September, 1923
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Trends in Photochemical Research By J.€&Mathews CNIVERSITY OF WISCONSIN, MADISON, \VIS.
T
H E history and progress of photochemistry have been During the progress of a fiercely contested battle it is not quite similar to the history and progress of other always wise to attempt to predict the h a 1 outcome, but it branches of science. First, we have scattered ob- is now clear that the original form of the photochemical servations, many of them imperfectly made, a gradual accu- equivalence law of Einstein, based upon the quantum theory, mulation of facts, attempts to work out relationships from is SO rarely in accord with observed facts as to render it of these facts, and, finally, t h e attempt to little value in itself. Any law which has provide a working theory which will be its sole foundation in the quantum theory of Planck is bound to be a subject for in accord with the observed facts. considerable dispute, inasmuch as this In recent years the interest of photo chemists has turned from the accumutheory itself is by no means fully accepted by many physicists. To those lation of facts regarding the synthesis and decomposition (photolysis) of comwho do not accept the quantum theory, the failure of the Einstein law and the pounds to a study of more fundamental radiation hypothesis of Lewis-Perrin is relationships. The number of known naturally the fulfilment of the expected, photochemical reactions now runs well Assuming the validity of the quaninto the thousands, and many of our tum theory, we soon find ourselves in ordinary reactions are undoubtedly more other difficulties. The Einstein law is dependent on radiation than we realize. To the mind of the writer there are supposed to apply only to the primary four fields of photochemistry that tranlight reaction, and the difficulty is that scend all others in present interest and the total amount of reaction as measimmediate usefulness. Before proceedured may be not just the amount of ing to them, however, it should be primary reaction which takes place, but pointed out that the photo chemist, may be either greater or less than that like tho colloid chemist, has gradually amount, due to secondary reaction or reenlarged his field so that the present actions. It thereiore becomes necessary boundaries are far wider than those to find a secondary reaction which is of a former generation. It is now J. H. MATHEWS exactly equivalent to the primary one, realized that visible liaht differs from since the latter can itself rarely be measthe ultra-violet, infra-rgd, heat, and electrical waves only ured quantitatively. A substance which will react with in frequency, and that they are all radiations of the same the product of the primary reaction quantitatively so as general character. From the shortest X-rays and rays from to give a true measure of the primary reaction, is called radioactive substances t o the infra-red, a t least, all radia- an “acceptor,” and the problem. resolves itself into finding tions are capable of producing photochemical changes, and suitable acceptors for the reactions to be studied. whether a certain reaction will be influenced by radiation Fraulein Puschl illuminated a mixture of bromine and is a question of wave length and of the specific reaction hexahydrohenzene (the acceptor) and determined the extent involved. While there are no authentic cases of photo- of reaction from time to time. She then calculated the chemical effects produced by electrical waves, the writer amount of bromine which should have been combined at sees no reason why such reactions are impossible, and these periods of time, using Einstein’s law, and found a indeed there seems to be ground for believing that some remarkable agreement between the observed and calculated instances of this kind will be discovered. It is reasonable values. Other acceptors, such as heptane, toluene, and to suppose that if certain systems are placed in a very hexane, were tried, but these gave too much bromine compowerful magnetic field so as to produce a condition of great bined; on the other hand, hydrogen proved to be an unstress or strain, the long electrical radiations might be able suitable acceptor since the amount of bromine actually comto produce changes in electronic configuration which would bined was only about one one-hundredth the amount rebring about chemical reaction. quired by the photo-equivalence law. The four principal fields of interest now occupying the The Lewis-Perrin radiation hypothesis, while appearing mind and attention of the photo chemist are: to be somewhat more hopeful than the original formulation of Einstein, has been shown to have many weaknesses and, (a) The photo-equivalence law of Einstein and the Lewis- in fact, some apparently insuperable difficulties. Perrin radiation hypothesis. Our story starts back in 1889 when Arrhenius deduced ( b ) The production of cold light. a relation for the chemical reaction which has been and still (c) The improvement of monochromatic light sources. ( d ) The laboratory synthesis of carbohydrate and protein is of considerable value, although the original theoretical material by the instrumentality of light. basis for the relation has been shown to be unsound. Stated mathematically, this relation is R.4DIATION HYPOTHESIS d- log _ -k A dT - T z The radiation hypothesis furnishes the present battle ground of most interest, and rightly so, because once we where k is the velocity constant of the chemical reaction, learn the ultimate relation between chemical activity and A is one-half of the energy required to change one molecule of frequency, many of our other problems will a t once be solved. 12. Electrochem., 24, 335 (1918).
