Reaction with carbonium ion

The reactions of carbonium ions occur via transition states having precise stereochemistry in which the electron pair of the attacking nucleophile must be colinear with the empty p-orbital of the electron-poor carbon atom. Thus, powerful stereoelectronic effects control these reactions. 

Discrimination can readily be observed betv/een the two possible modes of attack on a carbonium ion (195 196 and 195 - 197) when the nucleophile is part of the substrate. In such cases, the phenomenon of neighboring group participation is observed (for a review, see ref. 69). For example, solvolysis of the erythro-tosylate isomer 202 in acetic acid gave largely the erythro-acetate isomer 204 via the chiral bridged ion 203, whereas the threo isomer 205 yielded a racemic mixture of three products 207A and 207B via the achiral intermediate 206 (70). 

In the case of compound 216, treatment with boron trifluoride etherate gave a mixture of 217 and 218 (76). The transformation 216 - 217 must have occurred via the intermediate 219 as described above. The formation of 218 is the result of the migration of the methyl group followed by the loss of a proton (219 220 218) These two steps are equivalent to 200 201 and 1 95+1.  

Sometimes, several rearrangements occur consecutively. A spectacular case has been observed (76-79) in the acid-catalyzed transformation of 3-B-fried-elanol (221) into 13(18)-oleanene (222). In this case, 221 gave presumably the carbonium ion 223 which underwent six stereoelectronically controlled 

Conclusively, rearrangements of the Uagner-Meerwein type appear to be controlled by powerful stereoelectronic effects. 

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Formation of aromatic compounds involves either the dehydrogenation (by way of reaction with carbonium ions) of the cyclohexane compounds or, less likely, the cyclization of a triolefinic carbonium ion. 

Perhaps the most important single function of the solution environment is to control the mode of decomposition of reaction intermediates and hence the final products. This is particiflarly true in the case of electrode reactions producing carbonium ion intermediates since the major products normally arise from their reaction with the solvent. It is, however, possible to modify the product by carrying out the electrolysis in the presence of a species which is a stronger nucleophile than the solvent and, in certain non-nucleophilic solvents, products may be formed by loss of a proton or attack by the intermediate on further starting material if it is unsaturated. The major reactions of carbonium ions are summarized in Fig. 6. 

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

The relative rates of oxidation of phenylmethanes cover too small a range to be compatible with carbonium ion formation cf. the discussion on chromic acid oxidation of diphenylmethane, p. 295), and an initial reaction to give a radical plus Cr(V) followed by rapid transfer of a second electron to form Cr(IV) is more... 

As befits their status as compounds well-known to be in equilibrium with carbonium ions in suitable solvents, triphenylmethyl halides and related compounds give particularly unambiguous evidence of reaction involving ionic intermediates. In polar solvents they give... 

Carbonium ions are well-defined transition states in many reactions. Indeed, one of the earliest applications of MM method to the reactivity problem was concerned with carbonium ions. At present, only the Schleyer force field (26b) and its predecessor (253) are capable of handhng carbonium ions, although the parameterization principle used earlier can be readily improved upon to the present standards. Schleyer s measure of steric strain in carbonium ions, Ajtrain (the difference in steric energies of free carbonium ion and its... 

Peter Hervey Given was bom in 1918. He was educated at Oxford University, receiving a B.A. in Chemistry in St. Peter s Hall, Oxford, and the M.A. and D.Phil. in the Dyson Perris Laboratory under the direction of Professors D. LI. Hammick and Sir Robert Robinson (who was the Nobel laureate in chemistry for 1947). Given s thesis research dealt with carbonium ion reactions of aromatic hydrocarbons on cracking catalysts (1-. ... 

So far as vinyl monomers are concerned, ionic propagation proceeds with carbonium ions (cationic polymerization) or carbanions (anionic polymerization) at the chain ends. The study of the initiation process of radiation-induced ionic polymerization seeks to elucidate how these ions are formed from the primary ionic intermediates. Possible reactions... 

For many reactions, especially carbonium-ion type reactions, the zeolites and the amorphous silica-aluminas have common properties. The activation energies of the processes with both types of compounds change insignificantly, and both compounds have similar responses to poisons and promotors (1, 2). In general the zeolites are far more active than the amorphous catalysts, but ion exchange and other modifications can produce changes in zeolite activity which are more important than the differences between the activities of the amorphous and zeolitic catalysts ... 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

This type of carbon-carbon bond formation occurs through the interaction of double bonds with carbonium ions. It can be viewed as a sort of displacement reaction forming sigma C —C bonds, and it is for this reason that it is described in this Chapter. 

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