Spectrophotometric determination and kinetic studies of condensation of aromatic aldehydes with 2-thiobarbituric acid

Article abstract of DOI:10.1080/10426500307859

The condensation reaction of 2-thiobarbituric acid with aromatic aldehydes in ethanol has been investigated spectrophotometrically at 30-50°C. The reaction was catalyzed by HCl solutions. The reaction follows overall second order kinetics, first order eac

Full text of DOI:10.1080/10426500307859

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Spectrophotometric Determination and Kinetic Studies
of Condensation of Aromatic Aldehydes with 2-
Thiobarbituric Acid
a
a
H. A. A. Medien & A. A. Zahran
a
Ain Shams University , Cairo, Egypt
Published online: 27 Oct 2010.
To cite this article: H. A. A. Medien & A. A. Zahran (2003) Spectrophotometric Determination and Kinetic Studies of
Condensation of Aromatic Aldehydes with 2-Thiobarbituric Acid, Phosphorus, Sulfur, and Silicon and the Related Elements,
178:5, 1069-1080, DOI: 10.1080/10426500307859
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Phosphorus, Sulfur and Silicon, 2003, Vol. 178:1069–1080
Copyright �C 2003 Taylor & Francis
1
042-6507/03 $12.00 + .00
DOI: 10.1080/10426500390208901
SPECTROPHOTOMETRIC DETERMINATION
AND KINETIC STUDIES OF CONDENSATION
OF AROMATIC ALDEHYDES WITH
2
-THIOBARBITURIC ACID
H. A. A. Medien and A. A. Zahran
Ain Shams University, Cairo, Egypt
(Received August 12, 2002; accepted October 31, 2002)
The condensation reaction of 2-thiobarbituric acid with aromatic alde-
hydes in ethanol has been investigated spectrophotometrically at 30–
�
5
0 C. The reaction was catalyzed by HCl solutions. The reaction follows
overall second order kinetics, first order each in reactant. Activation pa-
rameters have been calculated from the dependence of the rate constants
on temperature. The rate of condensation increases with the presence
of electron donating groups on the aromatic ring of the aldehyde. The
rate-determining step involves dehydration of the aldol intermediate.
Based on this reaction, determination of 13 aromatic aldehydes in a
concentration range of 0.149–76 �g/ml is proposed.
Keywords: 2-Thiobarbituric acid; aromatic aldehydes; determination;
kinetic
Several aromatic aldehydes are present in some natural products. p-
Dimethylaminobenzaldehyde and syringaldehyde are useful reagents
1
2
in organic colorimetric analysis and in clinical chemistry. Primary
aromatic amines, sulfa drugs, and amino acids frequently have been
determined and kinetically studied with these reagents.3 Also, deriva-
tives of barbituric acid are valuable in medicine as soporifics, and they
belong to an important class of drugs used as hypnotics, sedatives, and
anesthetics. For all these reasons it is desirable to have an accurate
method for the analytical determination and kinetic measurements of
these compounds (e.g., aromatic aldehydes and derivatives of barbituric
acid) through their reaction.
,4
Address correspondence to H. A. A. Medien, Department of Chemistry, Faculty of
Science, Ain Shams University, Cairo, Egypt. E-mail:hmadian59@yahoo.com
1
069

1
070
H. A. A. Medien and A. A. Zahran
Many methods are cited in the literature for kinetic studies
of reactions of aromatic aldehydes and comprise reactions with
5
6
chlorodipentylborane, 3-methyl-1-phenylpyrazolin-5-one, phenyl-
7
8
9
hydroxylamine, pyridinium chlorochromate, and phenylhydrazone.
Other methods are available for kinetic reactions of aromatic
aldehydes.1
0–22
When aldehydes and 2-thiobarbituric acid are condensed through
a Knoevenagel condensation, alkylidene or arylidene derivatives of
2
3
2
-thiobarbituric acid are obtained. This reaction follows the mecha-
2
4
nism proposed in Perkin condensation. These derivatives have inter-
esting pharmacological expections, and industrial applications (dyes,
photographic sensitizers); they also are significant from the point
of view of food technology (test for the rancidity of food).25 These
derivatives also are used as an index of lipid peroxidation in plant
materials.26
The present investigation aims to study the condensation reaction of
aromatic aldehydes and 2-thiobarbituric acid (TBA) kinetically and to
study the effect of substituents on the reactivity. This work also aims
to introduce a spectrophotometric method for determination of the aro-
matic aldehydes.
EXPERIMENTAL
Apparatus
Visible absorption spectra were recorded on a Unicam SP 1800 record-
ing spectrophotometer. A SP 6-200 Pye-Unicam spectrophotometer was
used for the kinetic spectrophotometry. IR spectra were recorded (KBr)
1
on a Pye-Unicam SP 1000 spectrophotometer. H NMR spectra were
taken at 270 MHz, using CDCl3 as a solvent.
Materials and Reagents
All aromatic aldehydes used were of AR grade (BDH). Alcoholic solution
of 2-thiobarbituric acid (Aldrich) was freshly prepared for each analysis.
All other chemicals used were of reagent grade.
Analytical Procedure
A
calibration graph for each of the aldehydes [3-hydroxy-4-
�
3
� 3
methoxybenzaldehyde (10 M), p-hydroxybenzaldehyde (10 M),
p-dimethylaminobenzaldehyde (10 M), p-methoxybenzaldehyde
�
4

