Carbocation with aromatic stabilization

As a measure of their thermodynamic stability, the pAfR+ values for the carbocation salts were determined spectrophotometrically in a buffer solution prepared in aqueous solution of acetonitrile. The KR+ scale is defined by the equilibrium constant for the reaction of a carbocation with water molecule (/CR+ = [R0H][H30+]/[R+]). Therefore, the larger p/CR+ index indicates higher stability for the carbocation. However, the neutralization of these cations was not completely reversible. This is attributable to instability of the neutralized products. The instability of the neutralized products should arise from production of unstable polyolefinic substructure by attack of the base at the aromatic core. 

Because the initial electrophilic attack and carbocation formation results in loss of aromatic stabilization, the electrophiles necessary for electrophilic aromatic substitution must be more reactive than those that typically react with alkenes. Thus, chlorination or... 

The reactivity of aromatic side chains to undergo dealkylation is in line with the stability of the corresponding carbocations. This indicates the possible involvement of carbocations in dealkylation, which was proved to be the case. The intermediacy of the rm-butyl cation in superacid solution was shown by direct spectroscopic observation.228,229 Additional proof was provided by trapping the ferf-butyl cation with carbon monoxide during dealkylation 230... 

How do these cyclase enzymes control the precise regiochem-istry and stereochemistry of these multistep cyclizations The active site of pentalenene synthase consists of a hydrophobic cleft, which is lined with aromatic and nonpolar residues. It is thought that the carbocation intermediates might be stabilized by the formation of tt-cation interactions, with aromatic residues such as phenylalanine, tyrosine, and tryptophan. In pentalenene synthase, replacement of Phe-76 or Phe-77 by Ala gave > 10-fold reduction in activity, which suggests that they may stabilize carbocationic intermediates through Jt-cation interactions. 

The electrophile may be a positive ion or be a molecule that has a positive dipole. If it is a positive ion, it is attacked by the ring (a pair of electrons from the aromatic sextet is donated to the electrophile) to give a carbocation. This intermediate is a resonance hybrid as shown in 1, but is often represented as in 2. For convenience, the H atom to be replaced by X is shown in 1. Ions of this type are called Wheland intermediates, a complexes, or arenium ions. The inherent stability associated with aromaticity is no longer present in 1, but the ion is stabilized by resonance. For this reason, the arenium ion is generally a highly reactive intermediate, although there are cases in which it has been isolated (see p. 661). 

Most carbocations are high in energy, but some are higher in energy than others. There are four ways that carbocations can be stabilized interaction of the empty C(p) orbital with a nonbonding lone pair, interaction with a 1t bond, interaction with a cr bond (hyperconjugation), and by being part of an aromatic system. Hybridization also affects the stability of carbocations. 

Reaction of the alkyl carbocation with the pi electrons of the aromatic ring gives a resonance-stabilized cation intermediate ... 

In the reaction with benzene, experiments show that the product is a carbocation and not a bromonium ion. A bromonium ion such as 39 is not the intermediate. The aromatic stability of benzene is disrupted to form carbocation 40 it is an unstable intermediate, but 40 is somewhat stabilized, as shown in 40A this leads to sufficient stability so that the reaction generates this intermediate. The intact benzene ring is much more stable than the cyclohexadienyl carbocation, which is resonance stabilized but not aromatic. The electron potential model of arenium ion 40B indicates yellow-orange areas over carbons 3 and 5 and light blue over carbons 2, 4, and 6. The blue color indicates less electron... 

Although more stable than a typical alkyl carbocation because of resonance, the intermediate in electrophilic aromatic substitution is nevertheless much less stable than the starting benzene ring itself, with its 150 kj/mol of aromatic stability. Thus, the reaction of an electrophile with a benzene ring is endergonic, has a substantial activation energy, and is rather slow. 

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