14.S. B. MUNSON
3966
Effects of Structure on the Reactions of Hydrocarbon Ions
by M. S . B. Munson Esso Research and Engineering Company, Baytown Research and Development Division, Baytown, Tezas (Received M a y 16, 1967)
Mass spectrometric experiments on butane and isobutane a t pressures as high as 1.4 torr confirm the previously reported hydride-transfer reactions and the low reactivity of the butyl ions. Formation of C3H7+is also observed possibly by a methide-transfer reaction. Only very small differences are observed in the rate constants for reactions of several ions with butane and isobutane. From studies on mixtures of the butanes and butenes, it was possible to observe differences in reactivities between i-C4Hs+ and 1- or 2-C4H8+. The relative concentration of C3H3+ exhibits a peculiar pressure dependence which suggests that two types of this ion are also formed.
There has been a continued effort in these laboratories to make a systematic study of the reactions of hydrocarbon ions. The present paper is concerned with reactions of ions with the butanes and with reactions of the isomeric butene ions. A previous study on the reactions of ions in butane has been reported from these laboratories.' The butane and isobutane systems and mixtures of each of these compounds with olefins have been investigated to confirm our previous findings and extend the pressures of the experiments to about 1 torr, to check for differences in the reactivities of ions with the isomeric butanes, and to check for differences in reactivities of isomeric ions.
Experimental Section The instrument has been adequately described previously.2 The n-butane and isobutane used in these experiments were Phillips Research grade hydrocarbons (stated purity 99.95+ mole yo)for which no successful further purificat,ion was achieved. The purity of the other hydrocarbons was 99 mole % or better. The electron energy was approximately 800 v; the source temperature was 200 f lo", and the repeller was 5 v, field strength of 12.5 v/cm. Results and Discussion Reactions of Ions with Isomeric Butanes. Experiments were done on butane and mixtures of butane with a few per cent of other compounds: propylene, 1-butene, isobutylene, 1-pentene (6.4%), and 2,2,4trimethylpentane (3.5%). Experiments were also performed on isobutane and similar mixtures of isobutane The Journal of Phyeical Chemistrv
with these hydrocarbons. Let us first consider the reactions in the butanes. The major process is hydride transfer as indicated by the predominance of C4H9+ (predominantly sec-C4HB+).a C4H9+reacts slowly, if at all, with butane since the concentration of C4Ha+is substantially constant for pressures of 0.5-1.2 torr. Very similar results were noted for isobutane, the major difference being that 0.92 of the total ionization was present as C4H9+ (predominantly t-C4H9+)3in isobutane and 0.76 in butane. Figure 1 shows plots of the pressure dependence of the logarithms of the relative ionic concentrations for CzHs+and C8H7+in butane and in mixtures of butane with the olefins. Since the rate constants for reactions of the primary ions (those produced from direct ionization by the electrons) with the two butanes are large, the presence of a few per cent of added material should not change the rate constants for reaction of these primary ions. The points of different shapes indicate experiments on the various mixtures and it is apparent that the rates of reaction are the same in all of the mixtures. The linear plot in Figure 1 for the decrease in the (1) M. 5. B. Munson, J. L. Franklin, and F. H. Field, J.Phys. Chem., 6 8 , 3098 (1964).
(2) M. 5. B. Munson and F. H. Field, J. Am. Chem. Soe, 88, 2621
(1966). (3) (a) P. G. Ausloos, 8. G. Lias, and A. A. Scala, Advances in Chemistry Series, No. 58, American Chemical Society, Washington, D. C., 1966,pp 264-277; (b) M. 5. B. Munson, J. Am. Chem. SOC., 89, 1772 (1967).
