3214
L. G . s. BROOKER.4ND R. H.SPRAGUE [CONTRIBCTION NO.
820 FROM
THE
Color and Constitution. IV.l
KODAK RESEARCH LABORATORIES 1
The Absorption of Phenol Blue
BY L. G. S. BROOKER AND R. H. SPRAGUE Phenol blue is an un-ionized dye that may be represented by the resonance scheme Ia t----f Ib.
metrical dyes, employed in previous papers.1s2v3 For such a comparison I must be regarded as the structural cross between the cation of Bind(a, ~ Y I ~ ~ x ~ - N = ~ ~ = green o schedler’s (11) and the anion of phenol in: I dophenol (111), the iodide and the sodium salt of (b) bIenN= + C)=N--cI>-. which are shown in the formulas. Both of these ions are symmetrical, each being represented by It is of special interest from the theoretical stand- two identical extreme resonance configurations, point, because the two extreme resonance con- only one of which is shown in each case. figurations, I a and Ib, differ only as regards the terminal auxochromic atoms, the chromophoric I1 Me2NC>--N=o&Me2] Ichain being the same in each. Any difference of 111 [o=(=>=N-C>-o-],, stability that exists between the two configurations, therefore, may be ascribed solely to the If it were found that I absorbed exactly midgreater or less stability of MezN- and =O in Ia way between 11 and 111, i t could reasonably be compared to Me&=+ and -0- in Ib, bearing in argued that the degeneracy of the configurations mind that the opposite poles in Ib will exert a in I was as complete as in the parent symmetconsiderable electrostatic attraction on each rical dyes, and that Ia and I b were of equal other which will tend to make the configuration stability. If the observed absorption occurred relatively unstable. a t shorter wave length than that calculated from It is well known that the absorption of an I1 and 111, however, the interpretation would un-ionized dye is strongly affected by the solvent, be that Ia and I b differed in stability, and the and thus phenol blue gives solutions which vary amount of the difference between imax, obsd. and in color from reddish-violet in cyclohexane to X,, calcd, could be used as a measure of this deep blue in water. The values of Amax are energy difference. given in Table I for four solvents, and it is seen Compared with I, the absorptions of I1 and I11 that the absorption is shifted to progressively are relatively little affected by change of solvent. longer wave lengths as the dielectric constant of This is readily understandable, for since the exthe solvent is increased. The most reasonable treme resonance configurations of any symmetriexplanation of this is that although I b is, in the cal dye are identical, one cannot be stabilized by absence of solvent effects, less stable than I a be- any particular solvent more than the other. The cause of the coulombic interaction of the opposite values for Amax, are given in Table I and the charges of the dipole, the polar arrangement in points are plotted in Fig. 1. I b is increasingly stabilized by the dipole orientaTABLE I tion of the surrounding solvent molecules as the Approx. dielecA polarity of the solvent is increased. Thus stabitric X X A max. conmax. rnax. max. I Devialized, Ib tends to approach Ia in stability, and Solvent stant I I1 I11 calcd. tion the increasing degeneracy of the resonance struc- Cyclohexane 2 5520 ?. .. .. .. tures is accompanied by absorption a t longer Acetone 21 j820 7290 6420 6825 1005 wave lengths. 685 31 6120 7260 6400 6805 Methylalc. 65 80 6680 7280 6280 6745 The above argument has the limitation that it TVater gives no idea as t o how nearly Ia and I b approach The ionized dyes are both insoluble in cycloeach other in absolute terms. Further insight hexane, and comparisons of all three dyes were reinto this problem, however, is gained by using the stricted t o acetone, methyl alcohol and water. technique of comparing the absorptions of un( 2 ) Brooker, Sprague, Smyth and Lewis, ibid., 68, 1116 (1940). symmetrical dyes with those of the related sym(3) Brooker, White, Keyes, Smyth and Oesper, ibid., 63, 3192
[
(1) Part 111. THISJOURNAL, 63, 3208 (1941)
(1941).
'rr-Is ABSORPTION
Nov., 19-1I
OF PHENOL
BLUE
3213
TABLE I1 ,Mole fractions (C2), dielectric constants (D), densities of solutions in benzene (d), polarizations a t 25 O ( P J , molar refractions ( M R D ) ,polarizations a t 25" (Pm), observed moment ( f i ) and calculated moment ( p calcd.) Compound
CZ
I
0.000000 ,001226 .002475 ,003119 ,00585;i
d
D 2.2807 2.3382 2.4014 3.4415 2,5582
0.87233 ,87317 ,87406 ,87446 ,87635
Pr
Pi
The difference between, , , A, obs& for I and that calculated as the harmonic mean between I1 and I11 may be called the "deviation," and this varies considerably from solvent to solvent. I n acetone, configuration I b is less stabilized by the solvent than i t is in solvents of higher polarity, and the deviation is considerable, amounting to 1005 A. This is reduced to 685 A.in methyl alcohol and t o only 65 A.in water. I n this latter solvent, therefore, the stabilization of I b is so great that the degeneracy of configurations I a and I b is nearly complete. Thus, if the phenol blue molecule is sufficiently shielded against the effect of the attraction of the opposite poles in Ib, the degeneracy of the configurations reaches a very high level indeed, a conclusion which is interesting and perhaps surprising. The contribution of I b even in benzene is brought out by the fact that the observed dipole moment of 5.80 * 0.07 is much greater than the value of 2.4 * 0.5 calculated from Ia. We are greatly indebted t o Professor C. P. Smyth and Mr. P. Oesper for these values. Their data are given in Table 11. From the above conclusions it would appear that the lack of color of Michler ketone (IV), even in strongly polar solvents, is in all probability not due to the electrostatic attraction of (a)
Me2N=-C--Me?
/I
0
Iv (b) M e 2 S ( T > - C = a = & M e 2
I
-0
-
-
the charges in configurations of the type of IVb, but rather to the heavy stabilization of IVa be-
26,789 736 1 753 787
1
713
MRD
Pm(25O)
07
765 * 4 0
P
P calcd. ( X 10'9
( X 10'9
5.80 *0.17
2.4 1 0 . 5
J
CYCLOHEXANE SOLVENT
i,l_Iii'l I C !
ACETONE
METHYL LCOYOL
'
,
(
1
i
1~ 1
WATER I
l
1
L
l
I
-1
11
!,I
h
LA
- 1
,
I
I ,
~
I
If---' 1
!
1.;
+L
,
I
1
i 1