Spectrophotometric Determination and Kinetic Studies
� 3 � 3
1071
�
3
(
10 M), vanillin (10 M), syringaldehyde (10 M), 2,4-dihydroxy-
�
3
3
� 3
benzaldehyde (10 M), m-methoxybenzaldehyde (10 M), ben-
�
� 3
zaldehyde (5 � 10 M), p-chlorobenzaldehyde (5 � 10 M), p-nitro-
�
3
� 3
benzaldehyde (5 � 10 M), p-cyanobenzaldehyde (5 � 10 M), and
�
3
m-hydroxybenzaldehyde (10 M) was obtained by transferring (0.1–
1
1
.0 ml) aliquots in ethanol to test tubes followed by the addition of
.0 ml of the (0.1 M) ethanolic solution of 2-thiobarbituric acid (TBA)
and 1.0 ml of the (2 M) HCl solution. The mixtures are thermostated at
5
�
0 C in water bath for 15 min and then diluted to 10 ml with ethanol
and the absorbance of the resulting yellow color is measured at the
maximum wavelength of each aldehyde derivative.
The points on standard calibration curves each represent the out-
come of three determinations.
Kinetic Procedure
The rate of reaction of some aromatic aldehydes tested (viz:
p-methoxy-benzaldehyde, p-hydroxybenzaldehyde, benzaldehyde, p-
cyano-benzaldehyde, p-chlorobenzaldehyde, and p-nitrobenzaldehyde)
with 2-thiobarbituric acid in the presence of 2 M HCl was followed by
monitoring the increase in the absorbance of the product, at its wave-
length of maximum absorption with time.
Freshly solutions of (0.02–0.05 M) of all studied aldehydes, solution of
0.001 M) 2-thiobarbituric acid in ethanol, and (2 M) HCl solution were
(
prepared. To test the reaction order with respect to the aldehydes stud-
ied, mixing was carried out by withdrawing 1.0 ml, from each concentra-
tion of the aldehydes solutions (0.02–0.05 M) to 10 ml flasks containing
1
.0 ml of (2 M) HCl solution, 1.0 ml of (0.001 M) 2-thiobarbituric acid so-
lution and the appropriate volume of ethanol. A portion of the reaction
mixture was transferred to a quartz cuvette placed in a thermostated
�
(
30 C) cell compartment. A blank solution was similarly prepared with-
out aldehyde and the absorbance readings also were recorded and fol-
lowed as a function of time. A different sets of experiments were carried
out by withdrawing (0.5–3 ml) aliquots of 0.001 M (TBA) solution to dif-
ferent 10 ml flasks containing 1.0 ml of (0.05 M) aldehyde solution and
1
.0 ml of (2 M) HCl solution to test the order with respect to (TBA).7,28
Initial rates were determined by the method of initial rates,2
whereby the absorbance (A) was plotted against time for each run. The
initial slopes (dA/dt) for each run were determined, and used in the
following equation for calculating the rate of the reaction:2
7,28
dA
C∞ � Ci
A∞ � Ai
Rate = dt
�

