4290
K. TANNO, T. MIYAZAKI, K. SHINSAKA, AND S. SHIDA
Fragmentation of Excited Ions in the Radiolysis of Neopentane and Isooctane in the Liquid Phase
by Kuniyasu Tanno, Tetsuo Miyazaki,' Kyoji Shinsaka, and Shoji Shida Laboratory of Physical Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan (Received April 14, 1967)
The fragmentation of an excited ion in the radiolysis of neopentane and isooctane in the liquid phase was studied by using ion scavengers such as sulfur hexafluoride and methylamine. The yields of t-C4Hs+ion were measured with the result that G(t-C4H9+) = 1.0 in the radiolysis of neopentane and G(t-C4Hg+)= 0.36 in the radiolysis of isooctane. The ionic mechanisms of the radiolysis of neopentane and isooctane are discussed. The ionic C-C bond rupture in the radiolysis of several saturated hydrocarbons in the liquid phase was classified into three reaction steps, that is, the fragmentation of the excited ion, the fragmentation by neutralization, and the deactivation of the excited molecule formed by the neutralization.
Introduction The effect of density on the fragmentation of the excited ions in the radiolysis of isobutane and n-butane has been reported in a previous paper.2 The fragmentation of the excited butane ions was found to decrease with increasing density because of its collisional deactivation. Neopentane and isooctane have tertiary C-C bonds, which are much weaker than C-C bonds of butanes, and their radiolysis in the liquid phase has been undertaken in order to study the fragmentation of the excited ions at high density. Though the possibility of ionic dissociation in the liquid-phase radiolysis of neo-type alkanes has been discussed by several authors from the point of view of energetics3 and from the study using radical scavengers,* direct measurement of the yields of t-CdHS+ ion in the liquid phase has not been attempted. The present authors have been able to measure the yields of t-C4Hg+and CaH,+ ions in the gas phase by the ammonia m e t h ~ d . ~ . ~ Here f ~ the yield of the t-C4Hg+ ion in the liquid-phase radiolysis was measured by using methylamine as a detector of carbonium ion and sulfur hexafluoride as an electron scavenger.
Experimental Section Neopentane was supplied by the Takachiho Shoji Co.; its gas chromatographic analysis showed the The Journal of Physical Chemistry
presence of 0.05% n-pentane as an impurity. Isooctane (Eastman) was 99.7% pure and was passed through a soda lime column for the removal of water and carbon dioxide before use. Pentene-l, used as a radical scavenger, was 99.99% pure. Methylamine was obtained from a 40% aqueous solution by passing through traps at -78" and through a column of sodium hydroxide on a vacuum line. Sulfur hexafluoride, Matheson Co., was of high purity. The samples were irradiated with Cos" y rays at room temperature, at a dose rate of 6.2 X lo6 r/hr. The total doses were 2.85 X loz1ev/g for neopentane and 9.45 X lozoev/g for isooctane. The hydrocarbon products were analyzed by gas chromatography (dimethyl sulfolane column, 15 m, or bensyl ether column, 20 m).
Results and Discussion Fragmentation of Ions in the Radiolysis of Liquid Neopentane. The yields of isobutene, isobutane, and (1) Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, C h i k u w k u , Nagoya, Japan. ( 2 ) T. Miyasaki, J . Phys. Chem., 71, 4282 (1967). (3) T.F.Williams, Trans. FaTaduy Soc., 57, 755 (1961). (4) (a) T.Kudo and S. Shida, J . Phys. Chem., 67, 2871 (1963); (b) R. A. Holroyd and G. W. Klein, J . Am. Chem. SOC.,87,4983 (1965). (5) T. Miyazaki and S. Shida, Bull. Chem. SOC.Japan, 38, 2114 (1965). (6) T. Miyazaki and S. Shida, ibid., 39, 2344 (1966).
FRAGMENTATION OF EXCITED IONSIN RADIOLYSIS OF NEOPENTANE AND ISOOCTANE
propylene in the radiolysis of liquid neopentane are shown in Table I. For each lo00 moles of neopentane 70 moles of pentene-1 was added to act as a radical scavenger - in all the runs, in order to prevent isobutene from undergoing secondary reactions with the radicals. The yields of isobutene in the absence of sulfur hexafluoride and methylamine coincide approximately with the nonradical yields of 1.2' and Isobutene decreases sharply with the addition of sulfur hexafluoride. On the other hand, it increases upon the addition of methylamine even in the presence of sulfur hexafluoride. A possible mechanism of the ionic reactions may be considered as
429 1
+ SF6 +SFst-C4Hg+ + SF6- +t-C4Hg + SFB e
CaHlz+
+ SFs-
--3
CsHiz
(9) (10)
+ sF6
(11)
Therefore the formation of isobutene by neutralization (reactions 3 and 8) can be estimated from the decrement of isobutene by the addition of sulfur hexafluoride shown in Table I: G(3) G(8) = 0.9. The marked increase of isobutene by the addition of methylamine is due to the proton-transfer reaction of t-C4H9+ion with methylamine; a similar phenomenon was also observed amin the gas-phase radiolysis of the neopentane monia system.6
+
+
t-C&Ig+
+ CHiNHz
i-C4H8
+ CH,NH3+
(12)
We get from the increment of isobutene G(t-C4Hg+)
Ii-C4H, + CH4 The hydride ion transfer reaction of the t-CaHg+ ion with neopentane does not occur because it is an endothermic reaction. Since propylene is formed only in a small amount, reaction 5 can be neglected. I n the presence of sulfur hexafluoride as an electron scavenger the neutralization reactions (3), (4),(7)) and (8) may be inhibited; for example
Table I : Effect of SF6 and CHsNH2 in the Radiolysis of Liquid Neopentane" -Additiveb-----. SFs
0 50 70 100 70 70 70 70 70 70 0
CHsNHz
0 0 0 0 30 50 70 1.00
200 300 200
--
=
1.0
Since G(t-C4Hg+) = 1.76 and 1.7 in the gas phase by the ammonia method and the mass spectrum, respectively,B 60% of excited neopentane ion decomposes in the period of one vibration sec) before the collisions in the liquid phase, while most of excited butane ions decompose after sec.2 This is because the C-C bond of neopentane ion is much weaker than that of the butane ion. Holroyd and Klein measured the yields of radicals in the radiolysis of liquid neopentane and observed G(CH3) - G(C4Hg) = 1.6.4b This large difference in yield suggests that reaction 3 may be the main process in the neutralization of t-C4Hg+ion. Fragmentation of Ions in the Radiolysis of Liquid Isooctane. The effect of the additives on the radiolysis of liquid isooctane is shown in Table 11. For each 1000 moles of isooctane 50 moles of pentene-1 was added as Table 11: Effect of SF6 and CHaNH2 in the Radiolysis of Liquid Isooctane"
Gvalue
CaHs
i-OHra
i-C
