Solvents, acidic carbocations

The mechanism in Scheme 19 is somewhat speculative, and relies heavily on the assumption that axial attack of solvent is more energetically favorable than equatorial attack of solvent on carbocations similar in structure to 64. However, the stereochemistry of acid-catalyzed hydrolysis of two diastereomeric hexa-hydrophenanthrene 9,10-epoxides (65 and 67) provides support for this proposal. These two epoxides contain transfused cyclohexane moieties that restrict the geometry of each epoxide to a single conformation. Reaction of 65 with H+ yields a single carbocation conformation 66, in which the hydroxyl group is forced to occupy an axial position (Scheme 20). 

In summary, it appears that bromination usually involves a complex that collapses to an ion pair intermediate. The ionization generates charge separation and is assisted by solvent, acids, or a second molecule of bromine. The cation can be a P-carbocation, as in the case of styrenes, or a bromonium ion. Reactions that proceed through bromonium ions are stereospecific anti additions. Reactions that proceed through open carbocations can be syn selective or nonstereospecific. 

This type of reaction begins when a ir bond of an alkene donates an electron pair to an acid (H+)—an acid-base reaction where the alkene is a weak base. The Jt bond is broken as the new Br—H bond is formed, and the remaining carbon of the former double bond becomes a carbocation. The reaction of cyclohexene with acid to form secondary cation 294 illustrates this process. The cationic center then reacts with the nucleophilic gegenion (Br" from HBr) to produce bromocyclohexane. The latter portion of this sequence is analogous to the second step (coupling) of an Sjsfl reaction. The initial reaction usually involves formation of a solvent separated carbocation intermediate, but this depends on the solvent. A tight ion pair intermediate can react in the substitution step to give the same product. The net result of this cationic reaction is addition of H and Br across the jt bond. 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

In superacidic media, the carbocationic iatermediates, which were long postulated to exist duting Friedel-Crafts type reactions (9—11) can be observed, and even isolated as salts. The stmctures of these carbocations have been studied ia high acidity—low nucleophilicity solvent systems usiag spectroscopic methods such as nmr, ir, Raman, esr, and x-ray crystallography. 

It has been possible to obtain thermodynamic data for the ionization of alkyl chlorides by reaction with SbFs, a Lewis acid, in the nonnucleophilic solvent S02C1F. It has been foimd that the solvation energies of the carbocations in this medium are small and do not differ much from one another, making comparison of the nonisomeric systems possible. As long as subsequent reactions of the carbocation can be avoided, the thermodynamic characteristics of this reaction provide a measure of the relative ease of carbocation formation in solution. 

A wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

Ho, the acidity function introduced by Hammett, is a measure of the ability of the solvent to transfer a proton to a base of neutral charge. In dilute aqueous solution ho becomes equal to t d Hq is equal to pH, but in strongly acid solutions Hq will differ from both pH and — log ch+. The determination of Ho is accomplished with the aid of Eq. (8-89) and a series of neutral indicator bases (the nitroanilines in Table 8-18) whose pA bh+ values have been measured by the overlap method. Table 8-19 lists Ho values for some aqueous solutions of common mineral acids. Analogous acidity functions have been defined for bases of other structural and charge types, such as // for amides and Hf for bases that ionize with the production of a carbocation ... 

Carboxylic acids with an electron donating substituent in the a-position decarboxylate in a two-electron oxidation to carbocations (see chap. 7). These can react with the solvent (alcohol, acetic acid, water) or the unreacted carboxylate to ethers, esters, or alcohols (Eq, 14). In some cases the carbon skeleton rearranges, which is a clear indication of the cationic pathway. 

Carboxylic acids can be converted by anodic oxidation into radicals and/or carbo-cations. The procedure is simple, an undivided beaker-type cell to perform the reaction, current control, and usually methanol as solvent is sufficient. A scale up is fairly easy and the yields are generally good. The pathway towards either radicals or carbocations can be efficiently controlled by the reaction conditions (electrode material, solvent, additives) and the structure of the carboxylic acids. A broad variety of starting compounds is easily and inexpensively available from natural and petrochemical sources, or by highly developed procedures for the synthesis of carboxylic acids. 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

It was expected that values of ks/kp for partitioning of [1+] could be obtained from the yields of the products of acid-catalyzed reactions of [l]-OH and [2]. However, significantly different relative yields of these products are obtained from the perchloric acid-catalyzed reactions of [l]-OH and [2] in several mixed alcohol/water solvents.21 This demonstrates that the nucleophilic substitution and elimination reactions of these two substrates do not proceed through identical tertiary carbocation intermediates (Scheme 4). The observed... 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

Values of Kadd for the addition of water (hydration) of alkenes to give the corresponding alcohols. These equilibrium constants were obtained directly by determining the relative concentrations of the alcohol and alkene at chemical equilibrium. The acidity constants pATaik for deprotonation of the carbocations by solvent are not reported in Table 1. However, these may be calculated from data in Table 1 using the relationship pA ik = pATR + logA dd (Scheme 7). 

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