2538
RONALD L. WILLIAMS AKD ADOLFF. VOIGT
Recoil Reaction Products of Carbon-11 in Simple Aromatic Compounds1 by Ronald L. Williams and Adolf F. Voigt Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 60010 (Received September $0, 1068)
Carbon-11 was produced by the reaction lzC(y,n)llCin liquid benzene, toluene, and p-xylene, and the labeled products were analyzed by radiogas chromatography. A wide variety of products from methane to phenylacetylene was observed accounting for 20-35% of the total llC activity in the various systems. The remainder of the activity is shown to reside in higher boiling compounds. The production of analogous products in these systems is discussed, and the effect of the methyl groups on the product distribution is evaluated.
Introduction The reactions of atomic carbon in benzene have been investigated in a number of laboratories in recent years with a variety of reported These studies have included all three phase^,^'^ the effect of several scavenger^,^ and double labeling techniques.6 I n these results a limited number of low yield products were identified, significant polymerization was indicated, and a variety of yield values was reported. Some interesting hypotheses have been proposed, but the data have not been sufficient to decide among them. The present work has extended the scope of liquid aromatic systems to include toluene and p-xylene which provide interesting analogies with the benzene system.6 Yield values have been determined for an extended number of products and are discussed briefly by comp aring yield values of analogous products presumably produced by similar reaction routes. An estimate of polymerization yields determined by distillation techniques indicates the complexity of extended product analysis.
Experimental Section Liquid samples prepared by vacuum techniques were irradiated in the 70-MeV Iowa State University electron synchrotron, and the products resulting from the process IZC(y,n)IIC were analyzed by radiogas chromatography. Details of the experimental techniques have been discussed p r e v i o u ~ l y . ~Product ~~ identity was determined by the retention time of carrier compound injected onto the chromatograph column with the irradiated sample. For most of the products reported the identifications were confirmed by similar techniques on one or more additional columns with different retention properties. No changes in yield values were observed with carriers indicating complete elution of all the compounds reported. Phillips research grade benzene (99.94 mol yopurity), toluene (99.97 mol Yo purity), and p-xylene (99.90 mol yo purity) mere used without further purification. Eastman Organic Chemicals 2,2’-diphenyl-l-picrylhydrazyl (DPPI-I) was recrystallized from chloroformThe Journal of Physical Chemistry
diethyl ether mixture and used as a free-radical scavenger at a concentration of 0.001 mole fraction. To estimate the amount of IT-containing “polymer” formed, irradiated samples in their 0.2-ml bulbs were heated to various temperatures in a constant temperature bath and held for 5.0 min. The sample bulbs were removed from the bath and weighed, and the residual activity was counted and compared with the original, total activity in standard geometry above a 7.5 X 7.5 cm NaI(T1) scintillation detector. An additional 5.0min heat treatment at any step in the process did not change the residual activity at that temperature. I n a second experiment the bulbs were broken in the chromatograph in the usual manner and passed through a short column length (2.5 x 0.5 cm) packed with polyphenyl ether on Chromosorb P attached immediately downstream from the breaker. Empty tubing carried the more volatile portion to the gas-flow counter for the measurement of the fraction removed in each temperature interval. The 2.5-cm section was removed and counted. The yield values for the parent hydrocarbons were (1) Work was performed in the Ames Laboratory of the U. 8. Atomic Energy Commission; this is Contribution No. 2421. (2) (a) A. G. Schrodt and W. F. Libby, J . Amer. Chem. SOC.,78, 1267 (1956): (b) J. L. Sprung, S. Winstein, and W. F. Libby, ibid., 87,1812 (1965). (3) A. P.Wolf, R. C. Anderson, and C. 5. Redvanly, Nature, 176, 831 (1956). (4) B. Suryanarayana and A. P. Wolf, J . Phys. Chem., 62, 1369 (1958). (5) (a) A. P.Wolf, Ann. Rev. Nucl. Sci., 10, 259 (1960); (b) A. P. Wolf, “Chemical Effects of Nuclear Transformations,” Vol. 11, International Atomic Energy Agency, Vienna, 1961,p 3. (6) (a) E. P. Rack, C. E. Lang, and A. F. Voigt, J . Chem. Phys., 38, 1211 (1963); (b) D.E. Clark and A. F. Voigt, J . Amer. Chsm. SOC., 87,5558 (1965). (7) (a) R. T. Mullen, “The Chemical Interactions of Accelerated Carbon-14 Ions with Benzene,” Ph.D. Thesis, University of Callfornia, Berkeley, Calif., 1961; (b) R. M. Lemmon in “Chemical Effects of Nuclear Transformations,” Vol. 11, International Atomic Energy Agency, Vienna, 1961, p 27; (e) H. Pohlit, T. H. Lin, W. Erwin, and R. M. Lemmon, Abstracts, 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968, No. 0 125. (8) T. Rose, C. MacKay, and R. Wolfgang, J . Amer. Chem. Soc., 89, 1529 (1967). (9) G.F.Palino and A. F. Voigt, ibid.,91, 242 (1969).
