Ionization to carbocations

Nonbonded electron pair donors (w-donors) are expectedly readily protonated (or coordinated) with superacids. Remarkably, this includes even xenon, long considered an inert gas. The protonation of some 7T-, (T- and -bases and their subsequent ionization to carbocations or onium ions is depicted as follows ... 

Primary halides and methyl halides rarely ionize to carbocations in solution. If a primary halide ionizes, it will likely ionize with rearrangement. 

The previous section described an example in which a carbocation reacts with nucleophiles. In the conversion of 64 to 65, water is the nucleophile. Because an intermediate carbocation is formed in this reaction (66), it is possible to add a nucleophile to the reaction that is more reactive than water and change the course of the reaction in terms of product formation. If potassium iodide (KI) reacts with 64 in aqueous THF, iodide will not react via an Sn2 mechanism. Ionization to carbocation 66 occurs in the aqueous media, and both iodide and water are present as nucleophiles. Iodide is a stronger nucleophile than water (see Chapter 6, Section 6.7), so 66 reacts with iodide faster than it does with water. How is it possible to make such a statement The observed final product is 74 (l-iodo-2-methylpropane er -butyl iodide). This product can be formed only if iodide reacts as a nucleophile. The overall process is substitution (iodide substitutes for bromide) via a carbocation intermediate. 

If the solvent is water or if it contains water, the bimolecular (collision) processes between a neutral substrate and a charged nucleophile (such as nucleophilic acyl addition reactions and nucleophilic displacement with alkyl hahdes) are slower due to solvation effects. On the other hand, water is an excellent solvent for the solvation and separation of ions, so unimolecular processes (which involve ionization to carbocations see Chapter 11, Section 11.6) may be competitive. If the solvent is protic (ethanol, acetic acid, methanol), ionization is possible, but much slower than in water. However, ionization can occiu- if the reaction is given sufficient time to react. In other words, ionization is slow, but not impossible. An example of this statement is the solvolysis of alcohols presented in Chapter 6 (Section 6.4.2). Based on this observation, assume that ionization (unimolecular reactions) will be competitive in water, but not in other solvents, leading to the assumption that bimolecular reactions should be dominant in solvents other than water. This statement is clearly an assumption, and it is not entirely correct because ionization can occur in ethanol, acetic acid, and so on however, the assumption is remarkably accurate in many simple reactions and it allows one to begin making predictions about nucleophilic reactions. 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

The first step, which is rate determining, is an ionization to a carbocation (carbonium ion in earlier terminology) intermediate, which reacts with the nucleophile in the second step. Because the transition state for the rate-determining step includes R-X but not Y , the reaction is unimolecular and is labeled S l. First-order kinetics are involved, with the rate being independent of the nucleophile identity and concentration. 

The reactant must undergo ionization to form a carbocation intermediate in water with a rate constant of between 10 and 10 s in order to be able to conveniently monitor this reaction. 

Both Si—H and C—H compounds can function as hydride donors under certain circumstances. The silicon-hydrogen bond is capable of transferring a hydride to carbocations. Alcohols that can be ionized in trifluoroacetic acid are reduced to hydrocarbons in the presence of a silane. 

An alternative to Equation 10-1 would be to have Br2 ionize to Br and Br8, with a subsequent attack of Br on the double bond to produce the carbocation. The fact is that energy required for such an ionization of Br2 is prohibitively large even in water solution (AH0 > 80 kcal). One might well wonder why Equation 10-1 could possibly... 

Primary /3-aryl tosylates have also been shown to undergo solvolysis by two distinct pathways—aryl- and solvent-assisted.32 Tertiary /J-aryl tosylates, however, ionize to a stable carbocation and seem to require no assistance in isomerization.33... 

The solvolysis mechanisms of 2,2-dimethyl-3-pentyl- and l-(l-adamantyl)-propyl sulfonates appeal- to involve partial reversible ionization to the ultimate ion pair followed by competing elimination and solvent separation, substitution products being formed from the separated ions.27 The lifetimes of simple tertiary carbocations may be some 100 times shorter than previously thought several 3-(4-methoxyphenyl)-l,l-dimethylpropyl species hydrolyse in 50% aqueous TFE with rate constants estimated at some 3.5 x 1012 s-1.28 Much elimination was also observed.28 Two studies concerning proposed carbocation intermediates in enzymatic processes are reported.29,30... 

More recent measurements related to carbocation stabilities in strongly acidic media have involved rates of reaction rather than equilibria.52,54,72 75 Application of the X0 function to the correlation of reaction rates as well as equilibria mirrors the use of structure-based free energy relationships. Of interest is the access this gives to rate constants for (a) protonation of weakly basic alkenes and (b) acid-catalyzed ionization of alcohols to relatively unstable... 

Bridgehead bicyclo[2.2.1]hept-l-yl cation (1-norbornyl cation) has not yet been directly observed 1-chloronorbomane yields the stable 2-norbornyl cation in SbF5-SO2 solution.192 Thus, ionization to the bridgehead carbocation must be followed by a fast shift of hydrogen from C( 1) to C(2) (either intramolecular or intermolecular), the driving force for which is obviously the tendency to relieve strain in the carbocation. 

The reactivity of an alkyl halide in an SN1 reaction is dictated by the ease with which it ionizes to form a carbocation. Tertiary alkyl halides are the most reactive, methyl halides the least reactive. 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

The SN1 mechanism is a two-step process. In the first step, the alkyl halide ionizes to a carbocation and a halide ion. In the second, fast step, the carbocation combines with the nucleophile. The overall rate is independent of nucleophile concentration. If the halogenbearing carbon is stereogenic, substitution occurs with racemization. The reaction is fastest for tertiary halides and slowest for primary halides. The two mechanisms are compared in Table 6.2. 

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