POLY-HALOGEN METHANES

At the very beginning of our study we stated that when methane gas is acted upon by chlorine in the sunlight a mixture of four products is obtained resulting from the substitution of one, two, three or four chlorine atoms for an equivalent number of hydrogen atoms in the methane molecules. These four compounds are represented by the following formulas  

Di-chlor methane Tri-chlor methane Tetra-chlor methane 

The three poly-chlorine substitution products of methane, as represented above, are all known, but only two of them are of sufficient importance to be considered in detail. 

The tri-chlorine substitution product of methane is the common and very important anesthetic chloroform. It may be made by the method referred to, viz., by the direct chlorination of methane. This method is not, however, a practical one. The industrial preparation is from alcohol or acetone, by treatment with chlorine and an alkali. In the reaction with alcohol the chlorine acts, first, as an oxidizing agent, oxidizing the alcohol to aldehyde. The chlorine then acts as a substituting agent forming a tri-chlorine substitution product of the aldehyde. This tri-chlor aldehyde is then decomposed by the alkali and chloroform results. The steps in this reaction have been definitely proven, as follows  

In practice, the chlorination is effected, not by the use of free chlorine, as such, but by the use of bleaching powderj calcium hypochlorite. The preparation from acetone is by a similar chlorination. In the reaction which takes place, one of the methyl groups is substituted just as in the case of aldehyde, and then a similar decomposition by means of the alkali takes place. 

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Alkyl bromides and chlorides. Many simple alkyl bromides and chlorides, and poly-halogenated methanes and ethanes fall into this category, including some common solvents. All should be treated as potentially harmful, but in addition to those already listed above the following may be regarded as some of the more dangerous. 

In addition, the degradation of Cl and C2 poly halogenated substrates has been reported in non-aqueous (Kotsinaris et al. 1998), aqueous (Horanyi et al. 1982 Liu et al. 2000 Li et al. 2000 Chen et al. 2003) and mixed (Hori et al. 2003 Rondinini et al. 2004 Fiori et al. 2005) solvents, under different operating conditions. The reduction of CH2C12 on Ni and Cu in ACN resulted in the formation not only of methane and chloromethane, but also of ethylene, propene and buthene... 

Isomerism of Di-chlor Ethanes.—When, however, we study the constitution of the poly-halogen ethanes we find that isomerism occurs just as in the case of the propyl iodides and of the hydrocarbons above propane. In the case of ethane it is a fact that only one mono-substitution product of any type is known, thereby proving the symmetry of the ethane molecule and the like character of all six of the hydrogen atoms. When two hydrogen atoms are substituted by two chlorine atoms two dif event compounds are produced both having the composition C2H4CI2. From the constitution of the ethane molecule, that has been established by its synthesis from methane (p. 16), we can readily see how this may be explained as we may have two hydrogen atoms replaced by two chlorine atoms in two different ways, as follows ... 

A reaction similar to the overall copper-halogen exchange observed on addition of perhaloarenes to lithium dimethylcuprate is also given by pentafluorobenzene, pentachlorobenzene, 2,3,5,6-tetrachloropyridine (146), and tetrafluorobenzenes (144). A small excess of methyllithium was again required, otherwise the reactions were very slow. Formation of methylcopper during the addition was not observed with the poly-fluoroarenes, but a mechanism was proposed similar to that for the perhaloarenes (146). Equation (22) represents the overall reaction for pentafluorobenzene. In most cases, methane was evolved nearly quantitatively. 

The red algae (Rhodophyta) are the most prolific sources of halogenated organics in the marine environment, and this topic was recently reviewed (Fenical, 1975). At least six orders, representing some ten families of red algae are now known to produce a wide variety of structure types from halo-methanes (C,) to halogenated products derived from squalene (C30). Within this group are aromatic and acyclic compounds produced from acetate (poly-ketide) biosynthesis and monoterpenes (Cio), sesqui- (Cu) and diterpenoids (C2o)- The structures of well over 200 compounds have now been firmly established. 

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