11. KLEIS, 31. D. SCHEER, .4ND R. KELLEY
598
Disproportionation-Combination Reactions of Alkyl Radicals and Hydrogen Atoms at Low Temperatures
by Ralph Klein, Milton D. Scheer, and Richard Kelley .Vatio?!al Ihrreaic o,f Standards, Washington, D. C.
(Rewived October 10, 19GS)
The dispi,opoi.tioiiatioti combination reactions of s~c-butyl-sec-butyl and hydrogen atomsec-butyl t i a i ~heei~o1)sei’rrd in detail by the application of low tcmperature techniques. Tho ser-butyl radicals were prcparcd by the interaction of atomic hydrogen with condensed eis-2-htitcric. Scparation of thc see-butyl-sec-butyl from that of thc IT atom-sec-butyl tcm \vas cffected by control of diffusion in the matrix containing the czs-2hiitenc. .It 90°K., tlic ratio of disproportionation to combination for sfc-butyl-stc-butyl is 11. ‘rliat for TI atom-ser-butyl is 1.9. The effect of temperature in disproportionationrombinatioii reactions \va$ demonstrated by photolyzing azoisopropane as a soiirce of isopropyl radicals ovcr an cxtended temperature range. It is concluded from this, and othcr t>vidcncc,that the activation energy for cornbination exceeds that for disproportionation hy scl-rial hundred calories per mole.
pm-ature rcaction techniques to the study of disproportioriation and combination of radicals leads to some elegant rcsults ir~accessihleb y more coriveritional experirnents. Tlic inethods used havr been drscrihrd i i i a series of pupc?rs;.4-9 The esseiitial feature is the low tcmperutuw interaction betiveer1 tiydrogrri xtonix, gencrutcd in the gas phase, atid a corideiised reactant. In most of thesc studies the reactant has been ari olefin eiohcr in purc form or admixed with an inert tliluent~. The latter strongly influcmes the course of the reaction through its effwts 011 both concentrat,ion and diffwioii. I t has been s h o w i that the addition of atomic hydrogen to the olefin to form the radical occurs within a few molcc!ular diarnctcrs of the s u ~ f a c e . ~subsequent^ reac(1) A. F. Tri,tinaii-T)i(.kerison. A ~ L ~Rcpt. I , . (’hrm. S o
(2) ,J. A. Kerr a i d A . I;. Tr[)t,man-L)ickeitson, f’rogr. Hixoii, >I,1’. Stcfaiii, arrtl LI. R z w m ~ . I’roc. C ‘ h r m . S o r . , :$04 (IOfi2): (I)) 1’. S. 1)isoir. .I. 1’. Stefairi. and 11. Sewara. J . Am. Ch(,rn. Soc.. 8 5 , 2551 (lO(i3). (4) 11. Kleiii, 11. I). Scheer, end .J. G . Waller, J . I ’ h l p . Chem., 64: 1247 ( I9ti0). ( 5 ) 11. Klein and SI, 1). S ~ ~ h e et rX. d . , 66, “77 (1962). ( 6 ) It, Klein and SI. 1). Scheer. it&!., 67, 1874 (1963). ( 7 ) I < , Klein aiid SI. D. Scheer, J . rim. C h o n . S O C . , 8 0 , 1007 (1958). (41 A I . 11. Scheer arid I
