179731a0Nature1794562195704067317310028-0836195710.1038/179731a0ukNatureNatureNATUREnatureNature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public./nature/journal/v179/n4562issueJournal homeArchiveCurrent issueAdvance online publicationPrivacy policySubscribeNature Publishing GroupCurrent issue179731a0Competition in Atomic Halogenation Reactions
AU  - ACKERMAN, M.
AU  - CHILTZ, G.
AU  - DUSOLEIL, S.
AU  - GOLDFINGER, P.
AU  - MARTENS, G.
AU  - AUWERA, D. VANDERLaboratoire de Chimie physique moleculaire, Universite Libre de Bruxelles, Brussels. Jan. 18.IT has been shown recently1 that it is useful to consider a general mechanism for atomic halogenation reactions, which, in the case of the photochlorination of C2Cl4 and C2HCl5, is given by: It has also been shown that the usual steady-state treatment is adequate for calculating overall rate equations2, and that absolute reaction-rate theory and rough estimates of activation energies and entropies permit one to calculate approximately the overall rate constants1.
The rate of substitution (_D1, Table 1) and of several competition reactions (A2, E, F, Q) have now been measured3 and are summarized in the table with the previously measured rates of addition (AI, B) and of dehydrochlorination (C)1. Table 1 shows further that several overall rate constants which are measured directly may equally well be calculated from other measurements. The good agreement between these direct and indirect determinations is an extremely satisfactory control of the accuracy of the proposed mechanism ; it shows further that the error limits for activation energies and frequency factors are extremely narrow.
Further, as shown in Table 1, the life-time of the reaction chain in the addition reaction at low temperature (H) yields values of three rate constants of elementary steps (H, I, J).
Table l. CORRELATION BETWEEN THE RATE CONSTANTS OF CHLORINATION AND DEHYDROCHLORINATION OF CaCLt AND C,CL5H AND OF
COMPETITION BETWEEN THESE REACTIONS
Value of log (rate	Value of log (rate con-
	constants repre-	stants giving best
Type of rate measurement or combina-		Temperature	senting directly	agreement between
tion of measurements	Rate equation	interval	measured rates)	directly measured and
				constants)
A. (1) addition at low temperature				
(2) reaction of 01, with a mixture	W'W'XCl,)'"	360 -430  K.		
C,C1, + C.C1.H		370 -390  K.	- 660/Z1 + 2-56	- 660/2" + 2-56
(3) from substitution (X l)		370 -430  K.		
B. addition at high temperature	la1'2 kzihilkjc-i)1 '2(C]a)(CaCl4)1'!	520 -560  K.	+ 1,800/T - 1-17	+ 1.725/21 - 1 -20
0. dehydrochlorination	(IaJCz'kJk?)112 (CaClsH)1/2	400 -470  K.	- 2,500/T + 6-20	-2,450/2'+ 6-20
D {Vi (k k Ik Uf2J" rom substitution " (2) ' ' ' \B.E.*i*	/.l"(*,*.(Cl1)/*1'(O,Cl.H)+*,)->' i,(Cl1)	370 -430  K.	- 800/T + 3 -12	y~ 675/21 + 3-05
E. (1) from equality of addition and				
dehydrochlorination (2) (DIB)' F. (1) competition substitution 	* A'(CSC1SH) = fcsfc.tCCljMCl,)	400 -430  K.	- 4,600/T + 8-46	j--4,800/r+ 8-50
				1
dehydrochlorination (2) (CIDY 0. (1) competition addition substitu-	kJk CU)	400 -430  K.	- 2,890/T + 4-98	>  3.550/21 + 6 '30
				
tion	ksiCsCl jka' (C2ClfiH)	365 -385  K.	+ 1,400/r - 2-28	
(2) (B.CID'T				)  + 1.250/21 - 2 -20
(3) (0/B.E)'				I
\*J JfjJllH. Clj + CjCl, at low tempera-				J
ture in intermittent light		360 -420  K.	  150/2* + 4-6	- 150/21 +4-6
I. A.H.	k,			- 810/21 +7-2
J. I.F.	k.			- 4,360/T + 13-5
We intend to measure further the life-time of the reaction chain of the addition reaction at high temperature and of the dehydrochlorination, as well as further competition reactions, and to calculate the equilibrium constants. This should yield values for the eight rate constants which determine the reaction mechanism. A discussion of the measured values of activation energies and entropies in terms of reaction-rate theory will be useful after several similar systems have been studied (C2HC13 - C2H2C14 ; C2HaCl2 -C2H3C13; etc.).
The Institut pour la Recherche Scientifique dans l'lndustrie et l'Agriculture and the Fonds National de la Recherche Scientifique (Brussels) are thanked for their assistanceGosselain, , P. A., Adam, , J., and Goldfinger, , P., Bull. Soc. Chim. Belg., 65, 533, 544 (1956). Adam, , J., and Goldfinger, , P., ibid., 65, 566 (1956).ChemPortAdam, , J., Dusoleil, , S., and Goldfinger, , P., Bull. Soc. Chim. Belg., 65, 942 (1956).ChemPortAckerman, , M., Chiltz, , G., Goldfinger, , P., and Martens, , G., Bull. Soc. Chim. Belg. (in the press).