Reactions of Halogens and Hydrogen Halides

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

Reactions at a Group Viii Metal Center The Ni, Pd, Pt Triad 10.3.8.6. Reactions of Halogens and Hydrogen Halides... 

A number of reactions involve dissociation of a reactant as the first step. In many such cases, this step is reversible and fast enough compared with subsequent conversion of the dissociation products to be at quasi-equilibrium. Typical examples are gas-phase reactions of halogens or hydrogen halides. 

Among the classical (summarized in [11]) reactions of cyanoacetylenes (salt and n-complex formation, nucleophilic addition of, inter alia, amines and alcohols, Diels-Alder additions with 2 (see Section 2.22.2), addition of halogens and hydrogen halides, etc.) their polymerization might deserve a second look since the products formed - polyacetylenes - have attracted much attention during the last two decades. Thus 1, several of its derivatives, and 2 have been polymerized with anionic initiators (triethylamine, sodiiun cyanide, butyllithium) to give black, low-molecular-weight polymers claimed to have structures like 33 [38-40], which is obtained from 2 by treatment with butyllithium. 

Gattermann s reaction A variation of the Sandmeyer reaction copper powder and hydrogen halide are allowed to react with the diazonium salt solution and halogen is introduced into the aromatic nucleus in place of an amino group. 

Their physical properties are essentially those of the alkanes. It is the unsaturated linkages that dominate the chemistry and the main reaction is one of addition (e.g. hydrogen, halogen, and hydrogen halides) across the double bond to produce saturated compounds. This reactivity is utilized in the manufacture of long-chain polymers, e.g. polyethylene and polypropylene. 

It is fair to say the chemistry of 1-3 and their derivatives is dominated by reactions with nucleophiles. The molybdenum species Cp2Mo2(CO)4 does, however, also react with halogens and hydrogen halides (182, 183). 

Reactions of the organometallic compounds InCp and TlCp with halogens and hydrogen halides involve attack on the cyclopentadienyl group . However, I2 reacts with InCp by oxidative addition to yield Cplnl2 , which forms adducts with suitable donor ligands (e.g., 2,2 -dipyridyl). Loss of the cyclopentadienyl ligand accompanies... 

The kinetics of the gas-phase reactions of HBr with Cl2 and of HI with Cl2, Br2, and IC1 have been investigated by stopped-flow methods.27 The rate constants for the family of reactions can be rationalized if halogen and hydrogen halide molecules react to form a transition state in which the proton is near the centre of a triangle of three halogen atoms. In a study of the reaction ... 

PURPOSE OF EXPERIMENT Study some preparations, properties, and reactions of some halogens and hydrogen halides. 

The solvent effects on oxidative addition reactions to square planar iridium(I) complexes have drawn some comment. The general acceleration of the addition reactions of hydrogen, oxygen and methyl iodide to trans-[lrX(CO)(FFhQ)2] (where Xis a. halogen) by polar solvents, such as dimethylformamide, is taken by Chock and Halpern to be evidence for a polar transition state. ° Perhaps more interesting is the stereochemical result of the addition of alkyl and hydrogen halides to these iridium(I) complexes. "... 

As one of the enzymic reactions, asymmetric synthesis catalyzed by cyclodextrins has been studied in the past, but gave all the products in a low optical yield. We have already found a strong chiral induction for the chlorination of methacrylic acid in the crystalline cyclodextrin complexes. 100 % enantiomeric excess (e.e.) of (-)-2,3-dichloro-2-methyl-propionic acid and 88 % e.e. of its enantiomer were isolated in a- and 3-cyclodextrins, respectively. This paper describes asymmetric addition of gaseous halogens and hydrogen halides in the crystalline complexes comprising trans-cinnamic acid as a reactant and a- or 3-cyclodextrin as chiral matrix. Asymmetric bromination of menthyl cinnamate and of salts from the acid and several chiral amines have been reported, but gave low chiral inductions up to 2 16 % e.e.. 

Addition at the double C=C bond occurs via various mechanisms ionic, radical, or molecular. In this Section we consider briefly the mechanisms of heterolytic addition. Electrophilic addition reactions Adg are widely abundant. Halogens and hydrogen halides often add according to this mechanism. The addition of HX to the 7i-C=C bond in a polar medium where HX is ionized occurs in two steps... 

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