Carbocations isopropyl cation

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two methyl groups Al though allyl cation (H2C=CHCH2 ) is a primary carbocation it is about as stable as a typical secondary carbocation such as isopropyl cation (CH3)2CH-"... 

The two basic prerequisites to obtain high-molecular-weight polymers via cationic polymerization are the high nucleophilicity of the monomer and the relative stability of the carbocation to sustain propagation. The difficulty of ethylene and propylene to yield high-molecular-weight polymers in acid-catalyzed polymerization exemplifies this statement both have relatively low nucleophilicity and the derived ethyl and isopropyl cations have relatively low stability. 

Occasionally rearrangements from more stable to less stable carbocations occur, but only if (1) the energy difference between them is not too large or (2) the carbocation that rearranges has no other possible rapid reactions open to it.9 For example, in superacid medium, in the temperature range 0-40°C, the proton nmr spectrum of isopropyl cation indicates that the two types of protons are exchanging rapidly. The activation energy for the process was found to be 16 kcal mole-1. In addition to other processes, the equilibrium shown in Equation 6.7 apparently occurs.10 In the superacid medium, no Lewis base is available... 

Perhaps less obviously, the hydrocarbon also provides a reference for the carbocation. It is worthwhile examining the implications of such a reference, by considering briefly thermodynamic measurements of carbocation stabilities in terms of heats (enthalpies) or free energies of formation. Mayr and Ofial contrast our ability to measure the relative energies of tertiary and secondary butyl cations with the significant differences in relative stabilities of secondary butyl and isopropyl cations derived from different equilibrium measurements, namely, hydride, chloride, or hydroxide ion affinities. It is convenient to focus on this example and to assess the effectiveness of hydride affinities for comparing the stabilities of these three ions. 

It may be noted that the variation in increments shown in Scheme 12 represents differences in interaction energies between the OH group and the hydrocarbon fragment of the alcohol molecule. When pAR values are compared between different carbocations, they include the effect of the structural change upon the alcohol. Thus the difference in values for /-butyl and isopropyl cations underestimates the relative stabilities of the carbocations as measured by their HIAs by 1.5 log units (2.0/1.364). 

Protonated secondary thiols are stable even at higher temperatures. Protonated isopropyl thiol cleaves slowly at 0°C in HS03F-SbF5 (1 1 M) solution. No well-identified carbocations were found in the NMR spectra due to the instability of the isopropyl cation under these conditions. Protonated. sec-butyl thiol 55 cleaves to tert-butyl cation at this temperature [Eq. (4.33)]. 

Because of its resonance stabilization, the (primary) allyl cation is about as stable as a simple secondary carbocation, such as the isopropyl cation. Most substituted allylic cations have at least one secondary carbon atom bearing part of the positive charge. They are about as stable as simple tertiary carbocations such as the fm-butyl cation. 

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... 

Answer The solvents and leaving groups are the same. The SnI reaction will be faster via the more stable carbocation. The most stable carbocation and fastest SnI reaction is via the diphenylmethyl carbocation next is via the fert-butyl carbocation. The least stable isopropyl cation is the slowest, not expected to form at a significant rate. 

Substitution of the methyl group in the r-butyl cation by hydrogen thus causes an upfield shift of 10.4 ppm. Although the CMR shift of the carbocation center of the r-butyl cation is more deshielded than that of the isopropyl cation (by about 10 ppm), this can be explained by the methyl substituent effect, which may amount to 22 ppm. The tertiary butyl cation thus is more delocalized and stable than the secondary isopropyl. 

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. 

The H NMR spectrum of 155 in superacid had the right number of peaks to fit the symmetry of a static 2-adamantyl cation but the chemical shift value of the CH proton at the C(2) presumed carbocation center was only 5 H 5.1. This is 8 ppm to higher field than expected for typical static secondary carbenium ions such as the isopropyl cation."... 

The first reaction provides a route for the reduction of alkyl halides since the carbocation (isopropyl, in Rl) may be prepared from action of AICI3 on the corresponding alkyl halide. Reactions of the type Rl are also important in the process, catalytic cracking, in the manufacture of gasoline. They have also been studied in mass spectro-metric experiments [235]. Reaction R2 is one route to the preparation of carbocations under stable ion conditions. Reaction R3 is employed in the laboratory synthesis of the tropylium cation. Reaction R4, the (crossed) Cannizzaro reaction, is unusual in that it takes place under strongly basic conditions. The oxy dianion is an intermediate in the reaction of concentrated hydroxide with the aldehyde, R CHO. None of R R, or R may have hydrogen atoms a to the carbonyl groups. Formaldehyde (R = H) is readily... 

Nucleophilic capture of the spirooctadienyl cation opens the 3-member ring. This behavior characterizes many reactions of many other cyclopropane-containing carbocations, as well, y-radiolysis of perdeuterated propane forms CsD ions, most of which either transfer D or form isopropyl adducts. As the propane pressure is raised from 1000 mbar to 2000 mbar, however, the isopropyl/ -propyl adduct ratio falls from 30 1 to about 5.5 1. This implies the formation of corner-protonated cyclopropane, which reacts with nucleophiles as though it were an -propyl cation. With increased pressure, vibrationally excited protonated cyclopropane experiences more frequent nonreactive collisions, which deactivate it and slow down its rate of unimolecular isomerization to isopropyl cation. 

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. 

Olah has used the methods described above to make the methoxymethyl cation in solution. Although this cation can be drawn either as an oxonium ion or as a primary carbocation, the oxonium ion structure is the more realistic. The proton NMR spectrum of the cation compared with that of the isopropyl cation (this is the best comparison we can make) shows that the protons on the CH2 group resonate at 9.9 ppm instead of at the 13.0 ppm of the true carbocation. 

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. 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

---