RECOIL REACTION PRODUCTS OF " C
IN
2539
SIMPLEAROMATICS
Table I: Product Yields in Aromatic Systems" Benzene
Productb
Toluene
+ DPPH
Benzene
Methane 0.18 f 0.03 Ethylene Acetylene 4.68 4 0.14 Allene 0.14 f 0.04 Methylacetylene 0.16 f 0.05 (Vinylacetylene) 0.55 f 0.05 P-1 ;C T-1 (valylene) P-2; T-2( 1-penten-3-yne) (Diacetylene) 1.58 f 0.30 P-3(1,3-heptadiene-5-yne) Benzene 3.54 f 0.22 B-1 0.21 zt 0.04 B-2 0.28 f 0.03 Toluene 2.64 f 0.14 Cycloheptatriene 3.19 f 0.33 T-3 (3-methylcycloheptatriene) T-4(2-methylcycloheptatriene) T-5 (1-methylcycloheptatriene) 0.36 i 0.06 p-Xylene P-4(1,4- and 2,5-dimethylcy cloheptatriene ) 0.25 f 0.03 m-Xylene 0.15 f 0.04 o-Xylene 0.22 & 0.05 Ethylbenzene 0.53 4 0.13 Styrene 1.90 2s 0.26 Phenylacetylene 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene 0.30 f 0.07 B-3 p-Ethyltoluene p-Meth ylstyrene P-5; T-B(methylpheny1acetylenes) P-6 (2,S-dimethyls tyrene ) P-7 (2,S-dimethylphenylacetylene) 20.86 Total identified
0.43 i 0.06 4.59 0.13 0.14 0.62
f 0.24 4 0.03 f 0.05 2s 0.10
1.97 f 0.16 2.55 f 0.10 0.35 f 0.07 0.26 f 0.14 2.72 f 0.15 2.13 f 0.21
0.52 f 0.14
0.29 4 0.04 0.20 f 0.02 0.24 4 0.08 0.36 f. 0.09 2.88 4 0.15
p-Xylene
p-Xylene
+ DPPH
0.88 f 0.06 1.27 f 0.14 5.95 f 0.28 0.29 i: 0.08 0.86 f 0.20 0.50 f 0.08 0.14 =k 0.02 0.20 f 0.03 0.65 f 0.05 0.20 f 0.03
1.04 i 0.08 1.16 f 0.09 5.68 f 0.21 0.22 f 0.02 0.69 f 0.05 0.53 4 0.07 0.16 f 0.02 0.19 f 0.02 0.82 f 0.06 0.22 f 0.04
2.31 i 0.12
1.11 f 0.17
0.84 f 0.07
f 0.03 f 0.02 f 0.06 f 0.03
0.17 f 0.02 0.22 f 0.05 0.34 zt 0.06 0.76 f 0.05
2.40 f 0.18
2.06 & 0.10
0.97 zt 0.18
0.95 2s 0.13
1 . 0 5 f 0.04 1.15 f 0.14 3.65 i 0.15 3.77 f 0.18 0.94 zt 0.10 0.16 f t . 0 . 0 4 0.09 f 0.02
0.94 f 0.05 0.99 f 0.09 3.37 f 0.21 4.24 f 0.16 1.24 i 0.08 0.13 f 0.04 0.16 f 0.02
0.63 i 0.18
0.46 i 0.09
1.01 f 0.13 0.57 f 0.07 2.13 f 0.24
1.28 f 0.14 0.68 f 0.10 2.36 f 0.22
0.20 4 0.10d 0.50 f 0.06d
0.03 f 0.04d 0.30 f 0.03d
5.09 f 0.21 7.02 f 0.24
5.23 f 0.35 7.91 f 0.19
1.10 f 0.10
1.59 f 0.11
1 . 1 8 f 0.11 0.39 i 0.10
1.67 f 0.19 0.47 4 0.11
0.71 & 0.07 34.15
1.16 zt 0.16 35.78
Toluene
+ DPPH
0.49 f 0.03 0.65 i 0.06 5.41 i 0.24 0.15 f 0.04 0.55 f 0.07 0.46 f 0.07 0.08 f 0.02 0.10 f 0.03 0.67 i 0.13
0.51 f 0.02 0.64 2s 0.06 5.32 f 0.20 0.12 f 0.02 0.50 =k 0.05 0.61 f 0.04 0.09 i 0.02 0.11 f 0.03 1.25 f 0.06
0.59 f 0.13
0.48 f 0.06
2.70 f 0.16 0.11 0.14 0.29 0.77
0.29 f 0.06
20.97
25.77
26.69
'
" Error limits aire standard deviations. Products are reported in the order of elution from one of the columns used: 15 ft X 8 mm glass column of 20% Igepal C0880 on Anakrom solid support. This notation designates unidentified product in the various systems, e.g., P-1 = unidentified product number 1 in p-xylene system. Identities proposed on the basis of retention times are given in parentheThese yields include ortho and meta as well as para compounds, ses. determined by the usual gas-flow detection system and verified by collecting and counting various fractions of the parent effluent in a standard geometry.
Results Yield values in absolute per cent of the total carbon-11 activity produced are listed in Table I. Each system was studied over a range of total doseg (0.015--0.090 eV/molecule), and each product yield was examined for dose dependence. I n none of the observed products was the slope of a least-squares straight line through the yield values greater than the uncertainty in the slope. Therefore the reported yield values are the averages and standard deviations of the yield values over the entire dose range studied. The number of data points used in this analysis was from 10 to 20 for the various products
