SO2ELIMINATION IN DECOMPOSITION OF DIARYL SULFONES
853
Sulfur Dioxide Elimination in the Radiolytic Decomposition of Solid Diary1 Sulfones
by Larry Kevan, P. L. Hall, and E. T. Kaiser Department of Chemistry, University of Chicago, Chicago, Illinois
60637
(Received September 27, 1965)
y Radiolysis of solid p,p‘-ditolyl sulfone results, almost exclusively, in simple SO2 elimination with the production of an equivalent yield of p,p’-bitolyl. G(S02) = G(p,p’-bitolyl) = 0.05. Diphenyl sulfone and dibenzothiophene sulfone also show simple SO2elimination. However, the latter cyclic sulfone only gives G(S02) = 0.002 and is therefore 25 times more stable to radiation decomposition.
The radiolysis of organic compounds usually leads to a complex variety of products. This variety often obscures the mechanism and renders detailed interpretation difficult. However, we have found that solid p,p’-ditolyl sulfone undergoes, almost exclusively, simple SO2 elimination and produces an equivalent yield of p,p’-bitolyl upon y radiolysis. Diphenyl sulfone and dibenzothiophene sulfone also show simple SO2 elimination. However, the latter cyclic sulfone is 25 times more stable to radiation decomposition.
Experimental Section Materials. All sulfones were Eastman White Label grade. Diphenyl sulfone was recrystallized from ethanol-water until impurity peaks could no longer be discerned on its gas chromatogram. p,p’-Ditolyl sulfone was used without further purification. Its gas chromatogram was free of impurity peaks. Dibenzothiophene sulfone was recrystallized several times from absolute ethanol until pure. Our criterion for judging whether compounds were “pure” was based on the absence of detectable impurity peaks on gas chromatograms of saturated solutions in acetone taken a t the maximum sensitivity of our instrument. (See organic product analysis.) Sample Treatment. The purified sulfone was finely pulverized in a mortar and placed in a Pyrex vial fitted with a break-seal. The vial was degassed to torr and sealed off under vacuum a t 77’K. Normal sample size was 1 g. The vials were irradiated in a Cow source a t a dose rate of 0.7 Rlrad/hr.
Analysis. SO2analysis was carried out on a Wilkens Aerograph chromatograph, Model 661, using an electron-capture detector and a 2 ft X 1/8 in. silica gel column. At a flow rate of 40 ml/min Nz at 125’ the SO2 retention time was 3.2 min. Matheson SO2 was used for calibration. Calibrations were run with each group of samples. Peak areas were measured with a disk integrator. SO2 from irradiated samples was introduced in a vacuum line through the break-seal and pumped with a Toepler pump into an evacuated injection loop which was connected to the carrier gas flow system. No analysis for H2 was made. Organic products were analyzed by hydrogen flame ionization detection on an Aerograph Hy-Fi, Model 600C. A 5 ft X 1 / 8 in. column containing 5% SE30 in 30-60 mesh Chroniosorb W was used. Both H2 and Wz flow rates were 25 ml/min. For the products from diphenyl sulfone and dibenzothiophene sulfone a column temperature of 125’ was used, and for the p,p’-ditolyl sulfone products 150’ was used. A temperature of 200’ was used to look for higher molecular weight products. Irradiated sulfone from a sample vial was weighed out and dissolved in acetone (dimethyl sulfoxide was used as a solvent for dibenzothiophene sulfone) and diluted to a known volume. The solution was injected with a Hamilton 1O-pl syringe. Calibrations were carried out with similar solutions of standard compounds. Product yields are given as G values which refer to the number of molecules produced per 100 ev of radiation energy absorbed by the sample. Volume 70. Number 3 March 1966
L. KEVAN,P. L. HALL,AND E. T. KAISER
854
I
I
I
I
Results and Discussion
I
1
The SO2 yields and organic products formed by y radiolysis of p,p’-ditolyl sulfone, diphenyl sulfone, and dibenzothiophene sulfone are given in Table I. Yielddose plots are shown for the SO2 yields in Figure 1 and for the organic yields in Figure 2. Initial yields could be determined from the linear portions a t low dose which went through the origin in all cases except for the SO2 yield from diphenyl sulfone. Note that the analytical methods used allowed the SO2 yields to be measured down to 1 Mrad dose while the organic products could be measured only at somewhat higher doses. No other organic products were found and from our estimated limits of detection must have G < 0.005. Table I: Product Yields from Sulfone Radiolysis Parent sulfone
p,p’-Ditolyl sulfone ”
0
4
8
12
16
20
24
28
Dose (Megarads)
Diphenyl sulfone
Figure 1. Yield-dose plot for SO2 from ?-irradiated sulfones: 0, p,p‘-ditolyl sulfone; 0, diphenyl sulfone; x, dibenzot,hiophene sulfone.