EFFECTSOF STRUCTURE ON THE REACTIONS OF HYDROCARBON IONS
I
' O 1
'
I
'
I
'
I
'
1
3967
at pressures of 0.1-0.2 torr it is likely that the primary ions react before they attain that energy. However, the amount of CH3- transfer (or dissociative proton transfer) with CzH3+ was shown to increase relative to H- transfer when the ion energy was increased from quasi-thermal t,o only 1 v ; ~consequently, the reaction forming C3H7+may be dissociative proton transfer assisted by the translational energy of the Table I lists the rate constants for reaction of some of the primary ions formed by electron impact with the isomeric butanes. The rate constants are calcuTable I: Rate Constants for Reactions of Ions with Butanes k, 10-10 cc/molecule sec----n-GHiv-----c----i-CaH~LiteraThese ture data
7 7 -
Ion
> (GH,+) and the kinetics should be pseudo first order. I is the ion current, assumed to be proportional to ion concentration for all ionic species, k is the rate constant, parentheses indicate concentration, and t is the residence time of the ion calculated from simple electrostatics. On the other hand, the curve for the logarithm of the relative ion current for C3H7+as a function of pressure is sufficiently different in shape from the curve for C2H5+to correspond to a more complicated process. This curve for C3H7+indicates that it is being formed and consumed by ionic reactions with butane. The reaction may be a methide-transfer reaction for CI or C2 ions
R+
These data
(4)
or perhaps H2 transfer to C3Hj+. Similar behavior is observed for C3H7+ion isobutane, so similar reactions occur. The possibility of a methide-transfer reaction was suggested for p r ~ p a n ebut , ~ subsequent experiments attributed the products to other reactions.6 The maximum ion energy in these experiments is 2.5 v, but
lated from the slopes of the plots for the logarithms of relative ion current us. pressure for pressures of 0.030.2 torr, generally. Since the pressure measurement is direct and more accurate in these experiments, the present set of rate constants is to be preferred over the earlier values reported from these laboratories. The rates of reaction of CaH7+, C4HB+,and C4H9+with the butanes were calculated from the data at pressures above a few tenths of a torr and are less reliable. The upper limits indicated in Table I are obtained from the constancy of ionic concentrations at high pressures and the absence of higher molecular weight products. The structures of these primary ions are not known. However, since the primary ions are produced by 800-v electrons, it is reasonable to suppose that isomerization to the most stable structures is rapid. Even if the propyl ions which are produced by ion-molecule reactions in these isomeric butanes are formed with initially different structures, there is some evidence that isomerization of primary to secondary ions by (4) K. R. Ryan and J. H. Futrell, J. C'hem. Phys., 42, 819 (1965). (5) L. I. Bone and J. H. Futrell, ibid., 46, 4084 (1967).
Volume 7 1 , Number 12 JVoscmber 1967
XI. S. B. MUNSON
3968
H- shifts is very rapid even when compared to the times of these experiments.6 The butyl ions have been shown to have different structures3 and later in this paper differences in reactivities of the butene ions will be interpreted to show that these ions have different structures. Let us consider possible differences in the rate constants for the reactions of this set of ions with the two isomeric butanes. Only the butenes, which are expected to have different structures, show an appreciable difference in rate constants for reaction with these two isomers. There does, however, appear to be a small difference: the ions may react slightly more rapidly with isobutane than with n-butane. The average value of the ratio of the rate constants is 1.2. Sieck and Futrell’ find rate constants for reaction of C3Ha+ with butane and isobutane which are equal within experimental error (estimated at f10%). I n their experiments the residence times of the ions are more precisely determined than in the present experiments, but the pressure measurements are probably less reliable than the present ones. The two sets of data are not contradictory and they do indicate that the effect of this structural variation is small, if real. Borkowski and A4us100s,8 however, observed in radiation chemistry experiments that the ratio of rate constants Sor reaction of C3H7+with isobutane and C3H7+with n-butane was 0.67 in contrast to the value of 1.0 f 0.1 from the present set of experiments. There is no ready explanation for this discrepancy. One may expect with confidence that the C1 and Cz ions can abstract primary, secondary, and tertiary hydrogens from the isomeric butanes. ConsequentIy, the rate constants for reactions of the Cz ions with the butanes should be nearly the same because the polarizabilities of the butane isomers are essentially equaL9 For C3H6+,Hz- transfer from either butane and Htransfer from the tertiary carbon atom of isobutane are exothermic and H- transfer from a secondary carbon atom is about thermoneutral.10 Since there are energetically similar reactions for both butanes, one would not expect appreciable differences in rate constants. However, if the propyl ions have the secondary structure, as appears to be the case from radiation studies,8 then only one hydrogen is available for H- transfer in isobutane compared with four in n-butane. More detailed studies are necessary to decide whether this apparent preference for a tertiary hydrogen is real. Reactions of Isomeric Butene Ions. Figure 2 shows plots of the concentrations of butene ions (as fractions of the total ionization) as functions of pressure for butane and mixtures of butane with several olefins. The nonreactivity of the butene ions with butane is The Journal of Physical Chemistry
p (N
Figure 2.
