Synthesis, spectroscopic and molecular structures investigations of some carboxylated schiff bases

Article abstract of DOI:10.1016/j.molstruc.2004.06.025

A series of nine carboxylated Schiff bases (five of them are newly prepared viz. compounds 5-9), are prepared and characterized by various physico-chemical techniques. The molecular structures of synthesized Schiff bases are investigated by IR, UV-Visible, molar conductivities at different concentrations in two different solvents and by their pH values in ethanolic solutions.The IR spectra show absorptions due to =N⊕H- stretching and -N-H bending vibrations, the UV-Visible spectra indicates absorptions are due to protonated species. The molar conductivities, 0.1-0.6 Ω^-1 cm² mol^-1, prove that these compounds are weak electrolytes and are even weaker than tyrosine and phenylalanine, 2.5-13 Ω^-1 cm ² mol^-1. The melting points and pH values of Schiff bases are compared with those of some α-aminoacids and some related Schiff bases that have no COOH group in their structures. On the bases of these data, it was concluded that carboxylated Schiff bases exist in two forms, the ionized and the free base where the later is predominant. The ionized form is similar to the zwitterion of the α-aminoacid, in which a proton is transferred from COOH to the azomethine (-CHN-) group.

Full text of DOI:10.1016/j.molstruc.2004.06.025

Journal of Molecular Structure 705 (2004) 121–126
www.elsevier.com/locate/molstruc
Synthesis, spectroscopic and molecular structures investigations
of some carboxylated schiff bases
Salam J.J. Titinchia, Hanna S. Abboa, Ali A.H. Saeedb,
*
aDepartment of Chemistry, College of Science, University of Basrah, Basrah, Iraq
bDepartment of Chemistry, College of Science, University of Baghdad, Baghdad, Jadiriya, Iraq
Received 27 April 2004; accepted 21 June 2004
Abstract
A series of nine carboxylated Schiff bases (five of them are newly prepared viz. compounds 5–9), are prepared and characterized by
various physico-chemical techniques. The molecular structures of synthesized Schiff bases are investigated by IR, UV–Visible, molar
conductivities at different concentrations in two different solvents and by their pH values in ethanolic solutions.
4
The IR spectra show absorptions due to ZNH– stretching and –N–H bending vibrations, the UV–Visible spectra indicates absorptions
are due to protonated species. The molar conductivities, 0.1–0.6 UK1 cm2 molK1, prove that these compounds are weak electrolytes and
are even weaker than tyrosine and phenylalanine, 2.5–13 UK1 cm2 molK1. The melting points and pH values of Schiff bases are
compared with those of some a-aminoacids and some related Schiff bases that have no COOH group in their structures. On the bases of
these data, it was concluded that carboxylated Schiff bases exist in two forms, the ionized and the free base where the later is
predominant. The ionized form is similar to the zwitterion of the a-aminoacid, in which a proton is transferred from COOH to the
azomethine (–CHaN–) group.
q 2004 Published by Elsevier B.V.
Keywords: Carboxylated Schiff bases; Molecular structures; Zwitterionic iminium tautomers
1. Introduction
Recently very few studies are found to study and
investigate the behaviour of some carboxylic acids by
conductometric techniques like tartaric acid in which
ion-solvation behaviour was investigated by molar
conductance at different solvents mixtures [5]. Thermo-
dynamics parameters related to association and solvation
have also been determined.
Previously, it is well known that p-N,N-dimethylamino-
benzylidene-anilines (Scheme 1) give red colour in glacial
acetic acid or in aqueous sulphuric or hydrochloric acids
[1–4]. These Schiff bases form monoacid salts, these salts
were isolated as red to orange solids from their aqueous
sulphuric acid solutions, they have high melting points,
above 200 8C [2–4]. Upon neutralization with dilute sodium
hydroxide, the free Schiff base was recovered in every case.
In some cases, the rate of a chemical reaction can be
conveniently followed by monitoring the conductance of the
solution as a function of time and as the conductance
depends on the number of ions or ionic species in
the solution and therefore on the degree of ionization of
the electrolyte and hence on the molecular structure.
A clear understanding of the molecular structures of
carboxy Schiff bases is required, since they are the precursor
of many pharmaceutical products [6] and its important as the
carboxylic group increases the stability of the aldimine Schiff
base structure [7,8].
To achieve useful results a systematic study of synthesis,
characterization and molecular structural is investigated.
In the present work, structural studies of nine carboxylayed
Schiff bases are attempted by following their melting
points, IR and UV–Visible spectra, pH values of their
ethanolic solutions, and their conductivities in ethanol and
dimethylsulphoxide (DMSO).
* Corresponding author. Tel.: C964-1-7199650; fax: C91-133-227-4122.
E-mail address: aahsaeed@yahoo.com (A.A.H. Saeed).
0022-2860/$ - see front matter q 2004 Published by Elsevier B.V.
doi:10.1016/j.molstruc.2004.06.025

