Organic Reactions Involving Reactive Intermediates

Reactions of Alkenes—An Overview Organic Reactions Involving Reactive Intermediates Electrophilic Additions Hydroboration-Oxidation Oxidation... 

Such radicals or ion pairs are formed transiently as reactive intermediates in a very wide variety of organic reactions, as will be shown below. Reactions involving radicals tend to occur in the gas phase and in solution in non-polar solvents, and to be catalysed by light and by the addition of other radicals (p. 300). Reactions involving ionic intermediates take place more readily in solution in polar... 

The conversion of enol ether 80 to cyclic ketal 83 in water in 12% yield exemplifies the chemoselectivity possible with 14D9.79 Although 83 is the normal product of the acid-catalyzed hydrolysis of 80 in organic solvents, it is never observed in water because the highly reactive oxocarbenium intermediate is rapidly trapped by the solvent to give ketone 82 (via hemiacetal 81) as the sole product. The ability of the antibody to protect the reactive oxonium ion intermediate from hydrolysis and partition it toward a product that is not typically observed under these conditions (i.e., 83) mimics the capabilities of rather sophisticated enzymes. Extension to other reactions involving reactive, water-incompatible intermediates can be easily imagined. 

Although reactive intermediates are not stable compounds, they are important to our study of organic chemistry. Most reaction mechanisms involve reactive intermediates. If you are to understand these mechanisms and propose mechanisms of your own, you need to know how reactive intermediates are formed and how they are likely to react. In this chapter, we consider their structure and stability. In later chapters, we see how they are formed and ways they react to give stable compounds. 

The wealth of information on reactive intermediates is covered in textbooks of physical organic chemistry and the specialized books quoted above. Many reactions involving such intermediates will be discussed in Chapter 6. 

Many important organic reactions involve carbanions as nucleophiles. The properties of carbanions were introduced in Section 3.4.2, and their reactivity is discussed in more detail in Chapter 6. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base that can generate the reactive carbanion intermediate. Base-catalyzed addition reactions of carbonyl compounds provide many examples of this type of reaction. The reaction... 

Another general process involves the reaction of Pd(0) species with halides or sulfonates by oxidative addition, generating reactive intermediates having the organic group attached to Pd(II) by a ct bond. The oxidative addition reaction is very useful for aryl and alkenyl halides, but the products from saturated alkyl halides often decompose by (3-elimination. The a-bonded species formed by oxidative addition can react with alkenes and other unsaturated compounds to form new carbon-carbon bonds. The... 

The vast majority of organic radical reactions involve the radical as a reactive intermediate, since their values of k ) are so high, although we need to note that the second reaction need not be particularly fast it only has to be fast in relation to the first reaction. As a good generalization, the intermediate may be treated as a reactive intermediate if k(2)/k r) > 10-3. 

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