3296
COJIhICh’IC.\TIOSS TO
For reactions of R8SiH with OH- in %Yo ethanol a t 35” relative rates (k2) are, for (CzH&SiH = 1; I, lo3; 11, 10e105; (CH2)4Si(CH3)H, 10; ( C K Z ) ~ S ~ ( C H ~10-l. ) H , For (C2H5)3SiH a t 3 j 0 , kp = 0.1 mine-’, mole-’ 1.5p6 The corresponding disiloxane, bis- (1-silabicyclo[2.2.l]heptyl)oxide,3m.p. 76”, was isolated in 4070 yield (62 mg.) from the kinetics runs with I and perfectly linear pseudo first order plots were obtained. The 4-ring silane, 11, gives GOYo of the theoretical hydrogen a t an extremely fast rate and 40% a t a different rate which is smaller than the first by a factor of about three powers of ten. The latter rate has been shown to comprise hydrogen evolution from sym-di-n-propyldimethyldisiloxane which is formed by ring-opening of I1 (C-Si cleavage by OH-) in competition with the evolution of hydrogen from the 4-ring silane.? Thus, i t is clear that displacement of hydride ion by hydroxide ion proceeds far m o r e rapidly a t bridgeheada and 4-ring silicon atoms than a t silicon in ordinary a-cyclic and 5- as well as 6-ring silicon hydrides. These results comprise a complete reversal of the structure-reactivity relationships observed for displacements a t carbon when the same comparisons are made.g The above facts are in accord with the hypothesis outlined in our earlier comm~nication.~Thus, in the reaction of I1 with hydroxide the geometry of the Si(5) addition compound would approximate one of two (idealized) structures: (1) a = H, b = OH; (2) a = OH, b = H. C--
C
On the above model of Si(5) complex for I1 with hydroxide ionlo and similar models for I and other cyclic and acyclic hydrides, structure-reactivity relations resulting from steric factors are readily explained in terms of three factors: (I) ease of formation of Si(5) resulting from groups on silicon being “pulled back” in Si(4), away from the path of reagent attack. ( 2 ) I-Strain in Si(5) relative to ( 5 ) Relative rates for t h e 5 - a n d 6-ring silanes are based on d a t a in R . West, THISJOURNAL, 76, G O 1 5 (1954). (6) Salt effects on k2 resulting f r o m change in K O H concentration should not appreciably affect t h e above relative rates tabulation which covers t h e range 0.003 N t o 0.2 N KO€€; see J . E. Baines a n d C. Eaborn, J. Chent. SOL.,813 (1935). ( 7 ) In 9570 ethanol a t 3 5 . t h e &ring silane gives 6 0 % of t h e theoretical hydrogen with a n ammonia-ammonium iodide buffer (0.5 N NHa, 0.05 N XHrI) a t a r a t e which corresponds t o a half-life of 7.5 minutes. T h e same reagent gives no hydrogen in a reasonable time with t h e highly reactive triphenylsilane. (8) Energies of activation, E,, f o r (C2Hs)aSiH a n d I are 16 kcal./mole a n d 7 kcal./mole, respectively. T h e latter value was calculated from r a t e d a t a a t 0 a n d - 2 4 O using 0.0579 iV K O H . Relative rate for I a t 3 5 O is a calculated value based on t h e E.. (9) For a recent excellent summary of d a t a on effects of ring size o n reactivity, see 4 . Streitwieser, Jr., Chem. Rew., 56, 666-670 (1956). (10) There are, of course, four possible structures of Si(5) for I1 which n - o d d have t h e K S i - O H angle near 90°, b u t two of these w w d J ire energetically unfavornlile since t h e C-Si-C angle would he ronstrtrined from a n ideal anfile of l?O‘ to W90° hy t h e silacyclobiit.inr ?).4tem.
Vol. 79
THE EDITOR
Si(4).” (3) Steric strain in Si(5) relative to Si(4) due to increased “crowding” of groups in the former.12 (11) H. C. Brown and M. Borkowski, THISJ O U R N A L , 7 4 , 1894 (lQ52), a n d other earlier papers on t h e elegant I-Strain hypothesis. (12) Relative rate (h) for t-C,HBSi(CHa)2H is 10-2. Unpublished work of L. H. Sommer and W. P. Barie, Jr.
COLLEGE OF CHEMISTRY AND P f r u s r c s LEOH. SOMhfER THEPESNSYLVANIA STATE 0. F R A N C I S BEXNETT UNIVERSITY PAULG. CAMPBELL UXIVERSITY PARK, PENNSYLVAXIA DOSALDR. WEYENRLRG RECEIVED MAP 23, 1957
THE PREMIXED OZONE-HYDROGEN FLAME’
Sir: Mixing of pure ozone with combustible substances usually leads to self-ignition, explosion or detonation and $remixed pure ozone-fuel flames have not been reported. They are interesting not only because of the higher enthalpy content of the Os-fuel vx. the 02-fuel system, but primarily because the combustion kinetics in the flame front of the 08-systems can be expected to be substantially faster than for the corresponding 02-systems. We have described recently the combustion flame of ozone to oxygen.2 We also found that pure ozone can be mixed with pure hydrogen a t -78” and even a t 20” and 1 atm. without explosion and with no, or practically no, reaction for several hours. Pure ozone-hydrogen mixtures of any desired composition were prepared by mixing known volumes of the two gases. The mixtures were burned a t -78” initial temperature using the same technique as before.2 (The Reynolds numbers of all flames were below 2000.) The burning velocities were determined by the standard schlieren method. The same apparatus was used t o measure the burning velocity of the H2-02 system. The results are presented in Fig. 1. The experimental burning velocities a t 195’K. initial temperature and 1.0 atm., were for 6.0, 12.0, 18.2, 18.5, 25.0 and 100.0 mole yo 0 3 , respectively, 207 5, 664 f. 57, 1200 f 20, 1330 * 30, 1680 f 80 and 270 A 7 cm./sec. Mixtures of 94.0 to 75.0% H 2 (rest pure ozone) could be burned readily. These O3-H2 flames are brighter than the corresponding 0 2 flames but like the latter their luminosity is low. The stoichiometric mixture, i.e., 3.0 vol. Hz 1.0 vol. 0 8 , which could be appropriately called “super-knall gas,” burns very rapidly a t a velocity of 1680 * SO cm./sec., whereas “knall gas,” classically considered the fastest burning gas mixture, burns at 610 cm./sec. or 112.75 as rapidly. I n mixtures on the ozone-rich side of the stoichiometric point the flame front degenerates very rapidly to a detona-
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(1) This research was supported by t h e United States Air Force through t h e Air Force Office of Scientific Research of t h e Air Research a n d Development Command under Contract No. AFlS(600) 1475. Reproduction in whole o r in p a r t is permitted for any purpose of t h e United States Government. (2) A. G. Streng and A. V. Grosse, THISJ O U R X A L . 79, 1517 (1957) ; see also Pruc. l’llh Intvitntional .Cyinposi!i,n o i ! Coiitbi~clioiz, . \ i w 19-24, 1856: P ~ o r .Ii!leriidionol Oaoiit (-oijf#,rc,il
