[CONTRIBUTION FROM
THE
DEPARTMENT OF CHEMISTRY OF THE OHIOSTATE UNIVERSITY]
THE RELATION BETWEEN THE ABSORPTION SPECTRA AND THE CHEMICAL CONSTITUTION OF DYES. XVIII, THE EFFECT O F POSITION ISOMERISM ON THE ABSORPTION SPECTRA O F HALOGEN DERIVATIVES OF PHENYLAZOPHENOL WALLACE R. BRODE
AND
LA VERNE E. CHEYNEY
Received October 30, 1940
This paper is an extension of an investigation begun a number of years ago, concerning the relation between the absorption spectra and the chemical constitution of simple derivatives of phenylazophenol. In the previous papers of this series (I, 2, 3) it was shown that the positions and magnitudes of the absorption bands of the methyl, nitro, and nitro methyl derivatives were functions of the types, numbers, and positions of substituents present. In this investigation the methods and technique of the previous studies have been extended to the analogous chlorine and bromine derivatives of phenylazophenol; these included the eleven possible mono- and dichloro derivatives, in which only a single chlorine atom occurs on a benzene ring, and the eleven analogous bromine derivatives. The positions of substituent elements or radicals are indicated by the notations: o', m', 0 , o'm, etc., in accordance with the following formula:
D-N-N~OH m'
m
0'
o
In the mixed-substituent compounds involving methyl and nitro groups, the first symbol designates the methyl position; and the second, the nitro position. The unsubstituted compound is indicated in the tables by the symbol "Ph." These dyes were prepared from purified intermediates by standard coupling procedures ; they were recrystallized from appropriate organic solvents to constant melting points. The melting points of these compounds are given in a preliminary report presented at the Sixth Summer Conference on Spectroscopy (4). Those not previously recorded in the chemical literature were analyzed for their azo nitrogen content by the titanous chloride method of Knecht and Hibbert (5) as modified by Calcott and English (6). 341
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Three solvents of different chemical character were employed for the spectroscopic measurements: 95% ethyl alcohol, concentrated hydrochloric acid, and 3% aqueous sodium hydroxide. These solutions were prepared by the methods previously described ( 2 , 3 ) . The dye concentration was about 7.0 X 10-6 g. mole8 per liter in the alcohol and alkaline solutions and about 3.5 X 10-6 g. moles per liter in the acid solutions. It is t o be noted that the acid and alkaline solutions each contained a small amount of alcohol. The absorption spectra measurements in the ultraviolet were made with a Bausch and Lomb medium quartz spectrograph, equipped with a modified Hilger sector photometer; in the visible region a Bausch and Lomb Universal Spectrophotometer was employed. The methods and system of nomenclature have been previously described (3, 7, 8). The data were plotted with frequency in fresnels (vibrations/seconds X 10") as abscissae and extinction (log 2)as ordinates. Each curve was based on from fifty
I
to one hundred points. Due to lack of space, i t is not powible t o reproduce the sixtysix curves here in sufficient size t o show the variations. The frequencies and magnitudes of band maxima have been indicated in Figs. I, 11, 111, and IV. In these figures, however, for the sake of comparison, the experimental extinction values have been recalculated t o a common basis of concentration, the unit chosen being 1.5 X 10-4 g. moles per liter; these values are also calculated to a cell thickness of 0.5 cm., for ready comparison with previously reported values. The experimental thickness was 0.4 cm. in all cases. In any one solvent, the general shape of the curves is the same, and is quite similar to those previously reported. Significant differences, however, may be noted in both frequency and intensity. DISCUSSION OF DATA
A . Alcohol solutions. In this solvent the weak band in the violet region found by Brode (3) for the methyl derivatives, but absent in the nitro compounds (l), is present for about half the halogen derivatives. Its presence seems, however, to bear no simple relation to position of substitution. The third band in the extreme ultraviolet has been shifted towards higher frequency so that its maximum does not appear in the region studied. The principal band in the near ultraviolet is present in all cases and this study is confined to its characteristics. A general shift towards lower frequency with increase of molecular weight may be noted. In many cases, however, the shift is slight, and is within the limits of experimental error, so that no quantitative generalization may be stated. In Fig. I are plotted the variations of the principal band extinction and frequency shift with position of substitution for the substituting groups studied thus far. The general shape of the curve is very similar in all cases. Closer examination reveals several other interesting facts : 1. A nitro group increases the absorption band extinction in practically
ABSORPTION SPECTRA O F DYES
343
all positions studied, in the disubstituted (nitro methyl) compounds the values are considerably greater than those of any other derivatives. 2. The chloro and bromo curves run practically parallel throughout, with the bromo value being the higher in all cases. 3. The methyl curve, while similar in character to the others, does not bear any constant relation to them, since enhancement and depression effects appear to be of a lower order of magnitude.
