Ethyl carbenium ion

On the other hand, the primary carbenium ions ethyl+, propyF, and butyF, whose formation involves the oxonium methyl ylide, can be deprotonated into the corresponding olefins, but they can also undergo methylation and oligomerization into higher carbenium ions, leading to paraffins formation via H-transfer reactions and to aromatic via cyclization [119]. The latter transformation leads to polyring aromatics, which are too bulky to leave the catalyst because of the structure of SAPO-34. They cover sites and block pores and thus deactivate the catalyst. However, the catalyst should be partially deactivated to a primary formed propylene which could escape the pore system of the molecular sieve. 

Reaction scheme (bs basic site of the catalyst, R carbenium ion ethyl, R carbenium ion propyl ). Adapted from Alwahabi SM, Froment GF. Conceptual reactor design for the methanol-to-olefrns process on SAPO-34. Ind Eng Chem Res 2004 43 5112-22 Alwahabi SM, Froment GF. Single event kinetic modelingof the methanol-to-olefins process on SAPO-34. Ind Eng Chem Res 2004 43 5098- 111. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

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. 

The mechanism was then reexamined 25 years later in 1997 by Kappe. Kappe used H and C spectroscopy to support the argument that the key intermediate in the Biginelli reaction was iminium species 16. In the event, 5 reacted with 3a to form an intermediate hemiaminal 17 which subsequently dehydrated to deliver 16. Iminium cation 16 then reacted with 6 to give 14, which underwent facile cyclodehydration to give 15. Kappe also noted that in the absence of 6, bisureide 8 was afforded as a consequence of nueleophilic attack of 16 by urea (3a). This discovery confirmed the conclusion of Folkers and Johnson in 1933. As far as the proposal from 25 years earlier by Sweet and Fissekis, Kappe saw no evidenee by H and NMR spectroscopy that a carbenium ion was a required species in the Biginelli reaetion. When benzaldehyde (5) and ethyl... 

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. 

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,... 

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. 

Carbenium ions, especially from ethyl to butyl show remarkable dehydrogenation that gives rise to a characteristic accompanying pattern of peaks at m/z-2 and m/z-4, i.e., at the low-mass side of the corresponding peak ... 

Example The mass spectra of both acetone and butanone show typical acyiium ion peaks at m/z 43, whereas the signals in the spectra of isopropyl ethyl thioether (Fig. 6.9), of 1-bromo-octane, (Fig. 6.10), and of isomeric decanes (Fig. 6.18) may serve as examples for carbenium ion signals. The superimposition of both classes of ions causes signals representing an average pattern. The properties of larger carbenium ions are discussed in the section on alkanes (Chaps. 6.6.1 and 6.6.3). 

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. 

Other typical reactions of carbenium ions are alkene loss, provided sufficient chain length is available (Chap. 6.6.1), and dehydrogenation in case of the smaller ions such as ethyl, propyl, or butyl ion (Chap. 6.2.4.). 

A carbocationic species in which there is at least one pentavalent carbon atom (e.g., CHs ). 2. Traditional name for chemical species that are now referred to as carbenium ions. Considerable confusion exists in the literature with this term for carbocations. The -onium suffix usually refers to a higher covalency when compared to the neutral atom thus, CH5+ would be a true carbonium ion (in terms of the first definition). Additional ambiguity results when the term ethyl carbonium ion is used to describe both CH3CH2 and to R—CH2CH2. For these reasons, the terms carbocation or carbenium ion are now preferred. 

The Biginelli synthesis (Scheme 3) is an important route to dihydropyrimidilies, e.g. (25),46a with many variants of the original reactants now established. The mechanism has now been re-investigated using 1H- and 13C-NMR.46b The first step does not appear to involve aldol condensation or a carbenium-ion intermediate rather, condensation of benzaldehyde and urea gives an A-acyliminium ion intermediate (26), which then goes on to react with ethyl acetoacetate. 

Alkylation of methane, ethane, propane, and n-butane by the ethyl cation generated via protonation of ethylene in superacid media has been studied by Siskin,148 Sommer et al.,149 and Olah et al.150 The difficulty lies in generating in a controlled way a very energetic primary carbenium ion in the presence of excess methane and at the same time avoiding oligocondensation of ethylene itself. Siskin carried out the reaction of... 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

The ethyl ester 258 (Eq. 103) has been recovered unchanged after treatment with the boron trifluoride acetic acid complexin>, whereas cyclopropane 255 with an additional 2-methyl group opens under these conditions to provide y-butyrolactone 257112). Apparently the intermediate tertiary carbenium ion 256 is sufficiently stabilized by the trimethylsilylmethyl and the methyl group to be generated from 255. 

Hunger (2008) also monitored the H/D exchange of ethyl-d5-benzene with zeolite HY at temperatures up to 523 K. The combination of NMR and UV-vis spectroscopy was found to be useful in this case the progress of the exchange at the methyl groups of the side chain was evident from the NMR spectra, and carbenium ion intermediates were detected in the UV-vis spectra. 

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