Study on tautomerism reaction of p-amino benzylidene-p-hydroxy
aniline by UV spectroscopy
A.S.P.Azzouz
and
A.B.N.Al-Dabagh
Chemistry Department , College of Education , University of Mosul , Iraq
Keyword: Schiff base , Cis-trans isomers , tautomerism , equilibrium
constant .
Abstract
p-Amino benzylidene-p-hydroxy aniline (I) was synthesized from paminobenzaldehyde and p-aminophenol by a standard method . Its
structure was confirmed by chemical and physical methods . The former
method included , by using specific reagents for phenol and primary
amino groups . The latter method performed by the measurements of
melting point and UV-IR spectra of I .
Originally , the project was designed to study stability constants of
azo dye complexes formed by reactions of I with diazotized sulphanilic
reagent and other similar reagents . The unsuccessful study for complex
formation stated last , encourage the workers in this investigation to shift
the direction of project toward a clear answered for the reason of
impossible determination of stability constant value for the dye stated .
Hence the tautomerism study which happened in I in basic medium with
their experimental factors , such as pH , temperature and thermodynamic
parameters were outlined in this investigation .
Introduction
Tautomerism was an important area of chemical study1 , that had
long been challenge computationally , experimentally and intellectually .
Tautomerism interconversions2 had been investigated by chemists during
last decades . Tautomerism study had received renewed attention due to
its importance3 in determination of compound properties and their area of
applications . Such topic was regarded as a major topic in theortical
chemistry . For example tautomerisms of keto-enol4-5 , imine-enamine6-7
and many other systems7 .
1
The concept of tautomerism of carbonyl8 , analide9 , Schiff bases10
and benzoin11 had been extensively studied .
As a continuation of tautomerism reactions11-13 , performed in our
laboratory . This work describes the influence of alkali as sodium
hydroxide on UV spectrum of p-amino benzylidene-p-hydroxy aniline ,
which led to tautomerism reaction . The work describs the influence of
buffer solutions and temperature in the range 3-10 and (283-333)K
respectively , on tautmerism reaction in Schiff base under study .
Experimental
Pure p-amino benzaldehyde and p-hydroxy aniline were supplied
from Fluka chemical company . They were converted to Schiff base I by
using a standard method14 , or by mixing equimolar amounts of p-amino
benzaldehyde and p-hydroxy aniline in 20ml absolute ethanol . The
product collected was recrystallized from ethanol , yield a product having
a melting point of (168-170)ºC .
Optimal conditions for tautomerism of imine I
The proper solvent used in tautomerism study of I , was a mixure of
20% acetone – 80%ethanol . Originally , a 5×10-5M of solution I in a
solvent stated was prepared , followed by measurement of its UV
spectrum against a blank sample . Then after , to this solution, either
different 0.5M and 1M of Na2CO3 or 1M NaOH were added , followed
by measurements of their UV spectra in a similar way .
Preparation of buffer solutions
The following buffer solutions were used in tautomerism study as in
Table (1).
Table (1) : Compositions of buffer15 in the pH range 4-10
pH
4
5
6
7
8
9
10
Composition
0.2M CH3COONa (9ml) + 0.2M CH3COOH (41ml)
0.2M CH3COONa (32.25ml) + 0.2M CH3COOH (14.75ml)
0.2M CH3COONa (19ml) + 0.2M CH3COOH (9ml)
0.05M Borax (12ml) + (12.4g Boric acid +2.93g NaCl in liter)(188ml)
0.05M Borax (11ml) + (12.4g Boric acid +2.93g NaCl in liter)(29ml)
0.05M Borax (40ml) + (12.4g Boric acid +2.93g NaCl in liter)(10ml)
Equal Volume from 0.025M (Na2CO3 +NaHCO3)
2
Effect of buffers on tautomerism of I
In order to confirm the tautomerism of I , a similar 1ml of 10-4M of
imine I in the mixed solvent stated was prepared and diluted to 10ml with
any buffer solutions . The UV spectrum for each study was measured
versus blank solvent .
Effect of temperature on tautomerism of I
A similar study was adopted here to the last by measurements of UV
spectra at a temperature range between (10-50)ºC . The last resulted to
different values of equilibrium constants , followed by linear plots shown
later using Microsoft Excel 2007 .