in the several systems. The distribution of error limits indicates variable dose dependence as well as experimental scatter. The results of scavenger studies using 0.001 mole fraction DPPH in general show minor changes in yield values. The interpretation of these results as a criterion for hot us. thermal radical reactions must be qualified in view of the results reported by other workers using a variety of scavengers including DPPH.418 An evaluation of the extent of polymerization and build-up products in the benzene system is summarized in Table 11. More than 60% of the total carbon activity is retained on a 2.5-cm length of column a t 225" and is thus in compounds containing three or more benzene rings. I n the distillation experiment 32% remained in the uncrushed bulb at 226". The differVolume 78, Number 8 August 1989
2540
RONALD L. WILLIAMS AND ADOLFF. VOIGT
Table I1 : Results of Distillation Experiments A. Distillation from Sample Bulb"
T I'C
25 100 125 150 175 200 225
Activity remaining, %
94.7 60 53 39 36 33 32
Activity removed in temp inorement, %
5.3 35
7 14 3 3 1
B. Elution from 2.5-cm Column with Flow Countingb
T,O
C
25 25-100 100-225
Activity in short column, %
Not determined 78.0 61.2
Activity removed in temp interval, %
5.3 15.6 16.5
C. Summary Approximate % ' of total
Products analyzed and identified Two-ring compoundsc Others distilling to 225' (but not eluted from column in B) Residual activity a t 225'
20.8 16 28
shown in the present work are consistent with these proposals, and frequent use is made of a seven-member ring as an insertion intermediate which may fragment, stabilize, or undergo bimolecular reaction. On this basis the analogies among the three systems, benzene, toluene, and p-xylene, will be discussed with particular concern for the effect of the methyl substituents. The low yield of methane from benzene is consistent with previously discussed trends in paraffins and olefinsV6 I n the systems with methyl groups the increased hydrogen availability results in larger yields of methane consistent with the successive hydrogen abstraction route suggested by Wolf. I n the presence of radical scavenger the methane yield increases in all three systems. At present we have no explanation for this observation. Ethylene was identified in the toluene and p-xylene systems but not in benzene indicating that it is produced by reactions at the methyl sites. Comparison of the ethylene yields in the two systems shows a statistical increase with the second methyl group. One suggested reaction route for the production of acetylene from benzene is C atom insertion into a C-H bond followed by rupture of the two adjacent ring bonds (p to the llC) and pickup of a hydrogen atom.l0
35
" Average values for five determinations. A column section (2.5 X 0.5 cm) packed with polyphenyl ether on Chromosorb P was attached immediately downstream from sample breaker. This section was removed and its activity determined in a standard geometry. ' Carrier samples of these compounds were separated by mass detection, but poor resolution of the corresponding activity prohibited detailed analysis.