Dibenzothiophene sulfone
Initial product yields
G(S02) = 0.05 G(p,p’-bitolyl) = 0.05 G(unknown) = 0,004 G(S02) > 0.02“ G(bipheny1) = 0.05
G( Sot) = 0,002 G( biphenylene)b
’Initial
1
1
I
I
I
1
lo/ 9-
yield could not be determined but is estimated to be detection was several times greater than t h e expected G = 0.002; see text.
-0.05.
1
* Limit of
-
87c
a x
’s
6-
e
e
z
5-
0,
0 3
0
4-
32-
‘
-1 ov ’ 0
4
I 8
I 12
I 16
I 20
I 24
1
but were not f0und.l The data on p,p‘-ditolyl sulfone
28
(1) It has been pointed out by a referee that any reader familiar with the history of benzene radiolysis would immediately suspect “polymer” production. (See, for example, S. Gordon, A. R. VanDyken, and T. F. Doumani, J. Phys. Chem., 6 2 , 20 (1958).) This “polymer” (G = 0.5-1.0) found after y radiolysis of 3500 ml of benrene to a total dose of about 1 0 2 6 ev (about 50 hirads) consists primarily of CIZand ‘218 systems; biphenyl and various hydrogenated
Dose (Megarads)
Figure 2. Yield-dose plot for p,p’-bitolyl (0)from 7-irradiated p,p’-ditolyl sulfone and for biphenyl ( 0 )from 7-irradiated diphenyl sulfone.
The Journal
of
Physical Chemistry
I
SOz ELIMINATION IN DECOMPOSITION OF DIARYL SULFONES
suggest that a simple decomposition as shown by eq 1 is the net consequence of radiolysis. Ar-SOz-Ar +SOz
+ Ar-Ar
(1)
Diphenyl Sulfone. For diphenyl sulfone Figure 1 shows marked deviation from linearity in the SOz yield near the origin. It appears that, for higher doses, the SOz yield is 2-3 times less than the biphenyl yield of 0.05. This suggests that not all of the SOz was detected and that a significant amount was trapped in the irradiated crystals. After radiolysis, several irradiated diphenyl sulfone samples were heated to 100’ (mp 125’) for 1 hr. This treatment did increase the SO2 yields but did not give very reproducible results. We conclude that a simple decomposition, again represented by eq 1, also occurs in diphenyl sulfone. The yield of biphenyl from diphenyl sulfone is equal to the yield of p,p’-bitolyl from p,p’-ditolyl sulfone, and hence both sulfones appear to have about the same stability toward radiolytic decomposition. DibenxothiopheneSulfone. Dibenzothiophene sulfone was studied to see what effect a more constrained structure in which the S atom was in a ring would have on SO2 elimination. As in p,p’-ditolyl sulfone, the SOz yield is linear with dose to about 13 Mrads, after which it levels off, The initial SO2 yield, however, is only G = 0.002 which is 25 times less than the initial SO2yield found in p,p’-ditolyl sulfone. The more constrained cyclic structure appears to result in a definite and rather dramatic increase in the radiolytic stability of the molecule. This stability could be partially due to back reaction to re-form the parent sulfone and partially due to more efficient energy delocalization. The expected biphenylene product could not be detected became the limited solubility of dibenzothiophene sulfone in the solvent dimethyl sulfoxide reduced the sensitivity of analysis. The limit of detection for biphenylene under our analytical conditions was several times greater than the amount of biphenylene expected. It is interesting to compare our results on aryl sulfones with the radiolysis results of Ayscough, et al., on alkyl sulfones.* Product analysis of several irradiated dialkyl sulfones, R-S02-R, showed a small Hz yield which depended on the chain length of the alkyl group and smaller yields of SOz, Rz, and RH. Quantitative results are not given. This contrasts with the simple and predominant SO2 elimination we observe in diaryl sulfones. The radiolysis of aromatic compounds usually produces small yields of Hz and a number of different organic products. The Hz yields from benzene and biphenyl are 0.0363and 0.008,*respectively. When the