.t
additive), torr
Relative concentrations of butene ions.
shown by the constancy of the concentration of C4Hs+ for pressures above about 0.5 torr. It will be remembered from the data in Table I that the analogous ion in isobutane did indicate a rapid reaction. C4&+ is formed as a product ion in butane by Hz- transfer reactions. The data of Sieck and Futrell’ on Hztransfer reactions from partially deuterated n-butane to C3He+ indicate that the ratio 1-C4H8+/2-C4Hs+ should be about 0.7. Consequently, this nonreactivity of butene ions with butane suggests that neither 1-C4Hs+nor 2-C4Hs+ reacts with butane. In the mixture of n-butane and 6.5% propylene, there are notably more butene ions produced than in butane alone. These ions are probably produced by both Hz- transfer reactions’~~ GHa+
+ n-C4Hio
+C3He
-
+ C4Hs+
(5)
+ Ci”
(6)
and by Hz transfer reaction1l,l2
+
C ~ H I O + C3He
C4&+
The small decrease in relative concentration of C4H8+ in this mixture of butane and propylene for pressures above 0.8 torr is equal to the small amount of GH14+ produced by a collision-stabilized addition of butene ions to propylene (6) P. S. Skell and R. J. Maxwell, J . A m . Chem. SOC., 84, 3963 (1962). (7) See footnote a of Table I. (8) R. P. Borkowski and P. Ausloos, J . Chem. Phya., 40, 1128 (1964).
(9) H.H. Landolt and R. Bornstein, ”Zahlenwerte und Functionen,” Part 3, 6th ed, Springer-Verlag, Berlin, 1950. (10) Heats of formation of ions are taken from F. H. Field and J. L. Franklin, “Electron Impact Phenomena,” Academic Press Inc ., New York, N . Y.,1957, Table 45. (11) P. Ausloos and 9. G. Lias, J. Chem. Phys., 43, 127 (1965). (12) F. P. Abramson and J. H. Futrell, J . Phya. Chem., 71, 1233 (1967).
EFFECTS OF STRUCTURE ON
C4Hs+
THE
-
C3He
(C7H14+)*+ C4H10
REACTIONS OF HYDROCARBON IONS
(C?H14+)*
(7)
C7H14f+ C4H10*
Collision-stabilized addition of olefin ions to olefins was reported previously for eth~1ene.l~ The butene ions produced in this mixture of butane and propylene do not react with butane either. These observations support the previous statement that 1-C4H8+and 2-C2Hs+do not react with butane. The greater abundance of C4H8+ions in the mixtures of n-butane with the butenes is attributable primarily to charge-exchange reactions with these additives. Only the amount indicated by the zero pressure intercept can be attributed to direct ionization. The butene ions produced in the mixture with 2butene do not react rapidly with 2-butene and do not react a t all with n-butane. The decrease in relative concentration of C4&+ ions in the mixture for pressures above 0.5 torr can be attributed in a large part to a collision-stabilized reaction with butene, analogous to (7) and (8)
+ t-2-CdH8 (GHls+)* + C4HlO C4H8+
(C8H16+)* C4HlO*
+
+ C8Hl6'
(9)
(10)
I n addition, second-order products have been reported previously from reaction of butene ions with butene: C4H7+and C4H9+,14and C6Hg+.1491bThere is no evidence for any reaction of the 2-butene ions with butane. These collision-stabilized addition reactions are observed a t pressures of several tenths of a torr in the present experiments and could not be observed a t the lower pressures of the other experiments (of the order of 0.01 torr), It is apparent from Figure 2 that 1-C4H8+ reacts more rapidly in the mixture with l-butene than 2-C4Hsf does in the mixture with 2-C4H8. Only about one-half of the loss of C4Hs+can be explained by the formation of C8H16+ through reactions analogous to (10). However, it is known that second-order reactions of 1-C4H8+ with 1-C4H8are about four times as fast as second-order reactions of 2-C4Hs+ with 2-C4H8.14,15 Consequently, it is not possible to obtain an accurate balance on C4H8+ to decide whether or not reaction occurs with n-butane. The isobutylene ion reacts even more rapidly in the mixture of n-butane and isobutylene than does the I-butene ion in the mixture of l-butene and n-butane by a factor of approximately 3. Only about one-fourth of the decrease in concentration of C4H8+ above 0.2 torr can be accounted for by the formation of CSH16+ from collision-stabilized addition of olefin ions to olefins. I n addition, it is known that the cross section for second-order reactions of i-C4Hsf with i-C4H8 is about
3969
one-half the cross section for second-order reactions of 1-C4Hs+ with 1-C4H8.l6 Consequently, i-C4H8+ must react with n-C4H1,, in all likelihood by H transfer i-C4Hs+
+ n-GHlo
--f
t-C4H9+
+ sec-C4Hg (11)
for which the heat of reaction is zero or slightly negative. H- transfer to give sec-C4Hg+ and t-C4H9 is endothermic and less likely to occur. This difference in reactivity between the isobutylene ion and the 1- and %butene ions with butane suggests a difference in structure and a slow rate of isomerization. The discussion of distinguishable 1- and 2-butene ions is based primarily on the slightly different heats of formation of these ions and the present results provide no information about possible differences between them. As mentioned previously in this paper, the butene ions produced in isobutane react rapidly with isobutane. The reaction is i-C4Hs+
+ i-C4H10
---j.