ence, -28%, is attributed to compounds with boiling points considerably above 225" which decompose a t this temperature or are carried by the benzene vapor. Analysis of high boiling compounds, including two-ring compounds, is presently prohibited by the maximum operating temperature of the gas-flow counting cell.g
A second type of mechanism proposes C atom insertion into the ring followed by rupture of a and p bonds, either immediately followed by H pickup by the active fragment as in eq 2a, or after a hydride shift with no H pickup being required as in eq 2b.
*
HC;=CH
@a)
H&CH
(2b)
Discussion The obvious difficulties of a mechanistic treatment of these systems include the large number of products, the small percentage of the total activity identified, and the extreme fragmentations and rearrangements required for the production of some of the products. Several reaction intermediates have been proposed in previous discussions of the benzene system. Wolf4has suggested a seven-member ring as an insertion intermediate with alternative stabilization and rearrangement routes. Wolfgang* proposed several stable adducts with rbonded configurations and discussed the energetics of fragmentation of these intermediates. The results The Journal of Physical Chemistry
The results of Wolfll using mixtures of perdeuterated as well as selectively deuterated compounds indicate that C atom insertion in these types of reactions is more (10) Intermediates in brackets are highly reactive and are not intended to be detailed structures. Since only the labeled products of these reactions have been identified, the unlabeled fragments which must accompany them are not shown. (11) A. P. Wolf, Brookhaven National Laboratory, private communication, Aug 1987.
2541
RECOILREACTION PRODUCTS OF llC IN SIMPLE AROMATICS plausible for the formation of acetylene than the insertion of methyne, "CH, into the ring followed by similar rupture processes. If the compound contains a methyl group, additional pathways are available, including insertion into a sidechain C-H bond. QfH3
+
"C
--t
[0cH2kH]
I
I1
J
I FH-C-bH-CH]
3 -H
HC&H
I11
(3)
I n the methyl derivatives reactions similar to eq 2a and b will lead to methylacetylene, which is observed in very small yield in benzene and in considerably larger yield in toluene and p-xylene. This increase in yield is most probably due to C atom insertion into the ring bond adjacent to the side chain (eq 4). For a product
of carbon atom insertion into a ring C-H bond to contain the methyl group it would have to be a four-carbon compound such as 1,2-butadiene or butyne. The relative production of acetylene by reactions 1, 2, and 3 can be estimated as follows. The data on unscavenged systems are used; those from scavenged systems give similar results. For both reactions 1 and 2 there are six locations per benzene molecule for attack by C atoms so that the probability per location is 4.68/6 = 0.78%. The probability of ring insertion into toluene to form methylacetylene can be considered to be the difference in the yield of this product in toluene and benzene (0.55 - 0.16 = 0.3970) or half this difference for p-xylene [(0.86 - 0.16)/2 = 0.35%]. The average of these values, 0.3701,, can be used as the probability per location for reaction 2 in benzene, if it is assumed that the probability of this process is not changed by the presence of the methyl groups. The probability of reaction 1 is then 0.78 - 0.37 = 0.41%. The probability of reaction 3, side-chain C-H insertion, can be estimated by subtracting the expected yields for five locations around the ring or 0.78 X 5 = 3.90% from the total acetylene yield from toluene, 5.41%. If this value, 1.51%, is attributable to three C-H bonds, the probability per bond is 0.50%. A similar calculation for p-xylene gives 0.47% as the probability per bond. Other highly unsaturated products from similar reactions are observed in small to appreciable yields. Intermediate I from C-H insertion or I1 from insertion into a ring C-C bond can fragment to give a C4H3 fragment (111)which can pick up or lose a hydrogen to give vinylacetylene or diacetylene, both of which were
CH=C-CH=CH*
CHmC-CGCH
tentatively identified on the basis of retention time. The presence of a methyl group on the ring could change intermediate I11 to a methyl derivative (see eq 6) which on adding a hydrogen atom would lead to a pentenyne such as valylene or its straight-chain isomer.
