Halogen derivatives, solvents

The third group of solvents comprises the hydrocarbons and their halogen derivatives. They are not of interest for electrochemistry, since the solubilities and dissociation of salts, acids, and bases in them are low. Systems with protic or aprotic polar solvents are used in practice and have been investigated widely. 

A similar process may take place in the reduction of polar compounds with single bonds. A halogen, hydroxy, sulfhydryl or amino derivative by accepting an electron dissociates into a radical and an anion. In aprotic solvents the two radicals combine. In the case of halogen derivatives the result is Wurtz synthesis. In the presence of protons the anion is protonated and the radical accepts another electron to form an anion that after protonation gives a hydrocarbon or a product in which the substituent has been replaced by hydrogen. 

The mean problem when measuring many aromatics and their halogenated derivatives is their hydrophobicity. For that reason it is desirable that these compounds be measured in non-polar organic solvents. It was demonstrated, that the response of a Rhodococcus globerulus (old name Corynebacterium MBl) containing biosensor to 2,3-dihydroxybiphenyl in decane or octane remained constant for up to 60 min [119]. The use of -hexane in a biosensor system based on Pseudomonas putida increased the sensitivity for phenol fivefold, the measurement frequency sixfold, and operational stabihty was 7 h [113]. 

Over the past few years, it has been shown that the electroreduction of NiLn complexes is feasible in various solvents (DMF, acetonitrile, THF,. ..) at potential values depending on the nature of both solvent and ligand. The electroformed species reacts by oxidative addition with halogenated derivatives including organic halides (iodide, bromide, chloride) (equation 27). 

To these belong the hydrocarbons and their halogen derivs, such as benzene, pentane, toluene, chloroform, carbon tetrachloride, chlorobenzene, etc. Because of the inert nature of these solvents, no dissociation or other reaction can take place when a single acid or base is dissolved... 

Another method of preparation is as follows 1 33 parts of fluorescein are dissolved in 5 parts of ether and treated with 25 parts of selenium chloride in the same solvent. A yellowish-red precipitate separates, and after long stirring at the ordinary temperature the ether is distilled off. The residue is stirred with water, the mixture filtered and the residue now dissolved in sodium hydroxide. After further filtration the filtrate is treated with hydrochloric acid, which precipitates seleno-fluorescei n. Further purification is effected by solution in alkali and reprecipitation. A reddish-brown powder is obtained, soluble with fluorescence in alcohol, but insoluble in water. In concentrated sulphuric acid it dissolves to give an orange solution. Its alkali salts are very soluble in wrater, giving red solutions. This process may also be applied to phthalins, which are obtained by the reduction of phthaleins and their halogen derivatives. If the selenium chloride is replaced by the oxychloride similar products are obtained.2 In place of the phthalins specified in the patents quoted, their O-acetyl compounds or O-acetyl compounds of the phthaleins may be used in indifferent solvents. The products are different from those obtained by the action of selenium on fluoresceins in aqueous alkali solutions.3... 

Halogenated aromatic hydrocarbons such as chlorobenzene or mixtures with aliphatic or aromatic hydrocarbons or halogenated derivatives thereof were suitable reaction media. The volume of solvent was generally 5-20 times the weight of PVC. 

An apolar aprotic solvent is characterized by a low relative permittivity (sr < 15), a low dipole moment [ju < 8.3 10 Cm = 2.5 D), a low value ca. 0.0... 0.3) cf. Table A-1, Appendix), and the inability to act as a hydrogen-bond donor. Such solvents interact only slightly with the solute since only the non-specific directional, induction, and dispersion forces can operate. To this group belong aliphatic and aromatic hydrocarbons, their halogen derivatives, tertiary amines, and carbon disulfide. 

Hydrocarbons and their halogen derivatives have no tendency to gain or lose protons they are inert and exhibit no levelling effect. The dielectric constants are very low (2 to 6) and the ions associate, making conductance measurements of no value for determining the extent of protolysis. Proto-lytic equilibria are also complicated by association of the uncharged molecules themselves carboxylic acids, for example, exist as dimers in benzene. The same factors reduce greatly the solubilities of acids and bases in these solvents. 

In CCI4 and CI ICI3 the high formation rate of pyrocarbon is accompanied by the complete absence of polyyne formation in the respective solutions checked by the HPLC-DAD. No PAHs were found only a mixture of halogenated derivatives. Evidently, the large amount of Cl radicals produced by the plasmalysis of these solvents is sufficient to easily chlorinate the polyyne chains or to cause their cyclization into other products. The results of these experiments are discussed in more detail in Reference 48. 

Halogenated derivatives of 4-t-pentylphenol and 2,4-di-t-pentylphenol have been used as solvent aids in silver halide colour photographic materials (ref.90). 

A mixture which is to be examined is broken down into its constituents as far as possible by treating it with various substances, which either dissolve certain constituents of the mixture as such or convert them into soluble compounds. In effecting such solutions the following substances are frequently used Water, which removes from the mixture substances soluble in water a solution of hydrochloric acid, which removes basic substances insoluble in water a solution of sodium hydroxide, which dissolves acids insoluble in water concentrated sulphuric acid, which separates many oxygen compounds from hydrocarbons and certain halogen derivatives and organic solvents which, in certain cases, may dissolve certain constituents of the mixture and not others. 

However, another way exists by which an excited state of TAM dye leuco derivative may generate a radical cation, viz. by photoejection of an electron. If a solvated electron (Csoiv) forms, halogenated organic solvents would efficiently... 

---