THOMAS SMAILAND F. S. ROWLAND
456
The Primary Replacement Isotope Effect in Recoil Tritium Reactions with Isobutanel by Thomas Smail and F. S. Rowland Department of Chemistry, University of California,Irvine, California 9$6’64
(Received April $6, 1969)
The primary replacement isotope effect for the substitution of T atoms for H or D atoms in hydrocarbons has been measured as 1.25 i0.05 through comparison of recoil tritium reactions at the tertiary position in (CH3),CH os. (CH&CD and (CD&CH us. (CD3)sCD. The increased yield for replacement of B vs. D is suggested to result from the more rapid response of €1 atoms to changes in chemical binding during the interactions of the butane molecules with energetic T atoms. The isotope effect in the abstraction of H vs. D from the tertiary position in isobutane is 1.65 i 0.06. Alkyl substituent effects, including steric hindrance, are essentially absent in gas phase experiments with i-CdD10 ( R = 0.96; R = tertiary T per bond/primary T per bond) and smallwith GC4HI0(R = 0.87). A gas/liquid phase effect is observed which reduces the liquid phase value of R by 15-25%.
Introduction The reactions of the energetic tritium atoms from nuclear recoil have been extensively studied in recent years with a variety of hydrocarbons, with considerable emphasis upon the substitution reaction 1 initiated by tritium atoms with greater than thermal Isotope effects have been studied for this reaction T*
+ RH(D) +R T + H(D)
(1)
as well as for hydrogen abstraction and other substitution reactions, through comparison of results obtained with protonated us. partially and fully deuterated target molecule^.^-^^ The present paper describes the first measurement of the primary replacement isotope effect12for the replacement of H or D by energetic tritium atoms, as in (l), in the absence of other isotope effects.l1 Isotopic comparisons have been made earlier of the absolute yields of CHIT and CD3T from recoil T reactions with pure CH4 and pure CD4, respectively. l 3 I n such a comparison, one must consider three possible sources of isotopic differences: (1) primary replacement isotope effect, the variation in yield caused by the difference in the identity, H vs. D, of the replaced atom; (2) secondary isotope effect, the variation in yield attributable to the isotopic difference, CH, us. CD3, in the identity of the group to which the tritium atom becomes bonded during the reaction; and (3) moderator isotope effect, the variation in yield caused by differences in the rates of energy loss in nonreactive collisions with isotopic molecules, CH4 us. CD4, which are duly reflected in the number of collisions occurring in the energetic range, and in the yields of hot reactions resulting from those collisions. Both moderator and secondary isotope effects have been demonstrated to be substantial with each resulting in larger yields for the protonated m o l e c ~ l e . The ~~~ The Journal of Physical Chemistry
secondary isotope effect in the substitution of T-for-F in CH3F 21s. CD3F was shown to favor the former by competition experiments involving mixtures of methyl fluoride with cyclobutane’ or he1ium;g combination of these results with those found for the pure molecules alone permitted calculation of a moderator effect of 1.23 f 0.08, favoring more rapid energy loss (and therefore lower yield) for collisions of tritium with CH3F vs. CD3F.8 An isotopic preference for the replacement of H in CHaF vs. D in CD3F was also demonstrated in the same experiments, but involves simultaneously the possibility of both primary replacement and secondary isotope effects, and cannot be unequivocally assigned to either. The effects of moderator differences can be almost completely cancelled by competition experiments in which both competing molecules are exposed to the (1) This research has been supported by Atomic Energy Commission Contract No. AT-(11-1-)34, Agreement No. 126. (2) F a Schmidt-Bleek and F. 5. Rowland, Angew. Chem., Int. Ed., 3, 769 (1964). (3) R. Wolfgang, Progr. Reaction Kinetics, 3,97 (1965). (4) R. Wolfgang, Ann. Rev. Phys. Chem., 16, 15 (1965). (5) J. W. Root and F. 5. Rowland, J . Amer. Chem. Soc., 85, 1021 (1963). (6) H. C. Jurgeleit and R. Wolfgang, ibid., 85, 1057 (1963). (7) E. K. C. Lee and F. S. Rowland, ibid., 85,2907 (1963). (8) E. K. C. Lee, G. Miller, and F. 8. Rowland, ibid., 87, 190 (1965). (9) E. K. C. Lee, 3. W. Root, and F. S. Rowland, “Chemical Effects of Nuclear Transformations,” Vol. 1, International Atomic Energy Agency, Vienna, 1965, p 55. (10) C. C. Chou and F. S. Rowland, J . Amer. Chem. SOC.,88, 2612 (1966). (11) T. Smail and F. S. Rowland, J . Phys. Chem., 72, 1845 (1968). (12) The term “primary replacement isotope effect” has been proposed by us for comparisons of T* RH 2)s. T* RD, both leading
+ +
+ +
to RT; the term “primary substitution isotope effect” has similarly been proposed for comparisons of T* R H IJS. D* R H vs. H* RH. No measurement of a primary substitution isotopeeffect in the absence of other isotopic differences has yet been reported. (13) J. Cross and R. Wolfgang, J. Chem. Phys., 35,2002 (1961).
