Bromonium ion Halonium ions

The idea that a cyclic bromonium ion was an intermediate was a novel concept at the time of its proposal in 1937. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

Bromonium Ion (Section 6 16) A halonium ion in which the halogen is bromine (see halonium ion)... 

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

A detailed spectroscopic examination should settle the question of whether the ion has the open or the cyclic structure. In general halo-chromic salts lose their color when a covalent bond is established to the central carbon atom, but the bromonium ion might resemble the carbonium ion. Compounds of similar color but which are certainly not cyclic halonium ions are also known ... 

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. 

The rate of the return from bromonium ion to CT complex depends on the nature of the solvent and of the substituents bonded to olefin81,84. By using adamantylideneadamantane 33, halonium ions 34 are formed as stable (and isolatable) salts85, because of steric hindrance of the nucleophilic counter ion. Consequently, the products of addition, such as the dibromides 29, cannot be obtained. 

The formation of the halonium ion intermediate in halolactonizations and haloetherifica-tions is a reversible step. Therefore, initially, comparable amounts of the diastereomeric bromonium ions B and iso-B are produced from the unsaturated alcohol A of Figure 3.47. 

In Section 3.5.1 it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated olefins react with Br2 but not with Cl2 via halonium ions. Because C—Cl bonds are shorter than C—Br bonds, chloronium ions presumably have a higher ring strain than bromonium ions. Accordingly, a /3-chlorinated tertiary carbenium ion is more stable than the isomeric chloronium ion, but a /3-brominated tertiary carbenium ion is less stable than the isomeric bromonium ion. 

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

Anti stereochemistry results from the bromonium ion mechanism. When a nucleophile attacks a halonium ion, it must do so from the back side, in a manner similar to the SN2 displacement. This back-side attack assures anti stereochemistry of addition. 

A positively charged ion that has a positive charge (or partial positive charge) on a halogen atom. Typically, in a halonium ion the halogen atom has two bonds and bears a formal plus charge. (Specific chloronium ion, bromonium ion, etc.) (p. 773)... 

The reaction with A -bromosuccinimide or Af-bromoacetamide with or without added acid, on the other hand, leads predominantly to the same epoxide trans-6 as formed in the reaction with peracids. It is proposed that in the formation of cis-6 with acetyl hypobromite, hypobromous acid or A -chlorosuccinimide, nucleophilic attack of the halonium ion is fast. The epoxide ratio therefore is governed by the kinetically favored tran.v-halohydrin. However, in the reaction with A -bromosuccinimide or A -bromoacetamide the nucleophilic attack on the bromonium ion species becomes rate determining, attack of the nucleophile therefore follows the path of lowest activation energy. This is the attack on the c/.v-bromonium ion distant to the bulky tert-butyl group leading to the trans-diaxial bromohydrin, according to the Fiirst-Plattner rule, hence giving trans-6. 

Neighbouring halogen participation through halonium ions (p. 91) has been postulated to account for features of some substitution reactions. 5a-Bromo-6jS hydroxy steroids (17) are converted by thionyl chloride into the 5a-bromO"6 -chloro derivatives (19) with over-all retention of configuration [124], An intermediate 5a,6a bromonium ion (18) controls the stereochemistry of subsequent introduction of chloride ion. 

The last paper mentioned in this subsection is not a study of a reaction profile, but a contribution which cannot be categorised in any other section of this report, other than the current one, in view of its relevance to the general understanding of organic addition reactions. Halonium ions have played vital roles as intermediates in organic chemistry. Bridged species such as the bromonium ion were proposed in 1939 to explain the stereochemistry of Br2... 

Bromonium ion. See Halonium ion (R)- and (5)-2-Bromooctane, stereochemistry of hydrolysis of, 307—308, 319 A-Bromosuccinimide, reagent for allylic bromination, 371, 391 benzylic bromination, 415—416, 435 Brpnsted, Johannes, 134 Brpnsted acid. See Acidity Brpnsted base. See Basicity Brown, Herbert C., 228 Buckminsterfullerene, 410—411 1,3-Butadiene... 

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