Temperature carbocation rearrangement

The Focus On box in Chapter 8 on page 298 showed that when carbocations are generated in superacid solution, they undergo extensive rearrangements, usually forming a relatively stable tertiary carbocation. As an example, when 1-butanol is dissolved in superacid at — 60°C, the protonated alcohol is formed. Water does not leave at this temperature because the carbocation that would be formed is primary. When the temperature is raised to 0°C, water leaves but the carbocation rearranges rapidly to the more stable tert-butyl carbocation ... 

Primary cations can never be observed by NMR—they are too unstable. But secondary cations can, provided the temperature is kept low enough, sec-Butyl chloride in SO2CIF at -78 °C gives a stable, observable cation. But, as the cation is warmed up, it rearranges to the f-butyl cation. Now this rearrangement truly is a carbocation rearrangement the starting material is an observable car-bocation, and so is the product, and we should just look at the mechanism in a little more detail. 

We shall not treat cracking processes here due to the complexity of these high-temperature (usually around 500°C) reactions. However, cycloalkane dehydrogenation to aromatics (Appendix A2.4.4), alkane isomerization and olefin alkylation (leading to branched alkanes from linear ones) occur via such carbocation rearrangements. 

The extent of carbocation rearrangement is difficult to predict It depends on the alkene structure, the solvent, the strength and concentration of the nucleophile, and the temperature. In general, rearrangements are favored under strongly acidic, nucleophile-deficient conditions. 

It turns out that the high temperature and acidity required for acid-catalyzed alkene hydration causes exotie carbocation rearrangements and makes the reaction impractical for most applications. 

Examine the transition state for the hydride shift. Calculate the barrier from the more stable initial carbocation. Is the process more facile than typical thermal rearrangements of neutral molecules (.05 to. 08 au or approximately 30-50 kcal/mol) Is the barrier so small (<.02 au or approximately 12 kcal/mol) that it would be impossible to stop the rearrangement even at very low temperature Where is the positive charge in the transition state Examine atomic charges and the electrostatic potential map to tell. Is the name hydride shift appropriate If not, propose a more appropriate name. 

Studies reveal an advantage to using boron trifluoride in dichloromethane at reduced temperatures instead of Brpnsted acids in the organosilicon hydride reductions of a number of dialkylbenzyl alcohols.126 129 The use of Brpnsted acids may be unsatisfactory under conditions in which the starting alcohol suffers rapid skeletal rearrangement and elimination upon contact with the acid, and also in which the alcohol does not yield a sufficient concentration of the intermediate carbocation when treated with protic acids.126... 

The cyclopentyl cation (39) undergoes a rapid degenerate rearrangement which can be frozen out at cryogenic temperatures as shown by solid state CPMAS 13C NMR spectra.57 MP2/6-31G(d,p) calculations show that cyclopentyl cation has a twisted conformation 4058 in which the axial hydrogens are bend toward the carbocation center. This is due to the pronounced geometrical distortion caused by the hyper-conjugative interaction of the /i-cr-C-H-bond with the formally vacant 2pz-orbital at the C+ carbon of this secondary carbocation. 

Rearrangements of long-lived carbocations formed via ewfo-9-hydroxy-1,8,9,10,10,11,12-heptamethyltricyclo[6.2.2.02,7]dodeca-2(7),3,5,11 -tetraene (5) in FSO3H-SO2CIF-CD2CI2 at low temperature (-95 °C) were studied by H and... 

A long-established feature of the carbocation intermediates of reactions, such as SnI solvolysis and electrophilic aromatic alkylation, is a skeletal rearrangement involving a 1,2-shift of a hydrogen atom, or an alkyl, or aryl group. The stable ion studies revealed just how facile these rearrangements were. Systems where a more stable cation could form by a simple 1,2-shift did indeed produce only that more stable ion even at very low temperatures (see, e.g., Eq. 3). 

The reaction occurs via a carbocation intermediate. Therefore, it is possible to form both substitution and elimination products. Secondary alcohols with branching on the [f-carbon give rearranged products. The temperature must be kept low to avoid the formation of El product. 

Detailed wide-ranging studies are available on the addition of HC1 and HBr to alkenes. The most useful procedure is to react dry HC1 gas and the alkene neat or in an inert organic solvent. Water or acetic acid may also be used. Alkenes yielding tertiary or benzylic alkyl chlorides react most readily. Styrene, however, adds HC1 only at — 80°C to give a-chloroethylbenzene without polymerization.101 At more elevated (room) temperature polymerization prevails. HBr adds to alkenes in an exothermic process more rapidly than does HC1. Rearrangements may occur during addition indicating the involvement of a carbocation intermediate 102... 

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... 

The secondary butyl and amyl cations can be observed only at very low temperatures, and they rearrange readily to the more stable tertiary ions. Generally, the most stable tertiary or secondary carbocations are observed from any of the isomeric alkyl fluorides in superacidic solvent systems. 

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