Carbocations electron distribution

The electron-deficient region (in blue-green) of the allyl carbocation is distributed over both terminal carbons. 

In a recent extensive computational study [BLYP/6-31G(d,p) level, atoms in molecules (AIM) and NBO theories], DuPre has found charge distribution in cation 86, which prevents the development of unstable bridgehead carbocation. Electron delocalization results in nearly neutral atoms across the C—H—C bonding with the bridging hydrogen essentially in the Is electron configuration, and thus the proton is highly shielded in the NMR spectrum. 

The allylic carbocation intermediate displays what is called a resonance effect. That is the allylic carbocation can be drawn as two identical arrangements of atoms differing only in their electronic distribution. Each arrangement is called a resonance or contributing structure. These structures have no physical existence and they are not in equilibriim rather the real allyl cation is a hybrid of the two main contributing structures (hybrid also in electronic distribution) ... 

Problem 8.55 (a) Apply the MO theory to the C==C—C=C—C carbocation, considering the signs of the upper lobes of adjacent p atomic orbitals (f>) Indicate the relative energies of the molecular orbitals and state if they are bonding, nonbonding, or antibonding, (c) Show the distribution of the n electrons. ... 

Overall, ultrasound appears to favor the two-electron mechanism for the reaction, but the greatest effect of sonication upon product distribution was the substantial enhancement of alkene formation. Accordingly it was decided to examine a carboxylate electrooxidation system where there is no proton-loss pathway from the intermediate carbocation, namely using phenylacetate as a substrate. 

How the electrons are distributed upon fission of a C-C bond depends upon the ability of various groups attached to each of the C atoms to push electrons towards or pull them away from the site of fission (termed the inductive effect). For C atoms in virtually identical environments, such as in an n-alkyl chain, homolytic fission is most likely. However, electron-withdrawing groups (e.g. COOH, CHO, halogens) help to stabilize the formation of a carbanion by the C atom to which they are bonded. Conversely, electronreleasing groups (e.g. OH, NH2, OCH3, CH3) favour formation of a carbocation. 

The carbocation of Figure 5.19 can be written either as one of the two canonical forms (5.26) and (5.27) or using an electron smear (5.28) as shown in Figure 5.20. The smear is closer to reality since the molecular orbitals of the molecule will be distributed across the three carbons of the allylic cation. However, it will be able to react as if the positive charge were localised at either end. For each individual reaction, the nature of the other reactive species, the reaction conditions and the nature of the product will determine which way round the system will react. 

Since cations B, F and G do not undergo electron delocalization, they are less stable than the other cations Cations A, C, D and E have the positive charge distributed amongst four carbons, three of which are the same (ring carbons) Since carbocation stability follows the order 3 > 2° > 1°, cation C is the most stable one. Cation C is a tertiary benzylic cation, cations A, D, and E are either secondary or primary benzylic cations. Hence,... 

These results are explained by recognizing that one of the C=C units reacts with the acidic proton to generate a carbocation, 25, which is stabilized by two resonance contributors. Are the resonance contributors equal in energy To explain formation of 27 -i- 28 as the major products, 25B must be more reactive, so the electron density in the resonance contributors must be distributed in such a way that 25B has less electron density relative to 25A. This reaction can be analyzed in an alternative way. If there is a late transition state, the mqjor product will be determined by the relative stability of the products (see Chapter 10, Section 10.2.1). A disubstituted alkene is more stable than a monosubsti-tuted alkene. Both of these considerations must be examined. 

When 1,3,5-cycloheptatriene is heated with bromine, a stable salt is formed, cycloheptatrienyl bromide. In this molecule, the organic cation contains six delocalized tt electrons, and the positive charge is equally distributed over seven carbons (as shown in the electrostatic potential map in the margin). Even though it is a carbocation, the system is remarkably unreactive, as is expected for an aromatic system. In contrast, the cycloheptatrienyl anion is antiaromatic, as indicated by the much lower acidity of cycloheptatriene (pA"a = 39) compared with that of cyclopentadiene. 

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