1
072
H. A. A. Medien and A. A. Zahran
Where Ci and C∞ are the initial and final concentrations of the col-
ored reaction products respectively, calculated by using the molar ex-
tinction coefficients ε. Ai and A∞ are the absorbencies of the colored
species at the start and the end of the reaction. Since both Ci and Ai
are nearly equal to zero at the beginning of the reaction, then the above
equation reduced to:
dA
C∞
A∞
Rate = dt
�
Preparation and Analysis of Reaction Products
The synthesis of reaction products was undertaken using the same
methods cited in the literatures.2
9–30
To a solution of TBA (0.004 mmol)
in absolute ethanol, aromatic aldehyde (0.004 mmol) was added. The
�
reaction mixture was warmed at 60 C for 2 min and left at room temper-
ature overnight. The crystals obtained were filtered off, dried, and crys-
tallized from ethanol. When p-dimethylaminobenzaldehyde was mixed
with TBA, the product 5-(p-dimethylaminobenzylidene)-2-thio barbi-
�
turic acid was obtained, m.p. = 264 C, yield 95% (Found: C, 63.10; H,
�
1
5
3
3
.80; N, 14.80 calculated: C, 62.71; H, 5.92; N, 14.63); IR (KBr) cm
=
1
060, 2920 (NH amide), 1690, (C O); 1165 (C S). H NMR: � = 3.14–
.28 (s, 6H, 2CH3), 11.95–12.05 (s, 2H, NH), 8.15 (s, H, CH) 6.76–6.87
(
m, 2H, aromatic protons), 8.40–8.52 (m, 2H, aromatic protons).
Other products have been obtained using different aldehydes by the
same preparation method and their structures were confirmed by the
spectral data.
RESULTS AND DISCUSSION
Previous work31,32 showed that the reaction of an active methylene com-
pounds follow the general pattern of nucleophilic addition to a carbonyl
group, and Scheme 1 is proposed.
SCHEME 1

Spectrophotometric Determination and Kinetic Studies
1073
TABLE I Observed Rate Constants of the Condensation
Reaction Between p-Hydroxybenzaldehyde and
2-Thiobarbituric Acid in Presence of 2 M HCl at 303 K
1
03 [Ald] M
103 [TMB] M
103 kobs sec� 1
2
3
4
5
5
5
5
.0
.0
.0
.0
.0
.0
.0
0.1
0.1
0.1
0.1
0.05
0.2
0.3
0.36
0.52
0.73
0.90
0.46
1.18
2.69
The rate limiting step can be the formation of a metastable aldol (I)
or its dehydration to 5-arylidene-2-thiobarbituric acid, depending on
the substitution on aromatic ring and on the experimental conditions
solvent, pH).3
3,34
(
Preliminary reaction orders were determined from slopes of log (ini-
tial rate) versus log (concentration) plots. From the observed results, it
was shown that for all aldehydes studied the kinetics is second order
overall, first order with respect to each of the reactants, as shown in
Table I for p-hydroxybenzaldehyde reaction as an example, according
to the simple rate law:
d[product]/dt = kobs[aldehyde][TBA]
The study of the stoichiometry of the reaction indicates that the
products have a maximum absorption at a molar ratio of 1:1 (aldehyde:
TBA).
The observed rate constants kobs, calculated from the initial rate (Ri)
values are in good agreement with the values calculated from the first
order plots [ln(A∞ � At) against t] where At and A∞ are the values of
the absorbance at time t and at the end of the reaction respectively.
The second order rate constants calculated by dividing kobs values by
the concentrations of the aldehydes. Duplicate kinetic runs showed that
the rates are reproducible to within � 4%.
The kinetic results in Table II shows that the rate of the reaction
between different substituted benzaldehydes and 2-thiobarbituric acid
(
TBA) increases by the presence of electron releasing groups in the aro-
matic aldehyde ring, whereas electron-withdrawing groups suppress
it.
In fact, the addition of (TBA) to substituted benzaldehydes (first step)
is favored by electron withdrawing groups, and the dehydration step
(
second step) is favored by electron donating ones. The results for the

1
074
H. A. A. Medien and A. A. Zahran
) and the Activation Parameters
TABLE II Second Order Rate Constants (k
2
of the Reaction Between 2-Thiobarbituric Acid and Aromatic Aldehydes in the
Presence of 2 M HCl at 303 K
k2 l � mol� � sec� 1 1E#
1
1H#
1G#
1S#
Kcal � mol�
1
Cal � mol� 1 � K� 1
Aldehydes
303K 313K 323K
p-Methoxybenzaldehyde 0.25 0.32 0.40 3.82
p-Hydroxybenzaldehyde 0.18 0.25 0.32 5.60
3.22
5.00
8.32
11.11
12.9
8.77
18.51
18.71
18.95
19.78
20.9
� 50.47
� 45.2
Benzaldehyde
p-Cyanobenzaldehyde 0.03 0.06 0.10 11.71
p-Nitrobenzaldehyde 0.02 0.045 0.08 13.5
p-Chlorobenzaldehyde 0.065 0.12 0.17 9.37
0.12 0.23 0.30 8.92
� 35.1
� 28.63
� 23.5
19.32
� 34.82
reaction obtained (Table II) then can be rationalized considering that
the second step is rate-limiting and the first step is the faster of the
two. Therefore, the reaction process can be represented by the following
mechanism:2
9,30
The above equations lead to rate expression (1),
Rate = kK1K2[TBA] [Ald]
(1)