855
sulfone group is present, the strength of the C-S bonds, which are weak as compared to C-C bonds, and the molecular stability of SO2 are important factors which cause solely SO2 elimination to occur. More quantitative thermochemical consideration is precluded by lack of data for aryl sulfone^.^ G(SO2) = 0.0020.05 which is in the same range as the Hz yields from the pure aromatic hydrocarbons. Scission of C-H bonds seems to be unimportant in the radiolysis of diaryl sulfones. There is no evidence for terphenyls or higher polymeric products that would probably be produced by H atom reactions in analogy to benzene.Ib6 As stressed before in footnote 1, Cle- and Czr-containing compounds and higher polymeric products were not found under our experimental conditions, and it can be estimated that formation of C18-and Cz4-containing products occurs with yields that are less than 10% of the yields of SO2 and organic products that were observed. Although the yields of SO2 and the hydrocarbon product corresponding to material balance are small, the radiolytic elimination of SOz is unique in that a single, simple, specific molecular decomposition is the sole principal result of the y radiolysis of a rather complex type of organic compound. Also, cyclic aromatic sulfones show considerably more radiation stability than open-chain aromatic sulfones.
Acknowledgment. We wish to thank the Atomic Energy Commission and the Petroleum Research Fund for support of this research. biphenyls as well as partially hydrogenated terphenyls have been isolated. Presumably, the percentage of higher molecular weight components in the “polymer” increases with the total energy expended in the system. We found no indication in our studies that such “polymers” were produced in the radiolysis of solid aryl sulfones. Extensive gas chromatographic analysis of irradiated sulfones turned up only products mentioned in the text of this paper and no others. Surely, any polymeric species of from two to three aromatic residues or partially hydrogenated residues would he volatile enough to give noticeable peaks within 3 hr on chromatograms taken with a column temperature of 200°. Yet no peaks were found on such chromatograms otlier than the ones for the products mentioned in the text. This finding, coupled with at least a rough material balance found in the production of SO1 and diaryl in diaryl sulfone radiolysis, makes it improbable that “polymer” production takes place to any extent within an order of magnitude below the production of SO2 and Ar-Ar (Ar = aryl). (2) P. B. Ayscough, K. J. Ivin, J. M. O’Donnell, and C. Thomson, “5th International Symposium on Free Radicals,” Uppsala, Sweden, 1961, Preprint 4. (3) S. Gordon and Sf. Burton, Discussions Faraday Soc., 52, 88 (1952). (4) J. G. Burr and J. M. Scarborough, J . Phys. Chem., 64, 1367 (1960). (5) J. P. McCullough, D. W.Srott, and G. Waddington in “Organic Sulfur Compounds,” Vol. 1, N. Kharasch, Ed., Pergamon Press Inc., New York, N. Y . , 1961, p 20. (6) W. N. Patrick and M. Burton, J . Am. Chem. Soc., 76, 2626 (1954); T . Gaumann, H e h . Chim. Acta, 44, 1337 (1961).
Volume YO, Akmber 3 March 1966
