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Scientia Iranica C (2012) 19 (3), 605–618
Sharif University of Technology
Scientia IranicaTransactions C: Chemistry and Chemical Engineering
www.sciencedirect.com
Synthesis and electrochemical investigations oncertain pyrazolin-5-onesK. Ramana Kumar a, A. Raghavendra Guru Prasad b,∗, V. Srilalitha c, G. Narayana Swamy d,L.K. Ravindranath d
aMalla Reddy College of Engineering, Kompally, Hyderabad, A.P., Indiab ICFAI Foundation for Higher Education, Dontanpally, Hyderabad, A.P., Indiac C.M.R. Institute of Technology, Kompally, Hyderabad, A.P., Indiad Sri Krishnadevaraya University, Anantapur, A.P., India
Received 7 September 2011; revised 7 January 2012; accepted 28 February 2012
KEYWORDSPolarographic studies;1-(Toluenyl sulfonyl)-3-amino-4-(2′-substitutedaryl hydrazono)-2-pyrazolin-5-ones;
Kinetic parameters;Thermodynamicparameters;
Cyclic voltammetry;Mechanism of electrodereaction.
Abstract The electrochemical behavior of certain pyrazolin-5-ones was investigated at the droppingmercury electrode by employing DC polarography. The variables that influence the electrode processwereextensively studied. All compounds under investigation gave two well defined polarographic waves. Themechanism for the electrode process was proposed in acid, as well as in basic media. The results obtainedin polarography were compared with those obtained in cyclic voltammetry.
© 2012 Sharif University of Technology. Production and hosting by Elsevier B.V.Open access under CC BY-NC-ND license.
1. Introduction
The chemistry of pyrazolone derivatives has received muchattention, because of their interesting structural propertiesand applications to diverse areas [1]. Pyrazolin-5-ones are avery important class of heterocycles, due to their potentialpharmacological and biological applications [2–4]. It is alsowell known that they have been used as therapeutic agents,such as anti-inflammatory, antibacterial, antifungal, analgesic
∗ Corresponding author.E-mail addresses: [email protected] (K. Ramana Kumar),
[email protected] (A. Raghavendra Guru Prasad),[email protected] (V. Srilalitha), [email protected](G. Narayana Swamy), [email protected] (L.K. Ravindranath).Peer review under responsibility of Sharif University of Technology.
1026-3098© 2012 Sharif University of Technology. Production and hosting by Els
doi:10.1016/j.scient.2012.02.025
and antipyretic etc. [5,6]. However antifungal drug discoverycontinues to be a crucial area [7]. This is due to the fact that,in recent years, the pathogenicmicroorganisms have developedexcessive resistance to these drugs, due to widespread use [8].
Pyrazolin-5-ones are effective antifungal [9] agents. Pyraz-olin-5-ones function as potential cdc25 inhibitors and arethought to be a good lead scaffold for developing an anticancerdrug [10]. Some have shown preventive effects on myocardialinjury [11] and acute myocardial infarction [12], and have beenused in the treatment of cardiovascular disease [13].
In addition to these pharmacological applications, they havevital commercial significance. The design of photoluminescentlanthanide complexes of pyrazolones provide a suitable basisfor sensitized near-infrared luminescence [14], and thesematerials are used in different kinds of light conversion andlight-amplification devices [15,16]. The pyrazolin-5-ones areextensively used in the manufacture of commercial dyes, suchas photographic dyes, textile dyes etc. The application is basedon the fact that the pyrazolin-5-one system is an effectiveelectron acceptor [17], and can also act as a weak electron
evier B.V. Open access under CC BY-NC-ND license.
606 K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618
Table 1: Characteristics of pyrazolin-5-ones.
Sample no Substituent (R) Color Melting point (°C) Mol. wt. Elemental analysisFound (Cal) %
C H N S Cl
1 –H Orange 159–160 357 53.61 4.20 19.52 8.88 –(53.78) (4.24) (19.60) (8.98)
2 2′–CH3 Yellow 59–60 371 54.74 4.46 18.75 8.56 –(54.98) (4.62) (18.86) (8.64)
3 2′–OCH3 Yellow 67–68 387 52.61 4.39 18.01 8.09 –(52.71) (4.43) (18.08) (8.28)
4 2′–OH Black 251–252 373 51.35 3.98 18.65 8.51 –(51.47) (4.05) (18.76) (8.59)
5 2′–Cl Yellow 126–127 391 49.01 3.57 17.69 8.09 8.91(49.05) (3.61) (17.88) (8.19) (9.05)
Scheme 1: Synthesis 2-pyrazolin-5-ones.
donor [18]. It is also reported that iron complexes of certainpyrazolin-5-ones are environmental friendly and are potentialreplacements for some important hazardous brownish blackacid dyes [19].
