Carbonium ions intermediates

There are a few economical routes that can be employed for production of the largest-volume phosphines as specialty chemicals. The preparation of alkyl phosphines, where R > C2H, employs the addition of lower phosphines across an olefinic double bond. The reaction may be either acid-, base-, or radical-catalyzed. The acid-catalyzed addition probably proceeds through the generation of a carbonium ion intermediate which is attacked by the unshared... 

The mechanism of the alkylation reaction is similar to curing. The methylo1 group becomes protonated and dissociates to form a carbonium ion intermediate which may react with alcohol to produce an alkoxymethyl group or with water to revert to the starting material. The amount of water in the reaction mixture should be kept to a minimum since the relative amounts of alcohol and water determine the final equiHbrium. 

Vinyhdene chloride polymeri2es by both ionic and free-radical reactions. Processes based on the latter are far more common (23). Vinyhdene chloride is of average reactivity when compared with other unsaturated monomers. The chlorine substituents stabih2e radicals in the intermediate state of an addition reaction. Because they are also strongly electron-withdrawing, they polari2e the double bond, making it susceptible to anionic attack. For the same reason, a carbonium ion intermediate is not favored. 

The nature and stereospecificities observed in the rearrangement of (68b) to (69a) and (69a) to (70a) suggests that these rearrangements involve two discrete carbonium ion intermediates A and B (see Chart II). 

In contrast to the behavior of homoallylic alcohol (70a) when treated with methanesulfonyl chloride is pyridine, heating A -19-methanesulfonate (68b) in pyridine gives the 5)5,19-cyclo-6-ene (72). Vinylcyclopropane (72) is inert to the conditions used for converting vinylcyclopropane (73) to the A ° -B-homo-7)5-ol (70a). The latter results are only consistent with the existence of two discrete isomeric carbonium ion intermediates which give rise to isomeric elimination products. °... 

The reaction is generally thought to involve carbonium-ion intermediates but several puzzling features remain. Secondary aliphatic amines give nitrosamines without evolution of N2 ... 

In this paper, we study the stabihty of the carbonium ion intermediate formed in the cleavage of a glycosidic bond by lysozyme. It is found that the electrostatic stabilization is an important factor in increasing the rate of the reaction step that leads to the formation of the carbonium ion intermediate. Steric factors, such as the strain of the substrate on binding to lysozyme, do not seem to contribute significantly. 

The preparation of the less stable isomer (53b) of the oxazolone 53a involves a rather tedious procedure. It has been reported that 53a is rapidly isomerized to 53b in 48% hydrobromic acid saturated with gaseous HBr. In this way four azlactones have been converted into their isomers.It has been established, moreover, that the isomerization is radical-initiated and does not involve a carbonium ion intermediate. The isomerization can be reversed by pyridine. ... 

It has also been proposed that the ring-opened radicals may undergo ring-closure to a cyclobutane (Scheme 4.23).202,2 8 At this stage the only evidence for this pathway is observation of signals in the NMR spectrum of the polymer that cannot be rationalized in terms of the other structures. There is no precedent for 1,4-ring-closure of a 3-butenyl radical in small molecule chemistry and the result is contrary to expectation based on stcrcoclcctronic requirements for intramolecular addition (Section 2.3.4). However, an alternate explanation has yet to be proposed. The possibility of carbonium ion intermediates should not be discounted. 

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. 

This reversal has been demonstrated by both product and kinetic studies. In the absence of solvent nucleophilic assistance and of substituents favouring P-bromo-carbonium ion intermediates, the ionization of CTCs to bromonium (poly)bromide has been shown to occur not only for congested olefins, but more generally for "normal olefins both in aprotic chlorinated hydrocarbons and in protic solvents like acetic acid and methanol. 

An alternative explanation for the enhanced rates made its appearance. It was proposed that cr-participation in certain nonclassical ions provided a more satisfactory interpretation. This stimulated a detailed study of the norbornyl system, considered to provide the best available case for such nonclassical carbonium ion intermediates. The results failed to confirm the presence of significant -participation and supported the conclusion that the phenomena must be largely, if not entirely, steric in origin. 

The Diels-Alder orientation is correct ( para ) and the stereochemistry of (33) (32) should be right since the Pest conformation of the carbonium ion intermediate (35), with the large PhCO group equatorial, allows much better approach for Cl from the correct side. 

They also compared compounds which might be expected to give strong neighbouring group effects should a carbonium ion intermediate be formed, viz. 

When 3-methoxybenzyl acetate is photolyzed, however, the products expected from free radical coupling are decreased and a substantial amount of 3-methoxybenzylalcohol results. A carbonium ion intermediate has been proposed in the photolysis of this meta ester ... 

A number of possible unimolecular mechanisms can be envisaged for decarboxylation ranging from one involving a free carbanion intermediate [Eq. (4)], to one involving a free carbonium ion intermediate [Eq. (5)]. In... 

In order to overcome these difficulties a number of other kinds of supporting evidence for carbonium ion intermediates have been used. Even so, the status of many nucleophilic displacement reactions remains in doubt, and this type of reaction has probably been the subject of more mechanistic man hours of work than any other.211-212... 

They found that when the carbonium ion intermediate in a SN1 reaction has one or more beta-hydrogen atoms, an olefin is formed. 82... 

Fig. 10.1. Simplified mechanisms of the spontaneous and acid-catalyzed isomerization of arene oxides to phenols via a carbonium ion intermediate (benzene oxide (10.1) as the... 

The last years have seen a rapid development of the direct spectroscopic observation of stable carbonium ion intermediates in strongly acidic solutions. Thus many alkylcarbonium ions, previously suggested... 

Although HCo(CO)4 is a strong acid in aqueous solution and is capable of protonating even weak bases like dimethylformamide, there is no evidence that it protonates olefins in hydrocarbon solvents to form carbonium ion intermediates which might then rearrange by conventional 1,2-hydride shifts followed by proton elimination ... 

Acid-catalyzed reactions involve carbonium-ion intermediates. Their different product spectra can be used for detection of the probable reaction mechanism. 

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