T H E ULTRA-VIOLET ABSORPTION SPECTRUM OF FURFURAL BY FREDERICK H. GETMdN
The absorption of ultra-violet radiation by furfural was first studied by Hartley and Dobbiel in the course of an investigation of the absorption spectra of various heterocyclic compounds. Having prepared a highly purified sample at 764.9 mm.) they determined the absorption of furfural, (B.p. 161~-161.5~ produced by varying thicknesses of an alcoholic solution containing 0.096 gram, or I milligram-molecule, of furfural, C5H402,in 2 0 cc. of solvent. . The following table gives the wave-lengths expressed in micro-millimeters, pp, where general absorption was found to commence.
TABLE I
x
Thickness in mm.
1/x
25
372.3
20
346.7 346.7 339.9
15
IO
5 4 3
325.0 325.0
268.6 288.4 288.4 294.2 307.6 307 a6 318.3
314.1 308.7 323 e 9 I 308.7 323.9 From these results they concluded that alcoholic solutions of furfural exhibit strong general, but no selective absorption. Some years later, in a comparative study of the absorption spectra of 3 series of hetcrocyclic compounds as vapors, as liquids and in solution, Purvis2 re-examined alcoholic solutions of furfural and found, contrary to the statements of Hartley and Dobbie, that a N/rooo solution exhibits marked selective absorption, the head of the band occurring at X270pp. In view of these contradictory statements it appeared to be of interest to re-investigate the behavior of furfural toward ultra-violet radiation in both alcoholic and aqueous solutions. A Hilger quartz spcctrograph (size E6) with a wave-length scale was used in making the spectrograms. As a source of ultra-violet radiation a condensed spark between two electrodes, made of alloys of ferro-vanadoum and ferrochromium respectively, was used. This coinbination was found by the author several years ago3 to give an unusually large number of closely spaced lines many of which are characterized by exceptional brilliance. It has proven so 2
Hartley and Dobbie: J. Chem. SOC.73,598
* Purvis: Ibid. 97, 1655 (1910). a
Getman: J. Phys. Chem. 25,
150
(1921).
( I 898).
P. 11. GETMAX
3 93
satisfactory that he ventures here to reiterate its merits as a source of ultraviolet radiation. The furfural was prepared and purified according to the procedure outlined in a previous paper1 treating of the electrical conductance of various salts dissolved in furfural. The ethyl alcohol used was prepared by refluxing 95 per cent alcohol over lime and subsequently distilling, while in the preparation of the aqueous solutions, so-ca1led“conductance water” was used. In each series a mother solution containing 0.96 gram of furfural in roo cc. of solvent, i.e. a N / I O solution, was prepared by direct weighing, and from this N/ IOO and N j ~ o o osolutions were prepared by dilution. It should be pointed out that N / I O aqueous solutions of furfural are perfectly homogeneous, it having been shown by MainsZthat separation into two layers does not occur iintil a concentration of about 8 percent by weight of furfural is reached. The solutions were examined in a “Baly tube” provided with a I O ern scale graduated in millimeters. The thickness of the absorbing layer was diminished step by step until complete transmission was secured. The time of exposure was uniformly I O seconds throughout the entire seiies of experiments. The limits of abdorption in both the alcoholic and aqueous solutions were found to be identical. I t is probable that, with a spectroscope of greater dkpereive power, the limits of absorption in the alcoholic solutions would be found to lie nearer the visible portion of the spectrum in accordance with the law enunciated by Kundt that the limits of absorption of a solute are pushed toward the red region of the spectrum as the refractive index of the medium is increased. The limits of absorption in N/ro and N/IOOsolutions, together with the corresponding values found by Hartley and Dobbie, and Purvis, are given in the following table.
TABLE I1 (N/Io solution)
X (H & D)
Thickness in mm.
...
33
347 325
I3 2
I
3 16 (N/IOOsolution)
311
30
*..
I3
3 16
...
309
2
I
300
...
