Melvyn W. Mosher' ond Jay M. Ansell
Morsholl University Huntington, West Virginia 25701
Preparation and Color of AZO-Dyes
It has long been the goal of the chemical educator to bridge the dichotomy between the classical, but perhaps out-dated exercises of the organic laboratory and the students' theoretical classwork. To this end, a simple experiment for an undergraduate freshman or organic laboratory has been designed to relate the color of certain substituted azo-dyes to their visible absorption spectra. The experiment illustrates how the color of a number of azo-dyes arises and the shift of the color of these compounds by the introduction of suhstituents on the molecule of structure I. The method of preparation is the classic diazonation of aniline and coupling reaction as illustrated by eqn. (1).
upon the nature and position on the ring of the substituent and indicates the substituent's ability to attract or repel electrons, and p is a constant that is characteristic of the reaction series and is a measure of the sensitivity of this series to electron withdrawal or donation. The reaction constant, p, will vary with a change in reaction conditions and the nature of the reaction at the side chain upon which the reaction is taking place. The validity of the equation was originally restricted to meta and para substituted benzenes, hut has since been extended to include certain non-bulky ortho suhstituents on aromatic systems and most types of aromatic ring systems including heterocyclic compounds (4). The value for rho indicates something about the mechanism of the reaction. A positive value indicates that the reaction series is aided by electron withdrawal from the aromatic ring. Examples are the rate of addition of HCN to benzaldehydes (p = 1.49) and the rate of saponification of methyl benzoates in 60% ethanol (p = 2.46). A negative value indicates the reaction series is hindered by electron. withdrawal. Examples are the rate of free-radical sidechain bromination of toluenes ( p = -1.46) and the rate of hydrolysis of benzyl chlorides in 50% acetone ater a t 60°C (p = -1.69) ( I ) . In addition to the normal Hammett relation as illustrated in eqn. (2), other forms for finding a structural re-
In our organic classes, we have found that this is a useful way of introducing the Hammett Equation2 and structure reactivity relationships in general. The Hammett Eauation is one of the oldest. but most widelv used methods of the quantitative relatibnship between structure and reactivity in aromatic series (eqn. (2)) where k and ko are either rate or equilibrium constants for reactions of the substituted and the unsubstituted compound, respectively, a is a constant that is dependent 'To whom reprint requests should be addressed. Present address: Physical Science Department, Missouri Southern State College, Joplin, Missouri 64801. 2 More detailed reviews of the Hammett Equation can be found in references (1-3).
Figure 1 . Plot of visible absorption maxima in nanometers for arodyes (I) formed from substituted aniiines versus Hammett sigma constants. The equation for the least square line, excluding points forp-NO2 and PMeO, is y = - 1 8 . 7 8 ~ 480.8; standard deviation ot the slope is 1.03: correlation coefficient 0.986.
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Volume 52, Number 3, March 1975 / 191
Visible Absorption Maxima for a Number of Substituted Azo-Dyes of Structure I.a
absorption maxima (region 400-490 nun)
sigma constant for substituent
D-CH.
480b 470 475 476 486
0.00 +0.55 +0.39 +0.78 -0.17
m-Br m-C1 0x1 o-CH.
475 474 474 487
+0.23 +0.37 +0.40 -0.10
substituent P-H P-CF, p-Br p-NO1
a In all cases, the sigma values of the substituent are also listed. I n nm.
