Carbocations spectra

An interesting variant of the spectroscopic experiment is in situ investigations under catalytic conditions. For this purpose a special reactor cell has been designed equipped with quartz windows which allows the monitoring of the course of the reaction on the catalyst wafer. As an example, the transformation of ethene over HZSM-5 was studied up to 573 K using a scattered transmission accessory in which the sample is placed directly in front of the multiplier, so that the forward-scattered light almost completely impinges the detector [130]. At lower temperatures no substantial differences could be established between the carbocation spectra obtained in situ and ex situ. The spectroscopic discrepancies in the peak positions above 548 K are tentatively explained by the different... 

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

When compound B is dissolved in FSO3H at —78°C, the NMR spectrum shows that a carbocation is formed. If the solution is then allowed to warm to — 100°C, a different ion forms. The first ion gives compound C when quenched with base, while the second gives D. What are the structures of the two carbocations, and why do they give different products on quenching ... 

Non-Kolbe electrolysis may lead to a large product spectrum, especially when there are equilibrating cations of about equal energy involved. However, in cases where the further reaction path leads to a particularly stabilized carbocation and either elimination or solvolysis can be favored, then non-Kolbe electrolysis can become an effi-yient synthetic method. This is demonstrated in the following chapters. 

The complete dissociation of the hydrocarbons [l 2 ], [28" 2 ] and [40" 2 ] in DMSO has been demonstrated by quantitative generation of both Kuhn s carbanion [2 ] and carbocations [1" ], [28" ] and [40" ] as determined by UV-vis spectra (Table 6 and Eig. 4). However, since carbocation [24 ] has no absorptions at a wavelength region longer than 220 nm in the UV spectrum, there remained an ambiguity that this cation might have decomposed in the DMSO solution. A clue to this problem could be obtained by determination of the electric conductivity of DMSO solutions of hydrocarbon salts (Table 7) (Okamoto et al., 1990). 

The first two reaction types often lead to the formation of stable end-products, but (c) and (d) lead to the formation of new carbocations to which the whole spectrum of reaction types is still open. Most of these possibilities are neatly illustrated in the reaction of 1-amino-propane (11) with sodium nitrite and dilute hydrochloric acid [the behaviour of diazonium cations, e.g. (12), will be discussed further below, p. 119] ... 

However, an atom somewhat heavier than H might well be less susceptible to the perturbations that may disturb the latter as, for example, 13C which also generates an n.m.r. spectrum. Thus the 2-arylpropyl(cumyl) carbocations (56 produced from the corresponding tertiary alcohols in super acid —S02ClF/FS03H/SbF5— solution, cf. p. 181),... 

The quantitation of products that form in low yields requires special care with HPLC analyses. In cases where the product yield is <1%, it is generally not feasible to obtain sufficient material for a detailed physical characterization of the product. Therefore, the product identification is restricted to a comparison of the UV-vis spectrum and HPLC retention time with those for an authentic standard. However, if a minor reaction product forms with a UV spectrum and HPLC chromatographic properties similar to those for the putative substitution or elimination reaction, this may lead to errors in structural assignments. Our practice is to treat rate constant ratios determined from very low product yields as limits, until additional evidence can be obtained that our experimental value for this ratio provides a chemically reasonable description of the partitioning of the carbocation intermediate. For example, verification of the structure of an alkene that is proposed to form in low yields by deprotonation of the carbocation by solvent can be obtained from a detailed analysis of the increase in the yield of this product due to general base catalysis of carbocation deprotonation.14,16... 

Hydrolyses of alkyl halides and arenesulfonates have long been known to be micelle-inhibited (Gani et al., 1973 Lapinte and Viout, 1973, 1979) but now k+/k < 1, except for hydrolysis of methyl benzenesulfonate which involves extensive bond making in the transition state (Al-Lohedan et al., 1982b Bunton and Ljunggren, 1984). Thus values of k+/k < 1 seem to be characteristic of hydrolyses in the SN1-SN2 mechanistic spectrum which involve considerable bond breaking in the transition state, and k+/k is very low for hydrolyses of diphenylmethyl halides where the transition state has considerable carbocation character (Table 8). 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

Up to this point the CM model and the More O Ferrall PES diagram lead to essentially identical conclusions. The analysis by More O Ferrall (1970) of the elimination mechanism spectrum may be summed up by Fig. 27. The two axes represent C—H and C—X bond-breaking co-ordinates. The third axis, perpendicular to the plane of the paper, is the energy co-ordinate. Two of the diagonal corners represent reactants and products while the other pair of diagonal corners represent the carbocation and carbanion intermediates. All possible mechanistic pathways are simultaneously indicated on the energy surface. 

Attack by Cl, 111 I, 19 and RS + 2il is similar to that by Br there is a spectrum of mechanisms between cyclic intermediates and open cations. As might be expected from our discussion in Chapter 10 (p. 312), iodonium ions compete with open carbocations more effectively than bromonium ions, while chloronium ions compete less effectively. There is... 

Probably the first isolation of a triphenylmethyl carbocation salt was by Gomberg and Cone (68) who successfully prepared the perchlorate from the corresponding chloride. A direct synthesis from the carbinol was achieved at about the same time (69), and more recently the preparation of the perchlorate and tetrafluoroborate have been much improved (70). Anderson (7/) succeeded in recording the characteristic visible absorption spectrum of the ion in concentrated acids, and Fairbrother and Wright (72) observed the same absorption when triphenylmethyl bromide was ionised in benzene in the presence of stannic bromide. 

---