Isopropyl cation alkylation

Alkylation of benzene with the isopropyl cation then produces isopropylbenzene ... 

The value of pAR = -20.0 for the isopropyl cation is 2.1 log units more positive than the value derived from protonation of propene directly. In its favor, protonation of 2-butene occurs at a secondary rather than (as for 2-propene) a primary vinylic carbon atom, as is also true of formation of the cations correlated in Fig. 1. However, that correlation refers to benzylic rather than alkyl cations and there is no good reason to suppose their behavior is strictly comparable. 

The more negative pAR is preferred for two reasons. Firstly, as will be clear below, it is correlated better with the gas-phase stability of the isopropyl cation. Secondly, derivation of pAR 16.5 for the cyclohexyl cation from the rate constant for protonation of cyclohexene98 gives a value which, for a secondary alkyl cation, seems too close to that of the t-butyl cation (pAR = -16.4), even though the difference is increased by 2.1 log units if allowance is made for the more favorable geminal interactions of an OH... 

The observation of alkyl cations such as the ferf-butyl cation [trimethyl-carbenium ion, (CH3)3C+] 1 and the isopropyl cation [dimethylcarbenium ion, (CH3)2CH+] 2 was a long-standing challenge. The existence of alkyl cations in systems containing alkyl halides and Lewis acids has been inferred from a variety of observations, such as vapor pressure depressions of CH3C1 and C2H5CI in the... 

Propylation of propane by the isopropyl cation 31, for example, gives a significant amount (26% of the Q, fraction) of the primary alkylation product [Eq. (5.61)]. 

The strong competition between alkylation and hydride transfer appears in the alkylation reaction of propane by butyl cations, or butanes by the propyl cation. The amount of C7 alkylation products is rather low. This point is particularly emphasized in the reaction of propane by the terf-butyl cation, which yields only 10% of heptanes. In the interaction of isopropyl cation 31 with isobutane 2 the main reaction is hydride transfer from the isobutane to the isopropyl ion followed by alkylation of propane by the isopropyl ions (Scheme 5.20). 

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). 

Reaction of alcohols with benzene overNafion-H catalysts gave the corresponding alkylbenzenes (Table 5.11). When n-propyl alcohol was the alkylating agent, no n-propylbenzene was detected, and the only product obtained was cumene.112,187 This indicates the intermediacy of the isopropyl cation in the alkylation process. 

This parallelism is reflected in the proposed mechanism for the ionization of methane which shows that (a) the second step of the scheme invoives attack of an ethyl cation on methane, but the reaction cannot stop there, and goes on to (b), the third step, which involves attack of a secondary-isopropyl cation on methane. The primary and secondary alkyl cations are very strongly acidic species and are unstable under the reaction conditions. The condensation reaction essentially terminates with the much more weakly acidic tertiary-butyl ion. Alkane polycondensation and olefin polymerization side reactions producing stable, less acidic, tertiary ions obscured the simple alkylation reactions of the primary and secondary alkyl cations. Implicit in this mechanism, however, is that it is possible to react an acidic energetic primary cation (such as the ethyl cation) with molecules as weakly basic as methane and thus, the door was opened to new chemistry through activation of the heretofore passive, weakly basic, "paraffins" (20-24). 

H-beta zeolite proved to be an active and selective catalyst for alkylation of benzene with propene. In situ spectroscopic methods were applied to follow the formation and the evolution of surface intermediates and products,. It was found that when benzene is taken alone on the zeolite surface, its adsorption is reversible up to 473 K. On the contrary propene undergoes to several transformations even at 295 K. Isopropylbenzene behaves as propene, giving the same intermediates and products by decomposition at higher temperatures. Isopropyl cations formed upon chemisorption of propene on Broensted acid sites are the key intermediates for the alkylation reaction and are responsible for the faster deactivation via unsaturated caibenium ions formation. 

Which alkyl halide is obtained depends upon which intermediate carbonium ion is formed. This in turn depends upon the alkene and upon which carbon of the double bond hydrogen goes to. Propylene, for example, could yield an /i-propyl cation if hydrogen went to C-2 or an isopropyl cation if hydrogen went to C l. ... 