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inactive material to an active form, and T is the absolute temperature. According to this relation ‘ the temperature coefficient of chemical reaction should be diminished in a homogeneous system by an increase in concentration of the catalyst. Not only has this not been found to be true, but the actual existence of special activated molecules, such as Arrhenius postulated, seems a matter of much doubt. I n 1914 Marcelin formulated a relation much the same as that of Arrhenius, which has the form
- d_ T-
d log k
E -RTZ
E being defined as the “critical energy” which it is necessary that the molecule absorb, in addition to that already possessed, to render it active; and, following the suggestion of Lewis, this quantity has since been called the “critical increment.” Somewhat late? (1915) Rice deduced the more mathematically exact relation d-l o-g k
V,
-
-
V,
+ 1/2 ( R T )
R Tz Here, V , is the critical value necessary before reaction takes place and T,i is the value of the mean potential energy of the molecules undergoing reaction. Sext, Lewis and Perrin, independently, made the assumption that the source of the energy increment is infra-red radiation, and Ijewis pointed out that this quantity should be determinable from the temperature coefficient. If, for example, we know the value of the velocity constant for a reaction at 25” C. and 35” C., we should have k 1 1 l o g x =; (G - jus) Taking the hydrolysis of methyl acetate as an example, the increase in k for 10°C. is 2.5. From the relation above, E is 16,800 calories per mol, or 1.03 X ergs per molecule. E By the photo-equivalence law of Einstein ( K = -, where h hv is Planck’s constant, 6.547 x l O - * 7 erg second, v the frequency, E the heat absorbed to produce the reaction, and n! the number of molecules dissociated by light of frequency v), Lewis finds that for the infra-red radiation X = 7.5 p, corresponding to the absorption band for methylacetate, hv should be 0.262 X ergs, or 4 hv would furnish the quantity of energy E . Bccording to the radiation hypothesis of Lewis, catalytic reactions should be fully explained. The function of the catalyst, according to this hypothesis, is merely to absorb infra-red radiation of the proper wave length and pass it on to the reacting molecules. According to this hypothesis, when reactions take place in solution there should be a definite absorption band for the solute, and the position of this band should be capable of calculation. The decomposition of triethylsulfine bromide in nitrobenzene has been studied by von Halban.2 From his data H. A. Taylor, in Lewis’ laboratory, calculated the critical increment E and found it to be 28,630 calories, which corresponds to a value of 3.0 X l O I 4 for v or a wave length of 1.0 p. Taylor then proceeded to determine the location of absorption bands for triethylsulfine bromide in the spectral region 0.8 to 3.1 y, and found that there is but one distinct band in this region, a band appearing a t 1.05 p, which is very close to the value calculated from the critical increment. This excellent agreement is, however, a rather exceptional instance. In most cases where the hypot,hesis has been tested, the calculated and observed absorption bands do not at all coincide. The application of the radiation hypothesis to monomolecular gas reactions should afford an interesting test of the ralue of the hypothesis. This has been accomplished by dT
22. p h y s Chem., 67, 129 (1909)
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Daniels and Johnston3 for the decomposition of nitrogen pentoxide. They calculated the critical increment and found it to be 24,700 calories. The wave length required for the decomposition of nitrogen pentoxide should therefore fall a t 1.16 p, but, unfortunately for the theory, this substance is not decomposed by radiation in this region of the spectrum. There is a marked absorption band at 5.81 p and a less marked one a t 3.39 p. These values are about five and three times the required value of 1.16, respectively. The significance of this is not yet clear. Daniels and h e c k 4 have also investigated the decomposition of nitrogen pentoxide dissolved in various solvents. Here the agreement between experiment and theory is no better than for the case cited for the reaction in the gas phase. Summarizing the situation, we may say that during the past three or four years much attention has been devoted to this interesting hypothesis-an hypothesis which would be of the utmost value if found to be substantiated. However, it must be frankly admitted that the present outlook for the hypothesis, a t least in its present form, is not encouraging. It is quite possible that something fundamental is being overlooked and that eventually we may be able to deal with reactions in a satisfactorily quantitative manner. On the other hand, the fundamental postulates may be wrong.
