Halogenated carbocation

Olah GA, Mo YK (1976) Halogenated Carbocations.In Olah GA, Schleyer (eds) Car-bonium Ions. Wiley Interscience, New York, 5 2135... 

The focal point in this mechanism is the activation process that leads to the well-established Wheland intermediate. In order to address the mechanism of the activation of -complexes, we first recognize that most electrophiles (such as NO+ and N02+, various nitrating agents, halogens, carbocations, diazonium cations, sulfur trioxide, lead(IV), mercu-... 

One possibility is the two step mechanism of Figure 5 12 m which the carbon-halogen bond breaks first to give a carbocation intermediate followed by depro tonation of the carbocation m a second step... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation Methyl and ethyl halides do not form carbocations when treated with aluminum chloride but do alkylate benzene under Friedel-Crafts conditions The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds and these complexes are the electrophilic species that react with benzene... 

The regioselectivity of the second step is consistent with MarkownikofF s rule because a halogen atom can stabilize a carbocation by resonance. 

Vinyl and phenyl mfluoromethyl groups are reactive in the presence of aluminum chloride [10] Replacement of fluorine by chlorine often occurs Polyfluori-nated trifluoromethylbenzenes form reactive a,a-difluorobenzyl cations in antimony pentafluoride [11] 1 Phenylperfluoropropene cyclizes in aluminum chloride to afford 1,1,3-trichloro 2 fluoroindene [10] (equation 10) The reaction IS hypothesized to proceed via an allylic carbocation, whose fluoride atoms undergo halogen exchange... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

Figure 16.15 Carbocation intermediates in the nitration of chlorobenzene. The ortho and para intermediates are more stable than the meta intermediate because of electron donation of the halogen lone-pair electrons.

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

In vanadium-dependent haloperoxidases, the metal center is coordinated to the imidazole system of a histidine residue, which is similarly responsible for creating hypochlorite or hypobromite as electrophilic halogenating species [274]. Remarkably, a representative of this enzyme class is capable of performing stereoselective incorporation of halides, as has been reported for the conversion of nerolidol to various snyderols. The overall reaction commences through a bromonium intermediate, which cyclizes in an intramolecular process the resulting carbocation can ultimately be trapped upon elimination to three snyderols (Scheme 9.37) [275]. 

Another structural feature that increases carbocation stability is the presence, adjacent to the cationic center, of a heteroatom bearing an unshared pair," for example, oxygen," nitrogen," or halogen. Such ions are stabilized by resonance ... 

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 681), we invoke the Hanunond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes the carbocation by resonance ... 

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