lationship have been reported, for example, that reported by Ingraham (5) to relate the carbonyl absorption frequencies of substituted aromatic aldehydes with the sigma values of the various substituents. This relationship is illustrated in eqn. (3)
where v is the absorption maxima of the substituted compound, vo is the absorption maxima for the unsubstituted compound, a is the Hammett substituted constant, and p is the reaction constant. The results of plotting our classes' data using eqn. (3) are illustrated in Figure 1. It can be seen that even ortho substituents have been found to give as good a correlation as the more commonly used para and meta substituents. This allows the number and variety of anilines that can be used in this experiment for the preparation of the azodyes to be increased. In all cases, ortho sigma constants used were those calculated by Charton (51, while the other sigma constants were those found in Gould ( I ) . The table indicates the substituent, a, constant. and the absorption maxima determined for these compo"nds. A plot o? the absorption maxima for the azo-dye as a function of the Hammett sigma constant for the "arious substituents also produces a straight line (Fig. 2). Experimental
Place 1 g of the substituted aniline in a 50-ml beaker. Slowly add 25 ml of 0.5 M hydrochloric aeid (this can be prepared from 4 ml of concentrated hydrochloric aeid diluted to 100 ml with distilled water). Place the beaker in an ice bath and cool to W10'C. Add to this solution of the aniline hydrochloride, 1 g of sodium nitrite dissolved in 5 ml of water. Allow the mixture to stand at ice bath temperatures. Prepare a solution in a 250-ml beaker of 1 g beta-naphthol in 100 mi of 2% sodium hydroxide. Cool this solution to about 10°C by adding crushed ice slowly. Slowly pour with stirring the diazonium salt solution into the beta-na~htholsolution. Acidify the mixture till it is neutral to Litmus and filter the am-dye h i suetion filtration. It is important that the dye is neutral since these compounds are all acids, and with bases are converted into the anionic form. This form has no absorotion maxima in the reeion 470-500 nm, but has undergone a strong blue shift to the reglon 400-450 nm.
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Journal of Chemical Education
Figure 2. Plot of (u - v o ) / u o versus Hammett sigma constants for the visible absorption maxima of azadyss (I) formed from substituted anilines. The equation for the least square line, excluding P-NO2 andp-Me0. is y = -0.0414~ 0.00303: standard deviation of the slope is -0.00359: correlation coenicient 0.961.
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A small pea-sized partion of the dye is dissolved in either 95% ethanol or methanol. The color intensity is adjusted by dilution of a small aliquot with the solvent until the orange color is just visible. The visible absorption spectrum of the solution at every 20 nm is obtained from 700-320 nm. In the region of the absorption maxima (region 430-490 nm) the absorption maxima of the dye is obtained every 10 nm or less. A plot of the values is made and the absorption maxima of the dye is determined. This value is plotted on the class graph as a function of the substituent on the aniline. In this experiment, we bave encountered a laree deviation from the expected absorption maxima when suhstituents that interact mainly by resonance with the azo-linkage were used. These two substituents were p-methoxy and p-nitro (absorption maxima 462 and 476,~. res~ective1~). One major advantage of this experiment is the low cost and ease of availability of the starting anilines and other starting materials. The major drawback we have encountered is that Spectrometer 20 type instruments do not give accurate and re~roducible measurements of ~- the ~--. wavelength for the absorption and hence make determining the absorption maxima more difficult. We bave encountered deviations in the maxima of *8-10 nm for these dyes. This deviation will as a result cause many of the electron donators to have absorption maxima a t higher wavelengths than the unsubstituted dyes and many of the electron withdrawers to have absorption maxima at lower wavelengths than the unsubstituted dyes. As a result, a more expensive grade of scanning spectrometer is required. Hence, fewer machines would be available, and therefore would require longer time for the classes to complete their analysis. ~
Literature Cited (11 Gould, E. S., "Moehanism and Stmdur. in Organic Chemistry." Holt, Rinehsrt aod Winston, NeuYork 1939, pg22&227. (21 March, J., "Advanced Oqanie Chemistry: Reaction Mechanism and Structure," MeGrsu-Hill Bmk Co.. New York. 1968. pg 2-1. (3) JaKe'. H.H.,Cbm.Reu.. 53. 191 (1953). (4) Mosher, M. W. Shsrma. C. 8..and Chakrsharty, M. R., J. Mop. Reaownre. 7, 247 (1972). 151 Ingrahsm, L. L.. Couse. J., Bailey, G. F., and Stitt, F.. J. A m s r Chem. Soe., 74, 2291 (19521. (6) Chartoo, M.. J. Amsr. Chpm Sor., 86.2033(1964).