The diverse applications of pyrazolin-5-ones in differentfields have inspired the authors to investigate the reductionbehavior of these compounds. Particularly, the knowledgeof the electrochemical reduction of pyrazolin-5-ones is aprerequisite for understanding the metabolic pathway leadingto biological or pharmacological activity.
2. Experimental
2.1. Instruments and chemicals employed
The DC recording polarographic instrument manufacturedby ELICO Private Limited, Hyderabad, India, was used forpolarographic studies. The cyclic voltammetric unit employedfor the studies consists of an X–Y recorder (Model RE 0074),a PAR model 173 potentiostat, and a PAR model 175 universalprogrammer. pHmeasurements weremade using the ELICO pHmeter, Model L1-10, ELICO Private Limited, Hyderabad, India.
IR spectral details were obtained from a Perkin-Elmer 283spectrometer.
All reagents used were of analytical reagent grade procuredfrom Merck, India. The working solutions were preparedusing double distilled water. The Britton–Robinson buffer wasprepared from appropriate amounts of 0.04 M o-phosphoricacid, 0.04 M boric acid and 0.04 M acetic acid. The solutionsof desired pH values were prepared by the addition of anappropriate volume of 0.2 M sodium hydroxide solution.
2.2. Preparation of toluene sulfonyl hydrazide (Scheme 1 (I)) [20]
A solution of p-toluene sulfonyl chloride in acetone and anappropriate amount of hydrazine hydrate were treated with5% NaOH solution. The mixture was shaken vigorously forten minutes, cooled and poured into 1:1 HCl. The precipitateformedwas filtered, washedwithwater and recrystallized fromalcohol.
2.3. Preparation of substituted aryl diazonium chloride (Scheme 1(II)) [21]
The required amount of substituted aryl amine was dis-solved in a suitable volumeof dilute hydrochloric acid. The solu-tion obtainedwas cooled to 0 °C and a little excess of an aqueoussolution of sodium nitrite was added slowly. The addition of alittle excess of sodium nitrite solution stabilizes the diazoniumsalt.
2.4. Preparation of aryl diazonium cyano esters (Scheme 1(III)) [22]
The respective diazonium chloride solution was added toan ice cold solution of the mixture of sodium acetate andethyl cyano acetate solutions in methanol. The addition ofcorresponding diazonium chloride was continued until yellowcrystals were separated out. The crystals were filtered, washedwith water and dried.
2.5. Synthesis of 2-pyrazolin-5-ones (Scheme 1 (IV))
A mixture of appropriate amounts of diazonium cyanoester and toluene sulfonyl hydrazide in ethanol was refluxedfor six hours and cooled. The crystalline solid separated wasfiltered, washed with water, dried and recrystalised fromdimethylformamide (1:1). The melting points of the differentcompounds synthesized are presented in Table 1. As these
K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618 607
Figure 1: Plot of pH vs.−E1/2 for pyrazolin-5-ones. [pyrazolin-5-one] = 1×10−3 M;Medium=Dimethylformamide (40% v/v). I indicates first wave and II indicatessecond wave.
compounds are synthesized by already reported methods, theauthors have characterized these compounds by elementalanalysis, melting points and characteristic IR spectral bands.
The IR spectrum of the compounds showed bands charac-teristic of the cyclic carbonyl group at around 1670 cm−1 [23].The band at around 1555 cm−1 confirms the presence of>C=C–NH–N= vibration [24]. The bands mentioned above areattributed to the presence of the 2-pyrazolin-5-one structure.
2.6. General experimental procedure employed for polarographicand cyclic voltammetric studies
10 mL of the buffer solution of the required pH, 2.5 mL ofpyrazolin-5-one (1×10−2 M) and 10mL of dimethylformamidewere taken into the polarographic cell. The solution was madeup to a total volume of 25 mL with distilled water. Polarogramsand cyclic voltammogramswere recorded after deaerationwithnitrogen gas.