+
RECOIL TRITIUM REACTIONS WITH ISOBUTANE same tritium atom fl~x.7989~~ Secondary isotope effects can be eliminated by making comparisons of the substitution reaction with molecules which are identical except for one H us. D, as with CHX3and CDXs. Our present measurements have been carried out with a series of isotopic isobutane molecules and involve two sets of comparisons: (CH&CH us. (CHJ3CD; and (CD&CH us. (CD&CD. I n the first pair, the great majority of the energetic collisions occur with CH3 groups, and neither the substitution into the primary position nor the energy loss in nonreactive collisions should be appreciably affected by the presence of H or D in the tertiary position. Consequently, the tertiary H or D atom is exposed to essentially equivalent tritium atom fluxes, and the yield for replacement of each can be directly compared to the substitution for H in the primary position of the same molecule. An equivalent set of comparisons can be made for reactions in which both the substitution in the primary position and energy loss in nonreactive collisions occur with CD3 groups. The tritium content of the primary and tertiary positions in isobutane can be established by isotopic exchange of the former under conditions permitting retention of the tertiary activity.l67l6
Experimental Section Chemicals. Isobutane and 1,3-butadiene were Phillips Research grade. The deuterated isobutanes were supplied by Merck Sharp and Dohme of Canada and were stated to have a minimum isotopic purity of 98%. All gases were used without further purification, other than degassing at -196" just prior to filling of samples. 3He was degassed from a charcoal trap a t - 196" and was free of H T impurity. Sample Preparation. Gas samples were filled according to the usual procedure^.^-^ Liquid samples were prepared by successive condensation of butadiene and then isobutane into an evacuated capillary tube containing about 10 mg of LiF. The composition of the liquid samples was controlled through the use of calibrated volumes on the sample filling vacuum line. Sample Analysis. The distribution of tritium activity among the various radioactive products was determined by conventional radio gas chromatography. l7 HT/DT, CH3T, C2H3T,and C&T were separated using a 50-ft, I/4-in. od column of 30-40 mesh alumina which had been partially deactivated by coating with approximately 10% by wt propylene carbonate. The column was operated at 0" and 0.5 ml/sec He carrier flow rate. H T and DT were resolved with a 7-ft, 1/4-in. od column of y-alumina (40-50 mesh) coated with approximately 3.5% Fez03, operated a t 0.5 ml/sec He flow rate and 77°K. A "stripper" column of activated charcoal (10-ft, 1/4-in, od) operated at 25" was placed in front of the FezO3-A1203 column so that only isotopic hydrogens were
457 allowed into the low-temperature column. Full details of the preparation of these columns has been given.18 Separation of hydrocarbons through Cq was obtained with a 50-ft l/d-in. od column containing 30% by weight of di-n-butyl phthalate on 30-60 mesh chromosorb P. The operating conditions were 0" and 0.5 ml/sec He flow rate. This column was also used for the purification of the isobutane fraction from irradiated samples. Analysis of the Intvamolecular Location of Tritium in Isobutane-t. Otvos, et al.,19 demonstrated rapid primary hydrogen atom exchange for isobutane dissolved in concentrated sulfuric acid through a t-C4Hg+ intermediate, and this procedure was adapted for intramolecular analysis of the tritium positions in isobutane by Odell, et a1.16 The technique used in the present experiments is a modification of that described in detail