PRODUCTION OF COLDLIGHT This is a subject of great practical interest. Our modern lighting methods, in which visual radiation is produced by heating bodies to incandescence, are naturally very wasteful, since a large amount of energy is lost as temperature radiation. The firefly and glowworm have solved the problem somewhat better, though we humans would scarcely he satisfied with either of these lighting systems. The firefly produces substances which by oxidation produce light with little or no temperature radiation. We must at least admire his efficiency! What the firefly can do, man should be able to do, and far better. Undoubtedly, the time will come when light will be produced by controlled chemical reaction, and in a more economical way than by the systems now in use. I n one direction considerable improvement may be made, at least. Little expert knowledge is now used in the selection of paints and wall coverings in room interiors. Much of the light now absorbed and wasted may be saved and used by reflection, simply by a proper choice of wall covering. In addition, there is the possibility of using materials in paints and on paper or other surfaces that will emit light for many hours after receiving illumination during the day. It is felt that, in spite of all the excellent work done on phosphorescent sulfides, considerable work must still be done before we have the knowledge we need. There is distinctly a great field for research along this line in the hope of developing more efficient phosphorescent materials than are now known. Certainly, such an illuminating system, consisting of a soft glow emanating from the walls of n room, would be ideal from many points of view. Such illumination would not, only be restful to the eye, but would give possibilities of unique artistic effects. IhIPROVEhfENT O F
MONOCHROMATIC LIGHTSOURCES
The dependence of photochemical reaction on wave length (frequency) has already been mentioned. A moderate change in wave length may completely negative the effect desired. For example, oxygen may be changed to ozone or ozone to oxygen merely by a moderate change in wave length of the light illuminating the system. If unfiltered ultra-violet light from a quartz mercury-vapor lamp is used, the result will be an equilibrium between the ozone and oxygen, the a 4
J . A m . Chem. SOC.,43, 7 2 (1921). I b d . , 44, 757 (1922).
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position of which will be dependent upon the relative intensities of the two groups of wave lengths involved, and upon the temperature. Usually, photochemical reactions are sensitive to a comparatively narrow band in the spectrum. If a wide band is used, energy is therefore wasted, and an undesirable result may be produced. Now that light is playing an increasingly important role in the photosynthesis of many compounds, it becomes more important that sources of very intense monochromatic illumination be developed. The prebent practice is either to secure approximately monochromatic light by dispersion of light through a prism, letting the desired wave length fall upon a slit behind which the reacting system is placed, or by the use of filters designed to filter out the wave lengths not desired. I n eit)her case, the intensitj of the resulting illuminating beam is not great, particularly if it is anywhere near monochromatic. I n order that reactions may be carried out on anything like a commercial scale, better means of producing powerful, intense sources of such light must be developed. sPSTIIESIS O F CARBOHYDRATE AKD P R O T E I N M a T E R I A L
Much interest has lately been aroused in the synthesis of carbohydrates and protein material, particularly as a result of the fine investigations of Baly and his eo-workers in England and Baudisch in this country. As a result of these investigations the mechanisms of these photochemical processes are better understood, and while the work of the two groups is not entirely confirmatory, at least much progress
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has been made. It is believed that because nature seems to use chlorophyll for the synthesis of carbohydrates, it does not necessarily follow that this is the best of all possible catalysts, and it is the hope of the photo chemist that new and more effective catalysts may be found for this important photochemical process.