3. Results and discussion
3.1. Polarographic behavior of 1-(Toluenyl sulfonyl)-3-amino-4-(aryl hydrazono)-2-pyrazolin-5-one
1-(Toluenyl sulfonyl)-3-amino-4-(2′-substituted aryl hydra-zono)-2-pyrazolin-5-ones (1–5) exhibits two waves in theentire pH range of study (1.1–10.1). The wave height decreaseswith an increase in pH. A decrease in wave height withan increase in pH was observed for all compounds. An
inspection of the structure of the compounds suggests thatthe sites susceptible to reduction are exocyclic >C=N, cyclic>C=N and cyclic amide. However, exocyclic >C=N was moresusceptible to reduction than cyclic >C=N and cyclic amide.This was experimentally confirmed by the fact that 1-(Toluenylsulfonyl)-3-amino- 2-pyrzolin-5-one fails to undergo reductionunder experimental conditions. Hence, the polarographicreduction of 1-(Toluenyl sulfonyl)-3-amino-4-(substituted arylhydrazono)-2-pyrazolin-5-oneswas attributed to the reductionof the exocyclic >C=N group.
3.1.1. Effect of pH on half-wave potentialAll compounds (1–5) exhibit two well-defined waves in the
entire pH range (1.1–10.1) of study. As indicated in Table 2,the half-wave potentials of the first and second waves for allcompounds shift to more negative values with an increasein pH, in the range 1.1–7.1, and remain constant in alkalinemedium. The E1/2–pH plots (Figure 1(a)–(d)) have two linearportions. As revealed from Figure 1, the E1/2–pH relationshipfor the first wave is represented by:
(1) −E1/2 = 0.06 + 0.09868 × pH V vs. SCE;(2) −E1/2 = 0.04 + 0.08148 × pH V vs. SCE;(3) −E1/2 = 0.02 + 0.07879 × pH V vs. SCE;(4) −E1/2 = 0.08 + 0.08186 × pH V vs. SCE;(5) −E1/2 = 0.07 + 0.08372 × pH V vs. SCE.
The E1/2–pH relationship for the secondwave is represented by
(1) −E1/2 = 0.21 + 0.09135 × pH V vs. SCE;
608 K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618
Figure 2: Effect of pH on wave height (First wave). [pyrazolin-5-one] = 1 ×
10−3 M; Medium = Dimethylformamide (40% v/v).
Figure 3: Effect of pH on wave height (Second wave). [pyrazolin-5-one] =
1 × 10−3 M; Medium = Dimethylformamide (40% v/v).
(2) −E1/2 = 0.18 + 0.08169 × pH V vs. SCE;(3) −E1/2 = 0.12 + 0.08846 × pH V vs. SCE;(4) −E1/2 = 0.18 + 0.08761 × pH V vs. SCE;(5) −E1/2 = 0.21 + 0.08267 × pH V vs. SCE.
The half-wave potential value does not vary with pH in alkalinemedium. This observation suggests that the protons wereinvolved in the reduction process. The fractional value of P ,as indicated in Table 2, suggests that a heterogeneous protontransfer takes place in the reduction process [25].
3.1.2. Effect of pH on wave heightThe wave height for compounds (1–5) in different pHmedia
is presented in Table 2, and in Figures 2 and 3. The plots of waveheight vs. pH for the first and second waves, for all compoundsunder study, assume the shape of a dissociation curve. This typeof behavior is expected if the depolarizer undergoes chemicalcleavage in the acidic or alkaline medium.
The heterogeneous rate constant, K ◦
f ,h, and activation freeenergy change, 1G∗, at different pH values for compounds1–5 are presented in Table 2. The results revealed that theelectrode reactionwas turning increasingly irreversible with anincrease in the pH of the medium. This fact was confirmed by
Figure 4: Effect of mercury column height (h) on wave height (H). (I indicatesfirst wave, II indicates second wave). [pyrazolin-5-one] = 1 × 10−3 M;Medium = Dimethylformamide (40% v/v).
the decrease in K ◦
f ,h and the increase in the 1G∗ value with anincrease in pH.
3.1.3. Effect of mercury column height (h) on the wave height (H)
It is evident from Figure 4(a)–(c) that wave height (H)is linearly dependent on h1/2. The constant values of H/h1/2
indicate the diffusion controlled nature of the wave [26].
