Gas phase carbocation stabilities

The identity of r values for the gas-phase carbocation stabilities with those for the corresponding solvolyses provides important information regarding the general nature of the transition state as well as the intermediate of solvolytic process. 

Fig. 3.18. Gas phase carbocation stabilities relative to methyl cations in kcal/mol. Data from Y. Apeloig and T. Muller, in Dicoordinated Carbocations, Z. Rapopport and P. J. Stang, eds., John Wiley Sons, New York, 1997, Chap.2.

Robbins AM, Jin P, Brinck T, Murray JS, PoUtzer P (2006) Electrostatic potential as ameasure of gas phase carbocation stability. Int J Quantum Chem 106 2904-2909... 

A common and very valuable measure of gas phase carbocation stability is the hydride ion affinity (HI A), defined as AH° for the reaction in Eq. 2.15. This is simply the heterolytic analogue of the homolytic cleavage associated with the bond dissociation energy (BDE) just discussed. Just as a larger BDE implies a less stable radical, a larger HIA implies a less stable carbocation. And, just as with the BDE, the usefulness of the HIA is that it provides a number that can be compared directly for cations of dissimilar structure. This is less true of other measures of cation stability. 

Developments in the study of carbocations in superacid media over the past 30 years have been reviewed.1 The thermodynamics [AG(g)] of the reaction R+(g) + Rref OH(g) -> ROH(g) + R+ref(g) involving Rref = f-butyl and 21 R+ has been studied by high-level computation.2 A plot of AG(g) versus AG(solution) shows an excellent correlation, except for phenyl-substituted R+, which form a separate correlation family. The magnitude of the most positive surface electrostatic potential was proposed as an effective measure of the stability of gas-phase carbocations, with results presented for a number of structurally diverse cations.3 The electrostatic potential directly... 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

There is an excellent correlation between these data and the gas-phase data, in terms both of the stability order and the energy differences between carbocations. A plot of the gas-phase hydride affinity versus the ionization enthalpy gives a line of slope 1.63 with a correlation coefficient of 0.973. This result is in agreement with the expectation that the gas-phase stability would be somewhat more sensitive to structure than the solution-phase stability. The energy gap between tertiary and secondary ions is about 17kcal/mol in the gas phase and about 9.5 kcal/mole in the SO2CIF solution. 

One way of determining carbocation stabilities is to measure the amount of energy required to form the carbocation by dissociation of the corresponding alkyl halide, R-X - R+ + X . As shown in Figure 6.10, tertiary alkyl halides dissociate to give carbocations more easily than secondary or primary ones. As a result, trisubstituted carbocations are more stable than disubstituted ones, which are more stable than monosubstituted ones. The data in Figure 6.10 are taken from measurements made in the gas phase, but a similar stability order is found for carbocations in solution. The dissociation enthalpies are much lower in solution because polar solvents can stabilize the ions, but the order of carbocation stability remains the same. 

Table 14 contains the reaction enthalpies relative to the methyl cation from MINDO/3 calculations. The gas phase values demonstrate the following graduation of stability of the carbocations ... 

The Stability of hydrocarbon ions is discovered intuitively by observing whether the hydrocarbon ion can be isolated as a salt, for example, a sodium salt of the carbanion or a tetrafluoroborate salt of the carbocation. Conversely, a single hydrocarbon ion produced in the gas phase is obviously an unstable and short-lived species. Thus, many of the aliphatic carbocations in the gas phase are merely observable species but are not usable for synthesis. 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

Table 3 Relative stabilities" (in kcalmol of substituted bromonium ions and alkyl carbocations in the gas phase.11...

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

Model computational studies aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability for aza-PAHs derivatives were performed by density functional theory (DFT). Comparisons were made with the biological activity data when available. Protonation of the epoxides and diol epoxides, and subsequent epoxide ring opening reactions were analyzed for several families of compounds. Bay-region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (by the PCM method). 

Order of stability of fluoromethyl and fluoroethyl carbocations +CH3 < +CF3 < +CH2F < +CF2H +CH2CH3 +CF2CH3 +CHFCH3 Figure 1.7 Stability order of fluoroalkyl carbocations (gas phase). 

Our calculations did predict that a y methyl group should provide a small amount of stabilization for 4 in the gas phase. However, subsequent calculations that included solvation effects did find, in agreement with experiment, that a y methyl group on 4 should slightly depress the rate of carbocation formation." ... 

Significantly slower rates are found only for compounds that do not exhibit any aromatic ring or carbon-carbon double bond, and for aliphatic compounds with no easily abstractable H-atoms. Such H-atoms include those that are bound to carbon atoms carrying one or several electronegative heteroatoms or groups. (Note that the stabilization of a carbon radical (R ) is similar to that of a carbocation.) We will come back to such structure-reactivity considerations in Section 16.3, when discussing reaction of HO" with organic pollutants in the gas phase (i.e., in the atmosphere). 

After discussing the dehydration of methanol and formation of DME, we are able to illustrate a number of key theoretical concepts. The first is that carbocation fragments are found in transition states, rather than as stable intermediates. Furthermore, the nature of these species is different from what is predicted from gas-phase studies, experimental or theoretical. The cluster, i.e., the zeolite, controls the stabilization of this carbocationic fragment. Second, we see that each different reaction requires a different transition state, rather than formation of a transition state that can be converted in a number of possible reactions. (This latter view received some support as a result of different processes possessing very similar activation barriers.)... 

P-Fluonne or fluorine further removed from the cation center always inductively destabilizes carbocations [115, 116] No simple P-fluoroalkyl cations have been observed in either the gas phase or solution, and unlike the cases of the other halogens, there is no evidence for formation of alkyl fluoronium ions (5) in solution [117, 118], although (CH3)2F+ is long-lived in the gas phase [7791 The only P-fluonnated cations observed m solution are those that benefit from additional conjugativc stabilization, such as a-trifluoromethylbenzyl cations [772] and per-fluonnated allyl [729], cyclopropenium [772], and tropyliiim [727] ions... 

The gas-phase basicities of a-trimethylsilylstyrenes were determined by Mishima and coworkers by measurement of proton transfer equilibrium constants33. The basicity of a-trimethylsilylstyrene was found to be comparable to that of a-alkyl styrenes, which was taken to suggest that an o -trimethylsilyl group stabilizes a carbocation. 

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