Carbonium ions in radiation chemistry. Reactions of tert-butyl ions with hydrocarbons

Abstract

Butyl ions, generated by the y irradiation of neopentane, undergo hydride transfer reactions with alkanes having tertiary hydrogen atoms (/-C4H9+ + RH z'-C4Hio + R+) to form isobutane and a tertiary carbonium ion as products. By irradiating neopentane in the presence of mixtures of two reactant molecules, one deuterated and one nondeuterated, the relative rates of the hydride transfer reaction for 22 C5-C8 alkanes were determined with an unusually high accuracy, from the observed ratios of z'-C4H9D to z'-C4Hi0. Absolute rate constants for the reactions, based on a determination of the rate of depletion of the isobutane yield when ammonia is added, are given. The rate constants are generally very low [10-u-10-10 cm3/(molecule sec)], but are shown to depend on the heat of reaction and steric factors involving van der Waals interactions between the approaching ion and hydrogen atoms or methyl groups on the carbon atom ß to the tertiary carbon. The evidence indicates that the hydride transfer reactions of the /-butyl ion have an activation energy, E*.t < 0.15 eV. In another series of experiments, unsaturated hydrocarbons were added to neopentane-isopentane mixtures, and the rates of the condensation reaction of these additives with /-butyl ions were determined by measuring the rate of depletion of the isobutane yield. The rate constants for the condensation reactions are in the range 10~10-10~9 cm3/(molecule sec). Some insight into processes occurring in the radiolysis of neopentane can be gained from these results. It is shown, for example, that in the absence of reactive additives, not more than about 14% of the /-butyl ions undergoes processes resulting in the formation of isobutene.(1) This research was supported by the Atomic Energy Commission.(2) (a) F. W.

Kinetic mass spectrometric investigation of the reactions of t-C4H9+ ions with some simple polar molecules at thermal energies

“…Th e so lvation of N l-l t by N RI to for m (NR lhH + js also obse rv ed, as well as th e furth e r so lv a ti on of (C R IC OCRdNH t a nd (N H :1h H + by ac e tone: As stated in the Introdu ction, one of the o bj ectives of this study was to provid e a basis for co nvertin g th e relative rate constants for Process 1, which were previously meas ured in this laboratory [1] , to absolute values. Our ex perim e ntal valu e for Reaction 2 is lower than the value assumed in that study by a fac tor of approximately 4.4, and th e reported values s hould be adjusted accordingly.…”

“…R.S., Paris, France. 1 Figure s in brac ke ts ind icate th e li te ratu re re ferences at th e end of thi s papt:f. c u Ie . seco nd) for th e proto n tra nsfe r reaction involvin g t-C4 H;j a nd N H:l…”

“…In a rece nt arti c le from thi s la bora tor y [1] 1 th e rela· ti ve rates of c he mi cal reac ti on of t·C4 H;t ions with a va ri e ty of hydrocarb ons we re re ported. Th e meth od in vo lved the radi olysis of neope nta ne.…”

“…whi c h yie ld s t·C4H;t as th e maj or fragme nt ion, carri ed out in th e prese nce of s mall qu antiti es of mixtures of de ute rated a nd nonde ute ra ted hydrocarbons (RH2). The specifi c reaction investi gated was the hydride tran sfe r process (1) whi c h was monito red by de te rmining the isotopic co mpos iti on a nd y ield of th e stabl e c he mical e nd· produ ct . iso buta ne.…”

“…Although th e relative reactivities of so me 22 C5,CH alkan es we re de termin ed with a high prec is ion by thi s me th od. absolute rate coe fficie nt s for Process (1) could o nl y be estimated by ass uming the maximum theo re ti cal valu e (4.0 X 1O-!1 c m~/mol e · ·Support ed in part by th e U.S. Atomic Energy Co mmissio n. "Cues t Wor ker. 1970 -197 1.…”