Alkenyl ions

In addition it is possible that alkenyl ions are formed by loss of H2 from an alkyl ion by an approximately thermoneutral reaction. 

All of these data may be reconciled with the formation of substituted allylic (alkenyl) carbonium ions. Structures of this type can be derived from any substituted olefin by removal of a hydride ion from a carbon atom adjacent to the double bond (a-hydrogen). Since an olefin may be derived by proton removal from an alkyl carbonium ion, a substituted alkenyl ion may be derived from any alkyl carbonium ion precursor containing at least four carbon atoms and hence the spectra of their solutions should be very similar. Sulfur dioxide evolution from sulfuric acid solutions of olefins would accompany the formation of the alkenyl... 

The capability for alkenyl ion formation from different hydrocarbons and their derivatives follows the same sequence as observed in superacid solutions dienes>olefins>alcohols>paraffms. Decreasing the Brpnsted and the overall acidity has the consequence of slowing down the formation of alkenyl carbenium ions. Thus, protonation and hydride ion abstraction are the triggering steps. 

The methane chemical ionization mass spectra of cycloparaffins differ meaningfully from those of alkanes, although it appears that the same general types of reactions are involved in the production of the ions. The addition of a proton to a cycloalkane produces an (M + 1) ion with empirical formula C H2 +i, and extensive fragmentation of this ion to smaller ions with the same empirical formula also occurs. It may be presumed that the smaller ions are alkyl ions. It is also possible for the initial chemical ionization attack to result in hydride-ion abstraction to produce a (M — ion with empirical formula C H2 -i. Fragmentation of this ion occurs to produce smaller ions with the same empirical formula, which may be presumed to be alkenyl ions. Thus the chemical ionization spectra of cycloparaffins consists essentially of two series of ions, namely the alkyl series, C H2 +i, and the alkenyl series, C H2 -i. Both these series are incomplete in the sense that ions are not observed at all possible m/e values for all the compounds investigated. Spectra of two typical cycloparaffins are given in Table IV. 

In all the compounds studied, the (M — ions are the most intense, with relative intensities varying between 0.27 and 0.74 of the total ionization, depending on the structure of the cycloalkanes. These intensities are, on the whole, appreciably higher than the (M — 1) intensities found in alkanes, and for all the compounds investigated the sums of the intensities of the alkenyl ions are two to three times larger than the sums of the intensities of the alkyl ions. The 1 (or 3 ions for bicyclo-alkanes) are formed... 

The chemical ionization spectra of straight-chain 1-olefins consist almost exclusively of two series of ions, namely alkyl (C H2 +1) and alkenyl (C H2 i) ions. In this regard these compounds (and, indeed, mono-olefins in general) resemble the cycloparaffins discussed in the previous section. The intensities of the alkyl and alkenyl ions produced at the various carbon numbers in 1-decene, which serves as a typical 1-mono-olefin, are shown in Fig. 3. The intensities of the (M -f l)" and (M - 1) ions are small and decrease as the size of the olefin increases. Extensive fragmentation, which... 

The formation of the alkyl and alkenyl series of ions in the chemical ionization of mono-olefins may be explained in the same way as was the case with cycloparaffins namely, the alkyl ions result from an initial proton transfer from and the alkenyl ions result from an initial hydride... 

The alkenyl ion written in (14) has the ally lie structure, but alkenyl ions which are not allylic can also be formed in exothermic reactions. One can also write exothermic reactions for the formation of alkenyl ions which involve proton transfer to the olefin followed by loss of hydrogen molecule. All these reactions are strongly exothermic, and consequently extensive fragmentation of the (M H- 1)" and (M — I)" " ions is to be expected and is observed. 

Straight-chain 1-olefins studied. For olefins with fewer than 16 carbon atoms the alkyl ions are formed in greater abundance than the alkenyl ions, and this is contrary to the behavior found with the cycloparaffins, for which the intensities were greater (and usually appreciably... 

Fig. 4. Total alkyl and alkenyl ion intensities from straight-chain 1-olefins.

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