T H E PHOTODECOMPOSITIOX O F ETHYL IODIDE ICY THOMAS I R E D A L E
It is universally known that the alkyl iodides and many ot,her organic compounds containing iodine are unstable in the presence of light; but few quantitat,ive studies of this phenomenon have ever been made. Burke and Donnan' tried to find a relationship between the sensitivity of the alkyl iodides t'o light and their reactivity with silver nitratr. They considered that. the product,ion of iodine through decomposition by light v a s not a simple process and involved an oxidation of the hydrogen iodide first formed. Stobbe and Schmidt2 came to a similar conclusion. that oxygen was necessary for the photodecomposition of solutions of the alkyl iodides. Some years later Job and E m s c h ~ ~ i l l estudied r~ the photodecomposition of ethyl iodide anti found t'hat oxygen was not necessary for the reaction. According to their result,s the reaction is rather complicated: the final products being ethylene, ethane and iodine, and small amounts of but,ane and hydrogen. They argue that the first stage in the photochemical process is the detachment of an iodine atom from the molecule of ethyl iodide as the result, of the absorption of a light quantum. C2HjI hv + (C2HjI) + C2Hs (I)
+
+
They consider also, that another light react,ion takes place: the decomposition of hydrogen iodide, which result,s as the consequence of the original I atom acting as acceptor towards one of the hydrogen atoms in the ethyl radical.
+ (I) C7H.j + H I H I + i i v + H + (I)
C2HI.H
+
It, was with a yiew to settling one or two of the main points in this conflicting evidence t,hat the present research was undertaken. I t was considered that a study of the quantum efficiency of t,he decomposition Tvould settle the question as t o the nature of the primaly photochemical process. The reaction is interesting in view of its analogy to the decomposition of HI which has usually been investigated in the gaseous state,' though recently as a liquid by Bodenstein and L i n ~ e i g .It ~ was decided, however, in the first part of this investitigation to work with liquid ethyl iodide as the previous work mas carried out on this substance either as a liquid or as a solution in other organic liquids. J. Chem. Soc., 8 5 , 574 (1904). Z. aiss. Phot., 20, j i (1920). Compt. rend., 179,52 (1924). 4 Bodenstein: Sitz. Akad. Wiss. Berlin, 1918, 300,; Z. physik. Chem., 22, 23 (1897); Trautz and Scheifele: Z. wiss. phot., 24, 177, (1926). Tingey and Gerke: J. Am. Chem. Soc. 48 1838 (1926); Bonhoeffer and Steiner: Z. phyhk. Chem., 122, 287, (1926); Lewls: J. Phgs.'Chem., 32, 270, (1928). 5 Z. phgsik. Chern. 119, 123, (1926). 2
THE PHOTODECOb~POSITIOSOF ETHYL IODIDE
291
In 1926 the author in conjunction with Mr. IT, K,Madgin of Armstrong ('ollege, University of Durham, began some experiments to test one of the main points at issue-whether oxygen mere necessary for the photodecomposition. Carefully purified ethyl iodide was introduced into an all-glass apparatus and the dissolved gases removed by a process of successive coolings and evacuations with an oil pump. The ethyl iodide was then distilled at room temperature through a layer of purified phosphorus pentoxide into a tube of standard dimensions which was then sealed off. Comparisons were then made with a sample of purified ethyl iodide which had not been treated in this manner in a standard glass tube having access to atmospheric oxygen. Irradiation from a mercury vapour lamp under identical conditions seemed t o produce the same effect in the two cases, so far as the production of iodine was concerned. It would seem then that for light of wave lengths extending to about 313pp in the ultra-violet, oxygen is not necessary for the photodecomposition. This does not rule out the possibility that over a long period of time the oxidation of the H I may play some part in the reaction. But for the purpose of the present investigation this matter is not important. The initial stage of the decomposition should be a reaction of zero order for absorbed light of constant intensity; but owing to secondary absorption by the iodine accumulated in the system, the rate of reaction begins t o deviate from this law, The experiments to be described were therefore never carried beyond the stage where this secondary absorption effect was appreciable.