122
S.J.J. Titinchi et al. / Journal of Molecular Structure 705 (2004) 121–126
and voltage stabilized power supply supplied from WTEC,
Ltd. The minimization method of Fouss and Hisa [12] was
used for the analysis of the conductivity data.
Scheme 1.
3. Results and discussion
The elemental analysis of the synthesized carboxy Schiff
bases revealed their empirical formula. Table 1 shows the
analytical data, melting points, colour and yield for these
new compounds.
3.1. IR spectra
Scheme 2.
Table 2 shows the major IR bands in the spectra of
carboxylated Schiff bases 1–9, the characteristic features of
these spectra are: (i) The appearance of very broad strong
bands in the region of 3300–2300 cmK1, (ii) the appearance
of strong absorption in the region of 1690–1670 cmK1 and
(iii) the appearance of medium absorption in region of
550–530 cmK1. These absorptions are generally not
observed in the IR spectra of any kind of Schiff bases not
containing NH or NH2 group in their structures [13–19].
The absorption at 3300–2300 cmK1 is characteristic of
the quaternary ammonium salts [20] and accordingly,
this absorption is assigned to the stretching vibration of
aNH– in the IR spectra of our Schiff bases. Bending
vibration of N–H and its torsinol oscillation are located at
1690–1670 and 550–530 cmK1, respectively, [20]. The only
way, which leads to such absorptions in our Schiff bases, is
proton transfer from COOH group to the azomethine group
(–NaCHK) forming a di-polar ion structure similar to that
occurs in the a-amino acid (zwitterion ion), Scheme 3 (I).
Since some spectra of our Schiff bases show stretching
vibrations of O–H bond (Table 2), therefore it can be
suggested that the proton transfer is limited.
2. Experimental
Compounds 1–4 (Scheme 2) are well-known compounds
and were prepared according to standard literature
procedure as discussed elsewhere [9–11]. The new
carboxylated Schiff bases viz. compounds 5–9 were
prepared by refluxing for 90 min, equimolar amounts of
p-amino benzoic acid and the corresponding aldehyde in
ethanol. On cooling, a solid product in every case was
collected and recrystallized from ethanol. Ethanol and
DMSO were of ‘Fluka’ spectrosil grade, glacial acetic acid
was of ‘Analar’ grade BDH (99.6%), and they were used
directly without further purifications. The elemental C,
H and N analyses were performed on Perkin Elmer 240B
elemental analyzer. The IR and UV–Visible spectra were
measured on Pye Unican SP3-300 IR spectrometer and
Shimadzu UV-160 UV–Visible spectrometer, respectively.
IR spectra were measured as KBr disks. The pH values were
measured by Metrohm G32 pH-meter in ethanol solutions
(10K3 mol dmK3) at 298 K. The molar conductivities were
measured at 298.15 K (cell constantZ0.1796082 cmK1) on
a conductivity bridge Wyne Kerr Auto Balance precision
Bridge, type B 331 MK II, connected to a frequency
The iminium-phenolato zwitterions were also proved
by the IR and NMR spectra of phenolic Schiff bases
and iminium-phenolato tautomerizes to imine-phenol upon
reaction with carboxylates [21].
Table 1
Numerals, names, melting points, yield and analytical data of synthesized Schiff bases
Compd
no.
Name of compound/Stiochiometric
m.p.8C
(d)
Yield
%
Colour
Calcd (Found)
%C
%H
%N
7.33(7.62)
5
6
7
8
9
C11H13NO2 n-Butylidene-p-aminobenzoic acid
131
269
267
293
185
88
76
69
82
92
Yellow
powder
Yellow
solid
69.11 (68.98) 6.81(6.77)
64.74(64.41) 3.84(3.82)
C
14H10NO2Cl N-(p-Chlorobenzylidene)-p-aminobenzoic
5.41(5.40)
acid
C14H10N2O4 N-(p-Nitrobenzylidene)-p-aminobenzoic
Yellow
needles
Yellow
powder
Colourless
powder
62.22(62.01) 3.70(3.68) 10.37(10.41)
71.64(71.42) 5.97(5.81) 10.45(10.32)
acid
C
16H16N2O2 N-(p-N,N-Dimethylbenzylidene)-p-amino-
benzoic acid
C14H11NO2 Benzylidene-p-aminobenzoic acid
74.70(74.50) 4.90(5.10)
6.20(6.10)
d, decomposition.