FIG. Specific absorption indices (extinction) of the absorption maxima 0. the alcohol solutions (upper) and frequency of principal band maxima (lower). Substituent positions are indicated by 0 , m,o‘, etc. for CHa (-), C1 (- -) Br (-----) and NOz and CHs,N02(- - -) derivatives of phenylazophenol.
4. In every case the maximum extinction value of the monosubstituted series is that of the p’ isomer. 5. In the disubstituted series the maximum extinction values in every case are those of the p’o and p‘m isomers with the p’m values being greater than the p’o values. 6. The minimum extinction values for the chloro and bromo derivatives are those of the 0‘0 compounds.
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7. For the monosubstituted compounds the minimum extinction value is that of the 0’ isomer. 8. The average frequency position of disubstituted derivatives is less than the average frequency position of monosubstituted derivatives indicating a molecular weight effect.
FIG.11. Specific absorption indices (extinction) of the absorption maxima of the concentrated hydrochloric acid solutions (upper) and frequency of principal band maxima (lower). Substituent positions are indicated by 0,m, o’, etc. for CH, (-), C1 (- -) Br (-----) derivatives of phenylazophenol.
B. Hydrochloric acid solutions. In this solvent the first band noted for the methyl derivatives is entirely missing in the halogen derivatives. As in alcohol, the third band is shifted towards higher frequency, so that its maximum does not appear in the region studied. The second band, which is the principal one, is present in all cases, occurring in the visible region.
ABSORPTION SPECTRA OF DYES
345
Increase of molecular weight again produces a noticeable lowering of frequency, except in the m position, where little or no change is evident. In Fig. I1 are plotted the changes of the principal band extinction and frequency shift with position of substitution. (The original study on nitro derivatives did not include hydrochloric acid as a solvent, hence that information is lacking in this figure.) Again the curves are strikingly similar in shape, and a study of their characteristics leads to the following generalizations: 1. The chloro and bromo extinction effects are parallel, with the bromo value being the greater in all cases. 2. The methyl curve runs parallel to the halogen values and is greater in extinction. 3. The maximum extinction value for the monosubstituted halogen compounds is that of the p’ derivative and the minimum value is that of the 0’ derivative in all cases. 4. For the disubstituted halogen compounds the maximum extinction value is that of the p’m derivative followed by the p’o derivative and the minimum value is that of the 0‘0 derivative. 5. Halogen derivatives show greater maximum and minimum extinction effects than the methyl derivatives. 6. The average frequency values of the disubstituted derivatives are less than the monosubstituted derivatives, indicating a molecular weight shift of frequency. C. Sodium hydroxide solutions. In this solvent two strong bands are present in the ultraviolet region; however, as in the other solvents, the peak of the second band was not included in the region studied. The first band appears to be composed of two components, which together produce the observed principal band. Since this principal band varies considerably in shape with different compounds, it is difficult to draw any conclusions with regard to the relations between either frequency or magnitude of absorption and chemical constitution. In general, the frequency seems to be shifted toward lower values with increasing molecular weight. It has been found possible to analyze this principal band into two components (A and B) by the method described by Brode (3). By this means it is possible to determine approximately the frequency and magnitude of the component bands. The general shape of these curves lends support to the hypothesis that the two components of the principal band are produced by two forms of vibration of the molecule, which are present in a variable equilibrium, and that this equilibrium is influenced by the position of the substituting group. In accord with this hypothesis, it is apparent in Fig. 111 that the extinction values of one of the components (A) do not show the regularity of effectdemonstrated in the alcohol and hydro-
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chloric acid solutions of these dyes. However, the sums of the extinction values of the absorption bands (A B) show the same regular variation with position of substitution, as reported for the alcohol and hydrochloric
+
FIG.111. Specific absorption indices (extinction) of the absorption maxima of the “A” component (upper) and the “A” “B” components (lower) of the bi-component principal band observed in 3% aqueous sodium hydroxide. Substituent positions C1 (- -) and Br (-----) derivatives of are indicated by 0 , m,o‘ etc. for CHs (-), phenylazophenol.
