Carbocations halonium ions

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

Unlike a normal carbocation, all the atoms in a halonium ion have filled octets. The three-membered ring has considerable ring strain, however, which, combined with a positive charge on an electronegative halogen atom, makes the halonium ion strongly electrophilic. Attack by a nucleophile, such as a halide ion, opens the halonium ion to give a stable product. 

Orientation of Halohydrin Formation Even though a halonium ion is involved, rather than a carbocation, the extended version of Markovnikov s rule applies to halohydrin formation. When propene reacts with chlorine water, the major product has the... 

Addition of hydrogen halides involves formation of an open carbocation, not a cyclic halonium ion intermediate. The carbocation, which is sp-hybridized and planar, can be attacked by chloride from either top or bottom, yielding products in which the two methyl groups can be either cis or trans to each other. 

Bridged halonium ions resemble carbocations in that they are short-lived intermediates that react readily with nucleophiles. Carbocations are inherently unstable because only six electrons surround carbon, whereas halonium ions are unstable because they contain a strained three-membered ring with a positively charged halogen atom. 

In the presence of SbFs, inorganic halides such as NaCl and NaBr can serve as electrophilic halogenating agents [21b,28]. The halogenation of alkanes with dichloro- or dibromomethane has been achieved in the presence of SbFs (Eq. 13) [29], In this reaction, halonium ions are initially formed these in turn abstract hydride from hydrocarbons. Quenching of the resulting carbocations with halides leads to the desired haloalkanes. 

The methoxymethyl cation can be obtained as a stable sohd, MeOCHj SbF, Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. In super acid solution, ions such as CX (X = Cl, Br, I) have been prepared. Vinyl-stabilized halonium ions are also known. ... 

Polymer yields would be expected to depend on side-reactions of the propagating carbocation, and the formation of halonium ions with the solvent, CH3X, must be recognized as an important possibility [see reaction (6)]. Since the stability of these depends on the polarizability of X, CH3I should be most... 

Electrophilic additions of alkenes can be considered, in accordance with the Hammond postulate, to involve transition states resembling either the intermediate trivalent carbocations (or, in the case of halogenation, threemembered ring halonium ions) or the starting alkenes. In the first case, a new covalent bond is more fully developed in the transition state than in the latter case. The position of the transition state can vary from reaction to reaction and resemble either the starting alkene or the carbenium ion intermediate. 

Why does a carbocation exist here Not only is it 3°, it is also next to a benzene ring (benzylie) and therefore resonance-stabilized. This resonance stabilization would be forfeited in a halonium ion intermediate. 

Based on the previous analysis, 3-methyl-l-hexene reacts with mercuric acetate to give secondary carbocation intermediate 72. Is this carbocation unusual relative to previously discussed carbocations, such as 30 in Section 10.3 Note that mercury is a transition metal with d-orbitals that can donate electron density to the carbocation, stabilizing that positive center via what is known as back-donation (similar to what is observed with the halogens in the formation of halonium ions). Despite the manner in which is it drawn, this carbocation is not a three-membered ring, but rather a secondary carbocation that is stabilized by back-donation from the mercury atom. The dashed line (--) indicates significant coordination between the carbon and mercury. 

Reaction of the v electrons of the alkene with bromine (an electrophile) gives a bridged bromonium ion intermediate. This intermediate has some of the character of a carbocation (to account for the regioselectivity) and some of the character of a halonium ion (to account for the anti stereoselectivity). The secondary carbocation makes a substantial contribution to the stmcture of the resonance hybrid the primary carbocation is higher in energy and makes little contribution. 

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