3201
RECOIL REACTION PRODUCTS OF CARBON-11 IN C5HYDROCARBONS and the corresponding assigned a value of 350 entropy contribution is 3.94 eu. The entropy change 3N--8
in this mode is therefore -4.8 eu. 3N-7
ASintt
Sit -
= i
Xi is the residual intrinsic vibrational entropy z
change due to all the other modes. If all the other frequency changes are neglected, with the exception of the CF stretch (1100 cm-l) which is taken as the reac-
tion coordinate in the transition state, then ASintt = -1.8 eu. Thus, with a reaction path degeneracy of g = 4 the net change in entropy is -3.8 eu. From this we obtain log A (sec-l) = 13.1, which shows good agreement with the observed value of 13.3. Acknowledgment. We thank Dr. R. F. Hein of E. I. du Pont de h’emours Co. for a sample of 1,1,2,2-CzHd’.t. (45) S. W. Benson, private communication.
Recoil Reaction Products of Carbon-11 in C, Hydrocarbons1 by G. L. Jewett and A. F. Voigt* Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa (Receiued J u n e 14, 1971)
60010
Publication costs assisted by A m e s Laboratory, U . S. Atomic Energy Commission
Gaseous and liquid radioactive products up to Cg compounds produced by the 12C(y,n)Wreaction in liquid n-pentane, isopentane, pentene-1, cyclopentane, and cyclopentene were separated and analyzed by radio gas chromatography. Results were interpreted in terms of insertion reactions by the carbon atom and methylene radical, and the relative probabilities of fragmentation and stabilization reactioiis following their insertion in the parent molecules were determined. Experiments have been conducted to determine the yields of gaseous and liquid products from the reactions of recoil llC with the liquid hydrocarbons, n-pentane, isopentane, pentene-1, cyclopentane, and cyclopentene. The yields of the gaseous products were reported in earlier paper^;^^^ those of the higher boiling products have been presented4 but not published.
Experimental Section The technique used has been described516and further details are a ~ a i l a b l e . ~Briefly, small liquid samples, 0.2 ml, in glass bulbs were irradiated with 70-3IeV bremsstrahlung to cause the W ( Y , n ) W reaction. After approximately 20 miri to allow 150produced in the glass to decay, monitor counts of the bulbs were made with an NaI(T1) counter. The bulbs were broken in a stream of helium, and the products and parent compound were separated by conventional radio gas chromatography with the counting cell previously described.2 The liquids irradiated were Research grade obtained from Phillips Petroleum Go. with purities listed from 99.82 to 99.99 mol yo. They were used without further purification. I n the scavenger experiments with the aliphatic pentanes saturated solutions of iodine were used. In cyclopentane, several iodine concentrations below saturation were also used.
For the separation columns Chromosorp P was the solid support with the following liquid phases: 2-ethylhexyl acetate for the gases, tripropionin for C8 to C6 compounds, and Apiezon Oil for the highly unsaturated C6 compounds.
Results and Discussion Results given in Tables I and I1 are based on the total “C produced, following calibration techniques which have previously been describedaa Because of limitations of the chromatograph system, products with volatilities much less than that of the parent compounds were not carried through the column and detected. Thus products in which a carbene formed by carbon (1) Work was performed in the -4mes Laboratory of the U. S. Atomic Energy Commission, Contribution N o . 3036. (2) A . F. Voigt, D. E. Clark, and F. G. Mesich, “Cheniical Effects of Nuclear Transformations,” Val. I, International Atomic Energy Agency, Vienna, 1965, p 385. (3) D. E. Clark and A. F. Voigt, J . Amer. Chem. Soc., 89, 1528 (1967). (4) A. F. Voigt and G. L. Jewett, Abstracts, 152nd National Meeting of the American Chemical Society, New York, N . Y . , Sept 1966, No. R46. ( 5 ) G. F. Palino and A. F. Voigt, J. A m e r . Chem. SOC., 91, 242 (1969). (6) R. L. Williams and A. F. Voigt, J . P h y s . Chem., 73, 2538 (1969). (7) G. L. Jewett, Ph.D. Thesis, Iowa State University, Ames, Iowa, 1967.
