1461 No. 5069 DECEMBER 24. 1966 NATURE

E-+ CCI. --->- ECC1 3 + Cl- Formation of Nitrous Oxide THE mechanism of formation of nitrous oxide from am­ The tightly bound .CCI3 group will not be removable in monium nitrate is attributable to the recognized thermal the normal biochemical reaction and the enzyme molecule dissociation of ammonium salts followed in this instance will be inactivated. by nitration of ammonia and decomposition of the ensuing This process of enzyme inactivation could, in principle, nitramide. The mechanism of this latter stage has been account for the toxicity of carbon tetrachloride and studied by various workers, notably by Marlies and La similar compounds and also of many insecticides. The Merl. Further, the decomposition of methyl nitramido postulated mechanism would also explain the observation to tho oxide and methyl alcohol is on record 2. by Butlerl3 that carbon tetrachloride is reduced in vivo As the nitration process, for example, of benzene, is to chloroform. Thus a proportion of .CCls radicals may commonly accompanied by some oxidation, the possibility react with sites other than the enzyme and abstract, of its OCClllTcnce in the preparation of nitrous oxide with for example, a hydrogen atom from a hydroxyl group or formation of nitrogen peroxide is not surprising and from a sulphydryl group. The latter exchange does technically well recognized. It has also recently been the appear to be possible from energy considerations. subject of public concern.

D(RS-H) = 89 kcalJmole (ref. 14), .J. KENNER

D(CI3G-H) = 91·4 kcalJmole (ref. 9) 41 Burlington Road, I have found that the activation energy of the electron Withington, capture reaction in chloroform vapour Manchester 20. • Marlies and La Mer, J. Amer. Chem. Soc., 1i7, 1812 (1935). e-+ CHC1 3 --->- .CHCI. + CI- • Franchimont, A. P. N., and Umbgrove, Rec. Trav. CMm., 15,211 (1896).

is some 2·4 kcalJmole greater than that for the equivalent reaction with CC1., so this reaction must be expected to occur significantly more slowly . This may account for SOIL SCIENCE the lower toxicity of chloroform. Similarly, although the activation energy of the reaction Aromatic Hydrocarbons in Soil with trichlorofluoromethane is not known, the bond dis­ sociation energy D(Cl.FG-CI) can be calculatedl6 to be THE study of various soils by gas chromatography h<1>; about 74·5 kcal/mole, and it can be inferred that thc made it possible to identify several aromatic hydrocarbons

activation energy will be greater than that for CHCI3 , as well as thiophene in their aqueous extracts. This report where D(CI.HC-Cl) : 72 kcal/mole. This is compatible describes the identification of benzene, toluene, ethyl­ 16 with the finding that CFC1 3 does not initiate necrosis in benzene, p/m-xylene, a-xylene and naphthalene in the the rat liver. soi!. Four different types of soil, originating from the The properties of halothane, and other fluorine com­ AD horizon, have been investigated; m eadow soil, cul­ pounds, are less easy to understand. From calculations tivated soil, forest soil and pod so!. Each of these soils similar to those outlined above we might conclude that contains all the identified substa,nces, but their concen­ electron capture by the halothane molecule would result in trations which range from 1 to 5 p.p.m. vary from one l2 debromination. This is not observed in viva • It is soil to another. possible that the negative molecular ions of molecules Tho above mentioned compounds arc to be found ill containing several fluorine atoms are stable enough in twenty-nine substances released by gas chr·omatography. the physiological environment to become discharged by Idontification proceeds as follows. A kilogram of soil is other mechanisms before dehalogenation takes place. shaken with a litre of 0·1 normal hydrochloric acid; It is suggested, therefore, that if Slater' is right and these are loft in contact for 12 h and then filtered. The free radicals produced from carbon tetrachloride are filtrate is extracted with sulphuric ether and tho ether responsible for the initiation of liver damage, then the solution is then extracted with 0'1 normal sodium bicarb­ radicals are probably produced by a process of electron onate. The acid substances were thus eliminated. The transfer. This mechanism accounts qualitatively for ether is then evaporated so as to reduce the ethcr extract the relative toxicities of carbon tetrachloride, chloroform to 1 mi., and this is subjected to gaR chromatographic and trichloro-fluoro-methane but not for halothane. analysis. A gas chromatograph with double flame ionization is N. L. GREGORY used. The column is a 2 x 92 x 0·4 cm stainless steel tube packed with 6 per cent tricresyl phosphate on 60-80 mesh 'Chromosorb W'. Argon is used as carrier gas, National Institute for Medical Research, Mill Hill, with a flow rate of 45 m!./min and an inlet pressure of 14 lb./in.- gauge. During the analysis, the column is L ondon. heated to 800 C and then to 1320 C. R eceived November 11, 1966. Fig. 1 shows the chromatogram obtained during the , Slater, T. F .. Nature, 209, 36 (1966). analysis of cultivated soil and podso!. Table 1 shows the 'Buckel'nikova, N. S .• Sov. Phys. J.E.T.P., 7, 358 (1958). retention periods for the two temperatures applied. " Gregory, N. L ., thesis, Univ. London (1961). Thiophcne and toluene, together with the unknown , Lovelock, J. E., Nature. 189, 729 (1961). substance responsiblo for p eak 6, are quantita.tively the , Warhurst, E., Quart. Rev. Chem. Soc., London, 5, 44 (1951). most important. The latter compound is being further " ~arriot.t, J_. Thorburn, R., and Craggs, J_ D., Proc. Phys. Soc., B, 67, 437 (1954). investigated to identify it. The peaks 3, 8, \-I and II , Field, F. H .• and Franklin, J. L., Electron Impact Phenomena, 314 (Aca- corresponding respectively to benzene, p/rn-xylene and demic Press Inc., New York, 1957). a-xylene although not very sharp, arc <,!uite conclusive. ' Dorman, F. H., J. Chem. Phys" 44, 3856 (1966). 0 , Gaines, A. F., Kay, J., and Page, F. M., Trans. Farad. Soc., 62, 874 (1966). Naphthalene which appears as peak 29 at 132 C is also " Berry, S. ]f., and Reimann, C. W .. J. Chem. Phys., 38, 1540 (1963). quantitatively important. Fig. 1 shows tha t the cultivated U Pritchard, H. 0., Chem. Re!>., 52, 543 (1953). soil (A) contains more hydrocarbons than the podsol (E)_ "Chenoweth, M. B., and lIlcCarty, T•• P., Pharmacol. Rev., 15,673 (1963). Thiophene is due to the solvent used. " lIutler. T. C., J. Pharmacal. expo TIlerap., 134,311 (1961). This is the first recorded instance of idcntification "MackIe, H., and McClean, R. T. B., Trans. Farad. Soc" 58, 895 (1962). " Errede, L. A., J. Phys. Chem., 64, 1031 (1960)_ of aromatic hydrocarbons as natural compounds in soils. 11 Slater. T. F., Biochem. Pharmacol., 14,178 (1965). There are no data as to the origin of those substances.