S.J.J. Titinchi et al. / Journal of Molecular Structure 705 (2004) 121–126
123
Table 2
The major IR bands in the spectra of compounds 1–9 measured as KBr disks
Compd
no.
d,n/cmK1
n/O–H
n/Z4NH
d/N–H
n/CZN
Asym.
Sym.
Torsinol oscillation
N–H
C(–O)2
C(–O)2
1
2
3
4
5
6
7
8
9
–
3200–2400,m;br
3100–2300,s;br
3200–2400,s;br
3100–2500,s;br
3300–2400, vs;vbr
3100–2400,s;br
3100–2500,s;br
3100–2500,s;br
3200–2500,s;br
1670,s
1670,s;br
1690,s
1670,s
1680,s
1675,s
1670,s
1675,s
1670,s
1620,m
1585,s
1595,s
1590,s
1585,s
1600,s
1595,s
1585,s
1590,s
1590,s
1415,s
1405,s
1410,s
1415,s
1415,s
1420,s
1410,s
1420,s
1420,s
545,m
545,m
540,m
545,m
545,m
530,m
535,m
535,m
550,m
b
3600a,m;br
–
3320,s
1660,s
–
1620,m
1615,m
1620,m
1615,m
1620,m
1620,m
3390,s 3320,m
–
–
–
3360,w 3460,w
m, medium; br, broad; s, strong; vs, very strong; vbr, very broad; w, weak.
a
Overlapped with pyrrole N–H.
Overlapped with the absorption at 1670 cmK1
b
.
3.2. The electronic spectra
3.3. Conductivity measurements
Table 3 shows the longer wavelength absorption bands
in the electronic spectra of compounds 1–9 in ethanol and
glacial acetic acid. n-Butylidene-p-aminobenzoic acid,
compound 5 has only one absorption band in its electronic
spectrum, at 286 nm. This transition is certainly originates
from the lonely aromatic ring in its structure, since
aliphatic structures do not absorb light in this region. The
absorption at 286 nm is observed in the spectra of all
compounds at the same position and almost with close
intensity. Owing to these facts, the same moiety in every
Schiff base molecule must be responsible for this
transition, namely moiety (I) of Scheme 3. The longer
wavelength absorption band at 358–319 nm is not observed
in the spectrum of compound 5. Since all compounds
except compound 5 contain either benzaldehyde, substi-
tuted benzaldehyde or five-membered heterocyclic ring in
their structures, therefore the absorptions at 358–319 nm
(Table 2) may represent the local excitations of these
moieties. These transitions are affected by the nature of
substituent. In glacial acetic acid, these absorptions are
blue-shifted, whereas the absorptions at 270–288 nm are
red-shifted to 286–294 nm. Structure (II) of Scheme 3 may
be responsible for the transitions at 286–294 nm in glacial
acetic acid. The UV/visible absorption spectra of some
polymeric Schiff bases dissolved in concentrated sulphuric
acid showed a band near 300 nm indicating the presence of
iminium ion [22].
Owing to the fast hydrolysis of Schiff bases in aqueous
solutions [10,14,16,23], we used ethanol and DMSO as a
media for measuring the molar conductivities. Fig. 1
represents the variation of molar conductivities with molar
concentrations of tyrosine and phenylalanine as a standard.
The figure shows that molar conductivities increase
rapidly with increasing dilution, and their molar
conductivies lie in the range 2.5–13 UK1 cm2 molK1 at
concentrations of 0.001–0.00002 mol dmK3. On comparing,
the conductivities of the carboxylated Schiff bases with that of
strong electrolytes such as HCl and NaOH, it is clearly
indicatedthatthesecompoundshaveverymuchsmallermolar
conductivity and even much smaller than that of weak
electrolytes such as acetic acid [24]. According to these
information, we can conclude that the carboxylated Schiff
bases are weak electrolytes as tyrosine and phenylalanine.
Figs. 2–4 represents the variation of molar conductivities
with molar concentrations of compounds 1, 2, 6, 8 and 7 in
ethanol and DMSO solutions, the data of these figures
Table 3
The longer wavelength absorption bands in the electronic spectra of
compounds 1–9 in ethanol and glacial acetic acid
Compd In ethanol lmax/nm
(2/m2 molK1
In glacial acetic acid lmax/nm
(2/m2 molK1
no.
)
)
1
2
3
4
5
6
7
8
9
Sh. 325(490), 283(2130)
334(1500), 285(1500)
327(2000), 288(2200)
Sh. 319(850), 282(1800)
286(1100)
Sh. 302(1420), 290(1920)
Sh. 314(970), 292(3270)
–
–
288(1630)
Sh. 324(870), 284(1950)
358(1350), 270(1780)
Sh. 330(830), 285(3600)
Sh. 328(330), 285(2050)
Sh. 298(1670), 288(2070)
346(2300), 294(2000)
Sh. 295(1870), 286(2180)
Sh. 298(1200), 289(2100)
Sh. Shoulder.
Scheme 3.

