Formation of Carbocations

Carbocations can be formed in a number of ways, including heterolysis of a covalent bond and addition of an electrophile to a multiple bond. 

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 

Loss of halide can be assisted by the presence of a Lewis acid such as antimony(V) fluoride (reaction 5.7). In this or other polar aprotic solvents the earboeation formed has no nucleophile to react with except for the 

SbF5Cl anion, which regenerates the original halide. In these solutions, concentrations of cations can be achieved which are high enough to allow study by NMR spectroscopy. 

Electrophiles normally add lo the end of a multiple bond which gfyes (he more stable cation intermediate. 

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Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

Steps 4-5 Conversion of hemiacetal to carbocation These steps are analogous to the formation of carbocations m acid catalyzed reactions of alcohols... 

These reactions involve the intermediate formation of thiols, followed by condensation to the sulfides. The observation of isomerized products in suitable cases indicates the intermediate formation of carbocations, either by protolysis of alkanes by the superacid or reversible ionisation of the thiol products (149). 

The latter three of the above points are dealt with in the following parts (see parts 4.2-4.5). Experimental investigations of the inner structure of the cations can be supplemented by quantum chemical calculations 104 106). For example, the MINDO/3 method allows the heats of formation of carbocations to be calculated 107). A comparison of some calculated and experimental values (Fig. 6) shows that the reproduction quality of MINDO/3 varies. 

Jacobs et al. employed an acidic zeolite catalyst for the racemization of sec-alcohols, which occurs through the formation of carbocations [44] (Figure 4.19). The KR is catalyzed by CALB in the presence of vinyl octanoate as acyl donor. DKR takes place successfully in a biphasic system (octane/H2O, 1 1) at 60 °C. 

The use of trichloroimidates for the preparation of ethers is an effective method for O-alkylation of alcohols [27]. This method has found widespread use in the protection of alcohols as benzyl ethers since the corresponding trichlorobenzylimi-date is inexpensive and commercially available. The mechanism involves activation of the imidate with a catalytic amount of a strong acid (typically TfOH) which leads to ionization of the electrophile and the formation of carbocation which is rapidly trapped by an alcohol. For the preparation of sec-sec ethers, this protocol has been limited to glycosidation reactions, due to the SN1 nature of the reaction which often leads to diastereomeric mixtures of products [26],... 

With simple aliphatic amines, the initial diazonium cations (56) will break down extremely readily to yield carbocations (cf. p. 107) which are, for reasons that are not wholly clear, markedly more reactive than those obtained from other fission processes, e.g. RBr- R Bre. Where the prime purpose is the formation of carbocations, the nitrosation is better carried out on a derivative of the amine (to avoid formation of H20) under anhydrous conditions ... 

Figure 7.7 Free-energy diagrams for the formation of carbocations from protonated tertiary, secondary, and primary alcohols. The relative free energies of activation are tertiary < secondary primary.

The study of S -1 reaction in cyclohexane becomes difficult because of the ease with which the elimination reactions occur at the same time. However it has been shown that SN reaction will be sterically accelerated for an axial substituent, because the formation of carbocation will relieve the steric strain due to 1, 3 interactions which is absent with equatorial substituent. 

Electrochemical fluorination in anhydrous hydrogen fluoride (Simons process) involves electrolysis of organic compounds (ahphatic hydrocarbons, haloalkanes, acid halides, esters, ethers, amines) at nickel electrodes. It leads mostly to perfluori-nated compounds, but is accompanied to a high extent by cleavage and rearrangement reactions. The mechanism of the formation of carbocations according to Eq. (1) and Scheme 1 is assumed... 

Normura,. and Shima, H. (2008) Adsorption of hydrocarbons and formations of carbocations over zeolites smdied by IR spectroscopy. 

Step 2 Formation of carbocation It is the slowest step and hence, the rate determining step of the reaction. 

Positively charged amines that are structural analogues and are isosteric with putative carbocation intermediates in enzymic reactions. These compounds have proved their value in efforts to characterize enzyme mechanisms that proceed by the transient formation of carbocation intermediates. 

Scheme 1. Formation of Carbocation Intermediate Scheme 1. Formation of Carbocation Intermediate...

In the absence of either HPI or Co(acac)2, no appreciable conversion into products was obtained. EPR evidence for the formation of an aminoxyl radical intermediate was acquired, and a KIE of 3.8 determined . Generation of PINO in situ as the reactive intermediate was postulated . A subtle alternative enables the functionalization of hydrocarbons through the formation of carbocations as transient intermediates whenever PINO is formed and reacts in the presence of NO (e.g. 1 atm) . 

The diazotization reaction generates the same type of /i-hydroxy carbocation that is involved in the pinacol rearrangement. (See Section 5.6 in Part A for a discussion of the formation of carbocations from diazo compounds.)... 

Spiroorthocarbonates and spiroorthoesters are stable toward bases and nucleophiles but easily undergo cationic ROP. Cationic ROP of a spiroorthocarbonate (LXX) proceeds by initial formation of carbocation LXXI in which the carbocation center is stabilized by... 

Catalytic cracking proceeds through the formation of carbocations. The ease of formation depends upon the structural nature and stability of the carbocation generated. 

The relative rates of formation of carbocations are 3°, 2° allyl >r allyl, 2 >l°>CHy CONTROLLING FACTORS... 

Mechanistic studies established the formation of carbocation intermediate 5 stabilized by the conjugation with the phenyl ring. 

FIGURE 4.17 Energies of activation for formation of carbocations from alkyloxo-nium ions of methyl, primary, secondary, and tertiary alcohols. 

Diazonium salts can be regarded as combinations of carbocations R with N2 and, because of the considerable stability of nitrogen in the form of N2, we would expect diazonium salts to decompose readily with evolution of nitrogen and formation of carbocations. This expectation is realized, and diazonium salts normally decompose in this manner in water solution. The aliphatic diazonium ions decompose so rapidly that their presence can only be inferred... 

Extensive measurements of heats of formation of carbocations in the gas phase exist and there have been more limited measurements in solution for nonhydroxylic solvents.39 For comparison with equilibrium measurements in water, however, the most appropriate measurement would appear to be free energies of formation in aqueous solution. It is fortunate therefore that a convenient compilation of values of AGf(aq) at 25°C has been provided by Guthrie.38 This allows us for example to derive a value of AGf(aq) for a carbocation from a measurement of its pXR value, provided that the free energy of formation of the corresponding alcohol [R OH in Equation (1)] is known. 

The group contributions apply only to alkyl cations and are of limited practical value. However, apart from illustrating the application of group additivity contributions to energies of formation of carbocations, they offer a significant insight into comparisons of stability based on hydride ion affinities (HIAs) and pAlR values. 

HIAs is only O.lkcalmol-1. This reflects almost complete cancellation of contributions from the extra CH2 group in the butyl structure between the cation and hydrocarbon. It indicates that HIAs provide a good approximation to differences in stability between a carbocation center and the corresponding group contribution from a hydrocarbon, independently of structural variations at carbon atoms not attached to the carbocation center. Moreover, a comparison between two secondary carbocations leads to almost complete cancellation of the contributions from the parent hydrocarbons and from alkyl groups of the carbocations too far removed from the charge center to influence stability. One is very close therefore to a comparison of stabilities comparable to that between isomeric cations. It should be noted that such intrinsic stabilities are not expressed in heats of formation of carbocations because they include uncanceled contributions from more remote portions of the structure. 

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