by Rosenberg.la Isotopic exchange of hydrogen was carried out in sealed glass ampoules, maintaining the relative amounts of isobutane (600 cm-ml) and concentrated sulfuric acid (8 ml) and the gas/liquid volume ratio (1.4) in the ampoule constant to within i5%. The exchange rate between primary tritium in isobutane and acid protons was determined for these standard experimental conditions, as indicated in Figure 1. In the first set of experiments, isobutene (0.5% x isobutane) was added to accelerate the primary exchange reaction, and exchange was carried out for a total time of 10-12 hr. In the second set of experiments, no isobutene was added, and exchange was carried out for total times of 20-26 hr. The amounts of isobutane and concentrated sulfuric acid were such that about 1% of the original primary tritium atoms would remain in the isobutane when isotopic equilibrium was reached between hydrocarbon and original acid. About halfway through the exchange period (5-6 hr with isobutene; 10-15 hr without) the isobutane was transferred into a fresh batch of acid and the exchange continued. With two such batches of acid, removal of primary tritium was more than 99.9% complete. The exchange reaction was followed by measuring the specific radioactivity of the gas chromatographically purified isobutane-t, i.e., counts per minute of radioactivity per unit area of mass peak under standard conditions. Typical measurements are given in Table I. (14) J. W. Root, W. Breckenridge, and F. S. Rowland, J.Chem. Phys. 43,3694(1965). (16) A. Odell, A. Rosenberg, R. Fink, and R. Wolfgang, ibid., 40, 3730 (1964). (16) A. Rosenberg, Ph.D. Thesis, Yale University, 1064. (17) J. K. Lee, E. K. C. Lee, B. Musgrave, Y . N. Tang, J. W. Root, and F. S. Rowland, Anal. Chem., 34,741 (1962). (18) J. W.Root, Ph.D. Thesis, University of Kansas, 1964. (19) J. W. Otvos, D. P. Stevenson, C . D. Wagner, and 0. Beeck, J . Amer. Chem. SOC.,73,6741 (1951). Volume 74, Number 2 January 22v1970
458
THOMAS SMAILAND F. S. ROWLAND
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HOURS EXCHANGE Figure 1. Primary tritium exchange in i-CdHsT: i-C4HS added; 0, no i-C& added.
X, 0.5%
-2.51
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2
HOURS EXCHANGE Figure 2. Primary tritium exchange in i-C&T(O) and i-CaDgT ( X ), each with 0.5% i-CaHs present.
Table I : Relative Specific Activity of i-CaHgT us. Exchange Time in Concentrated HzS04 -Relative
Time, hr
2.00 5.00 10.3 20.0 30.0
specific act.,------= 100 a t zero time i-C4HIo f 0.5%C& i-C4HlO Only
10.8 9.56 9.81 9.37 8.62
rt: 0.18 rt: 0.18 It 0.17 It 0.14 Az 0.14
31.2 9.93 9.43 9.54 9.21
& 0.27 f 0.18 32 0.18 f 0.18 & 0.17
Exchange of Tertiary Hydrogen Atoms. Study of the long term behavior of (CH&CT in sulfuric acid revealed a slow exchange of the tertiary tritium with the acid. Without added isobutene, the tertiary exchange reaction appeared to have an induction period under our experimental conditions of approximately 20 hr. The addition of 0.5% isobutene removed the induction period, and accelerated the rate of tertiary exchange. The first set of experiments, involving added isobutene in each case, has therefore been corrected for the loss of tertiary tritium activity which occurred during the 10-12 hr necessary for complete removal of the primary tritium atoms. These corrections ranged as large as 10% for the estimate of tritium activity remaining after complete primary exchange. The second set of experiments was conducted after the detailed measurements of the rate of tertiary exchange, and required no correction since the primary exchange was completed while tertiary exchange was still negligible (
