F-butyl carbocation

In our discussion of the SnI reaction above, we proposed the f-butyl carbocation as a reasonable intermediate formed by loss of bromide from f-butyl bromide. We now need to explain the evidence we have that carbocations can indeed exist, and the reasons why the f-butyl carbocation is much more stable than, for example, the -butyl cation. 

The f-butyl carbocation is relatively stable as far as carboca-tions go, but you would not be able to keep it in a bottle on the shelf The concept of more and less stable carbocations is important in understanding the SnI reaction, but it is important to realize that these terms are all relative even stable carbocations are highly reactive electron-deficient species. 

Although the ESCA spectrum of the f-butyl carbocation shows C Is levels for both the carbenium carbon atom and the three methyl carbon atoms, the ESCA spectrum of the trityl cation (PhsC ) suggests that all of the carbon Is electrons have the same binding energy. Explain this result. 

There is evidence that carbocations do play a role in the reaction of some diazonium ions. For example, the products of nucleophilic addition to f-butyl carbocations are essentially the same whether the carbocations are produced from aliphatic amines or from solvolysis of the corresponding alkyl halide. The decomposition of the diazonium ion to form a carbocation should become less favorable as the stability of the carbocation decreases, and the formation of an ethyl cation from ethyldiazonium ion in the gas phase has been calculated to be endothermic by 13.9kcal/mol. The decomposition of a 1° alkyldiazonium ion to N2 and a 1° carbocation in solution may be energetically more favorable, perhaps even exothermic, but it is questionable... 

Solution f-Butyl alcohol is a tertiary alcohol thus it reacts by an Sjjl mechanism. As in all Sfjl reactions, the rate-determining step involves formation of a carbocation in this case, the f-butyl carbocation. The rate of this step does not depend on which acid is used, so all of the reactions proceed at equal rates. 

There are less data related to carbocation lifetimes as compared to radical lifetimes. Yet, some extensive studies by Mayr, Richards, and others have provided much insight into substituent effects on their lifetimes. In general, the lifetimes are extremely short in water. For example, Toteva found that the f-butyl carbocation has a lifetime of only lO" s in water. Hence, although we consider tertiary carbocations stable, they are clearly not persistent in this medium. Secondary carbocations are even more reactive toward addition of water, and many secondary derivatives undergo concerted hydrolysis in water that avoids formation of the carbocation reactive intermediate. The primary 4-methoxybenzyl carbocation inter-... 

At temperatures above -40 °C, the cation rearranges to f-butyl carbocation. Give mechanisms for all the hydrogen and carbon shifts described here. 

There is direct evidence, from ir and nmr spectra, that the f-butyl cation is quantitatively formed when f-butyl chloride reacts with A1CI3 in anhydrous liquid HCI.246 In the case of olefins, Markovnikov s rule (p. 750) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride247 and 1-chloroadamantane248 alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole but not benzene.249 It was noted on p. 337 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates.250... 

Electron deficiency at a carbon causes drastic deshielding. This is observed for the sp2 carbons typical of carbocations [79], In such systems, the sp2 13C chemical shift range may approach 400 ppm relative to TMS. If the positive charge is dispersed in a carboca-tion, e.g. by resonance, the electron deficient carbon will be more shielded. The following comparison of f-butyl-, dimethylhydroxy- and dimethylphenyl-carbenium ion illustrates this ... 

Developments in the study of carbocations in superacid media over the past 30 years have been reviewed.1 The thermodynamics [AG(g)] of the reaction R+(g) + Rref OH(g) -> ROH(g) + R+ref(g) involving Rref = f-butyl and 21 R+ has been studied by high-level computation.2 A plot of AG(g) versus AG(solution) shows an excellent correlation, except for phenyl-substituted R+, which form a separate correlation family. The magnitude of the most positive surface electrostatic potential was proposed as an effective measure of the stability of gas-phase carbocations, with results presented for a number of structurally diverse cations.3 The electrostatic potential directly... 

Solution of antimony pentafluoride in anhydrous HF was found to be an efficient medium for alkylation reactions of fluoroolefins. Compounds, known to give stable carbocations in superacidic media are excellent alkylating agents. For example, f-butyl chloride (57) reacts with TFE, giving the corresponding alkane 58 ... 

So far we have added heteroatoms only—bromine, nitrogen, or sulfur. Adding carbon electrophiles requires reactive carbon electrophiles and that means carbocations. In Chapter 17 you learned that any nucleophile, however weak, will react with a carbocation in the S>jl reaction and even benzene rings will do this. The classic S>jl electrophile is the f-butyl cation generated from t-butanol with acid. 

In Chapter 17 we showed you that it is possible to run the NMR spectra of carbocations by using a polar but nonnucleophilic solvent such as liquid SO2 or SOC1F. Treating an alkyl halide RX with the powerful Lewis acid SbFs under these Conditions gives a solution of carbocation the carbocation reacts neither with solvent nor the SbF5X counterion because neither is nucleophilic. We know, for example, that the chemical shifts in both the l3C and NMR spectra of the f-butyl cation are very large, particularly the 13C shift at the positively charged centre. 

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. 

Diem et al. investigated the photolysis of adamantyl iodide as a po ible source of carbocationic initiation for the polymerisation of isobutene. Althou no polymer was obtained with this simple tystem (or with f-butyl iodide as photolyte), addition of iodine scavengers such as zinc, zinc fodide or both together gave some polymerisation, indicating that the carbocations produced in the photolysis of alkyl iodides possess a modest initiating power if generated in the presence of isobutene. 

Product B must arise from a Friedel-Crafts alkylation with the f-butyl cation as intermediate This comes from the loss of carbon monoxide from the acylium ion. Such a reaction happens oniv when the simple carbocation is stable. 

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. 

Oligomerization After the primary reaction forms a Cg+ carbocation, a second olefin reacts to form a higher molecular weight hydrocarbon (e.g., C12) and another f-butyl cation. Further reactions can result in even larger products (e.g., CifiS, etc.). 

Organic compounds with covalent bonds to electronegative elements may dissociate to form carbocations, especially if the cation is stabilized by the inductive effect, as in the f-butyl cation, or by resonance, as in the cumyl (2-phenylprop-2-yl) cation. This can happen slowly in polar solvents such as water. The SN1 reaction of t-butyl halides is an example (reaction 5.6). The slow and rate-determining heterolysis of the halide is followed by a rapid reaction of the f-butyl cation with water to give the alcohol product. -Cl HiO... 

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