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H. Yousefi, A. Yahyazadeh / Chinese Chemical Letters 23 (2012) 685–689
H2SO4, NaNO2
N2 HSO4
X
X
NH2
0- 4 oC
N2 HSO4
OH
N
OH
1) pH: 7.8 - 8.6
2) pH: 5.5 - 6
N
+
X
O
O
X
X: 1= -OC2H5, 2= -NHCOCH3, 3= -CH3, 4= -H, 5= -Cl, 6= -F, 7= -COCH3, 8= -NO2
Scheme 1. The azo compounds synthesized from 4-benzyloxyphenol.
Diazotization of p-substituted aniline derivatives was carried out according to the method reported in Ref. [9]. 4-
Benzyloxyphenol (0.600 g, 3 mmol) dissolved in sodium hydroxide (0.300 g, 10 mmol) and water (10 mL). This
solution was cooled in an ice-bath 0–5 8C and the diazonium solution of aniline derivative that previously prepared
was added drop-wise over 20 min with vigorous stirring. The pH of the reaction mixture was maintained at 7.8–8.6 by
adding 2.5% sodium hydroxide solution. The mixture was then stirred for 1 h at 0–5 8C. After completion of the
reaction the pH was maintained between 5.5 and 6 by simultaneous addition of 10% hydrochloric acid solution. In the
end, the resulting solid was filtered, washed thoroughly with cold water and dried. Recrystallization from EtOH–H2O
ended in pure crystals of the dyes.
4-(Benzyloxy)-2-((4-ethoxyphenyl)diazenyl)phenol (1). Red crystals, yield 89%; mp: 129–130 8C; FT-IR (KBr): n
3470 (OH), 3080 (Aro.-H), 1460 (N N) cmÀ1; 1H NMR (CDCl3): d 1.50 (t, 3H, J = 6.8 Hz), 4.16 (q, 2H, J = 6.8 Hz),
5.13 (s, 2H), 6.96 (d, 1H, J = 8.8 Hz), 7.00 (dd, 1H, J = 8.8, 2.8 Hz), 7.08 (d, 2H, J = 9.2 Hz), 7.38 (t, 1H, J = 7.2 Hz),
7.42 (dd, 2H, J = 7.6, 7.2 Hz), 7.49 (d, 2H, J = 7.6 Hz), 7.52 (d, 1H, J = 2.8 Hz), 7.86 (d, 2H, J = 9.2 Hz), 12.50 (b, 1H,
OH). Anal. calcd. for C21H20N2O3: C, 72.40; H, 5.79; N, 8.04. Found: C, 72.45; H, 5.81; N, 8.10.
Supplementary data are also available for the other products shown in Scheme 1.
2. Results and discussion
From the spectra obtained, it can be drawn that these dyes may exist in two tautomeric forms of which the
deprotonation leads to a common anion, as shown in Scheme 2. The infrared spectra of the dyes 1–8 in (KBr) showed
strong absorption bands at 3410–3470 cmÀ1 related to the hydroxyl group. The 1H NMR spectra recorded in CDCl3 at
room temperature showed a broad singlet peak at 12.30–12.60 ppm for the hydroxyl proton (OH). The spectral data
generally lead to the conclusion that, these dyes exist in the azo form in the solid state and chloroform.
The absorption spectra of dyes 1–8 were recorded in various solvents at a concentration of 10À5–10À6 mol LÀ1, and
the results are summarized in Table 1. It was found that the absorption maxima of these dyes are strongly solvent
dependent and varied strikingly by changing the solvent. Stated another way, the more polar the solvents, the greater
are the bathochromic shifts. Regarding this fact, the following order was discerned for the influence of solvents on the
increment of lmax: DMSO > DMF > acetonitrile > methanol > acetic acid > chloroform.
As an example, the spectral shifts of dye 8 in various solvents are outlined in Fig. 1. As can be seen in Fig. 1, the
spectra of this dye show one maximum at 429 nm in chloroform and one at 434 nm in acetic acid, while in DMSO,
DMF, acetonitrile and methanol a new more intensive band appears in the range of 448–470 nm (Fig. 1). Thus, the
absorption curves almost pass through an isosbestic point located approximately at 434 nm indicative of an
equilibrium established between either two tautomeric forms (azo and hydrazone) or between one tautomeric form and
the anionic form. In proton donating solvents such as acetic acid, chloroform and methanol, the dyes demonstrate a
O
N
O
N
H
N
H
N
Benzyl
Benzyl
O
Ar
O
Ar
(hydrazone)
(azo)
Scheme 2. The tautomerism in the dyes.
Yousefi, Hessamoddin
