Halide additives

REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION MARKOVNIKOV S RULE... 

Regioselectivity of Hydrogen Halide Addition Markovnikovs Rule... 

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES... 

Sections See Table 6 6 Hydrogen halide addition to alkenes proceeds by elec... 

A proton and a halogen add to the double bond of an alkene to yield an alkyl halide Addition proceeds in ac cordance with Markovnikov s rule hy drogen adds to the carbon that has the greater number of hydrogens halide to the carbon that has the fewer hydro gens... 

The order of reactivity of the hydrogen halides is HI > HBr > HCl, and reactions of simple alkenes with HCl are quite slow. The studies that have been applied to determining mechanistic details of hydrogen halide addition to alkenes have focused on the kinetics and stereochemistry of the reaction and on the effect of added nucleophiles. The kinetic studies often reveal complex rate expressions which demonstrate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or Itydrogen... 

There is usually a competing reaction with solvent when lydrogen halide additions to alkenes are carried out in nucleophilic solvents ... 

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

As in the case of hydrogen halide additions, this mode of attack should lead to anti addition. 

Mixed halide additions are most satisfactorily interpreted as proceeding via a halonium complex (1), in which the less electronegative halogen approaches from the less hindered a-side. The energetic preference for a pm-planar transition state leading to the diaxial product (2) is sufliciently great that the... 

We can gain a general understanding of the mechanism of hydrogen halide addition to alkenes by extending some of the principles of reaction mechanisms introduced earlier. In Section 5.12 we pointed out that carbocations are the conjugate acids of alkenes. Therefore, strong acids such as HCI, HBr, and HI can protonate the double bond of an alkene to fonn a car bocation. 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

Osmium carbonyl (Os3(CO)i2) acts as a photoinitiator of vinyl polymerization [20], which can function without a halide additive. The mechanism of photoinitiation is by a hydrogen abstraction from monomer to pho-... 

No mechanism for cracking in N2O4 has been established . In organic media crack velocities are similar to those obtained in distilled water. Lowering the water content results in lower velocities. Not all authors attribute failures in organic liquids to the residual moisture . Furthermore, part of the fracture may be transgranular . Water additions to methanol increase crack velocities as do halide additions. In oils velocities are similar to those in organic liquids and distilled water. 

TABLE 5. Survey of sulfonyl halide additions to unsaturated systems and hydrogen halide eliminations to unsaturated sulfones... 

Low energy ion-molecule reactions have been studied in flames at temperatures between 1000° and 4000 °K. and pressures of 1 to 760 torr. Reactions of ions derived from hydrocarbons have been most widely investigated, and mechanisms developed account for most of the ions observed mass spectrometrically. Rate constants of many of the reactions can be determined. Emphasis is on the use of flames as media in which reaction rate coefficients can be measured. Flames provide environments in which reactions of such species as metallic and halide additive ions may also be studied many interpretations of these studies, however, are at present speculative. Brief indications of the production, recombination, and diffusion of ions in flames are also provided. 

Addition of Alkyl Halides (Addition of Haiogen, Carbon)... 

Cleavage of epoxides with hydrogen halides Addition of hypohalous acids to alkenes... 

In contrast to the work of Peterson (38) with 3-hexyne in trifluoroacetic acid (vide supra), no trimeric adducts were observed in the hydrogen halide additions to propyne (48). 

As a consequence of facile homolytic cleavages, sulfonyl halides (I > Br > Cl F unsuitable) are able to add to unsaturated C—C systems. To prevent (or reduce) competing polymerizations, the additions of sulfonyl chlorides have been recommended to be carried out in the presence of copper(I/II) salts (Asscher-Vofsi reaction ). Comprehensive surveys have been published on the resulting j8-halogeno sulfones (or their vinyloguous compounds) as well as on their dehalogenation products (vinyl sulfones, 1-sulfonyl-l, 3-dienes, etc.). Table 5 reviews a series of sulfonyl halide additions and facile hydrogen halide eliminations. 

The diminished stereospecificity is similar to that noted for hydrogen halide addition to phenyl-substituted alkenes. 

Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity.64 However, the (J-selenyl halide addition products readily rearrange to the isomeric products.65... 

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