Experimental The source of radiation was a quartz mercury vapour lamp run on a I I O volt D.C. circuit v i t h a burner voltage of 80 volts and a current of 2 . 5 amperes. It was enclosed in a circular metal box, blackened on the outside, and with an aperture 1.8 cms. in diameter in the front arc. K h e n first used the lamp was quite new and gave lines below z z o p p . These spectral measurements and others of a more quantitative character were made with a Hilger quartz spectrograph. The magnitude of the adsorption by ethyl iodide in any spectral region could be estimated with fair accuracy by varying the times of exposure on a photographic plate, taking the Schwarzchild constant as unity.' Accurate measurements of the absorption and of the radiant energy from the lamp were carried out with a Moll thermopile and galvanometer. Ethyl iodide in alcohol has an absorption band2 at about 260pp and it also absorbs at 3 6 5 p p when a column of the liquid several centimetres long is employed. Job and Emschwiller state that it begins t o decompose with light of wave length qropp, but the absorption in this region is very slight, and the decomposition would only be noticeable with a long column of the liquid. The filt,er employed for isolating the 365 p p region was that described by Kinthe? and made by using a I cm. thickness of a 0 . 0 3 7 ~neutral solution Baly and Riding: Proc. Roy. SOC.,113.4, 709 (1927). Crymble, Stewart and Wright: Ber., 43, 1183 (1910). Z.Elektrochemie, 19, 389 (1913).
292
THOMAS IREDALE
of diamond fuchsin.’ The spectral transmission of this filter was tested with the quartz spectrograph. Transmission of the 313pp and 334pp lines was negligible, the small amount of 4 0 4 p p and red lines were not absorbed appreciably by the ethyl iodide and the infra-red radiation was shut off in the usual way by water screens. By the use of a quartz lens and screen and appropriate apertures in front of the lamp, a nearly parallel beam of light was obtained passing the water screens and the filter. The radiation from the lamp was carefully measured at different times during the course of a photochemical decomposition. Owing to the narrow dimensions of the light beam attempts were made to measure its average energy by rotating the thermopile to different positions at right angles to the beam. The variations in the galvanometer readings for these different positions were, however, very slight. The amount of 3 6 5 p p light in the beam was found by replacing one of the water screens by Tinther’s 3 13pp filter, a dilute solution of nitrosodimethylaniline and potassium chromate, which absorbs the 36 jpp rays. The thermopile was calibrated in two ways: (I) with a Hefner lamp. There were several reasons, however, for doubting the accuracy of this particular instrument.
(2) with the radiation from the blackened surface of a tin box-Leslie cube-containing boiling water, using appropriate screens and apertures. 9 0 7 of ~ the possible radiation was allowed for in calculating the energy reaching the thermopile, which was covered with a rock-salt plate during these calibration tests, and tested at different distances from the radiating surface The surface density uf the radiation was calculated with the aid of the usual equation,* using the theoretical value for the Stefan-Boltzman ~ o n s t a n t . ~ For the working distance of galvanometer and scale calibration with the Hefner lamp gave 119 ergs per sq. cm. per sec. for I cm. deflection;l with the Leslie cube, 1 1 2 , 114, 1 1 2 , 113 ergs per sq. cm. per sec. for several distances of the thermopile. The average of these figures, 116 ergs per sq. cm per sec., was used in all subsequent calculations of radiant energy. The ethyl iodide was purified by shaking with dilute alkali and successive quantities of water, drying over calcium chloride, and distilling, the fraction boiling a t 7zo-73’C/760 mm. being collected and preserved in a dark bottle over silver foil. I n the experiments the ethyl iodide was contained in a cylindrical cell, 5.45 cms. long, on the ends of which were cemented circular quartz plates. The thermopile was covered with a similar quartz plate, as the correction for surface reflection at the front of the cell would amount to ssc. The amount of light absorption by the ethyl iodide was calculated from the spectrographic figures-about 85% for this particular Gray: J. Phys. Chem., 31. I732 (1927). Lummer and Pringsheim: Ann. Physik, (3) 63, 399 (1897). 3 Millikan: “The Electron,” 238. Gerlach: Physik. Z., 14, 577 ( 1 9 1 3 ) .