T h e Journal of Physical Chemistry, Vol. 76, N o . 21, 1071
3202
G. L. JEWETTAND A. F. VOIGT
Table I: Products from Pentanes and Pentene 7 -
Product
Methane Ethane Ethylene Acetylene Propane Propylene Allene Methyl acetylene Butanes Butenes Butadiene n-Hexane 2-Methylpentane 3-Methylpentane 2,2-Dimethylbutane Total hexanes Total hexenes Hexadienes Hexyne-1 Unidentified product Methyl iodide a
*Pentane
n-Pentane
+ 11
6.36 3= 0.12 1.14 f 0.07 8.35 i 0.25 17.5 f 0.5 1.00 A 0.03 4.44 f 0.20 1.09 2.62 i 0.18 0.65 2Z 0.03 2.84 f 0.02
3.78 i 0.02 0.81 f 0.10 7.21 f 0.01 18.6 =k 0.1 0.27 f 0.04 3.92 & 0.02
8.8 f 0 . 3 6 . 2 f 0.1 3 . 3 f 0.4
6.7 f 0.2 4 . 2 f 0.2 2.0f0.1
18.3 14.9 f 0.5
12.9 12.0 f 0 . 3
2 . 0 7 f 0.11 0.26 i 0.04 2.22 f 0.07
Per cent yield fromIsopentane
Isopentane
6.46 =k 0.03 1.75 f 0.15 12.5 A 0.5 21.7 & 0.14 1.07 f 0.02 4.21 f 0,02 0.97 f 0.08
+ 11
3.21 & 0.03 1.17 i0.15 11.0 i 0.06 22.1 f 0.07 0.40 i 0.05 3.49 0.14
0.72 rt 0.10 1.26 =t0.07
0.61
+ 0.19
Pentene-1
1.47 f 0.03 0.35 i 0.05 4.16 f 0.08 17.4 rt 0.1 0.17 f 0.01 2.66 i 0.04 1 . 3 7 h 0,04 2.31 f 0.06 0.18 f 0.03 1,29i 0.05 2.71 f 0.06
*
9 . 0 f 0.2a 7.73i0.04 2.15 =k 0.10 17.9 12.4 & 0 . 4
7.7 0.20 5.10+0.05 1.10 zk 0.15 13.9 7.9 32 0 . 1
5.14 i 0.03 12.0 i 0.5 1.67 i 0.35 4.85 i 0.04
Includes 2,3-dimethylbutane.
Table 11: Products from Cyclopentane and Cyclopentene -Per cent yield from-----Cyclopentane Ia
,--
Product
Methane Ethane Ethylene Acetylene Propylene Allene Methylacetylene Butenes Butadiene Kexene-l Hexadienes Methylcyclopentane Methylenecyclopentane Methylcyclopen tenes Cyclohexene Bicyclo [3.1.0]hexane 1,3-Cyclohexadiene Benzene 2 Unidentified products Methyl iodide
+
Cyclopentane
7.6 f 0.4 0 , 4 3 f 0.06 3.22 2Z 0.13 16.4 0.5 1.22 & 0.22 1.59 =!= 0.08 1.15rt0.05 1.15 f 0.02 0.53 f 0.07 3.0 & 0.3 2 . 3 f 0.2 16.3 A 0.5 6 . 3 f 0.3 4 . 3 f 0.6 5.2 f0 . 1
a Yield decreases with increase in iodine concentration. iodine concentration.
1.1-0.8.