t-C4H9+
+ t-C4Hg
(12)
since there is no product other than C4H9+ of sufficient concentration to account for the decrease in the concentration of C4H8+. The rate constants for reaction of the butene ions are essentially the same in isobutane and in the mixtures of isobutane with the additives, except for the mixtures with l-butene and 2-butene. Small amounts of collision-stabilized olefin addition products were observed. The fact that the rate constant for the disappearance of the ions of m/e 56 in the mixture of isobutane and isobutylene is the same as that in isobutane supports the idea that charge exchange with isobutylene and H2- transfer with isobutane produce the same butylene ions, i-C4H8+. The rate constant for the disappearance of m/e 56 in the mixture of isobutane and l-butene is 1.4 X cc/molecule sec (assuming that the reactant is isobutane). This value is lower than that given in Table I by an amount which is probably greater than the experimental error and suggests that some of the butene ions have different reactivities (and possibly different structures). The rate constant for the disappearance of ions of m/e 56 in the mixture of isobutane and 2-butene is 0.34 X 10-*0 cc/molecule sec (assuming the neutral reactant to be isobutane). This difference of a factor of 7 is well beyond experimental error and shows clearly that the butene ions from 2-butene are different from the butene ions from isobutane. The reactions analogous to (12) (13) F. H. Field,
J.
Am. Chem. sot.,
1523 (1961).
K ~ J. C h~m . phus.. ~ 45. 706 ~ (1966). ~ . (15) R. Fuohs, 2. Naturforsch., lba, io26 (1961). (14) I.
Volume 7 1 , Number 1.9 hTol;ember1967
M. S. B. MUNSON
3970
rX-+ 2-C4H8+
+ i-C4Hlo{
I
lX+
+ sec-C4Hp
t-C4H9+
(13a)
..os1
I
I
I
1
I
I
I
Q&4%
sec-C4He+
A 6.4% .).
I
I
I
I
I
I
I
1
I
_.
06.
+ t-C4Ho (13b)
are both endothermic and the reactions should be less favored than (12). These observations suggest that the isobutylene and 2-butene ions have the different structures that one would intuitively assume and that the rate of interconversion is slow. Because of the already established differences in reactivities of l-C4Hs+ with 1-C4H8 and 2-C4H8+with 2-C4Hs, it is not possible to be certain that these results indicate any difference between these two ions. Reactivities of C3H3+. I n these butane systems there is another indication of isomeric ions of different reactivities-the very unusual pressure plot of the relative concentration of CaH3+shown in Figure 3. This initial steep decrease to an essentially constant value is unique among the ions in these mixtures. This behavior suggests strongly that there are two types of C3H3+ions formed in butane-one which is as reactive as the majority of the other ions and another which is essentially unreactive. Equally surprising is the nonreactivity with olefins and the lack of hydride transfer of the tertiary hydrogen in 2,2,4-trimethylpentane. The only compound with which reaction occurs is isobutylene, but it is not possible to identify the products. Essentially the same results are observed for isobutane, an initial steep decrease to an essentially constant value. The final concentration of C3H3+ in isobutane is slightly higher than in n-butane, 0.03 and 0.02. This difference may represent a difference in the ratios of the two forms of C3H3+produced in the two gases. These data may be taken as support for the suggestion that there is a C3H3+ion of energy lower than the ac-
The Journal of Phycrieal Chemistry
I
I
. .. 0 6.4% A
$Ol 0
1
I
I
I
I
40
20
I
Ea
I
P(tatall, torr
I
I
80
to0
IN
Figure 3. Relative concentration of C ~ H+I as a function of pressure in n-CdHlo and certain mixtures with n-C~Hlo.
cepted value of 270 kcal/mole.16 It is not possible in the present series of experiments to establish what are the reactions of CaH3+. However, possible reactions are
-r
C3H5+
C3H3+
+ n-CdHx
4C4H9+
+ C4H8
(14a)
+ C3H4
(14b)
Similar reactions have been reported for C3H3+ ions with propane.6 Both reactions are exothermic if the heat of formation of C3Ha+is 270 kcal/mole. If there is a nonreactive form of C3H3+ and reaction 14 is prohibited for energetic reasons, then perhaps the heat of formation of this lower energy species is less than 250-255 kcal/mole. This suggestion is admittedly highly speculative, but perhaps it will lead to more definitive experiments. Acknowledgments. The author is grateful to Drs. J. L. Franklin and F. H. Field and to the reviewer for helpful comments about this paper. ~~
~
~~
~
(16)G. B. Kistiakowsky and J. V. Michael, J. Chem. Phye., 40, 1447 (1964).
