Electron delocalization 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 is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

Section 11 14 Benzylic carbocations are intermediates in SnI reactions of benzylic halides and are stabilized by electron delocalization... 

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... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl 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). 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations. ° If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown in the above examples, is a discrete reaction intermediate. If a carbon-carbon double or single bond participates in the departure of the leaving group to form a carbocation, it may be that a nonclassical carbocation is involved, but there is no necessary relation. In any particular case, either or both of these possibilities can be taking place. 

In the interaction of a pair of atomic orbitals, two electrons form a bond and four electrons form no bond (Sect. 1.1). The snbstitnted carbocations are stabilized by the electron delocalization (hyperconjngation and resonance) through the interaction of the doubly occupied orbitals on the snbstitnents with the vacant p-orbital on the cation center. The exchange repulsion (Sect. 1.5) is cansed by four electrons. Now... 

There is also substantial stabilization of [4+] by electron delocalization from the cyclic a-vinyl group. This is shown by a comparison of the thermodynamic driving force (p Tr lies between —7.8 and —8.5) and absolute rate constant (ks = 1 -6 x 107 s 1) for the reaction of [4+] in 25% acetonitrile in water with the corresponding parameters for reaction of the resonance-stabilized l-(4-methoxyphenyl)ethyl carbocation in water (p Tr = — 9.4and s= 1 x 108 s Table 5). 

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. 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations.95 If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown... 

The bond-type connected may be used whenever a treatment of bonds with all participating electrons delocalized is adequate to the problem, e.g. as is the case with nonclassical carbocations [37] ... 

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. 

In water, N3 is much less reactive in aromatic nucleophilic substitution than expected from its reactivity toward carbocations, that is, its N+value. Ritchie (43) initially developed his N+ scale from nucleophilicities toward preformed carbocations and the scale fits the data for nucleophilicities toward many electrophiles, regardless of their charge. However, in water, and similar hydroxy lie solvents, the nucleophilicity of azide ion, relative to that of other anions, seems to be related to the carbocation-like character of the electrophile. An acyl derivative with its sp2 carbonyl group is somewhat akin to a carbocation stabilized by an alkoxide group, >C=0 <-— >C+-0 , just as a triarylmethyl carbocation is stabilized by electron delocalization into the aryl groups and azide ion is a good nucleophile toward these electrophiles. As compared with anions such as OH- or CN , azide ion, in water, is very reactive toward carbocations and in deacylation but is relatively unreactive toward dinitrohaloarenes (44). 

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. 

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