The other, less well-identified products could be expected t o have similar formation paths. The relatively high yield of labeled parent compound in aromatic systems has also been reported previo u ~ l y . * ~Proposed ~*~ reaction routes include (1) *C insertion into a ring C-C bond, hydride shift, expulsion of C atom to return to six-membered ring
8+
c
(7)
(2) *C iniiertion into a ring C-C bond, expulsion of C-H followed by pickup of H atom by the C6Hs radical
+
CH
(3) *CH insertion into ring C-C bond, expulsion of C-H
I n the methyl derivative, expulsion of a carbon atom with an attached methyl group will produce the simpler homolog, benzene from toluene, toluene from p-xylene, which leads to an interesting comparison. A probabilVolume 73, Number 8 August 1909
2542
RONALD L. WILLIAMSAND ADOLFF. VOIGT
Table I11 : Probability of Expulsion Reaction Reactant
Bz
Product
Conditions
Yield, %
No./moleoule
Probability/bond
To1 XYl
Bz To1 XYl
Unscav. Unscav. Unscav.
3.54 f 0.22 2.70 f 0.16 2.40 rt 0.18
6 5 4
0.59 f 0.04 0.54 f 0.03 0.60 =t 0.05 Av 0.58 i 0.04
To1 XYl
332
To1
Unscav. Unscav.
0.59 rt 0.13 1.11 i 0.17
1 2
0.59 + 0.13 0.55 =t0.09 Av 0.57 rt 0.11
XYl
BZ To1 XYl
Scav. Scav. Scav.
2.85 f 0.10 2.31 =t 0.12 2.06 f 0.10
6 5 4
0.47 f 0.02 0.46 f 0.02 0.52 f 0 . 0 3 Av 0.48 rt 0.03
To1 XYl
BZ To1
Scav. Scav.
0.48 =k 0.06 0.84 rt 0.07
1 2
0.48 rt 0.06 0.42 rt 0.04 Av 0.45 i 0.05
Bz To1
ity of insertion followed by expulsion can be calculated on a per bond basis by dividing the appropriate yield by the number of such possible reactions per molecule. As shown in Table 111, the probability of the reaction occurring is the same within experimental error whether the leaving group is C-H or C-CHs. The addition of a ring methyl group to the parent occurs in all cases. The most probable mechanism for this reaction is the insertion of methylene-l'C into a C-H bond. However, degradation s t ~ d i e s on ~ ~ toluene-"C produced by the action of energetic I4C atoms on benzene have shown that 12-14% of the lahel is in the ring, not in the methyl group. A different mechanism is required for the explanation of this part of the toluene yield. A possibility would be ring insertion by carbon atoms followed by collapse with the ejected carbon atom remaining attached to the ring by one bond and subsequently picking up hydrogen atoms to form toluene. It is, perhaps, incorrect to apply the results of the degradation studies to the present work since in that work much larger radiation dose is given to the sample (1-2 compared to -0.05 eV/molecule). Under such conditions products of more disruptive mechanisms might be expected. Since such mechanisms need to be invoked for only a small part of the yield, the yields for the different compounds can be compared on the basis of the simpler methylene insertion. Such a comparison is shown in Table IV along with similar data on the products of methylene insertion into a side-chain C-H. It can be seen that in both the unscavenged and scavenged systems, the probability of the reactions occurring in one of the toluene ring positions is considerably greater than for the similar reaction in benzene. It appears that there is a strong preference for para addition, with ortho and meta following in that order but not widely different. The probabilitJy for this reaction occurring The Journal of Physical Chemistry
in p-xylene is similar to that for the ortho and meta positions in toluene, considerably larger than the probability for benzene. The idea of directed attack by methylene on substituted benzene is surprising since methylene is known to be a nondiscriminating reagent and one would expect that energetic methylene would show even less discrimination. Very little has been published on the reactions of methylene with substituted benzenes. Terao and ~Shida13 ~ ~ ~ studied ' ~ the photolysis of ketene in toluene in the gas phase at a few centimeters pressure. Their results are quite different from those of the present experiments, but the conditions were also quite different. The methylene which appeared in addition products was distributed as follows: m-xylene 40010, p-xylene 25%, ethylbenzene 25%) and o-xylene -0%. The absence of o-xylene was attributed to steric hindrance. Rleemvein, et aZ.,14reported that in the photolysis of diazomethane in anisole nearly equal amounts of the 0-, m-, and p-methoxytoluenes were produced, which would indicate an approximately twofold preference for p a m insertion. The only agreement among the results of these very different experiments is that para insertion appears to be favored above the expected value of 20% of the total yield for the three isomers. It is obvious, however, that no conclusions can be drawn from these few results. Methylene additions to the side chain forming ethylbenzene from toluene and p-ethyltoluene from p-xylene are also observed in good yield. As expected, the reaction is more probable in xylene than in toluene, but not by a factor of 2. As shownin Table IV, the yield per bond is 1.22 in toluene and 0.85 in xylene. (12) R. Visser, C. S. Redvanly, F. L. J. Sixma, and A. P. Wolf, Rec. Trav. Chim., SO, 533 (1961). (13) T. Terao and S. Shida, Bull. Chem. Soc., Jap., 37, 687 (1964). (14) H, Meerwein, H. Disselnkotter, F. Rappen, H. v. Rintelen, and H. vande Vloed, Ann., 604,161 (1957).
