Mass spectrometer, carbonium ions

We also met the unusual cation CH5, This cation shares eight electrons among five bonds and has a full outer shell like that of the ammonium ion NH4. We call CH5 a carbonium ion. The three ions formed in the mass spectrometer have only three bonds to the positively charged centre, only six electrons in the outer shell, and are electron-deficient. We call these ions carbenium ions and we may call both types carbocations. Table 17.1 gives a summaiy of the two types of carbocations. 

Since then, the mass spectrometer has gradually developed into a versatile and sophisticated tool for the study of ion-molecule reactions, and has provided most of the information at present available on the chemistry of gaseous carbonium ions. Furthermore, the mass spectro-metric studies were successfully correlated with the results of kinetic investigations on the radiolysis of gaseous organic compounds, that involves, as suggested by Lind in his pioneering work of 1912, the intervention of ionic processes. 

Unfortunately, from the point of view of the physical organic chemist, the mass-spectrometric approach suffers from certain intrinsic limitations. In the first place, the range of pressures accessible to the investigator is severely limited, and most of the available data refer to experiments carried out at pressures well below one torr. In the second place, the mass spectrometer detects only charged species, and the neutral molecules, which represent the final products of the carbonium-ion reactions and are of prime concern to the physical organic chemist, cannot be determined at all. Finally, since the structure of the ionic species, that are analysed exclusively according to their m/e ratio, cannot be directly deduced from mass spectra, it is difficult to discriminate isomeric ions, and to study the isomerization reactions of the carbonium ions, which play such an important role in their solution chemistry. 

This also provides a reasonable explanation for the extensive rearrangements which are found to occur in carbonium ions produced in mass spectrometers. See papers by S. Meyerson and P. N. Rylander, J. Am. Chem. Soc., 78, 5799 (1956) S. Meyerson and H. M. Grubb, ibid., 79, 842 (1957) and S. Meyerson, J. Chem. Phys., 27, 1116 (1967). 

The behaviour of these carbonium ions and the energetics of their formation in the mass spectrometer have been discussed by Maccoll et and analogies... 

The problem was again taken up in general terms at the Chemical Society Symposium on the Transition State (Maccoll, 1962). Here, the behaviour of carbonium ions in the mass spectrometer and in solvolytic reactions was examined in relation to the behaviour of the virtual carbonium ions postulated in the gas-phase elimination reaction. Such properties as the ease of formation of carbonium ions and their rearrangements were examined, and, where data were available, a linear relationship was found between the activation energy for elimination and the heterolytic bond dissociation energy. This relationship is shown in Fig. 6. 

It would appear from the foregoing that there is a class of gas-phase reactions for which the transition state is best represented as having an essentially carbonium-ion pair character. In this way the effect of substitution at or near the centre of reaction can be interpreted, and the vast body of theory in the literature of physical organic chemistry used for the purpose of predicting rates of gas-phase reactions. In addition, the known properties of carbonium ions, as determined by the mass-spectrometer, can be invoked—as indeed they were in discussions of the SN1 and El reactions in polar solvents (Evans, 1946)—to correlate the effects of substituents in gas-phase eliminations. The advantage of studies in the gas-phase lies in the fact that the behaviour of a single molecule can be observed, without the added complication of the cooperative effect of the solvent. But gas-phase studies may, in turn,... 

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