~
KOTES
Sept., 1963
1923
TABLEI1
INTRTNSIC VISCOSIW DATA Configuration M, 330,000 Atactic 140,000 Atactic 44,000 Atactic 14,500 Atactic Atactic 214,000 Atactic 87,000 €3-4" Atactic 37,000 B-6" 480,000 Isotactic C'-2Ab 200,000 C'-2Bb Isotactic Isotactic 130,000 D-2b 13-4b Isotactic? 34,500 calculated from [7]in cyclohexane a t 25'. [TI in dl./g.
FracOion
A-3" A-4" A-6 a A-7,8a B-2"
an
Cyclohexanone 92"
I-Chloionaphthalene 74'
[v I 1.05 0.68 0.37
[VI
0.99 .64 .37 .20
ancalculated from [ T ]
(9) F. Danusso, G . RIoraglio, V. Flannella, and G. Netta, Rend. Accad. Nas. Lsncez, 25, 510 (1958). (10) S.J. Y o 0 and J. B. Kinsinger, J . Chem. P h y s . , S6, 1371 (1962). (11) C, A , J. Hoeve, t b z d . , 35, 1266 (lQ61).
I
[PI
0.48 .33
0.54 .36 .23 1.31 .76 .67 .30
[t,
.22 1.02 .66 .58 .28 in decalin at 135°.6c
TABLE I11 POLYPROPYLENE AT THE FLORY TEMPERATURE (I?) Configurae (Pof'/M)1/2 tion Solvent ("C.) K x 104a a x 101' 18.2 0.50 475 Atactic 1-Chloronaphthalene 74 17.2 0.50 475 Atactic Cyclohexanone 92 12.0 0.50 475 Atactic Phenyl ether 153 19.2 0.47 Atactic PhenyI ether 145 16.8 0.50 475 Atactic Isoamyl acetates 34 13.2 0.50 475 Isotactic Phenyl ether 145 0.54 11.2 Isotactic Phenyl ether 153 a K = [ 7 ] / M c e . @ taken as 2.1 x 10z1
g. Values of fof were calculated from the familiar expression for freely rotating chains. The isotactic polymer a t its 0 temperature is found to be 10% larger in dGUz than its atactic counterpart at its 0 temperature. This makes approximately a 30y0difference in the volume pervaded by the two under similar conditions and reflects the larger coil size of the isotactic species. These coil dimensions undoubtedly reflect the difference in the time averaged conformational states available to the tvvo species. Differences in the distribution of conformations should be more exaggerated in poor solvents where segment-segment interactions are favored and would tend to decrease in importance in good solvents where a more random flight conformation is assumed. In view of the number of conformational states available to each species, a single parameter such as dgu2does not permit one to infer that the isotactic chain has more internal conformational order than the atactic because one parameter is insufficient to make such a judgmont.1° For instance, the increased size of the isotactic chain in solution could reflect either: (a) a larger number of preferred or unpreferred conformations at or about the trans position, (b) some degree of internal helical structure retained from the crystal state helix, or (c) higher neighbor interactions or any combination thereof. Additional information on the distribution of conformations is needed t o resolve this problem. It is noted that neither species of polypropylene is a highly expanded chain as both have somewhat smaller unperturbed dimensions than those of either atactic or isotactic poly~tyrene,~ and much smaller than the calculated unperturbed dimensions of polyethylene. l1
Phenyl ether 153'
Phenyl ether 145'
@ouz, M)'/2
x 101' 955 935 830
( P o u ~ / P o f q '/2
930 910
1.99 1.93
2.02 1.96 1.75
The temperature coefficient of the unperturbed dimeinsions (d In 8ou2/ cl In T = -0.043) for the atactic polymer is slightly less than that reported for polyisobutylene.12 A similar coefficient for the isotactic species is impractical to obtain by this method as, at lower Flory temperatures, liquid-crystal separation occurs, and a t higher temperatures, degradation of polymer becomes a serious experimental problem. (12)
C. 8.J. Boeve, i b i d . , 32,888 (1960).
PRIMARY PROCESSES I N THE PHOTOLYSIS OF ETHYL NITRATE BY R. E. REBBERT Natzonal Bureou of Standards, Waahzngton 26,D . C. Recesred February 25, 1083
Recent photochemical studies1 have indicated that nitroethane decomposes by an intramolecular elimination process to give ethylene as CZHSSOZ-t kv --+ CzH4
+ HONO
I
in addition to a dissociation into an ethyl radical and nitrogen dioxide as CzHsNO?