Instrumentations
1. The m.p of imine I was measured by using 30 SMP apparatus
manufactured by Bibby scientific limited 2003 .
2. The UV spectra was measured by using Shimadzo UV-1650
spectrophotometer using a quartz cells of dimensions 1×1×3 cm3 .
This connected to a water bath model EyEL4 type NTT-2200p ,
manufactured by Rikakikai Co. Ltd. Tokyo.
3. The IR spectra was measured for solid sample by using FT-IR
spectrophotometer Tenser-27 Bruker company by KBr disk method .
4. The pH of any solution was measured by WTW 82362-Weilheim
pH720 apparatus .
Results and Discussion
The chemical structure of imine I was confirmed by using similar
physical method , namely by using melting point and UV-IR spectra .
Results collected here were compared with other salicylidene ortho meta
and p-hydroxy anilines whose had similar structures and were well
agreed16 with our study .
A preliminary study was started by the measured of UV spectrum of
I in mixed 20% acetone-80% ethanol solvent as in Fig. 1 which showed
the following absorption bands as Table 2 .
3
Fig. 1 : Original UV spectrum of 5×10-5M of imine 1 in 20% acetone-80% ethanol
These bands have molar extension coefficient values of more than
1000 units and confirmed three π  π* transitions resulted from three
different chromophoric linkages, namely two aromatic rings and one
azomethine linkage . This was in disagreement with EL-Bayoumi17
finding on benzylidene aniline who has similar structure to imine I .
These variation of results could be interpreted due to the presence of
NH2 and OH substituents on para positions of aromatic rings of imine
I , which could cause such expected different results .
A previous study13 showed that addition of NaOH solution to
2-hydroxy-1-napthylaldehyde resulted to zwitter ion formation that
led to keto tautomer by resonance process .
On the same basis the workers added solutions of 0.5M and 1.0M
base Na2CO3 and 1.0M NaOH to 0.5×10-5M of I in 20% - acetone 80% ethanol mixed solvent as in Fig. 2 which showed the following
data :Table 2 : Absorption spectra of imine I in 80% acetone – 80% ethanol mixed
solvent
a) Original spectrum
b) After addition of bases
Conc.of base
a-
λ(nm)
333.0
250.0
4
A
1.188
0.818
Σmax Liter.mole-1.cm-1
23.76×103
16.36×103
0.5M Na2CO3
b-
1M Na2CO3
1M NaOH
234.0
321.0
259.6
251.0
242.0
318.8
259.0
250.2
246.2
320.4
259.6
251.4
243.0
0.248
1.057
0.919
0.983
0.972
0.724
0.591
0.644
0.669
0.704
0.668
0.708
0.680
4.96×103
21.14×103
18.38×103
19.66×103
19.44×103
14.48×103
11.82×103
12.88×103
13.38×103
14.08×103
13.36×103
14.16×103
13.60×103
Table 2 showed three UV bands of original spectrum , had
completely and suddenly changed to four UV bands after addition
of Na2CO3 and NaOH bases . In literature18-19 it was confirmed
that cis and trans keto tautomers appeared at longer wavelength as
compared with enol tautomer . Hence , it was suitable to
mentioned in this respect , that UV band appeared after addition of
bases as a result of tautomerism reactions in imine . These
appeared tautomers in imine I in a direction of increasing
wavelengths were in order of trans keto , cis keto , enol and imine
respectively . These tautomerism reactions happened in I in the
presence of bases were in agreement20-21 and with mechanism of
base catalysed21 reactions , as shown in Scheme I .
Influence of buffers in the range of pH 4-10 on tautomerism of I
In order to certain the mentioned tautomerism reactions happened in
I , this led to the preparation 10-4M solutions of imine in a buffers having
pH range 4-10 in the same mixed solvent . Results collected were
tabulated in Table 3 .
