S.J.J. Titinchi et al. / Journal of Molecular Structure 705 (2004) 121–126
125
Table 4
Comparison between melting points and pH values of compounds 1–9 and of some a-aminoacids and related Schiff bases. (pH in ethanol at fixed
concentrations 0.001 mol dmK3
)
Our Compd no.
m.p./8C
decomp.
pHa Value
Related Schiff
base no.
m.p.b/8C
a-amino-acid
name
m.p/8Cc
decomp.
pHa value
1
2
3
4
5
6
7
8
9
197
163
197
187
131
269
267
293
185
5.80G0.05
5.72G0.02
5.72G0.11
5.78G0.04
5.92G0.01
5.99G0.05
6.22G0.08
5.98G0.04
5.96G0.11
10
11
12
13
14
15
16
17
18
Liquid
86
Glycine
232
297
277
238
260
318
320
Alanine
Liquid
Liquid
Liquid
161
Histidine
Arginine
Cystein
Tryosine
Phenyl-alanine
6.57G0.02
6.57G0.02
91
79
51
Compounds 10, 11, 12 and 13 are thiophene-2-, pyrrole-2-, furan-2- and thiophene-3-carboxylideneaniline, respectively. Compound 14 is n-butylidene-aniline.
Compound 15 is p-chlorobenzylidene-p-N,N-dimethyl-aminoaniline. Compound 16 and 17 are p-nitrobenzylidene and p-N,N-dimethylamino-benzylideneani-
line, respectively. Compound 18 is benzylideneaniline.
a
Each value is the average of at least two readings.
b
Values taken from Refs. [9,10,23].
c
Values taken from Ref. [27].
our Schiff bases with those analog which have not containing
COOH group in their structures, (ii) to compare their melting
points with the melting points of some a-aminoacids and,
(iii) their pH values in ethanolic solutions with the pH values
of two aminoacids in the same solvent and concentration
(0.001 mol dmK3). Table 4 represents these data, three
important arguments can be observed firstly, the melting
points of compounds 1–9 are much higher than those of their
analogs, which contain no COOH group in their structures,
and this means that the chemical structures between them are
entirely different. Secondly, the melting points of com-
pounds 1–9 are comparable with those of p-N,N-dimethyl-
aminobenzylideneanilines mono-acid salts [1–4] to some
extent with the melting points of a-aminoacids, all are high
and decompose without melting (Table 4) which means that
compounds 1–9 may have structures of an ionic character
similar to that of a-aminoacids (Scheme 3). Thirdly, the pH
values of their ethanolic solutions are close to the pH values
of the ethanolic solutions of phenylalanine and tyrosine
(Table 4) but they are slightly acidic, this is probably because
the proton transfer from COOH to the azomethine group is
not complete. The melting points and pH data represent a
further support to our suggestion from being compounds 1–9
as a mixture of free and ionized forms, where the former is
predominant.
The strong absorption band in IR spectra at
l670–l690 cmK1 are assigned to NH bend in the iminium
tatuomer which is not obtained in any Schiff bases
not containing NH or NH2 group in their structures.
In addition to other two absorption bands in the region of
550–530 cmK1 for torsinol oscillation N–H and
3300–2300 cmK1, which is characteristic of the quaternary
ammonium salts.
The absorption band at 270–288 nm in the electronic
spectraofallcompoundswithnearlysameintensitysuggested
the formation of the same moiety. Thus, this transition is due
to the iminium tatuomer of the carboxylated anion, which is
red-shifted to 286–294 nm in the presences of glacial acetic
acid due to the protonation of the carboxylated anion.
Based on the conductivity measurements it is concluded
that the carboxylated Schiff bases are less ionized than
aminoacids and it may existed in two forms, the ionized form
(zwitterion) and the free unionized form. The latter form is
predominately existed. This is further consolidating our
suggestion of the existence of both free and ionized forms.
It is known that amino acids are zwitterionic compounds both
in the solid and in solution. In solution, the zwitterion is in
equilibrium with both its conjugated base and its conjugated
acid and the position of equilibrium depends on the acidity of
the medium. On comparing our carboxylated Schiff bases
with amino\ acids we can further extended our support.
4. Conclusion
The main purpose of this study is concerns to investigate
the molecular structure and to allege the prototropic
equilibrium between some N-aryl imines possessing a
carboxyl substitutent and their zwitterionic iminium
tautomers. The postulate is based on the assignment of IR,
UV–Visible spectral absorptions and molar conductivities
that provides a good evidence for the interpretations of
postulated tautomeric equilibrium.
References
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[2] A.A.H. Saeed, Indian J. Chem. 17A (1979) 105.
[3] A.A.H. Saeed, Indian J. Chem. 17B (1979) 462 and references cited
therein.
[4] R.L. Reeves, W.F. Smith, J. Am. Chem. Soc. 85 (1963) 724.
[5] B.J. Ishwara, H.R. Shivakumar, J. Mol. Liq. 111 (2004) 101.