+ h~ +CzHs + X02
I1
It seemed plausible then that the photolysis of ethyl nitrate should also give an intramolecular elimination of acetaldehyde as CaHbOSOz
+ hv
---f
CH3CHO
+ HONO
I11
besides a dissociation into an ethoxy radical and nitrogen dioxide as CzHbOSOz
+ hv
+C2HjO
+ SO2
IV
The only pliotolysitr of ethyl nitrate reported in the (1)
R. E.Rebbert and N, Slagg, Bull. Sot, Chim. BeEpeg, 71, 709 (1962).
1924
NOTES
Vol. 67
TABLE I PHOTOLYSIS OF ETHYL NITRATEIN Time, min.
Additive
Pressure of additive, mm.
Roa
Roo
1 .. ... (0.37) (0.89) 3 .. ... 0.41 ,10 10 ... 1.1 .18 30 .. ... 1.4 .36 GO .. ... 1.0 .67 120 .. ... 0.40 1.3 0.05 5.3 0.71 30 NO2 33 NO2 0.25 7.3 0.92 1.3 37.7 .79 30 NOz 1.4 .. .. 30 0 2 12.2 .. .. 30 0 2 30 NO 0.8 .. .. 30 NO 2.2 0.0 0.36 12.2 30 NO .. .. 2.1 0.0 1.1 30 CzH4 12.0 30 CzHa Temperature, 28.5 I 1.5'; pressure of ethyl nitrate, 25 mm.
..
literature is the work of Gray and Style.2 They interpreted their results completely in terms of the dissociation process IV. Their work was done mainly at 95' and at 2600 A. with conversions of 4%. The present work was conducted mainly a t room temperature and at 3130 A. with conversions of approximately 0.25%. Experimental The gas phase experiments were carried out in a mercury-free system composed of a 3-1. spherical Pyrex vessel with a quartz window on one side. The light source was an Osram-200 high pressure mercury arc in conjunction with Corning filters 7-54 and 0-53. The gaseous products were measured in a gas buret and analyzed on a mass spectrometer. The ethyl nitrite, acetaldehyde, and methyl nitrite were analyzed on a Perkin-Elmer gas chromatograph (Model 154) equipped with both a thermal conductivity and a flame ionization detector. A diisodecylphthalate column was used at 30". Calibration was done in terms of peak heights since the acetaldehyde and ethyl nitrite peaks were not completely separated. The quantum yield measurements are based on the assumption that sPo0 = 1.0 for diethyl ketone in the gas phase a t 106". A Bausch and Lomb monochromator (Model 33-86-40) was used to measure the relative absorpiion for diethyl ketone and ethyl nitrate in the gas phase a t 3130 A. The few liquid phase experiments were performed in a quartz cell (2.5 X 0.05 cm.) with a volume of about 0.3 cc. This cell was equipped with two outlets. One outlet was covered by a break seal and the other was sealed off after filling. An Osram100 high pressure mercury arc was used as the light source in conjunction with Corning filters 7-54 and 0-53. The cell was immersed in a water bath in a dewar with double quartz windows. The ethyl nitrate was from Eastman Organic Chemical Co. and was fractionally distilled in a spinning band column. Nitric oxide and nitrogen dioxide were from the Matheson Company. The nitric oxide was distilled from silica gel a t Dry Ice temperature. The ethylene was Phillips research grade.
Results The main products for the gas phase photolysis at low conversions were ethyl nitrite, acetaldehyde, nitrogen dioxide, and oxygen. Methyl nitrite, carbon monoxide, hydrogen, nitric oxide, and carbon dioxide were minor products. No attempt was made to detect or analyze nitromethane, formaldehyde, or nitrous oxide. The peak due to nitromethane would be lost on the chromatograph in the excess ethyl nitrate peak. Xitrogen was produced but the results were not consistent. The results are given in Table I. The rates of formation of methyl nitrite are not accurate and they (2)
J. A. Gray and p, W,G,Style, Trans. Faradag SOC.,49,
52 (1953).
