JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Conversion of Azo-6-aminouracils to Azo-barbituric Acids
acid (BAazo-1 – 4) were recrystallized from ethanol-diethyl ether
mixture.
NMR (125 MHz, DMSO-d6, d/ppm): 178.19 (>C=O), 169.50
(>C=O), 125.62 (Ar-C), 80.81 (Ar-C), 79.93 (Ar-C), 116.93
(-C-N-), 28.24 (N-CH3), 27.37 (N-CH3). Anal. Calcd for
C12H11N5O4: C, 47.22; H, 3.63; N, 22.94. Found: C, 47.09; H,
3.79; N, 23.09. Determination of pKa values of BAazos (1 - 4):
The acid dissociation constants of the BAazos were determined
by following UV-vis spectroscopic method. The concentration of
the dyes (BAazos 1 – 4) in the buffer solution of pH 2.0–12.0 was
maintained at 1x10-5 mol/dm3. The ionic strength, I = 0.1 of all the
solution was maintained with KCl solution. The pKa values of all
the as-prepared dyes were calculated from the obtained spectra
following two well-known spectrophotometric methods: the
half-height and the modified limiting absorption from the absorp-
tion –pH relationship, as reported from our laboratory.22 More-
over, it is the Henderson–Hasselbaltch acid-base equation which
could be expressed as in Eq. (1) which relates pH and pKa to the
equilibrium concentration of anionic (L-hydazo) and neutral form
(HLhydrazo) of our investigated hydrazone dyes of BAazos.
Physical and spectral data of the products are given as fol-
lows.
1,3-dimethyl-5-(phenylazo)-barbituric acid (BAazo-1):
A light yellow solid (79%) with a melting point of 253 ± 1 °C. IR
(KBr pillet, u/cm-1): 3446 (bs, -OH), 3114-2936 (bw, -NH), 1719,
1674 (s, >C=O), 1641 (s, >C=O---H), 1515 (s, -N-N=C), 1461 (s,
-N=N-), 1363 (s, -C-N-), 1273 (s, -C-O), 1094 (s, -N-N-), 954,
808, 750, 469, and 416 (others). 1H NMR (CDCl3, d/ppm): 14.65
(s, 1H, NH), 7.58-7.43 (m, 5H, Ar-H), 3.43 and 3.40 (s, 6H,
N-CH3). 13C NMR (125 MHz, DMSO-d6, d/ppm): 160.39 (>
C=O), 158.69 (>C=O), 150.62 (>C=O), 141.38 (Ar-C), 129.70
(Ar-C), 126.19 (Ar-C), 116.79 (-C-N-), 28.09 (N-CH3), 27.18
(N-CH3). Anal. Calcd. for C12H12N4O3: C, 55.38; H, 4.65; N,
21.53. Found: C, 56.01; H, 4.73; N, 21.65. 1,3-dimethyl-5-(p-
methylphenylazo)-barbituric acid (BAazo-2): A light yellow
solid (63%) with a melting point of 249 ± 1 °C. IR (KBr pillet,
u/cm-1): 3446 (bs, -OH), 3109-2937 (bw, -NH), 1718, 1673 (s,
>C=O), 1639 (s, >C=O---H), 1507 (s, -N-N=C), 1444 (s, -N=N-),
1358 (s, -C-N-), 1278 (s, -C-O), 1084 (s, -N-N-), 952, 818, 747,
pH = pKa + log{[L-hydrazo]/[HLhydrazo]}
(1)
1
460, and 413 (others). H NMR (CDCl3, d/ppm): 14.70 (s, 1H,
CONCLUSIONS
NH), 7.47-7.23 (m, 4H, Ar-H), 3.43 and 3.39 (s, 6H, N-CH3), 2.38
(s, Ar-CH3). 13C NMR (125 MHz, DMSO-d6, d/ppm): 160.45
(>C=O), 158.68 (>C=O), 150.52 (>C=O), 139.09 (Ar-C), 135.87
(Ar-C), 130.15 (Ar-C), 116.79 (-C-N-), 28.06 (N-CH3), 27.14
(N-CH3), 20.56 (Ar-CH3). Anal. Calcd for C13H14N4O3: C, 56.93;
H, 5014; N, 20.43. Found: C, 57.13; H, 5.22; N, 20.59. 1,3-
dimethyl-5-(p-chlorophenylazo)-barbituric acid (BAazo-3):
A light yellow solid (65%) with a melting point of 248 ± 1 °C. IR
(KBr pillet, u/cm-1): 3444 (bs, -OH), 3115-2956 (bw, -NH), 1715,
1673 (s, >C=O), 1644 (s, >C=O---H), 1508 (s, -N-N=C), 1450 (s,
-N=N-), 1363 (s, -C-N-), 1275 (s, -C-O), 1081 (s, -N-N-), 952,
807, 750, 450, and 419 (others). 1H NMR (CDCl3, d/ppm): 14.61
(s, 1H, NH), 7.52-7.38 (m, 4H, Ar-H), 3.43 and 3.39 (s, 6H,
N-CH3). 13C NMR (125 MHz, DMSO-d6, d/ppm): 160.19 (>
C=O), 158.63 (>C=O), 150.40 (>C=O), 140.51 (Ar-C), 129.89
(Ar-C), 129.59 (Ar-C), 118.46 (-C-N-), 28.13 (N-CH3), 27.21
(N-CH3). Anal. Calcd for C12H11ClN4O2: C, 48.91; H, 3.76; N,
19.01. Found: C, 49.05; H, 3.63; N, 18.92. 1,3-dimethyl-5-(p-
nitrophenylazo)-barbituric acid (BAazo-4): A light yellow
solid (57%) with a melting point of 292 ± 2 °C. IR (KBr pillet,
u/cm-1): 3447 (bs, -OH), 3117-2946 (bw, -NH), 1726, 1678 (s,
>C=O), 1643 (s, >C=O---H), 1515 (s, -N-N=C), 1460 (s, -N=N-),
1368 (s, -C-N-), 1249 (s, -C-O), 1086 (s, -N-N-), 955, 863, 749,
It is concluded that azo-uracil derivatives containing
–NH2 groups at their 6-positions could be converted into
the corresponding barbituric acid derivatives by following
simple acid assisted hydrolysis process in aqueous medium
and characterized spectroscopically. The presence of >
C=O group instead of 6-amino group at the six position of
as-prepared azobarbiburic acid makes it hardly responsive
to solvatochromism. Similar to azouracils, they also exhibit
structural emissions. In addition, their solution behavior to-
wards acids and bases is totally in opposite direction, for
example, the methanolic solution with wavelength maxima
at 392 nm for BAazo-2 shifts towards blue at 358 nm in
presence of base whereas the wavelength maxima remains
unaffected in presence of acid; the completely opposite
phenomenon was observed for azouracil derivatives. The
determined acid dissociation constant (pKa) values in-
crease according to the following sequence: BAazo-2 > 1 >
3 > 4. Moreover, such knowledge of conversion reaction
may enrich the chemistry of azo-pyrimidine derivatives for
their potential applications in different field of sciences.
ACKNOWLEDGEMENTS
Authors are grateful to the Department of Science and
Technology (DST), Govt. of India, for financial support
(grant no. SR/NM/NS-1176/2012(G)&(C)) and to the De-
1
490, and 418 (others). H NMR (CDCl3, d/ppm): 14.55 (s, 1H,
NH), 8.34-7.66 (m, 4H, Ar-H), 3.45 and 3.42 (s, 6H, N-CH3). 13
C
J. Chin. Chem. Soc. 2016, 63, 000-000
© 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9