Intramolecular hydrogen bonding of azo dyes in aqueous solution.

in 0.1 M ionic strength aqueous solution at 120 for the following: 2,4-dihydroxy4'-nitroazobenzene (I), 2,4-dihydroxy-4'-sulfonateazobenzene (II), and p-(2hydroxy-l-naphthylazo) benzenesulfonic acid (III). The reported values of k231 were 4O X 105, 3.6 X 105, and 3.6 X 106 M-1 sec-1. They attributed the much lower values of k23, for these acids than the value3 k23, 2 X 109 M-1 sec-1 kn,' for the superficially similar reaction HP042+ OHI F043+ H20 to strong intramolecular hydrogen bonding in the protonated azo dyes that is absent in the hydrogen phosphate dianion. The greater electronegativity of the substituent NO2 in (I) than S03 in (II) accounts qualitatively for a greater decrease in charge on the azo nitrogen of (I) with a necessarily weaker intramolecular hydrogen bond in (I) and consequent larger value of k23,. These values of k23, for three azo dyes raise the question whether trends of pKaM = loglo(aHI [A2-]/ [HA-]) and k23, in a larger sample of azo dyes can be correlated with electrostatic effects of ring substituents on the electronegativity of the azo nitrogen atom that acts as a proton acceptor in the intramolecular hydrogen bond. Since resonance stabilization would promote a stable planar structure for the hydrogen bond about equally in all these azo dyes, this effect may be neglected. Materials and Methods.-2,4-Dihydroxy-4'-nitroazobenzene (I), 2,4-dihydroxy4'sulfonateazobenzene (II), 4-(2-thiazolylazo)-resorcinol (IV), 6-methyl4-(2-thiazolylazo)resorcinol (V), and 4-(2-pyridylazo)-resorcinol (VI) were purchased from Aldrich Chemical Co., Inc., and 4-(m-nitrophenylazo)-resorcinol (VII) from Eastman Kodak Co. We