TRE
GASPHASE"
RHS RCH~CHO ( X 108 cc. (STP)/min.)
(0.32) .074 .094 .13 * 21 .42 .17 0.19
.. ..
..
.. .*
..
0.32
..
RCzHaONO
...
3.8 3.8 4.4 5.0 4.9 4.7 6.1 6.2 6.8 4.0 3.6 8.1 8.3 7.0 4.6 4.0
2.6 2.7 3.1 3.7 4.5 3.5 3'2 1.4 3.8 5.4 3.8 3.9 2.6 2.4 2.0
ROHSONO
0.24 .28 .29 .64 .73 .76 .86 .78 .72 .72 .73 64 .38 .14 .72 .55
.
should be regarded as relative rates only. The quantum yields of ethyl nitrite and acetaldehyde formation extrapolated to zero time are 0.139 and 0.094, respectively. The rates in Table I may be converted to quantum yields by multiplying the results by 37.6. I n the liquid phase at 0 O , oxygen is no longer a product. Ethyl nitrite and acetaldehyde are still the main products with small amounts of methyl nitrite. The distribution of products a t low conversions is acetaldehyde, 53.6%; ethyl nitrite, 46.0%; and methyl nitrite, 0.4%.
Discussion These results provide evidence to indicate that there are three primary processes occurring
CzH60N02
+ h~ +CH3CHO + HOKO CzH50 + NO2 +C2H50NO + 0
I11 ITT
v
Evidence for process V is the formation of ethyl nitrite which is not appreciably reduced by the addition of various radical scavengers such as ethylene and oxygen. The reduction (about 20%) that does take place can be assigned to the inhibition of the reaction
C2H60 +NO
--j
C~HSONO
(1)
in the presence of these scavengers. In this regard, it is important to note that oxygen did inhibit the formation of methyl nitrite in the photolysis of nitromethane.' Additional evidence for process V is the formation of oxygen which is completely inhibited by the addition of nitric oxide and ethylene. This indicates that oxygen atoms are responsible for the formation of oxygen, probably by the reaction
0+n.0,-302+N0
(2)
The low yield of oxygen a t very lorn conversions can be explained tentatively by a scavenging of the oxygen atoms by the various radicals present. With time, the nitrogen dioxide concentration increases and reaction 2 becomes more important and, consequently, the rate of oxygen formation increases. At still longer times, the concentration of nitrogen dioxide reaches a steady
NOTEB
Sept., 1963 state but molecular oxygen is consumed by reactions such as 0 2
+ 2 N 0 +2 5 0 2
(3)
+ R +ROz
(4)
and On
KO*. The addition of nitrogen dioxide does increase the rate of production of oxygen. Some additional oxygen is formed, of course, by the photolysis of the nitrogen dioxide itself. However, a t low concentrations of added nitrogen dioxide (0.05 mm.) this correction to the rate of formation of oxygen is relatively small, cc. (STP)/min. Thus, after this about 1.5 X correction is made, the rate of formation of oxygen with added nitrogen dioxide is 3.8 x 10-3 cc. (STP)/min., which is the ,same as the rate of formation of ethyl nitrite a t very low conversions. The formation of ethyl nitrite and oxygen atoms by the photolysis of ethyl nitrate is not too surprising. The equivalence of process V is reported to occur in the photolysis of inorganic nitrate in the solid phase and in solution^.^ It is also reported to occur in the yradiolysis of inorganic nitrates. The evidence presented would seem to indicate that it also occurs for the gas-phase photolysis of organic nitrates. Evidence for process IV is the large increase in the rate of formation of acetaldehyde and ethyl nitrite in the presence of small amounts of nitric oxide. A large addition of nitric oxide brings about a reduction in the rate of formatiion of both products. The increase can be accounted for by the reactions 1and 5.