Spectrophotometric Determination and Kinetic Studies
1075
Since, [Ald] � [TBA], then, Eq. (1) becomes:
Rate = kK1K2[TBA]
From Eq. (2), one can conclude that
kobs = kK1K2[Ald]
(2)
(3)
Eq. (1) shows that the reaction follows second order kinetics, first
order each in aldehyde and (TBA). The second order rate constants
were calculated using the equation, k2 = kobs/[Ald].
Effect of Varying [HCl]
As mentioned above, the reaction involves an addition step followed
by dehydration step, which is the rate determining one. The addition
step is enhanced by addition of an acid, which facilitate the attack of
(
TBA) molecule to give the intermediate leading to enhancing the rate
of the reaction.
For all aldehydes studied, the reaction was found to be catalyzed by
addition of HCl solution. The values of kobs increase in [HCl] from 0.1 M
up to 2 M, after which the value of kobs becomes constant.
Effect of Substituents
An attempt has been made to apply the Hammett’s equation for
the reaction of different substituted benzaldehydes (p-OCH3, p-OH,
p-CN, p-Cl and p-NO2) at 303 K using the � values (Hammett’s po-
lar substituent constants) for these aldehyde substituents as shown
in Figure 1. The negative � value (� 1.0217) (r = 0.982) means that
electron-repelling groups enhance the rate of the reaction. This is be-
cause the electron repelling groups enhance the dehydration step of
the aldol formed (I) and stabilizes the cationic intermediate which is
formed after losing a water molecule to give the final product.
Activation Parameters
The rates of the reaction of seven different benzaldehydes were deter-
mined at different three temperatures: 303, 313, and 323 K. The activa-
#
tion energies 1E , determined from the slopes of the Arrhenius plots of
�
1
#
#
log k2 versus T and the other thermodynamic parameters 1H , 1S
#
and 1G calculated at 303 K and are given in Table II.
The entropy of activation values are negative as expected for bimolec-
ular or ter-molecular reactions. In this study the entropy of activation
#
1
S is substituent dependent. Since all the substituents are located in
para position, direct steric interaction is unlikely, so that resonance

1
076
H. A. A. Medien and A. A. Zahran
FIGURE 1 Hammett plot for the reaction of substituted benzaldehydes with
thiobarbituric acid at 303 K.
and/or inductive effects are the operating factor. Within the series,
electron-releasing substituents in the aromatic aldehyde, through their
resonance and/or inductive effects, localize the formal charge on the T.S.
This favors the formation of more ordered transition states reflecting
high and negative entropies of activation. On the other hand, the pres-
ence of electron withdrawing substituents in the aldehyde alters their
nature, leading to much more delocalization of the charge in T.S., and
hence reflecting low and negative entropies of activation.
#
#
The linear correlation between 1H and 1S (Figure 2) for the re-
action of different substituents of benzaldehyde with (TBA) shows that
all the compounds react by the same mechanism.3
5,36
The value of the
isokinetic temperature � obtained was 360.45 K, (r = 0.998). This value
is not the real temperature used in the kinetic runs.
#
The constancy of 1G may be explained on the basis of an isokinetic
relationship that exists for a series of compounds of slightly different
structures but undergoing reaction essentially by the same mechanism;
#
#
the 1G may be more or less constant with relative changes in 1H and
1
#
37
S as pointed out by Leffler.
The mechanism for the title reaction is common for all members in
the series as indicated by (a) the good straight-line plot of 1H versus
#
#
1
S , (b) the linear plots of log k2 at 323 K against log k2 at 303 K
(
gradient, 0.657, r = 0.987).

