Halomethanes reactions

In addition to the chloroacetates, as shown by comparisons with the estimates in Table V, the half-lives for both lindane and mirex in the natural water samples would have been considerably longer than those observed, had reaction with solvated electrons in bulk solution been the dominant mechanism for photoreaction. The higher efficiency of these halocarbon reactions may be attributable to sorption of the chloroacetates on the NOM, which permits more facile electron capture. Other possible pathways for reactions of sorbed halocarbons include direct photoreduction by excited states of the NOM, which, like solvated electrons, also are quenched by oxygen. Alternatively, the enhancement may involve other direct electron-transfer mechanisms such as amine-halomethane reactions. These alternative possibilities are examined in the following section. 

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

O Paleta, Ionic Addition Reactions of Halomethanes with Fluoroolefins... 

Ionic Addilion Reactions of Halomethanes aiih Ruoroolcfins... 

Halofluoroalkenes may be prepared by using fluorodihalomethanes or fluoro-halomethanes in olefination procedures similar to those descnbed above. Ruoro-trichloromethane treated with tris(dimethylamino)phosphine forms the corresponding phosphomum salt, which can then be used in the Wirtig procedure. The reaction depends on the nature of the solvent in tetrahydrofuran, little olefination if any occurs however, when benzomtrile is added to the mixture, ylide formation is promoted [50] (equation 48) (Table 19). 

With methane the reaction produces a mixture of halomethanes and a HX. 

Chlorination reactions. Chlorination of hydrocarbons has been carried out in Japan, chlorination of toluene in the United States, chlorination of tetrachloropentane in the former U.S.S.R. to give octachlorocy-clopentane, and chlorination of propanoic acid in France to give chloro-propanoic acid. Chlorination of methane by irradiation to give lower halomethanes was found to be cost-effective. Chlorination of various amorphous polymers such as polypropylene, polybutadiene, and PVC, has also been carried out. 

For n = 1, this is the standard synthesis of [l.l.l]propellane 40a.20,21 Also for n = 3, this method makes [3.1.1]propellane 40c accessible with reasonable effort.22 Although [2.1.1]propellane 40b was detected by its IR spectrum at liquid nitrogen temperature,23 it could not be obtained as a stable compound via this route.22 The addition of halomethanes across the central bond by a radical chain mechanism is common to 40a and 40c. The addition of carbon tetrabromide to 40c afforded a 45% yield of 41.18 Likewise, a number of bicy-clo[l.l.l]pentane derivatives 42 were obtained by reaction of the corresponding halomethanes with 40a.17,24... 

Because of the high volatility of cyclopentene, the above procedure was not attempted, and after the reaction the butadiene and cyclopentene were distilled into a 2-liter bulb to which was added a measured amount of fluorotrichloromethane as an internal standard for GLPC analysis. The amount of unreacted butadiene was obtained by comparison of the area ratio of butadiene and halomethane in the analysis with the ratio in a... 

Robinson, G. N Q. Dai, and A. Freedman, Reactions of Halomethanes with y-Alumina Surfaces. 2. X-ray Photoelectron and Temperature-Programmed Reaction Spectroscopic Studies, J. Phys. Chem. B, 101, 4947-4953 (1997). 

In line the with the chemistry of dialkylzinc [36], the zinc homoenolate is inert to carbonyl compounds in a variety of solvents, Eq. (33). Slow addition accurs only in an HMPA/THF mixture. When the reaction is conducted in halomethane in the presence of Me3SiCl, however, a very rapid addition reaction occurs [33], Control experiments indicate that the acceleration is due to the activation of the carbonyl group by Me3SiCl. The activating effect of the chlorosilane disappears in ethereal solvents. 

KINETICS AND MECHANISM OF HOMOGENEOUS REACTIONS OF HALOMETHANES WITH CHLORINE ATOMS... 

TABLE 4. Room temperature rate constants and preexponential A factors (in cm3 molecule 1 s 1), as well as activation energies and reaction enthalpies (in kJ mol 1) for reactions of chlorine atoms with methane and halomethanes. 

The rather fast reaction rate of halomethanes with Cl atoms suggests that this process may play a primary role in the removal of halomethanes from the troposphere and results in the formation of HC1 or 1C1 molecules. These degradation pathways do not lead to bromine or iodine atoms but to relatively stable molecules, which may initiate a different bromine and iodine cycles in the marine boundary layer. The atmospheric lifetime of IC1 is probably controlled by its sunlight photodissociation to iodine and chlorine atoms. Another possible degradation pathway of IC1 may be the hydrolysis to hypoiodous acid IOH, which may further be dissolved in seawater. 

Saini HS, Attieh JM, Hanson AD (1995) Biosynthesis of Halomethanes and Methanethiol by Higher Plants via a Novel Methyltransferase Reaction. Plant Cell Environ 18 1027... 

Alkylation of fluoroolefins is probably the most studied reaction of that kind. Different aspects of this process, such as reaction with formaldehyde and hexamethylenetetramine in HF [3], or condensation of halomethanes with fluoroethylenes, catalyzed by AlCl3 [27] have been reviewed. A recent development in this field mostly associated with the introduction of new catalysts, such as SbF5 [7] or aluminum chorofluoride AlClxFy [12], was covered in two other review articles. Here a brief description of some of these reactions is given. 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

The solvent-induced geometrical distortions can be very important to understand the properties and the reactivity of systems in solution for example, SN2 reactions on halomethanes, which proceed by a simple activated mechanism in the gas phase, are described by a double well mechanism in polar solutions, where stable precursor complexes appear [1], In addition, for some systems important regions of the conformational space become accessible only if solute-solvent interactions are taken into account. An outstanding example is provided by (poly)peptides whose conformations are often described in terms of the two backbone dihedral angles <.f> and tp (see Figure 3.1). 

Equations such as (36)-(40) suggest that radical-radical recombination could result in undesirable organohalogen products. However, such reactions are likely only when the initial halomethane concentrations are very high. Under normal circumstances radical-radical recombination would be unlikely, simply because the radicals are at low concentration. Further, once produced they would decompose by radiolysis under continued irradiation. Halogenated products, other than stoichiometric chloride and bromide ion, have not been detected [13], Even at intermediate doses, dehalogenation to inorganic halogen anions was stoichiometric. 

One of us, as well as Bottoni, recently investigated homolytic substitution reactions of silyl radicals at the halogen atom of halomethanes (CH3X, X = Cl, Br, I), and the chalcogen atom in methanechalcogenols (CH3EH, E = S, Se, Te) and alkyltellurols, with expulsion of alkyl radical (equations 12 and 13)40-43. 

The amino group at the 5-position on the 3//-l,2,3-triazolo[4,5-d]pyrimi-dine ring was converted into a halogen atom by treatment with isopentyl-nitrite in halomethanes. 5-Halotriazolopyrimidines, unsubstituted at the 7-position, reacted with butyllithium to give 7-butyl-6,7-dihydro-3-phenyl-3//-l,2,3-triazolo[4,5-d]pyrimidines, whereas in the 7-substituted-5-halo-triazolopyrimidines a halogen-metal exchange reaction took place and... 

Dissociation of halomethanes, particularly CF4, by collisions with rare gas active species has also been studied under various experimental conditions. The cross sections of each product ion in the M+/CF4 (M = Ar, Ne, He) reaction system from thermal to 50 eV have been determined using guided ion-beam techniques72. It was found that the energy dependence of the cross sections can be understood by considering the energies needed to access specific electronic states of CF4+ 72. 

---