Electrons, delocalization Electrophilic additions

For another example of how delocalized electrons can affect the product of a reaction, we will compare the products formed when isolated dienes (dienes that have only localized electrons) undergo electrophilic addition reactions to the products formed when conjugated dienes (dienes that have delocalized electrons) undergo the same reactions. 

Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions. Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete n electron delocalization (resonance). Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved. For example, benzene reacts with bromine to form bromobenzene. 

The allylic cyclohex-2-enyl radical has its unpaired electron delocalized over two secondary carbon atoms, so it is even more stable than the unsubstituted allyl radical. The second propagation step may occur at either of the radical carbons, but in this symmetrical case, either position gives 3-bromocyclohexene as the product. Less symmetrical compounds often give mixtures of products resulting from an allylic shift In the product, the double bond can appear at either of the positions it occupies in the resonance forms of the allylic radical. An allylic shift in a radical reaction is similar to the 1,4-addition of an electrophilic reagent such as HBr to a diene (Section 15-5). 

AS you continue your study of organic chemistry, you will notice that the concept of having delocalized electrons is invoked frequently to explain the behavior of organic compounds. For example, in Chapter 8 you will see that having delocalized electrons causes certain dienes to form products that would not be expected on the basis of what you have learned about electrophilic addition reactions in Chapters 3-6. Electron delocalization is such an important concept that this entire chapter is devoted to it. 

The relative rates at which alkenes A, B, and C undergo an electrophilic addition reaction with a reagent such as HBr illustrate the effect that delocalized electrons can have on the reactivity of a compound. 

As we look at more examples, notice that the first step in all electrophilic additions to conjugated dienes is addition of the electrophile to one of the sp carbons at the end of the conjugated system. This is the only way to obtain a carbocation that is stabilized by resonance (i.e., by electron delocalization). If the electrophile were to add to one of the internal sp carbons, the resulting carbocation would not be stabilized by resonance. 

Formation of this bond interrupts the cyclic system of it electrons, because in the formation of the arenium ion the carbon that forms a bond to the electrophile becomes sp hybridized and, therefore, no longer has an available p orbital. Now only five carbon atoms of the ring are s hybridized and still have p orbitals. The four it electrons of the arenium ion are delocalized through these fivep orbitals. A calculated electrostatic potential map for the arenium ion formed by electrophilic addition of bromine to benzene indicates that positive charge is distributed in the arenium ion ring (Fig. 15.2), just as was shown in the contributing resonance structures. 

The delocalized n bonding system confers stability on the structure and also gives benzene and related compounds a particular chemistry. The presence of a stable structure means that reactions involving benzene will tend to retain or restore this structure where possible. As in alkenes, the presence of such an electron-rich region in the molecule makes arenes susceptible to electrophilic attack. However, alkenes undergo electrophilic addition, but the interaction of benzene and other arenes with electrophiles results in electrophilic substitution reactions, in which the structure of the aromatic ring is preserved (Figure 20.34). 

Figure 15.5 Calculated electrostatic potential maps for the arenium ions from electrophilic addition of bromine to (a) methylbenzene (toluene) and (b) trifluoromethylbenzene. The positive charge in the arenium ion ring of methylbenzene (a) is delocalized by the electron-releasing ability of the methyl group, whereas the positive charge in the arenium ion of trifluoromethylbenzene (b) is enhanced by the electron-withdrawing effect of the trifluoromethyl group. (The electrostatic potential maps for the two structures use the same color scale with respect to potential so that they can be directly compared.)...

Electrophilic Addition Reactions, Stereochemistry, and Electron Delocalization... 

The reactions of organic compounds can be divided into three main types addition reactions, substitution reactions, and elimination reactions. The particular type of reaction a compound undergoes depends on the functional group in the compound. Part 2 discusses the reactions of compounds whose functional group is a carbon-carbon double bond or a carbon-carbon triple bond. We will see that these compounds undergo addition reactions, or, more precisely, electrophilic addition reactions. Part 2 also examines stereochemistry, thermodynamics and kinetics, and electron delocalization—topics that can be important when trying to determine the outcome of a reaction. 

Delocalized electrons play such an important role in organic chemistry that they will be a part of all the remaining chapters in this book This chapter will start by showing you how delocalized electrons are depicted. Then you will see how they affect things that are now familiar to you, such as values, the stability of carbocations, and the products formed from electrophilic addition reactions. 

Because of its large delocalization energy, benzene is an extremely stable compound. Therefore, it does not undergo the electrophilic addition reactions that are characteristic of alkenes except under extreme conditions. (Notice the conditions that Sabatier had to use in order to reduce benzene s double bonds on page 332.) Now we can understand why benzene s unusual stability puzzled nineteenth-century chemists, who did not know about delocalized electrons (Section 8.1). 

The reactivity of the phosphorus ylide 1 strongly depends on substituents R R. For preparative use R often is a phenyl group. When R or R is an electron-withdrawing group, the negative charge can be delocalized over several centers, and the reactivity at the ylide carbon is reduced. The reactivity of the carbonyl compound towards addition of the ylide increases with the electrophilic character of the C=0 group. R R are often both alkyl, or alkyl and aryl. 

The mechanism of substitution on an electron-rich benzene ring is electrophilic substitution, electrophilic attack on an atom and the replacement of one atom by another or by a group of atoms. The fact that substitution occurs rather than addition to the double bonds can be traced to the stability of the delocalized 7T-electrons in the ring. Delocalization gives the electrons such low energy—that is, they are bound so tightly—that they are unavailable for forming new cr-bonds (see Sections 2.7 and 3.12). 

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