Carbenium ions methyl

Forty years after the initial proposal, Sweet and Fissekis proposed a more detailed pathway involving a carbenium ion species. According to these authors the first step involved an aldol condensation between ethyl acetoacetate (6) and benzaldehyde (5) to deliver the aldol adduct 11. Subsequent dehydration of 11 furnished the key carbenium ion 12 which was in equilibrium with enone 13. Nucleophilic attack of 12 by urea then delivered ureide 14. Intramolecular cyclization produced a hemiaminal which underwent dehydration to afford dihydropyrimidinone 15. These authors demonstrated that the carbenium species was viable through synthesis. After enone 13 was synthesized, it was allowed to react with N-methyl urea to deliver the mono-N-methylated derivative of DHPM 15. 

According to semiempirical MNDO/PM3 calculations, the Te-methylated isomer 20 is energy-disfavored relative to 19. Moreover, the C—Te" bond in 20 is elongated (3.53 A) to such an extent that its stmcture should better be described as the carbenium ion 20a [94JCS(P2)2341]. 

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. 

Scheme 8.—Formation of ) -D-Galactosylmethyl Carbenium Ion from y -D-GaIactosyl-methyl-4-(nitrophenyl)triazene.

In the copolymerization of isopropenylferrocene with a-methyl-styrene at 0°C, it appears that the stable carbenium ion of isopropenylferrocene acted as an inhibitor for the polymerization. 

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

The data are summarized in Table II. They have been normalized to kx x s i for each zeolite catalyst. In general it is seen that the7transfer of an ethyl group (E,E E,X) occurs faster than that of a methyl group (X,E X,X). This is in agreement with the indicated mechanism for transalkylation (Figure 4) which involves a benzylic carbenium ion intermediate. In the case of methyl transfer, this is a primary cation,... 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

TABLE 6. Effect of the methyl group at the electrophilically attacked vinylic carbon by p-methoxydiphenylmethyl carbenium ion... 

Without having the thermochemistry data at hand, it is not trivial to decide which pair of products will be preferred over the other. In general, the formation of the higher substituted and/or larger carbenium ion is preferred, because it can more easily stabilize a charge. However, the tendency is the same for the radicals and one may expect loss of ethyl to be favored over loss of methyl, for example. Thus, the formation of both the ionic and the radical fragments are of decisive influence on the final distribution of products. 

As with acylium ions and carbenium ions before, the series of homologous im-monium ions is part of the mass spectrometrist s tool box. They can easily be recognized in the mass spectra and have even-numbered m/z values (Tab. 6.6). In the El spectrum of iV-ethyl-iV-methyl-propanamine the series is completely present from m/z 30 up to m/z 100. 

Methyl shift in carbenium ions is expected to require three elementary steps (ring closure, ring opening, hydride shift), enabling the positive charge to move... 

It is important to note that, while the primary product Cg carbenium ions that are formed (after reaction with 2-butene or 1-butene) are secondary, they can undergo hydride shift or methyl shift and form a tertiary carbenium ion in each case. In that case the driving force is diminished for either of the two tertiary Cg carbenium ions to abstract a hydride ion from i-butane since this now becomes a transition from a large tertiary carbenium ion to a smaller tertiary carbenium ion. Nevertheless, this hydride transfer can still occur due to the high ratio of i-butane to tertiary Cg carbenium ion that exists in the reaction medium. At the same time the tertiary Cg carbenium ion may get alkylated with another butylene molecule to make the more stable C12 carbenium ion, which would then lead to heavies. 

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

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