Cation carbocation

Organic cations (carbocations and onium ions) are important reactive intermediates in organic synthesis. From an experimental point of view, it is noteworthy that the manner in which we carry out reactions of organic cations is different from that for carbanions (Scheme 1). Usually, carbanions are generated and accumulated in a solution in the absence of electrophiles. After the generation process is complete, an electrophile is added to the solution of the pre-formed carbanion to achieve a desired transformation. In contrast, organic cations are usually generated in the presence of nucleophiles. This is probably... 

Therefore they are carbor cations. Carbocations have only six electrons in their valence shell... 

Step 1 results in the formation of an organic cation. One carbon atom in this cation has only six electrons in its valence shell and carries a charge of -hi. A species containing a positively charged carbon atom is called a carbocation carbon + cation). Carbocations are classified as primary (1°), secondary (2°), or tertiary (3°), depending on the number of carbon atoms bonded directly to the carbon bearing the positive charge. All carhocadons are Lewis acids (Section 2.6) and electrophiles. 

It has been determined that impurities such as moisture can accelerate this reaction, as AlCb-promoted reactions conducted under strictly anhydrous conditions can occur at a significantly lower rate of reaction. Due to this observation, it was subsequently discovered that the addition of cocatalysts can accelerate the rate of this reaction, such as oxygen, proton-releasing substances (ROH), Bronsted acids, and cation/carbocation producing substances. For example, the F-C alkylation of arenes with alkenes and alkynes promoted by AICI3 was found to be cocatalyzed by trace amounts of moisture effecting an increase in both the rate and yield of the reaction whereas the opposite was observed with use of FeCb as the cocatalyst. 

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

Finding snch acids (called snperacids ) turned out to be the key to obtaining stable, long-lived alkyl cations and, in general, carbocations. If any deprotonation were still to take place, the formed alkyl cation (a strong Lewis acid) would immediately react with the formed olefin (a good TT-base), leading to the mentioned complex reactions. 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

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. 

During this time I suggested (in 1972) naming the cations of carbon compounds carbocations (because the corresponding anions were named carbanions ). To my surprise, the name stuck and was later officially adopted by the International Union of Pure and Applied Chemistry for general use. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

One of the major contemporary chemical controversies in which I was inadvertently involved developed in the 1950s, surprisingly over the structnre of a deceptively simple seven carbon-containing bicyclic carbocation, the 2-norbornyl (bicyclo[2.2.1]heptyl) cation. The in-... 

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

Clearly there was no lack of devoted adversaries (perhaps a more proper term than enemies) on both sides of the norbornyl ion controversy. It is to their credit that we today probably know more about the structure of carbocations, such as the norbornyl cation, than about most other chemical species. Their efforts also resulted not only in rigorous studies but also in the development or improvement of many techniques. Although many believe that too much effort was expended... 

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

The transition state is closer m energy to the carbocation (tert butyl cation) so Its structure more closely resembles the carbocation than it resembles tert butyloxonium ion The transition state has considerable carbocation character meaning that a significant degree of positive charge has developed at carbon... 

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

As carbocations go CH3" is particularly unstable and its existence as an inter mediate m chemical reactions has never been demonstrated Primary carbocations although more stable than CH3" are still too unstable to be involved as intermediates m chemical reactions The threshold of stability is reached with secondary carbocations Many reactions including the reaction of secondary alcohols with hydrogen halides are believed to involve secondary carbocations The evidence m support of tertiary carbo cation intermediates is stronger yet... 

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