UTILIZATION OF RADJAKT ENERGY OF SUN Another photochemical problem of great interest, and one which has excited the imagination of the scientists of many generations, is the possible utilization of the enormous quantities of radiant energy coming from the sun. Waste places, now useless for any purpose, may eventually become the sites of great manufacturing plants, once the secret of transforming radiant energy into chemical energy is solved. Here we have an unlimited and almost constant source of energy of enormous magnitude, and one which by its successful utilization will tend to neutralize in a large measure the effect of our diminishing coal and oil supply. While the coal and oil problem may not be a serious one for the present generation, surely the time will come when other sources of energy supply must be found and utilized if civilization is not to perish. Knowing that hundreds of billions of horse power of radiant energy are constantly coming to the earth from the sun, it is not strange that the photo chemist casts a speculative eye toward the possible utilization of this enormous amount of energy, a very considerable portion of which is now entirely wasted.
Occurrence of Levo-Menthone in Pine Oil' By Augustus H. Gill I V f A S S A C H U S E T T S I N S T I T U T E OF
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HE pine oil known as "Spinol" was manufactured by dry distillation of wood, the heat being controlled by a circulating oil bath, the three fractions being turpentine, pine oil, and impure resin oils known as tar oils. This had been redistilled so as to free it from the turpentine fraction; it had stood for about fourteen years, giving ample opportuiiity for resinification. The physical characteristics of the sample were: Specific Gravity a t 15.5'' C. 0.9856
Refractive Index a t 20' C. 1 4868
Optical Rotation a t 26" C -70 20'
The oil mas distilled under 4 mm. pressure, a t first small quantjtios of water and terpenes distilled over, and a large portion, about 60 per cent, distilled over between 74" and 79" C. Fraction between 79" and 100" C. was 10 per cent. By repeated fractional distillation of the first fraction a t atmospheric pressure, two distinct fractions were isolated, one boiling a t 202' to 203" C. which constituted 8 per cent of the original oil, while the other had a boiling point of 208.5" to 209.5" C. and constituted 20 per cent of the oil. The fraction boiling between 79" to 100" C. at 4 mm. was gluey to the touch and was suspected to contain terpineol; attempts to convert it into terpin hydrate by the method of Tieniann and Schmidt2were in vain; hence, the oil contained no terpineol, thus differing in its most important constituent from the pine oils so far studied. The fraction boiling at 202" to 203" C. was thought to be fenchyl alcohol. It was optically inactive. It was oxidized to its corresponding ketone fenchone by nitric acid.s The inactire Eenchone thus obtained had a boiling point of 193" C. and an odor resembling camphor. The inactive fenchone 1 2
*
Recexved July 10, 1923. Be7 , 28, 1781 (1895). A n n , 263, 131, 146 (1S91).
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was next converted into semicarbazone with semicarbazide hydrochloride, having a melting point of 173" C., differing from the active form which has a melting point of 182' to 183" C. Thus, the presence of inactive fenchyl alcohol was conclusively proved. The second fraction, which was levo-rotatory, having an optical rotation -20" 28', was identified as I-menthone by converting it into semicarbazone, melting point 184O C. This was the first time that I-menthone was isolated from oils of the Pinus family. A study of the resinous matter in the oil was made by steam distilling the oil, when the first half, which distils over, contains all the fenchyl alcohol and Z-menthone, and an appreciable quantity of resinous matter. The other half, which remains in the flask, consists of a viscid liquid, an examination of which showed an acid value of 2.7, saponification value of 17.2, and an iodine value to be 65.6. These values are in very close agreement with those found for rosin oil, except for the fact that the rosin oils so far studied are strongly dextro-rotatory, the value varying from 30 to 50, while this sample under examination was levo-rotatory, having a value of about -14". It is possible that some of the menthone in the oil may have polymerized or resinified, thus giving a levo-rotation. It gave the Renard, Liebermann-Storch, and Halphen reactions for rosin oil. This pine oil differs from others in its high menthone content and absence of terpineol; besides fenchyl alcohol it probably contains small quantities of terpenes and sesquiterpenes, which have changed into the resinous body, the remainder constituting a large percentage of the rosin oil, with small quantities of rosin spirit and colophony. Acknowledgment is due 11. J. Karasji by whom some of the work was performed.
