Nucleophiles carbocation reactivity

A similar picture holds for other nucleophiles. As a consequence, there might seem little hope for a nucleophile-based reactivity relationship. Indeed this has been implicitly recognized in the popularity of Pearson s concept of hard and soft acids and bases, which provides a qualitative rationalization of, for example, the similar orders of reactivities of halide ions as both nucleophiles and leaving groups in (Sn2) substitution reactions, without attempting a quantitative analysis. Surprisingly, however, despite the failure of rate-equilibrium relationships, correlations between reactivities of nucleophiles, that is, comparisons of rates of reactions for one carbocation with those of another, are strikingly successful. In other words, correlations exist between rate constants and rate constants where correlations between rate and equilibrium constants fail. 

All of the mechanisms that have been presented so far have involved the reaction of electrophiles with nucleophiles. Carbocations and carbanions have been encountered as intermediates. In this chapter the chemistry of a new reactive intermediate, called a radical (or free radical), is presented. A radical is a species with an odd number of electrons. After a discussion of the structure of radicals, including their stability and geometry, various methods of generating them are described. Next, the general reactions that they undergo are presented. Finally, specific reactions involving radical intermediates are discussed. 

Based on the flavonoid reactivity pattern (figure 2), the dichotomy between electrophilic and nucleophilic characters is strongly related to pH. The more acidic the pH is, the more cationic charges prevail. Thus, the balance between flavylium cations (AH+) and hydrated forms (AOH) is pH-dependent as well as the interflavanic bond cleavage yielding the flavanol cations (F+). In addition, acetaldehyde protonation is also controlled by pH. Therefore, it can be expected that reactions involving flavylium, flavanol or protonated acetaldehyde cations are favored at lower pH values. Based on carbocation reactivity, these cationic species can be classified as follows ... 

So far we have added heteroatoms only—bromine, nitrogen, or sulfur. Adding a carbon substituent to a reluctant aromatic nucleophile requires reactive carbon electrophiles and that means carbocations. In Chapter 15 you learned that any nucleophile, however weak, will react with a carbocation in the SnI reaction benzene rings are no exception. The classic S l electrophile is the f-butyl cation, which is generated from ferf-butanol with acid. 

A,A/-dialkylhydrazones, can act as C- or A/-nucleophiles. Their reactivities have been measured by reaction with a range of benzhydryl cations, Ar2CH+, as reference electrophiles with known E values. Kinetic reaction of the carbocations at the (terminal)... 

Under superacidic, low nucleophilicity so-called stable ion conditions, developing electron-deficient carbocations do not find reactive external nucleophiles to react with thus they stay persistent in solution stabilized by internal neighboring group interactions. 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

One of the most important and general trends in organic chemistry is the increase in carbocation stability with additional alkyl substitution. This stability relationship is fundamental to imderstanding many aspects of reactivity, especially of nucleophilic... 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is, we are not comparing the stability of the two to each other. Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic. Cycloheptatrienyl cation, although aromatic, is still a carbocation and reasonably reactive toward nucleophiles. Its special stability does not imply a rock-like passivity, but rather a much greater ease of formation than expected on the basis of the Lewis structure drawn for it. A number of observations indicate that cycloheptatrienyl cation is far-more stable than most other car bocations. To emphasize its aromatic nature, chemists often write the structure of cycloheptatrienyl cation in the Robinson circle-in-a-ring style. 

In this section, the emphasis is on carbocation reactions that modify the carbon skeleton, including carbon-carbon bond formation, rearrangements, and fragmentation reactions. The fundamental structural and reactivity characteristics of carbocations toward nucleophilic substitution were explored in Chapter 4 of Part A. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

How Does Structure Determine Organic Reactivity Partitioning of Carbocations between Addition of Nucleophiles and Deprotonation... 

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