Table 3 : UV absorption bands of 10-4M imine I at buffers having pH
range 4-10 .
pH
Natural
4
λ(nm)
328.4
231.6
412.6
321.6
224.0
A
0.438
0.365
0.202
0.573
0.360
5
Σmax Liter.mole-1.cm-1
4380
3650
2020
5730
3600
5
7
8
10
412.2
322.0
232.6
230.8
323.8
325.8
234.2
327.4
234.6
0.202
0.564
0.446
0.536
0.351
0.625
0.478
0.701
0.567
2020
5640
4460
5360
3510
6250
4780
7020
5670
Table 3 showed the following results :1. The original UV spectrum of imine I having two bands in
equilibrium with each other . These related to the presence of I in
enol and keto forms with extinction values of 4380 and 3650
Liter.mole-1.cm-1 respectively . The greater Σmax value for enol as
compared with keto means an increase abundance of enol tautomer
over keto tautomer in solution .
2. At pH values 4, 5 imine I showed another third band at wavelength
412.6nm or 412.2nm with molar extension value of 2020 . This
attributed to the formation of nitriliun or oxonium ion as as result
of combination10 of H+ with nitrogen azomethine or with oxygen
of phenol group in imine . The rest of UV bands were related to the
occurance of I in enol and keto forms , with an increase in
concentration of enol as compared keto at these two pH values .
3. At pH7 , imine I showed two UV bands for enol and keto forms
with Σmax values of 5360 and 3510 respectively . This was in
agreement with upper previous paragraph .
4. Imine I had shown two UV bands at pH 8 or 10 related to keto and
enol tautomers respectively .
This last statement was confirmed with bathochromic shift of keto
band into a range of wavelength (325.2-327.4)nm and the
appearance of new extinction coefficient values of 7020 and 6250
Liter.mole-1.cm-1 for keto and enol tautomers respectively . In
summary statements , at pH 8 or 10 the tautomerism of I enol to its
cis and trans keto tautomers .
6
)a)
)b)
)c)
Fig. 2 : UV spectrum of 5×10-5M after addition of :a- 0.5M Na2CO3 , b- 1M Na2CO3 , c- 1M NaOH
7
CH
H
H 2N
O H +OH
N
C
N
K3
Enol-Amine
NH2
K2
O
O
Cis keto
K4
H
NH 2
C
N
CH
HN
Trans keto
N
OH
Imine
Scheme I : Mechanism of tautomerism reactions in imine I after addition of a
base
Influence of temperatures on tautomerism of I
In this respect a mixture from 5×10-5M from I with 0.5M Na2CO3 in
the mixed solvent mentioned , was prepared . The UV spectra for mixture
were recorded at temperature range (10-50)ºC ,versus blank sample . The
record spectrum was shown in Fig. 2a , having four peaks , as illustrated
in Table 4 .
Table 4 : Absorbances of mixed imine with Na2CO3 at temperature range
(10-50)ºC
tc0
A1
1nm
A2
2nm
A3
3nm
A4
4nm
10
0.840
251
0.300
334.6
0.581
259.6
0.604
242.2
20
0.782
251
0.460
323.0
0.601
259.8
0.629
242.2
30
0.728
251
0.551
321.4
0.671
259.6
0.659
242.2
40
0.655
251
0.700
321.4
0.708
259.8
0.669
242.2
50
0.590
251
0.718
321.2
0.767
262.4
0.710
242.2
These four wavelengths λ1 , λ2 , λ3 and λ4 were related to tautomers
enol , trans keto , cis keto and new imine product tautomer respectively .
Actually , the last tautomer was formed by tautomerism of type
eneamine
imine reaction .
These λ1 , λ2 , λ3 and λ4 having absorbances values A1 , A2 , A3 and
A4 respectively . When the last absorbances were plotted versus the
temperature (t) resulted to a linear plots with correlation coefficient range
values R2 = 0.955 – 0.9971 .
8
a
b
c
d
Fig. 3 : The relationships between absorbances versus t for
a) A1 versus t
b) A2 versus t c) A3 versus t d) A4 versus t
9
Careful examination of plots showed an inverse relation-ship between
reactant absorbance A1 for enol tautomer with temperature , where as
products absorbances A2 , A3 and A4 showed a direct relationships .
These results were expected and agreed with practical experiments .
The thermodynamic parameters ΔGº , ΔHº and ΔSº for tautomerism
reactions of I as shown in Sheme 1 , were estimated by using Hartman22
etal for tautomerism reaction as an example A1
A2 .
Hence the equilibrium constant K can be calculated by equation (1)
K = C2/C1 = A2/A1 . α1/ α2 … (1)
C1 and C2 = concentrations of reactant and product in the equilibrium
reaction .