‘Getman: J. Phys. Chem. 28, ZIZ (1924) * Mains: Chem. nnd Met. Eng. 26, 779 (1922).
...
297
...
TIlE ULTR.4-VIOLET ABSORPTIOK SPECTRUM O F FURFURAL
3 99
The spectrograms of N / ~ o o ofurfural solutions were found to reveal, in addition to general absorption, the existence of a well-defined and persistent band. The absorption curve corresponding to this band is plotted in Fig. I together with that determined by Purvis, the former being the full line curve and the latter the dotted. It is apparent that the two curves are essentially identical, the differences being no greater than the probable experimental errors of the method. The position of the head of t8heabsorption band is the same in each, via., X270pp, or frequency 3 70.
-i
3
a 3 6 I FIG. 2
The marked absorbing power of minute quantities of furfural for ultraviolet radiation is further brought out by the spectrogram shown in Fig. 2 , where the six successive spectra have the following significance: ( I ) spectrum of source, ( 2 ) absorption spectrum of 50 mm. of solvent, (3), (4), ( 5 ) and (6) absorption spectra of solutions of furfural containing I part of solute in roo, 1000,10,000and IOO,OOO parts of solvent respectively. It will be seen that only in the last and most dilute solution are the shorter wave-lengths transmitted. It is undoubtedly to the persistence with which furfural absorbs ultra-violet radiation that we must attribute Hartley and Dobbie's failure to tliscover its selective in addition to its general absorption. Although freshly distilled furfural is almost colorless, it develops color on exposure to light, or on prolonged standing, passing from a pale straw-colored liquid, through various shades of yellow and orange to a dark brown liquid which is almost opaque to light. It seemed of interest to ascertain whether, as the visible absorption spectrum of furfural changes, a corresponding change tnkes place in its ultra-violet absorption spectrum. I n order
400
F.
H. GETMAN
to settle this point, a flask containing a sample of pure and nearly colorless furfural was exposed to the direct rays of the sun for two weeks, a t the expiration of which time the liquid had acquired a deep amber color. A series of aqueous solutions were prepared from this substance and their ultra-violet absorption spectra examined in the manner already described. It was found that tbe deep band in the N / ~ o o osolution remained unaltered, but that in the more concentrated solutions the general absorption was greater than in corresponding solutions of freshly distilled furfural. This is shown in the following table where the comparative values of the limits of absorption of varying thicknesses of N/ro solutions of the pure and yellow modification of furfural are given.
TABLE I11
(N/Io soIution) Thickness i n mm.
(Pure)
(Yellow)
IO0
3 70 3 70 345 335 3 16
420
50 IO
5 I
In other words, exposure to light appears t'o cause an increase in the absorption in the more concentrated solutions in the direction of the longer wavelengths, the yellow color being due to the selective absorption of the violet and a portion of the blue region of the visible spectrum. It seems highly probable that the instability of furfural as shown, by its tendency to polymerize, is intimately connected with its marked absorbing power for ultra-violet radiation. If it be true, as the advocates of the theory of dynamic isomerism maintain, that no organic compound shows an absorption band unless the possibility of tautomerism exists within the molecule, the existence of selective absorption in furfural might be explained by an isorropic process such as is represented by the following formulas:
/O\
c.1I II -c - c-c
1_
-c /O\ cI -c I = c-
This, however, is purely conjectural and no satisfactory explanation of the existence of an absorption band in furfural or other organic compounds can be advanced until a rational theory of the absorption of radiation has been developed. I n this connection it is of interest, however, to note that Baeyerl 'Ber. 10, 1358 (1877).
THE ULTRA-VIOLET ARSORPTION SPECTRUM OF FURFURAL
40 1
as the result of his study of the chemical behavior of furfural, advanced three possible structural formulas of the compound, as follows: H-Cj
H-
/o\
40 l-C-H
- -H
Hillside Laboralory. Waniford, Cnnn.
H-C H-L
/o\ =
40
C-C-H
L-H
/o\
C
40
C-C-H