+ KO +CHDCHO + HNO
(5)
A rough estimate of the ratio kl/ks can be obtained from the ratio of ethyl nitrite to acetaldehyde forrned in the presence of iiitric oxide and corrected for the amount of both substances formed a t very low conversions. This gives h / k s = 3.3, which is a reasonable estimate in view of McMillan’s6 value of k 6 / k , = 6.6 at 26’ for the reactions (CH3)zCHO
+ NO
+ (CHJzCHONO
+ (CH3)ZCO
It is also in agreement with the estimate of 2 to 3 for the ratio k l / k s made in the photolysis of nitroethane.’ The formation of small amounts of niethyl nitrite can be accounted for by process IV followed by the reactions CzHsO +CH3
Thus, the rate of oxygen formation decreases. There is the possibility that oxygen may be iorined by the secondary photolysis of the product NOz. However, a t low conversions (below O.l%, which is equivalent to an experiment of about 12-min. duration), it can be estimated from blank experiments that less than 25% of the oxygen could be formed in this may. Moreover, a t higher conversions, where presumably more KO2 is present, less oxygen is formed. I n agreement with this, Gray and Style12who carried out experiments in which the conversion was 4%, did not report any oxygen while KO,was the predominant oxide of nitrogen present. It is also difficult to reconcile the small change in oxygen production reported in Table I for a lchange in NO2 pressure from 0.05 to 0.25 mm., if oxygen is formed only from the photolysis of
C2H60
1925
(6)
+ HNO (7)
(3) G. K.Rollefson and M. Burton, “Photochemistry and the Mechanism of Chemical Reactions,” Prentice-Hall, Inc., New York, N. Y., pp. 140, 375. (4) T.-H. Chen and E. R. Johnson, J . P h y s . Chem., 66, 2249 (1962). ( 5 ) G. R. RiIcMillaz., J . Am. C h e n . Soe., 88, 3018 (1961).
CH3
+ CHzO
+ KO2 +CH3ONO
(8)
(9) The formation of acetaldehyde as a major product a t low conversions and the fact that acetaldehyde is not appreciably reduced in the presence of an excess of ethylene are evidence in favor of process 111. A rough calculation of the relative importance of each process and of the quantum yield associated with each process can be made if we assume that the rate of formation of ethyl nitrite a t zero time is due to process V, that the rate of formation of acetaldehyde a t zero time is due to process 111, and that the additional ethyl nitrite and a,cetaldehyde formed in the presence of S O , plus the methyl nitrite formed under the same conditions is due to process IV. Thus, we have a111 = 0.094, @pv = 0.139, and @IV = 0.240. The quantum yield of processes I11 and V may be slightly high. However, the quaiitum yield of process I V probably is low since the back reaction C2H60
+ KOz +CzH60N02
(10) This makes the total
must take place to some extent. quantum yield -0.5. I n the liquid phase, ethyl nitrite is produced but no oxygen is found. It is difficult to decide whether process V occurs but the oxygen atom is somehow consumed, or if process V does not occur and ethyl nitrite is formed in some other manner. The very small quantity of methyl nitrite indicates that reaction 8 does not occur to any large extent in the liquid phase. This is to be expected in the liquid phase where the excess energy of the ethoxy radical is collisionally deactivated. Process I11 and IV both seem t o occur, but it is difficult to estimate the contribution of each. Acknowledgment.-The author wishes to thank Dr. Peter Ausloos for suggesting this problem and for his many helpful suggestions. We wish to acknowledge financial support of this research by a grant from the United States Public Health Service, Department of Health, Education and Welfare.
-
COMPLICATING FACTORS IN THE GAS PHAS.E PHOTOLYSIS OF AZOMETHANE BYR. E. REBBBXTAND P. J. AUSLOOS National Bureau of Standards, Washington 86,D . C. Ileceined ;March 18, 196s
I n a recent publication on the gas phase photolysis of azomethaiie, Toby and Weiss’ suggested a new ethaneproducing reaction 2CH3Nz * CzHa
+ 2N2
(1) On the other side, Rebbert and Ausloos2 presented evidence for the foriniation of ethane by a unimolecular elimination from azomethane
CH3NzCH3
+ h~ +CzHe + Nz
(1) S. Toby and B. H. Weiss, J . Phya. Cham., 66, 2682 (1962). (2) R. E. Rebbert and P. Ausloos, ibid., 66, 2253 (1962).
(I)