A1 and A2 =Absorbances of reactant and product respectively .
α1 and α2 = molar extension coefficient of reactant and product
respectively .
CT = Total concentration of reactant .
CT = C1 + C2 … (2) , Applying Beer law to (2)
d = path length of cell .
In a previous study22 , it was assumed that α1 and α2 values do not
changed by the variation of temperature . When the three equilibrium
reactions shown in Scheme (1) applied to equation (4) , the results were a
three plots outlined in Fig. 4 of R2 range values 0.9442-0.968 with values
of α1/ α2 , α1/α3 equal 0.5541 , 1.7039 and 0.3998 respectively .
10
a
b
c
Fig. 4 : The relationships of tautomerism reactions of I as :a) A1 versus A2
b) A1 versus A3
c) A1 versus A4
The equilibrium constants K2 , K3 and K4 in imine I were calculated
as shown in Table 3 . The variation of K2 , K3 and K4 with temperatures ,
facilitate the evaluation of enthalpy ΔHº for tautomerism reactions using
integrated Vant Hoff equation of the form :11
The plots of lnK2 , lnK3 and lnK4 versus T-1 showed a linear plots as in
Fig. 5 .
a
b
c
Fig. 5 : In relationship of lnK versus T-1 for :a) lnK2 versus T-1
b) lnK3 versus T-1
c) lnK4 versus T-1
12
The rest of thermodynamic parameter ΔGº and ΔSº were calculated
from equations (6-7) of the forms :ΔGº = ΔHº - ΔSº
…(7)
Table 5 : The thermodynamic parameters ΔGº , ΔHº and ΔSº for
tautomerism in I
G20
G2
KJ.mole-1
KJ.mole-1
H 20
H 2
KJ.mole-1
KJ.mole-1
S 20
S 2
J.mole-1.K-1
J.mole-1.K-1
1
T
A 
K2  2 . 1
A1  2
lnK2
0.0035
0.1978
-1.6204
3.81257
19.26688
54.60
0.0034
0.3259
-1.1211
2.73100
18.73139
54.60
0.0033
0.4193
-0.8691
2.18938
2.23091 18.73586
0.0031
0.5921
-0.5240
1.36359
18.45616
54.60
0.0030
0.6743
-0.3940
1.05805
18.69671
54.60
1
T
A 
K3  3 . 1
A1  3
lnK3
0.0035
1.1785
0.1642
-0.386.33
10.02295
36.78
0.0034
1.3095
0.2696
-0.65674
10.12037
36.78
0.0033
1.5704
0.4513
-1.13688
0.0031
1.8417
0.6107
-1.58921
9.92354
36.78
0.0030
2.2150
0.7952
-2.13544
9.74512
36.78
1
T
A 
K4  4 . 1
A1  4
lnK4
0.0035
0.2874
-1.2468
0
G30
G3
KJ.mole-1
KJ.mole-1
0
-1.18092
0.3215
-1.1347
18.77740
H 30
H 3
KJ.mole-1
KJ.mole-1
10.00804
0
9.96400
54.60
0
54.60
S 30
S 3
J.mole-1.K-1
J.mole-1.K-1
36.78
0
36.78
G40
G40
H 4
H 40
S 40
S 4
KJ.mole-1
KJ.mole-1
KJ.mole-1
KJ.mole-1
J.mole-1.K-1
J.mole-1.K-1
2.93354
7.936.67
2.51067
0.0034
0
2.76413
13
17.67
7.86737
7.94404
0
17.67
17.67
0.0033
0.3619
-1.0163
2.56020
7.91690
17.67
0.0031
0.4083
-0.8957
2.33086
7.86435
17.67
0.0030
0.4811
-0.7316
1.96465
7.67493
17.67
The positive signs of all
parameter means that all three
tautomerism reactions of I were enothermic or occurred after absorption
of heat .
has a negative signs . This means that tautomerism reaction of k 3
equilibrium constant occurred spontaneously .
and
positive signs as explained by a non spontaneous reactions
performed .
They have equilibrium constants values less than unity .
have
to be
Finally all values of
,
and
have a positive signs .
These mean that all tautomerism reactions resulted in more random of
products .
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