+
acid solutions. A study of these curves permits the following generalizations : 1. As noted in the other solvents, the chloro and bromo curves are practically parallel throughout, with the bromo value being the higher in all cases.
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ABSORPTION SPECTRA OF DYES
2. The methyl curve is roughly parallel to the other two, but shows general significant variations, such as a slightly greater average extinction. 3. In all cases the maximum extinction value of the monosubstituted compounds is that of the p’ derivative. 4. In the disubstituted compounds, the maximum extinction value in all cases is that of the p‘o derivative. This differs from the other solvents, where the p’m value is the greatest. 5. In the chloro and bromo series, the minimum extinction value is that of the 0‘0 compound, which is in agreement with the results observed in other solvents. 6 . In an attempt to classify the equilibrium condition between the “A”
( ALJ While the arrangement shows
and “B” components the proportion of A has been plotted Le. against position of substitution (Fig. IV).
~
a70
A A+B.
-. -
010
o’m m‘m m
p’m Ph
m’
p’
d
0’0
o
$0
m’o
“A-B” EQUILIBRIUM I N DILUTE AQUEOUS A SODIUM HYDROXIDE, I.E., A + B CHa (-)1 Cl (- -) and Br (-----)
FIG. IV. PROPORTION OF “A”
IN THE
a definite trend, the results are not as satisfactory as those reported for the methyl derivatives (3). SUMMARY
The study of the influence of character and position of substituents on the absorption spectra of phenylazophenol has been extended to include twenty-two mono- and di- halogenated compounds in which not more than one halogen is attached to each ring. These measurements have been made in the visible and ultraviolet regions of the spectrum from 400 to 1400 f (750to 215 mp) in three solvents; 95% ethyl alcohol, concentrated hydrochloric acid, and 3% aqueous sodium hydroxide. There is a marked uniformity with regard to the intensity of extinction and the frequency shift as a result of the position of the substituent group in which para substitution enhances the extinction. There is a regular shift of maxima position in which the substitution of greater mass shows greater deviations from the mean. Substitution in the p’ position produces
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BRODE AND CHEYNEY
an increase in extinction in the order, methyl, chloro, bromo, iodo, and nitro, where the methyl shows the least effect; while substitution in the o’ position, and in particular 0’0,produces a marked decrease in both extinction and frequency of the absorption band. In accordance with the method of analysis of the bi-component alkali band into “A” and “B” factors, it is to be noted that the A values fail to conform with data from other solvents. However, the sum of A and B conforms reasonably well with the data obtained in other solvents. The A analysis of the alkali data through the study of the ratio of A indicates +
that it is possible to classify the various types of substituent placing through a study of these data. COLUMBUS, OHIO. REFERENCES BRODE,Ber., 61, 1722 (1928). BRODE,J . Am. Chem. SOC.,61,1204 (1929). BRODE,Bur. Standards J . Research, 2,501 (1929). BRODE,Proc. Sixth Summer Conference on Spectroscopy, John Wiley and Sons, N. Y., 1939, p. 128. (5) KNECHTAND HIBBERT,“New Reduction Methods in Volumetric Analysis”, Longmans, New York, 1926. (6) CALCOTT AND ENGLISH, Ind. Eng. Chem., 16, 1042 (1923). (7) GIBSONAND OTHERS, J . Optical SOC.Am., 10, 169 (1925). (8) GIBSONAND OTHERS,Bureau of Standards Sci. Papers 440, 18, 121 (1922).
(1) (2) (3) (4)
