Carbon tetrachloride, bond dissociation

As chlorination proceeds from methyl chloride to carbon tetrachloride, the length of the C—Cl bond is decreased from 0.1786 nm in the former to 0.1755 nm in the latter (3). At ca 400°C, thermal decomposition of carbon tetrachloride occurs very slowly, whereas at 900—1300°C dissociation is extensive, forming perchloroethylene and hexachloroethane and Hberating some chlorine. Subjecting the vapor to an electric arc also forms perchloroethylene and hexachloroethane, as well as hexachlorobenzene, elementary carbon, and chlorine. 

The 119Sn chemical shift of dimethyltin dichloride in carbon tetrachloride and other non-polar solvents remains practically invariant to large changes in concentration. It has a value of ca. +140 ppm. This indicates the ease with which the molecules are able to dissociate into discrete tetrahedral species in solution as a result of the very weak inter-molecular Sn... Cl bonds which exist in crystalline dimethyltin dichloride. (55) On the other hand, a chemical shift-concentration study of trimethyltin formate in deuterochloroform solution (56) has revealed a dramatic change in chemical shift from +2-5 ppm for a 3 M solution to + 152 ppm on dilution to 0-05 m in the same solvent. This has been attributed to self-association of monomeric tetrahedral trimethyltin formate molecules, [3]. As the concentration is increased, five-coordinate oligomeric or polymeric species, [4], could be formed. These are known to exist in the solid state. (57)... 

Carbon tetrachloride could be formed by the abstraction of a chlorine atom from a hexachloroacetone molecule by a trichloromethyl radical tetrachloroethylene could then result from the dimerisation of dichlorocarbene radicals produced from the dissociation of pentachloroacetonyl radicals. Haszeldine and Nyman identified trichloroacetyl chloride and octachloropropane as products of the liquid phase photolysis, suggesting a primary step of Type 3 involving rupture of the carbon-halogen bond. Photolysis in the liquid phase was found to be very slow, and this has been attributed to cage effects and recombination of radicals formed in the primary step. 

In so far as the rate of formation of radicals reflects their stability or reactivity the findings of Hart and Wyman are instructive. In carbon tetrachloride the rate of decomposition of benzoyl peroxide was twice as fast as that of biscyclopropanoyl peroxide. Ingold and coworkers have found that in the photodecomposition of benzoyl and biscyclopropanoyl peroxides, in carbon tetrachloride at 298 K, the phenyl radicals produced reacted faster (7.8 x 10 M s ) than the cyclopropyl radicals (1.5 X 10 M s ). These results are consistent with C-H bond dissociation energies for benzene (llOkcalmol) and cyclopropane (106kcal mol ) which implies that the cyclopropyl radical should be less reactive than the phenyl radical. In subsequent work they also showed that at ambient temperatures radical reactivities decreased along the series /c = Ph > (Me)2 C=CH > cyclopropyl > Me. Table 4 records the absolute rate constants for the reaction of these radicals with tri-n-butylgermane. 

Therefore, it is important to examine the reactivity of distannene 14, which does not undergo dissociation into stannylenes in solution. Distannene 14 reacts with carbon tetrachloride and phenylacetylene to afford the corresponding adducts, respectively, which retain the Sn—Sn bond (Scheme 2.9.5). ... 

Others8-10 have reported that a photoinduced charge transfer is involved in the dissociation of the C-halogen bond in the series of alkyl halides, carbon tetrachloride, chloroform, methylene dichloride, methyl chloride and methyl bromide. The activation of these was by UV laser irradiation of the halides as films adsorbed on a silver(lll) surface. The process is proposed to follow the path outlined in equation 1 in which a substrate electron brings about the dissociation of the RC1. Again the electron transfer process involves the formation of the radical anion of the halo compound that collapses into halide and the corresponding alkyl radical. 

In methanol, Cl and 7tH+ ions exhibit a retarding effect. The stabilities of nickel(o)-phosphine complexes have been assessed these seem to depend more on the size of the phosphine and electronic effects on bond strengths are of secondary importance. In the oxidative addition of aryl halides to nickel(o)-phosphine complexes the reaction appears to proceed via an initial slow dissociation step to give Ni(PR3)2 which then attacks the organic species. The mechanism of oxidative elimination of these nickel species thus contrasts with that for the platinum(o)-phosphines where the dissociation of the ligand is rapid and the rate-determining step is that involving the redox interaction. The oxidation of tetrahedral cobalt(i) complexes with carbon tetrachloride has been described ... 

This reaction was reinvestigated recently in carbon tetrachloride solution at 20 kHz.57 Sonolysis of the solvent was not detected and, if present, does not induce any isomerization. Of the several alkyl bromides tried, f-butyl bromide is the most efficient, a result which is explained by its higher volatility and the lower dissociation energy of the C-Br bond. It was concluded that the rate limiting step is actually the sonolysis of the carbon-halogen bond inside the bubble.58... 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

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