RECOILREACTION PRODUCTS OF
IN
SIMPLEAROMATICS
2543
Table IV : Methylene Additions to Aromatic Hydrocarbons Reactant
Bz To1 To1 To1 To1 P-XYl To1 P-XYl Bz To1 To1 To1 To1 P-XYl To1 P-XYl
Product
Conditions
Yield, %
Bonds/ molecule
Probability/ C-H bond
To1 P-XYl m-Xyl 0-Xyl ZXyl's 1,2,4-Trnb E t be p-Et to1 To1 P-XYl m-Xyl 0-Xyl ZXyl's 1,2,4-Tmb E t bz p-Et to1
Unscav. Unscav. Unscav. Unscav. Unscav. Unscav. Unscav. Unscav. Scav. Scav. Scav. Scav. Scav. Scav. Scav. Scav.
2.70 i 0.16 0.77 f 0.03 1.05 f 0.04 1.15 f 0.14 2.97 i 0.15 2.13 f 0.24 3.65 f 0.15 5.09 + 0.21 2.31 f 0.12 0.76 f 0.05 0.94 f 0.05 0.99 zf 0.09 2.69 f 0.11 2.36 =t0.22 3.37 f 0.21 5.23 i 0.35
6 1 2 2 5 4 3 6 6 1 2 2 5 4 3 6
0.45 f 0.03 0.77 i 0.03 0.52 f 0.02 0.57 f 0.07 0.59 zf 0.03 0.53 f 0.06 1.22 f 0.05 0.85 f 0.04 0.38 f 0.02 0.76 i 0.05 0.47 i 0.03 0.50 f 0.05 0.54 f 0.02 0.59 f 0.06 1.12 i 0.07 0.87 zf 0.06
Table V : Cz Additions t o Aromatic Hydrocarbons Reactant
Bz To1 XYl Bz To1 XYl
Bz To1 XYl Bz To1 XYl Bz To1 Bz To1 a
Product
Phac (Me phac (Di me phac Phac (Me phac (Di me phac Sty Me sty (Di me sty Sty Me sty (Di me sty E t bz E t to1 E t bz E t to1
Conditions
Unscav. Unscav. Unscav. Scav. Scav. Scav. Unscav. Unscav. Unscav. Scav. Scav. Scav. Unscav. Unscav. Scav. Scav.
Yield,
1.90 0.26 1.10 i 0.10 0.71 f 0.07 2.88 f 0.15 1 . 5 9 f 0.11 1.16 f 0.16 0.53 i 0.13 0.50 f 0.06 0.39 i 0.10 0.36 f 0.09 0.30 f 0.03 0.47 i 0.11 0.22 =t0.05 0.20 i: 0.10 0.24 f 0.08 0.30 zf 0.04
Bonds/ moleoule
Probability/ C-H bond
6 5 4 6 5
0.32 f 0.04 0.22 f 0.02)" 0.18 zf 0.02) 0.48 f 0.03 0.32 i 0.02) 0.29 f 0.04) 0.09 i 0.02 0.10 f 0.01 0.10 i 0.03) 0.06 f 0.02 0.06 f 0.01 0.12 f 0.03) 0.04 f 0.01 0.04 i 0.02 0.04 f 0.01 0.06 f 0.01
4 6 5 4 6 5 4 6 5 6 5
Products in parentheses only tentatively identified; see Table I (footnote c ) .