124
S.J.J. Titinchi et al. / Journal of Molecular Structure 705 (2004) 121–126
Fig. 3. The variation of molar conductivities with molar concentrations of
Fig. 1. The variation of molar conductivities with molar concentrations of
tyrosine and phenylalanine in ethanol at 298 K.
compound 8 in DMSO at 298 K.
equimolar mixtures of some Schiff bases and substituted
acetic acids were studied in relation to the nature of the state of
protonation indicate that, in low polarity solvents, weak acids
can only partially protonate the Schiff base, while in a more
polar medium the protonation percentages are higher [26].
shows that: (i) The molar conductivities of these
compounds, 0.1–0.6 UK1 cm2 molK1 at concentrations of
(0.78–13.8)!10K4 mol dmK3, are smaller by about 23 times
than that of tyrosine and phenylalanine, (ii) the molar
conductivities increase rapidly with increasing dilution and
(iii) the dielectric constant of the solvent has no effect on the
results (Fig. 4). Due to (i) and (ii) we can consider that
compounds 1, 2, 6, 7 and 8 are weak electrolytes and weaker
than tyrosine and phenylalanine. The variation of molar
conductivities with molar concentrations in ethanol and
DMSO of compound 5 (Fig. 5) seems to be unusual since the
molar conductivities increase slightly with increasing con-
centration, however the values 0.25–0.45 UK1 cm2 molK1
are still much smaller than those of tyrosine and phenyl-
alanine. Based on these results, it can be concluded that our
Schiff bases are less ionized than aminoacids. They may be
existed in two forms, the ionized species I of Scheme 3
(zwitterion) in an appreciable amount and, predominately
existed in the free (unionized) form.
3.4. Melting points and pH values
According to the IR, UV–Visible and molar conduc-
tivities data on our carboxylated Schiff bases, we have
decided to compare the followings: (i) the melting points of
This study is in agreement with the X-ray data, FTIR
and CPMAS spectroscopy of Schiff base derived of 2,20-
dihydroxybiphenyl-3-carbaldehyde with n-butylamine (which
contains less acidic group than carboxylic group, i.e. phenolic
OH) in which they obtained two symmetry-independent
zwitterions in unit cell [25]. Also the NMR spectra of
Fig. 4. The variation of molar conductivities with molar concentrations of
compound 7 in ethanol and DMSO at 298 K.
Fig. 2. The variation of molar conductivities with molar concentrations of
Fig. 5. The variation of molar conductivities with molar concentrations of
compounds 1, 2 and 6 in ethanol at 298 K.
compound 5 in ethanol and DMSO at 298 K.

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.
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