3.4-1.7. 1.2 dz 0 , 2 16.6-11. sa,* 7.0-2.2. 7.8-4. O a 4,4-2.50
2.89 32 0.08 0.14 rt 0.02 1.28 & 0.07 14.3 =I=0 . 3 0.20 =t0.01 0.86 f 0.01 0.56 & 0.02 0.23 f 0.05 1 . 7 9 2 ~0.02 3.2 i 0.2 1.28 f 0.11 6.9 f 0 . 3 2.18 f 0.09 3.7 f 0.5 3.6 A 0 . 1 2 . 0 Jr 0.2 3.5 i 0.4
6.8-8. lC May include some 1,s-hexadiene.
atom insertion into one molecule of parent reacts with a second molecule to form a large product, e.g,, CI1from Cj, could not be observed. The formation of products of this kind is a possible explanation for the rather low total recovery found for several of the compounds. The genera1 reaction scheme shown in Figure 1 applies to reactions in n-pentane, isopentane, and, with minor The Journal of Physical Chemistry, Vol. 76, N o . $1, 1971
4.4 i 0.9 0.33 f 0.02 2.39 f 0.11 17.0 =t0 . 3 0.79 f 0.07 0.75 f 0.1 I . 6-0.25"
Cyclopentene
Yield increases with increase in
changes, in cyclopentane. For the unsaturated molecules, reaction with the ir system of the double bond must also be considered. The relative contribution of alternate routes to the same product was not established in these experiments, and it is probable that mechanisms other than the ones shown also contribute. For example, although ethylene is shown only as a
RECOILREACTION PRODUCTS OF CARBON-11 IN C5HYDROCARBONS "C t RCH,
-
[RCH? "CH]-RCH
='ICH2
3203
Table I11 : Reaction Pathways, Pentanes
----__ Pathway
Pure
Yields, %--------IsopentaneScavScavPure enged enged
Reacting as C Acetylene Unsaturated C3 Unsaturated Ca Unsaturated Ce Total % Stabilization
17.5 8.2 2.8 15.0 __ 43.5 34.5
18.6 6.0 2.2 12.0 38.8 30.9
21.7 5.2 1.3 12.4 40.6 30.5
22.1 3.5 0.6 7.9 34.1 23.2
8.3
7.2
12.5
10.9
1.1 1.0 0.6 1__ 8.3 21.0 87.2
0.8 0.3 0.3 12.9 __ 14.5 90.3
1.8 1.1
1.2 0.4 0 13.9 __ 15.5 89.7
7-n-Pentane-
"CH,
ItH
Reacting as CH Ethylene
"CH,
Figure 1. Reaction pathways:
+ RCHa.
product of methyne insertion followed by cleavage, an alternate path involving carbon atom insertion, cleavage, and hydrogen abstraction is quite probable. Certain points can be established. The formation of the olefin from the preceding paraffin, e . g . , hexene-1 from n-pentane, most likely involves insertion of the bare carbon atom, followed by stabilization of the carbene.* Similarly, the most probable reaction for the formation of the hexanes from pentane is methylene i n ~ e r t i o n . ~Thus one can classify (Table 111)reactions of bare carbon atoms and methylene into fragmentation, yielding acetylene, ethane, CB,and C4 products, and stabilization, yielding Cs hydrocarbons. Products which can logically be attributed to reactions of methyne and methyl radicals are also listed in Table I11 without implying that these are the necessary pathways. For the reactions attributable to the carbon atom the percentage of products resulting from stabilization ranges from 23 to 34, while for the reactions of methylene, this percentage is 84 to 90. It is certainly not unexpected that the carbon atom carries with it more energy so that after insertion, stabilization is less likely than would be the case for methylene. It is of interest to examine the yields attributable to methylene insertion for statistical behavior since such examination has previously been made for products from 14Crecoil experiments.1° I n n-pentane the formation of the three hexanes on a purely statistical basis would give the ratio n-hexane :2-methylpentane :3methylpentane = 3:2:1. The observed ratios are 2.66: 1.88:1 for the unscavenged and 3.33:2.1: 1 for the scavenged system, probably a statistical distribution within experimental error. Wolflo reported this ratio as 3.2:2.1 : 1for 14Cresults with n-pentane. The four hexanes possible from isopentane were not completely resolved, but for the three fractions separated the expected ratio would be 3-methylpentane : (2-methylpent ane 2,3-dimet hylbut ane) :2 ,2-dime t hylbutane = 6: 5 : 1. The unscavenged system gave
+
Reacting as CHI Ethane Saturated Cs Saturated C4 Saturated Cs Total % Stabilization Reacting as CHa Methane Methyl iodide Total Total recovered
_ .
