Electron delocalization allylic carbocations

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

FIGURE 10.2 Electron delocalization in an allylic carbocation. (a) The tt orbital of the double bond, and the vacant 2p orbital of the positively charged carbon, (b) Overlap of the tt orbital and the 2p orbital gives an extended TT orbital that encompasses all three carbons. The two electrons in the tt bond are delocalized over two carbons in part (a) and over three carbons in part (b). 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

Conjugation stabilizes the allyl carbocation because overlap of three adjacent p orbitals delocalizes the electron density of the n bond over three atoms. 

We have already drawn resonance structures for the acetate anion (Section 2.5C) and the aUyl radical (Section 15.10). The conjugated allyl carbocation is another example of a species for which two resonance structures can be drawn. Drawing resonance structures for the allyl carbocation is a way to use Lewis stmctures to illustrate how conjugation delocalizes electrons. 

The trae stracture of the allyl carbocation is a hybrid of the two resonance stractures. In the hybrid, the Jt bond is delocalized over all three atoms. As a result, the positive charge is also delocalized over the two terminal carbons. Delocalizing electron density lowers the energy of the hybrid, thus stabilizing the allyl carbocation and making it more stable than a normal 1° carbocation. Experimental data show that its stability is comparable to a more highly substituted 2° carbocation. 

Allylic carbocations, free radicals, and carbanions are resonance stabilized. In each case the stabilization is the result of delocalization of the positive or negative charge or the free radical. Resonance forms differ in the position of electrons and charge but not atoms. Every atom in an allylic carbocation, free radical, or carbanion possesses a p-orbital and the pi-electrons and charges or unpaired electrons are delocalized throughout these orbitals. 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

Allylic and benzylic cations have delocalized electrons, so they are more stable than similarly substituted carbocations with localized electrons. An allylic cation is a carbocation with the positive charge on an allylic carbon an allylic carbon is a carbon adjacent to an sp carbon of an alkene. A benzylic cation is a carbocation with the positive charge on a benzylic carbon a benzylic carbon is a carbon adjacent to an sp carbon of a benzene ring. 

Because the given reagent is an acid, start by protonating the molecule at the position that allows the most stable carbocation to be formed. By protonating the CH2 group, a tertiary allylic carbocation is formed in which the positive charge is delocalized over two other carbons. Then move the tt electrons so that the 1,2-methyl shift required to obtain the product can take place. Loss of a proton gives the final product. 

Tertiary carbocations are more stable than primary ones, but powerful stabilization is also provided when there is genuine conjugation between the empty p orbital and adjacent it or lone pair electrons. The allyl cation has a filled (bonding) orbital containing two electrons delocalized over all three atoms and an important empty orbital with coefficients on the end atoms only. It s this orbital that is attacked by nucleophiles. The curly arrow picture tells us the same thing. 

The carbocation formed in this step is an arenium ion or cyclohexadienyl cation, also known as a a-complex. It is an allylic carbocation and is stabilized by the electron delocalization represented by resonance among the contributing structures ... 

Allylic carbocations, like allylic radicals (Section 8.6), have a double bond next to the electron-deficient carbon. The allyl cation is the simplest allylic carbocation. Because the allyl cation has only one substituent on the carbon bearing the positive charge, it is a primary allylic carbocation. Allylic carbocations are considerably more stable than comparably substituted alkyl carbocations because delocalization is associated with the resonance interaction between the positively charged carbon and the adjacent tt bond. The allyl cation, for example, can be represented as a hybrid of two equivalent contributing structures. The result is that the positive charge appears only on carbons 1 and 3, as shown in the accompanying electrostatic potential map. 

Benzylic carbocations, radicals, and anions resemble their allylic counterparts in being conjugated systems stabilized by electron delocalization. This delocalization is describable in resonance, valence bond, and molecular orbital terms. 

Allylic radicals are stable for the same reason that allylic carbocations are stable (Section 8.13). Like an allylic carbocation, an allylic radical has two resonance forms. One form has the unpaired electron on the left and the double bond on the right, and one form has the unpaired electron on the right and the double bond on the left (Figure 12.1). Neither structure is correct by itself the true structure of the allyl radical is a resonance hybrid of the two. In molecular orbital terms, the unpaired electron is delocalized, or spread out, over an extended tt orbital network rather than localized at only one site. Thus, the two terminal carbons share the unpaired electron. 

Benzylic and allylic halides readily undergo SnI reactions as well, because they form carbocations that are stabilized by electron delocalization (Section 8.13). 

If the two resonance contributors of the allylic carbocation formed in an SnI reaction are not mirror images (as they are in the preceding example), two substitution products will be formed. This is another example of how electron delocalization can affect the nature of the products formed in a reaction (Section 8.16). 

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