Alkyl ions

In a variation on this approach, p-chlorobenzaldehyde is rst condensed with 2-aminopyridine. Reduction of the resulting iff base (62) affords the corresponding secondary amine. Alkyl-ion with the usual side chain affords the antihistamine, chlor-ramine (64). ... 

We have previously shown (8) that the chemical ionization spectra using methane as reactant are generated by the combination of dissociative proton transfer from CH5 + and hydride ion abstraction and alkyl ion... 

If we assume that rearrangement to form a secondary alkyl ion can occur,... 

Reactions 8 and 9 represent and are typical of all the dissociative ionizations producing the observed fragment alkyl ions. Reaction 9... 

A possible alternative mechanism for the formation of fragment alkyl ions comes immediately to mind—namely, beta fission of the Ci8 ion formed in Reactions 6 and 7 to form a smaller alkyl ion and an olefin. Thus we write as a typical example ... 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

It is of interest to compare the relative intensities of all the alkyl ions formed from a singly branched paraffin with the intensities of the corresponding ions formed from normal paraffins. Table III lists the ratios of the relative intensities of the several alkyl ions formed from 2-methyldecane (compound 1, Table II) and normal undecane. Within the limits of accuracy and detail with which we are concerned, the intensities of all the ions except the Ci0H2i+ (m/e = 141) are the same. The low value of the ratio for m/e = 127 may be ascribed to a greater... 

This reaction exemplifies the important process of methyl removal, which becomes even more significant in the case of multiply branched paraffins. The rather large exothermicity of Reaction 11 results from the fact that a secondary carbonium ion is formed. The beta fission process can be illustrated using reactions in 8-ra-hexylpentadecane (compound 2) as an example. Table II shows that the ions formed by C-C fission at a branch point (Ci4+, m/e = 197 and Ci5+, m/e = 211) have intensities appreciably larger than the other alkyl ions in the same region... 

In proceeding to a consideration of the chemical ionization mass spectra of more highly branched paraffins, it will be most convenient to consider separately the several different classes of alkyl ions found in the spectra—i.e., MW — 1+, MW — 15+, MW — 29 +, etc. We can see from Table II that a considerable amount of variation in the relative intensity of the MW — 1 ions (always the highest mass ion for which an intensity is given in the table) occurs. However, we shall show that the observed MW — 1 intensities can be approximately accounted for in terms of the concept of localized electrophilic attack by the reactant ion. First, however, we must consider the energetics of two processes which may be important in generating the spectra of branched paraffins. One of these is the abstraction of a primary hydrogen by the reactant ion. As a typical example we may write... 

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

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

The above discussion has been concerned with H transfer reactions to alkyl ions. In addition, there is mass spectrometric (16) and radiolytic (18) evidence that H can also be transferred to smaller olefinic ions (CnHm +)... 

The relative probabilities of Reactions 24, 25, and 26 were, respectively, 1.00, 0.25, and 0.12 at a hydrogen pressure of about 1 atmosphere (9). These numbers could be derived either by analyzing the stable alkanes formed in the unimolecular decompositions (Reactions 24-26) or from the products of the hydride transfer reactions between C5Hi2 and the alkyl ions. Elimination of H2 from protonated pentane may also occur, but it is difficult (although not impossible) to establish this reaction through neutral product analysis. 

All alkyl ions tested demonstrate a comparable behaviour independent of the sign of their charges. The decrease of the reaction enthalpies AH (11) with the change from the methyl to the ethyl cation (AAH (ll) = 165 kJ mol-1) and from the ethyl to the but-2-enyl cation (AAH°(11) = 117 kJ mol-1) corresponds to the increase of stability of these carbenium ions, which are expressed by the difference of their heats of formation (AAH f = —118 and AAHj = —42 kJ mol-1 90)) and of their hydride ion affinity (AHIA = 176 and 126 kJ mol-1 91)), respectively. 

A further result of the calculations presented is that, in addition to the but-2-enyl ions, the small alkyl ions are suitable models for the ionic chain ends as well. 

The initiation reaction in the polymerization of vinyl ethers by BF3R20 (R20 = various dialkyl ethers and tetrahydrofuran) was shown by Eley to involve an alkyl ion from the dialkyl ether, which therefore acts as a (necessary) co-catalyst [35, 67]. This initiation by an alkyl ion from a BF3-ether complex means that the alkyl vinyl ethers are so much more basic than the mono-olefins, that they can abstract alkylium ions from the boron fluoride etherate. This difference in basicity is also illustrated by the observations that triethoxonium fluoroborate, Et30+BF4", will not polymerise isobutene [68] but polymerises w-butyl vinyl ether instantaneously [69]. It was also shown [67] that in an extremely dry system boron fluoride will not catalyse the polymerization of alkyl vinyl ethers in hydrocarbons thus, an earlier suggestion that an alkyl vinyl ether might act as its own co-catalyst [30] was shown to be invalid, at least under these conditions. 

Mass spectra of OPEOs, their acidic metabolites, OPEC and halogenated derivatives in El and Cl modes have been reported [80-82]. The most prominent ions formed under Cl resulted from alkyl ion displacement and olefin displacement, and those formed under El resulted from benzylic cleavage [82]. 

In order to rationalize the data from a non-classical point of view one would have to propose that the relative stability of the edge- and corner-protonated species is different in norbornyl than in alkyl ions and that they are far apart in energy or that only the latter is an intermediate on the energy surface. There are no obvious reasons to believe that this is the case so that the exchange studies favour the classical ion. 

In a polymerization system, not only tertiary alkyl ions but also ions of the allyl type, because of their stabilizing resonance, would be formed readily. Hence, some hydrogenation and dehydrogenation of the primary polymer (e.g., RCH2CH2CH=CHR ) would occur in the following manner ... 

Clearly, Reaction 2 is favored over Reaction 1. In extension of Reaction 2 to higher homologs, Reaction 3 corresponds to the union of the alkyl ion, R+, with propylene or higher n-alpha-olefins (1-C H2 ), to give the secondary ion, R— CH2—CH—(CH2) -3— CH3, with the approximate energy values listed above. 

The catalytic isomerization reation of olefins is caused by either an associative or a dissociative mechanism. The associative mechanism involves either dissociative mechanism involves allylic intermediates, as described in Scheme 2, where M—H and M represent active sites. 

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