_ .
0.7 18.9 __ 22.5 84.0
_ .
3.8
6.4
... __
... __
..* __
6.4
3.8
6.4
3.2 5.4 8.6
79.2
64.3
82.0
69.1
6,4
~
the ratio 3.6 :4.2 : 1 and the scavenged system 4.6 : 7.0 : 1. The 3-methylpentane, formed by attack a t one of the two adjacent methyl groups, apparently has less than a statistical yield, possibly for steric reasons. The results from cyclopentane can also be classified as due to carbon atoms and methylene reactions (Table IV) showing definite but less striking differences. I n this case 50% of the products of carbon atom reactions result from stabilization processes. The production of ethylene can be attributed to methylene without involving hydrogen loss or pickup and it has been included in the methylene products. Since the ethylene yield is much lower here than in the aliphatic compounds, this change is not drastic. Results in parentheses were obtained if ethylene was not included as a methylene fragmentation product. A similar analysis has been made for the two compounds with double bonds (Table V). This may not be as valid a comparison as the other cases since reactions occur at the double bond as well as a t the CH bonds. It is noted that a considerably smaller fraction of the total yield is attributable to methylene reactions, and the fraction of the "C which remains unbound until it has picked up enough hydrogen to form methane has become very small. ( 8 ) G. Stocklin and A. P. Wolf, J . A m e r . Chem. Soc., 85, 229 (1963). (9) See reviews by A. P. Wolf, Advan. P h y s . Org. Chem., 2, 201 (1964); R.Wolfgang, Progr. React. Kinet., 3, 97 (1965). (10) A. P. Wolf, "Chemical Effects of Nuclear Transformations," Vol. 11, International Atomic Energy Agency, Vienna, 1961,p 3.
T h e Journal of Physical Chemistry, Vol. 7 6 , N o . $1, 1971
G. L. JEWETTAND A. F. VOIGT Table V : Reaction Pathways, Pentene and Cyclopentene
Table IV : Reaction Pathways, Cyclopentane Pathway
---Yields, Pure
%--Scavenged
--Yields,
Pathway
Reacting as C Acetylene Diunsaturated Ca, Cq Unsaturated Ce Total Stabilization
16.4 3.2 20.5 __ 40.1 51
17.0 1.6 18.6 37.2 50
Reacting as C Acetylene Diunsaturated Ca, Ca Polyunsaturated Cs Total Stabilization
Reacting as CH, Ethane Ethylenec Monounsaturated Cs, Ca Saturated Cs Total % Stabilization
0.4 3.2 2.5 19.4 25.5 76 (87)a
0.3 2.4 1.8 19.4 23.9 81 (90)s
Reacting as CH Ethylene
Reacting as CH, Methane Methyl iodide Total Total recovered
~
7.6 I _
7.6 73
~
4.4 7.3 11.7 73
See text.
In both scavenger systems in which methyl iodide was determined, isopentane and cyclopentane, its yield more than compensates for the reduction in methane yield. Thus the iodine must react in part with a
The Journal of Physical Chemistry, Vol. '76,N o . 81, 1971
Reacting as CHs Monounsaturated Ca, Ca Monounsaturated C6 Diunsaturated noncyclic C6 Total % ' Stabilization Reacting as CHa Saturated C I - C ~ Methane Total Total recovered
%-
Pentene
Cyolopentene
17.4 6.4 16.5 40.3 41
14.3 3.2 9.0 26.5 34
4.2
1.3
4.1 5.0
0.2 14.3 3.3 17.8 99
__
9.1 55 0.7 1.5 ___
2.9 -
2.2
2.9
56
49
precursor to the methyl radical. Other iodinated products such as methylene iodide would not have been detected, which may help to account for the lower total recovery in the scavenged systems.