Spectrophotometric Determination and Kinetic Studies
1077
#
#
FIGURE 2 The linear relationship between 1H and 1S .
At studied temperatures, an electron-withdrawing substituent in the
para position of the aldehyde decreases the reaction rates, while a sim-
ilarly positioned electron releasing or donating substituent increases
it. This behavior could be explained by the observation that the ratio
kp-OCH /kp-NO was 12.5, 7.1 and 5.0 at 303, 313 and 323 K respectively
3
2
(
Table II). Consequently, the variation of the rate constant depends on
the nature of the substituent in the aldehyde. The order of decreasing
reactivity of substituted aldehyde in ethanol is p-OCH3 > p-OH > H >
p-Cl > p-CN > p-NO2. This order of decrease in magnitude of the sub-
stituent effect is expected on the basis of both inductive and resonance
effects.
Determination of the Aldehydes Tested
Effect of Time and Temperature
The absorbance of the color due to reaction of the aldehydes tested
with excess (TBA) in the presence of 2 M HCl in ethanol had reached
�
a maximum intensity after 15 min at 50 C. It is observed that the

1
078
H. A. A. Medien and A. A. Zahran
TABLE III Statistical Analysis of the Calibration Graphs for the
Determination of Some Aromatic Aldehydes by Reaction with
2
-Thiobarbituric Acid in the Presence of 2 M HCl
�
max Concentration
ε
�
Slope Intercept
(b) (a)
� 1
)
Aldehydes
nm range �g /ml (l mol 1 cm
3
-Hydroxy-4-methoxy-
benzaldehyde
425
7.6–76.0
10900
0.0144 � 0.0164
p-Dimethylaminobenzaldehyde 460 0.149–1.49
19670
15730
14850
16650
15380
520
0.132 � 0.0003
0.086 � 0.024
Syringaldehyde
440
1.82–18.2
Vanillin
400 1.521–15.215
0.0976
0.004
p-Methoxybenzaldehyde
p-Hydroxybenzaldehyde
m-Hydroxybenzaldehyde
Benzaldehyde
400
410
410
360
360
420
360
355
400
1.36–13.60
1.22–12.2
1.22–12.2
10.6–53
6.93–69.25
1.38–13.8
5.95–59.5
7.55–75.5
1.36–13.6
0.122 � 0.009
0.126 � 0.018
0.042
0.0052
0.005
0.0
550
p-Chlorobenzaldehyde
1840
12200
980
1560
6610
0.0133 � 0.01
2
,4,-Dihydroxybenzaldehyde
0.089 � 0.01
p-Cyanobenzaldehyde
p-Nitrobenzaldehyde
m-Methoxybenzaldehyde
0.008
0.0103
0.003
0.009
0.048 � 0.023
absorbance is nearly constant with increasing the temperatures from
�
5
0 to 70 C.
Accuracy and Precision
Table III shows linear regression analysis of the data obtained from
calibration graphs of the aldehydes tested from the relation A = a + bc,
where A is the absorbance at the relevant peak in a 1.0 cm quartz
cell, a and b are the intercept and the slope of the calibration graphs,
respectively, and c is the concentration of the aldehyde in �g/ml in the
final solution.
Five replicate determinations at different concentration levels of
p-hydroxybenzaldehyde as a representative aldehyde were carried out
to test the precision and accuracy of the proposed method. The recovery
was 96.98%. The relative standard deviation R.S.D. (n = 5) at 12.2 �g/ml
and the percentage relative error were 0.96 and 0.43 respectively. The
figures obtained point out to the good accuracy and repeatability of
the method. The ruggedness of the proposed method was studied by
evaluating the coefficient of variation (five replicates) at 12.2 �g/ml for
p-hydroxybenzaldehyde determination during one and five days. It was
found to be 0.96 and 1.023% respectively.
Selectivity
The effects of various compounds on the determination of 10� 4
p-hydroxybenzaldehyde were studied. The results are summarized in
M

Spectrophotometric Determination and Kinetic Studies
1079
TABLE IV Effect of Various Foreign Species on the Determination
�
4
of 10 M of p-Hydroxybenzaldehyde at the Optimum Conditions
Substance
Tolerance Limit Ratio (mol/mol)
Formaldehyde, acetaldehyde
Acetone, benzoic acid
Aniline
100
50
30
Table IV. The tolerance limit was defined as the concentration of added
substance causing less than � 3% relative error.
Conclusion
In summary, 13 aromatic aldehydes have been shown to be assessable
with a simple analytical method. The present method of determination
offers the advantages of: (a) reagent availability and stability; (b) rea-
sonable selectivity in the presence of many other C O containing com-
pounds; (c) less time consumption; and (d) high sensitivity compared to
other spectrophotmetric methods.
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