Use SpartanView to compare electrostatic potential maps of the ethyl cation, the isopropyl cation, and the ierf-butyl cation. How does the number of alkyl groups attached to the positive carbon change the potential at this atom What does this imply about the direction of electron transfer between an alkyl group and the positive carbon ... 

In 1979, Olah and Meidar found that alkylation of toluene offered solely a mixture of isopropylto-luenes. No n-propyltoluenes were detected. This was explained as proceeding by the initial formation of the Cl-protonated n-propyl chloride (2 Scheme 2), which rapidly rearranges to the isopropyl cation, followed by reaction with the aromatic ring. 

Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Let s compare the isopropyl cation with the iso-propenyl cation. In the latter, the central C has two cr bonds, one it bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp3)-H cr bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp3)-H cr bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp2)-H cr bonds on the left. Because C(sp2) orbitals are lower in energy than C(sp3) orbitals, the... 

We consequently undertook the infrared and Raman26, 34 spectroscopic study of the tetramethylethyl cation 154 and for comparisons a series of alkyl cations with known static structure, such as the f-butyl, f-amyl, and isopropyl cations 1, 3 and 2. The nearly identical spectra of the ions and the evident planarity (or close to planarity) of the carbocation centers suggest that the tetramethylethyl cation is classical , similar to the static ions used for comparison. 

For the norbomyl cation (22), on the other hand, the average 13C chemical shift for C—1,2 was found at 126.1 ppm (at -150°C where all other rearrangements except for the degenerate alkyl shift are frozen out )55)- Thus, C-1,2 absorb by ca. 80 ppm further upfield that the corresponding carbons of the equilibrating ion (25). A similar discrepancy (85 ppm) is estimated on the basis of isopropyl cation and C—1 of norbomane as a model. Some doubt, however, remains whether these acyclic ions are good approximations of the hypothetical 2-norbornyl cation. 

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two alkyl groups. Although allyl cation (CH2=CHCH2 ) is a primary carbocation, it is about as stable as a typical seconciary carbocation such as isopropyl cation, (CH3)2CHE... 

El elimination, 196-198, 201 Friedel-Crafts alkylation, 451 53, 479 SnI nucleophilic substitution, 143-146, 315-320,331 isopropyl cation, 141, 224 methyl cation, 141 iert-pentyl cation, 929 rearrangements, 187-189, 201, 219-220, 319-320,331,452,479 structure, bonding, and stability, 140-143, 162... 

The NMR spectra -. To calculate the averaged shift of the C and C atoms for the rapid equilibration of epimeric 2-norbomyl cations Olah used an isopropyl ion as a secondary carbocation model. Kramer observes that the signal of the C atom in the tert-butyl cation spectmm is in a lower field than for an isopropyl cation. Kramer means that the positive charge on the secondary cation centre is lower than on the tertiary one this contradicts the fact that tertiary alkyl carbocations are more stable than secondarv ones. Hence he concludes that averaged shifts cannot be calculated from the NMR spectrum of the isopropyl cation since it is not a typical secondary carbocation. 

In view of alkylation of benzene hypropene, Flego et al. [798] investigated by UV-Vis and IR spectroscopy the adsorption and co-adsorption of these two hydrocarbons on H-Beta, which proved to be an efficient alkylation catalyst. It was claimed by the authors that isopropyl cations, which formed on the Bronsted acid sites were the key intermediates of the reaction (cf. also Janin et al. [671 ] and Macedo et al. [672]). 

Protonated cyclopropane has been reported in the gas phase" "" to be ca 8 kcal mol" in energy above the isopropyl cation. The bent bonds of the cyclopropane ring are susceptible to electrophilic attack leading to the expectation that cyclopropane will be more basic than saturated alkanes and that protonation will occur on the C—C bond, i.e. the edge-protonated isomer will have the lowest energy. There is, however, considerable evidence from solution chemistry that corner-protonated cyclopropanes exist as intermediates in 1,2-alkyl shifts in carbocations. There have been several reviews of protonated cyclopropanes " and, in the current work, only the very recent theoretical work will be reviewed. 

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