‘
THE PHOTODECOMPOSITION O F ETHYL IODIDE
2 93
length of reaction cell. I t mas also estimated from the change in energy of the light beam on passing through the ethyl iodide. Owing to the slightly divergent nature of the beam and the change in refractive index of the medium, measurements were made across a constant area of the beam, and an average figure for the absorption was obtained which agreed very well with that calculated from the percentage of 36 j p light ~ in the beam and the spectrographic data. Very little of the infra-red radiation passing the water screens could therefore have beeen absorbed by the ethyl iodide. The iodine liberated through the decomposition was estimated by titration with a very dilute solution of sodium thiosulphate. This analytical method was checked with the aid of solutions of known iodine content, and can be made very exact. The rate of reaction' of the iodide with the thiosulphate would be negligible in this case. The errors in this analytical method are certainly less than those involved in the measurement of the light absorption d more accurate spectrophotometric method was not available.*
TABLE I Wave length of light: 3 6 j p p Period of decomposition Hours 10.0
r j.0 14.0
,
4.75
(
4.0
f
4.0
4i 1.35
2.6j
~
S o . of auanta ahsorbid per sec. by C2HJ
x
10-14
S o . of quanta abs. by GHsI during period of decomp.
x
3 ' 74
.04
1.17 I . 13 0.89
1.28 1 3 . 2 1
0.70
1.00
10-18
3.41
0.91
3.18
0.89
3.13
0.98
3
I
1
0.90 0.jo
28
.08
J
2.2;
I . I j
0.93
2.83 3 .oo
13 0.96
I1 .04 .16) 2 . 2 0
5.62 1 . j
I ,
x
Molecules division Quanta
10-18
1.04 I
S o . of mols. of CiHJ decomposed
1.94
0.89
1.21
2.46
I
2
( 2 . 5
1.06 I .06 0.90
9.0
1.02
4.0
I
[ 4.5 ~
.30
60
0.8j
3 I4 1.91
0.92
Slator: J. Chem. Soc., 85, 1286 (1904). and Linweig: Z. physik. Chern., 119, 123 (1926).
* Bodenstein
.oo
I
.03
294
THOMAS IREDALE
The results of measurements of the quantum efficiency are given in Table I. The intensity of the absorbed light was not varied very widely. The magnitude of the variation can be gauged from the second column S o accurate temperature control was attempted. The temperature of the laboratory varied from 18-23'C. The number of molecules of C2HJ decomposed was equated to the number of iodine atoms formed, a quite reasonable procedure in view of the simplicity of the results. Kithin the limits of experimental error it seems that the quantum mechanism is a very elementary one, Einstein's law of the photochemical equivalence being valid for this special case The quantum efficiency is slightly higher for ultraviolet light below 36;pp. An average value was obtained for the region 248-36jpp, which was worked out for the quantum corresponding to wave-length 3 10pp from the considerations of the data for mercury vapour lamps,' the present lamp being I O O hours old when these particular measurements were made. It is obviously an approxiination, but adequate means vere not available in the laboratory for testing the spectral distribution of energy from the lanip I t would seem that with the exclusive use of light of the shorter nave lengths, the quantum efficiency would become identical with that of the €11 decomposition T A B L E 11 Wave length of light: 248-365pp Average quantum corresponding to 3 ~ o p p Period of decomposition
Hours
S o . of quanta absorbed by CSHLIX 1 0 - l ~
S o of mols of CIHJ decomposed x IO-^^
11olecules Quanta
2.83
1.2
2.75
1.1
I . j0
1.3
2.3i
I.2
I .66
1.2
Discussion of Results
The results seem to shoiv that the photodecomposition of liquid ethyl iodide is a very simple process, one quantum effecting the detachment of an iodine atom from one molecule This rules out the second postulate of Job and Emschiviller-the formation and decomposition of HI; unless we imagine some collision mechanism nhereby the energy furnished by the light in excess of that found from thermal measurements is effective for the deconiposition. The heat of dissociation of C2HJ is, according t o Bowen* 40.000 calories, whereas the energy furnished by light of \yave-length 36jpp is 7 2 , 0 0 0 calories per gram-molecule This leaves 3 2 , 0 0 0 calories to be accounted for which is only half the heat of dissociation of H I J. Phys. Chem., 30, 1427 (19261. Trans. Faraday Soc., 21, j44 (1926:
THE PHOTODECOMPOSITIOS O F LTHYL IODIDE
295
Comparisons of thermochemical and photochemical data, however, may not always be satisfactory. because the former are based on measurements made on a system of molecules in statistical equilibrium, whereas the latter are calculated from the results of selective light artion, and may only apply to molecules in special quantum states. .Isin the case of HI the decomposition of C2H:I results in the formation of an excited I atom, the excess energy probably being dissipated as kinetic energy through collisions. This analogy is not unreasonable in view of the fact that a non-ion linkage has been attributed t o HI. Studies of the photolysis of these alkyl iodides is being continued. Vniveraity of Sydney,
S.S.W',, Auslralia.
'Franck and Iiuhn: 2. Physik: 43, 164 (19271
