Of carbocations

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. 

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... 

A wide variety of carbocations and carbodications, including those that are aromatically stabilized as well those as stabilized by heteroatoms, were reported in the nearly 200 publications on the topic during my Cleveland years. 

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... 

On the basis of my extensive study of stable, persistent carbocations, reported in more than 300 publications, I was able to develop the general concept of carbocations referred to in Chapter 9. Accordingly, in higher-coordinate (hypercoordinate) carbonium ions, of which pro-... 

There is ample evidence from a variety of sources that carbocations are mterme diates m some chemical reactions but they are almost always too unstable to isolate The simplest reason for the instability of carbocations is that the positively charged car bon has only six electrons m its valence shell—the octet rule is not satisfied for the pos itively charged carbon... 

FIGURE 4 13 The order of carbocation stability is methyl < primary < second ary < tertiary Alkyl groups that are directly attached to the positively charged car bon stabilize carbocations... 

Free radicals like carbocations have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that release electrons Consequently the order of free radical stability parallels that of carbocations... 

The rate at which alcohols are converted to alkyl halides depends on the rate of carbocation formation tertiary alcohols are most reactive primary alcohols and methanol are least reactive... 

Like tertiary alcohols secondary alcohols normally undergo dehydration by way of carbocation intermediates... 

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

The reactivity order parallels the ease of carbocation formation Increasing rate of elimination by the El mechanism... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

Clearly the steric crowding that influences reaction rates in 8 2 processes plays no role in Stvfl reactions The order of alkyl halide reactivity in 8 1 reactions is the same as the order of carbocation stability the more stable the carbocation the more reactive the alkyl halide... 

This carbocation does not receive the extra increment of stabilization that its benzylic isomer does and so is formed more slowly The regioselectivity of addition is controlled by the rate of carbocation formation the more stable benzylic carbocation is formed faster and is the one that determines the reaction product... 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

Reaction with hydrogen halides (Sec tion 4 7) The order of alcohol reactiv ity parallels the order of carbocation staiiility RjC" > R2CH > RCHj" > CHj" Benzylic alcohols react readily... 

This IS a frequently used proce dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability R3C > R2CH > RCH2 Benzylic al cohols react readily Rearrangements are sometimes observed... 

Steps 4-5 Conversion of hemiacetal to carbocation These steps are analogous to the formation of carbocations m acid catalyzed reactions of alcohols... 

Fnedel-Crafts alkylation Alcohols in combination with acids serve as sources of carbocations Attack of a carbocation on the electron rich ring of a phe nol brings about its alkylation... 

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

This bicychc carbocation then undergoes many reactions typical of carbocation inter mediates to provide a variety of bicychc monoterpenes as outlined m Figure 26 7... 

Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl pyrophosphate produce a rich variety of structural types m the... 

These reactions involve the intermediate formation of thiols, followed by condensation to the sulfides. The observation of isomerized products in suitable cases indicates the intermediate formation of carbocations, either by protolysis of alkanes by the superacid or reversible ionisation of the thiol products (149). 

---