Rearrangements of carbonium ions

The rearrangement of carbonium ions that readily occurs according to the thermodynamic stabiUty of cations sometimes limits synthetic utility of aromatic alkylation. For instance, the alkylation of ben2ene with / -propyl bromide gives mostly isopropylben2ene (cumene) much less... 

A number of alkyl and aryl migrations, which may be tenuously regarded as similar to carbonium-ion rearrangements, have been summarized by P. Brown and Djerassi (1967). The close similarity of the mass spectra of norbornyl chlorides have been explained by rearrangement of carbonium ions (Bunton and Del Pesco, 1969). 

Fig. 33. Potential energy curves for three types of carbonium ion rearrangements. The-energy maxima represent transition states for (i) ionisation of C-X bond 2) rearrangement of carbonium ion ...

This is not our first encounter with the transfer of hydride ion to an electron-deficient carbon we saw much the same thing in the 1,2-shifts accompanying the rearrangement of carbonium ions (Sec. 5.22). There, transfer was intramolecular (within a molecule) here, it is intermolecular (between molecules). We shall find hydride transfer playing an important part in the chemistry of carbonyl compounds (Chap. 19). 

The rearrangement of carbonium ions was first postulated, by Meerwein (p. 160) in 1922, to account for the conversion of camphene hydrochloride into isobornyl chloride. Oddly enough, this chemical landmark is the most poorly... 

This alkyl carbonium ion, an electrophile, attacks the electron rich aromatic ring to give the alkylbenzene. The fact that carbonium ions are involved means it is to be expected they would undergo rearrangement to the most stable carbonium ion. It is this rearrangement of carbonium ions that accounts for the production of tert-pentylbenzene as the major product in (a)-(d), In each case, the tert-pentyl CH,... 

The hydrolysis of epoxides is pH dependent and can occur through acid, neutral, or base-promoted processes. Because the acid and neutral processes dominate over environmentally significant pH ranges, the base-catalyzed process can often be ignored. The reaction products resulting from epoxide hydrolysis are diols, and to a lesser extent, carbonyl products, which result from the rearrangement of carbonium ion intermediates. 

The development of the chemistiy of the so-called long-lived carbonium ions combined with that of the NMR method has opened new opportunities for investigating the rearrangements of carbonium ions it has become possible to obtain the kinetic characteristics of the very 1,2-shift processes — the key step of numerous rearrangements. 

In the framework of the present review the problem of the medium effects on the rates of rearrangements of carbonium ions has two main aspects. Firstly, in order to establish the relationships between the structure of long-lived carbonium ions... 

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