The reaction by which styrene is produced from toluene and p-methylstyrene from p-xylene is most likely C atom insertion into the C-H bond followed by stabilization as the olefin. Such reactions have been observed in many aliphatic systems. I n this case the yield from xylene is close to twice the yield from toluene. On a per bond basis the yield of styrene from toluene is 1.25 (unscavenged) and 1.41 (scavenged) while the comparable values for methylstyrene from xylene are 1.17 and 1.32. The formation of phenylacetylene from toluene (and of methylphenylacetylene from xylene) by C atom insertion in a side-chain C-H bond requires that two hydrogen atoms be lost, which appears to be a rather unfavorable process. To our knowledge the comparable aliphatic reaction
R-CHs
+ C*
[RCH2--C*-H]
+ RC=C*H
+ 2H
has only been reported15 in the case of methane (R = H) which is not quite comparable.ls However, in the aromatic systems the yield of this reaction (R = CaHe, P-CH~C~H is ~appreciable, ) 1-201,) and this mechanism may account for part of that yield. Another mechanism is discussed below. The addition of labeled CZH, fragments to the aromatic ring results in a group of products which are (15) (a) C. Mackay and R. Wolfgang, J. Amer. Chem. Soc., 83,
2399 (1961);(b) G.Stooklin and A. P. Wolf, ibid., 85,229 (1963).
(16) NOTEADDED IN PROOF.We have recently determined yields of similar products from aliphatic molecules, hexyne-1 (2.3%) from n-pentane and heptyne-1 (2.0%) from n-hexane.
Volume 73, Number 8 August 1969
2544 observed in each of these systems (see Table V). The only one of these which has been reported previously is phenylacetylene from b e n ~ e n e . ~I n recently reported studies on phenylacetylene from the action of accelerated 14C atoms on solid benzene,7c it was shown that -96% of the label is in the side chain. The major portion of this product which is side-chain labeled would appear to be formed from a C2H, fragment interacting with the aromatic ring. The formation of a CZH fragment is shown in eq 1-3; less unsaturated fragments could result from hydrogen pickup or abstraction. The formation of phenylacetylene from benzene and the homologous reaction in toluene and xylene involve the replacement of a hydrogen by the C-CH entity (eq 10) 1
I n toluene such a reaction might involve either replacement of a hydrogen atom yielding methylphenylacetylene or of the methyl radical in which case phenylacetylene would result. This constitutes a second possible mechanism for the formation of phenylacetylene from toluene. Pickup of hydrogens by the C-CH entity before it interacts with the parent compound or similar pickup by an intermediate after this reaction could lead to styrene and ethylbenzene from benzene and to their methyl derivatives from toluene and xylene. As seen in Table V, when the yields for these products are compared on a per bond or per position basis, the probabilities are fairly constant in many cases, but appreciably different in others. There is some uncertainty in com-
The Journal of Physical Chemistry
RONALD L. WILLIAMS AND ADOLFF. VOIGT pound identification in some of these cases, and further interpretation does not seem justified. The yield of cycloheptatriene in the benzene system very likely results from the stabilization of the ring intermediate. However, the low yield of the methylcycloheptatrienes from toluene is difficult to explain. Carrier compounds for definite identification of the low yield products T-2 through T-4 (methylcycloheptatrienes) and P-4 (dimethylcycloheptatrienes) are presently not available. Additional conclusions can be drawn from the experiments on the fate of the remainder of t)he llC atoms. As shown in Table 11, Section C, 20.8% of the total activity has been adequately identified. Two-ring compounds such as biphenyl, diphenylmethane, fluorene, and phenylcycloheptatriene account for an additional IS%, but in this work they were not resolved sufficiently for the determination of accurate yield values. Another fraction (28%) was removed in the distillation experiment and is attributed to compounds containing three or more rings which either decompose under these conditions or are carried by the benzene vapor. This 28% fraction could not be eluted from the 2.5-cm column suggesting that the column provides considerable protection from decomposition. The final fraction (35%)) which was not removed from the sample bulb, was contained in a visible, viscous residue suggesting high-molecular-weight polymers. The maze of possible products and reactions involved in polymerization requires the development of techniques different from those used in this study.
Acknowledgments. The authors are very grateful to Dr. Alfred Bureau and the other members of the Iowa State University synchrotron staff.
