C protonated ion 470 formed first rearranges to C—H protonated ion 469 (energy barrier = 9.6 kcal mol-1) and then dissociation to sec-C4H9+ + H2 takes place. 

If the ethyl cation would have reacted with excess ethylene, primary 1 -butyl cation would have been formed, which irreversibly would have rearranged to the more stable. sec-butyl and subsequently ferf-butyl cations giving isobutane as the end product. 

The l-(l-methylcyclopropyl)ethyl cation 48 undergoes degenerate rearrangement at -80 °C, whereas it is a static classical ion at -95 At -80 °C, the NMR absorptions... 

From the dynamic NMR studies it was shown that the activation barrier for the degenerate equilibrium is 7.4 kcalmoT at -112 This activation barrier is similar to those observed for other 1-cyclopropylethyl cations. The l-(c/.s-l,2-dimethylcyclo-propyl)ethyl and l-(2-methylcyclopropyl)ethyl cations 51 and 55 have an activation barrier for the degenerate rearrangement of less than 5 kcalmol The degenerate equilibrium in all these cases may proceed through the substituted cations as intermediates vide suprdf. ... 

The reaction of ethylene with ethane is of major scientific significance. The only C4 product, formed in 78% selectivity, in this reaction is normol-butane (Scheme 4) which does not isomerize under these conditions with the HF-TaF5 catalyst (eq. 4) or under similar conditions with the HF-SbF5 catalyst (30). This means that the primary ethyl cation is alkylating a primary ethane position (path a) and that there is no classical free primary normal-butyl cotion formed (path b) because such a cation, as would be generated from n-butylchloride, would yield exclusively isobutane upon rearrangement as shown in one experiment carried out where n-butyl chloride with hydrogen in the acid yielded only isobutane (eq. 5). 

Similarly, Siskin " found that when ethylene was allowed to react with ethane in a flow system, only n-butane was obtained. This was explained by the direct alkylation of ethane by ethyl cation through a pentacoordinated carbonium ion (equation 127). The absence of a reaction between ethyl cation and ethylene was explained by the fact that no rearranged alkylated product (isobutane) was observed. 

An experimental study also exists with respect to the C4H1C cations. Using the pulsed electron-beam MS technique, Hiraoka and Kebarle observed two species in the ion-molecule reaction of ethane and the ethyl cation. They identified them as the 59 2-H-n-butonium and the 60 2-C-n-butonium cation. C-C protonated ion 60 formed first rearranges to C-H protonated ion 59 (energy barrier = 9.6 kcal mol ) before dissociation to xcc-QH/ -1- If. 

Primary alkyl halides do react in superacids, but ordinarily the products are 2° or 3° carbocations arising from rearrangements. Nevertheless, there is evidence for the ethyl cation in mixtures of CH3CH2F and SbFs at low temperature in SO2 solution as well as in a H3PW12O40 pseudoliquid phase. There are also indications for formation of the ethyl cation in a reaction of ethene with methane and a HF-TaFs catalyst and in the solvolysis of ethyl tosylate in concentrated H2S04. In addition, a 1° carbocation-brosylate ion pair was proposed as an intermediate in the El... 

An analogous calculation for the methyl groups or the averaged methine carbons in the triply degenerate rearrangement (23) of l-deuterio-l-(p,P-dimethylcyclopropyl)ethyl cation [9] which is a two site fast exchange... 

I-(2,3-Dimethylcyclopropyl)ethylcation. The l-(2,3-dimethylcyclopropyl)-ethyl cation [127] undergoes a fast triply degenerate rearrangement (Olah et al., 1982). The structure is not controversial and therefore this system serves as a model cation for the investigation of equilibrium isotope effects in cyclopropylmethyl-type cations (Siehl and Koch, 1985). 

The first attempt to detect a bridged ethyl cation in solution was that of Roberts and Yancey in 1952. These workers showed that deamination of ethylamine-l- C in aqueous solution yields ethylene and 38% ethanol that contains 1.5% ethanol-2-Clearly the bridged ion is not an important intermediate in this reaction. It has also been shown that solvolyses of ethyl-l- C toluene-p-sulfonate in acetic acid, formic acid, and 75% dioxane-water and of specifically deuterated ethyl toluene-p-sulfonate in trifluoroacetic acid and 96 % sulfuric acid " proceed without appreciable rearrangement. It seems that bimolecular solvolysis is the predominating reaction in all these solvents. However, solvolysis of deuterium-labeled ethyl toluene-p-sulfonate in fluorosulfuric acid yields a product with 30-40 % rearrangement, " and it is possible that a bridged ion is an intermediate in this reaction. 

Our discussion of ethyl cation illustrates many issues that are universal in carbocation chemistry. As already mentioned, carbenium ions are especially prone to rearrangement. Often, several different but similar structures can equilibrate rapidly via such rearrange-... 

Alvarez-Idaboy, Eriksson and LuneU have also studied the hole-catalyzed ethylene trimerization [52]. They find a stable complex between the 1-butene radical cation and ethylene that can rearrange with an activation energy of 9.2 kcal mol to the 1-hexene radical cation. In this case, there is apparently no 1-ethyl-tetramethylene radical cation intermediate. Once again, the results of the calculations are in accord with experimental findings. 

The meso-ionic 1,2,3,4-oxatriazoles (286) yield phenol by acidic hydrolysis, phenyl azide by alkaline hydrolysis, and the 5-ethyl 1,2,3,4-oxatriazolium cation with triethyloxonium tetrafluoroborate. The rearrangement 286 -> 288 is achieved with boiling ethanolic ammonia. 

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

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