3.1.4. Effect of concentration (C) of the depolarizer on the waveheight (H)
The effect of concentration of 1-(Toluenyl sulfonyl)-3-amino-4-(substituted aryl hydrazono)-2-pyrazolin-5-ones on
K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618 609
Table
2:Po
larograp
hic
characteristics
and
kine
ticpa
rameters
of1-(Tolue
nylsu
lfony
l)-3-am
ino-4-(2
′-sub
stitu
ted
hydr
azon
o)-2-p
yraz
olin-5-one
.[Pyraz
olin-5-one
]=
1×
10−3
M;Med
ium:Aq
ueou
sdimethy
lform
amide(40%
v/v).
pH−E 1
/2V
vs.S
CEW
avehe
ight
H(cm)
1E 1
/2/
1pH
(mV)
αn a
Num
bero
fprotons
(P)
D×
106
(cm2s−
1 )I×
103
K◦ f,h
(cm
s−1 )
1G∗
(Kcalm
ole−
1 )
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eIw
ave
IIwav
eIw
ave
IIwav
eIw
ave
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ave
IIwav
eIw
ave
IIwav
eIw
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IIwav
e
–H
2.1
0.22
0.38
4.8
5.0
0.07
940.08
38
0.60
0.48
0.80
500.67
994.82
55.23
52.66
72.77
82.67
2×
10−3
1.26
8×
10−4
4.57
25.36
14.1
0.37
0.53
3.6
3.8
0.60
0.48
0.80
500.67
992.71
43.02
42.00
02.11
16.02
9×
10−5
5.84
3×
10−6
5.55
36.15
86.1
0.55
0.71
2.6
2.4
0.53
0.41
0.71
110.58
071.41
61.20
61.44
41.33
31.50
9×
10−6
4.58
3×
10−7
6.50
86.81
78.1
0.75
0.94
1.3
1.2
0.41
0.32
0.55
010.45
320.35
40.30
20.72
20.66
71.25
4×
10−7
6.75
3×
10−8
7.15
27.31
310
.10.75
0.94
1.3
1.2
0.41
0.32
0.55
010.45
320.35
40.30
20.72
20.66
71.25
4×
10−7
6.75
3×
10−8
7.15
27.31
3
-CH3
2.1
0.24
0.40
5.0
5.3
0.11
950.10
44
0.63
0.52
1.27
20.91
785.23
55.88
32.77
82.94
41.74
7×
10−3
7.22
3×
10−5
4.68
25.50
74.1
0.39
0.55
3.8
4.1
0.63
0.52
1.27
20.91
783.02
43.52
02.11
12.27
83.35
3×
10−5
2.68
2×
10−6
5.70
66.35
96.1
0.57
0.73
2.8
2.7
0.57
0.45
1.15
20.79
421.64
21.52
71.55
61.50
06.44
4×
10−7
1.75
1×
10−7
6.72
87.06
68.1
0.77
0.94
1.5
1.5
0.45
0.37
0.90
90.65
300.47
10.47
10.83
30.83
34.82
5×
10−8
2.16
4×
10−8
7.39
97.60
710
.10.77
0.94
1.5
1.5
0.45
0.37
0.90
90.65
300.47
10.47
10.83
30.83
34.82
5×
10−8
2.16
4×
10−8
7.39
97.60
7
-OCH
3
2.1
0.27
0.43
5.3
5.5
0.09
980.09
26
0.68
0.57
1.14
740.89
215.88
36.33
52.94
43.05
68.38
9×
10−4
2.82
8×
10−5
4.87
25.74
94.1
0.42
0.58
4.1
4.3
0.68
0.57
1.14
740.89
213.52
03.87
22.27
82.38
91.22
4×
10−5
7.92
7×
10−7
5.96
66.67
56.1
0.60
0.76
3.1
2.9
0.62
0.50
1.04
620.78
262.01
31.76
11.72
21.61
11.82
4×
10−7
4.04
9×
10−8
7.05
57.44
58.1
0.80
0.97
1.8
1.7
0.50
0.41
0.84
370.64
170.67
90.60
51.00
00.94
41.15
4×
10−8
5.11
9×
10−9
7.77
07.98
010
.10.80
0.97
1.8
1.7
0.50
0.41
0.84
370.64
170.67
90.60
51.00
00.94
41.15
4×
10−8
5.11
9×
10−3
7.77
07.98
0
–OH
2.1
0.23
0.39
5.8
6.0
0.08
870.09
10
0.61
0.49
0.91
500.75
387.04
57.53
93.22
23.33
32.56
7×
10−3
1.19
1×
10−4
4.58
35.37
74.1
0.38
0.54
4.6
4.8
0.61
0.49
0.91
500.75
384.43
14.82
52.55
62.66
75.77
8×
10−5
5.45
1×
10−6
5.56
56.17
66.1
0.56
0.72
3.6
3.8
0.55
0.42
0.82
500.64
622.71
43.02
42.00
02.11
11.32
6×
10−6
5.13
5×
10−7
6.54
26.78
78.1
0.76
0.93
2.3
2.5
0.43
0.34
0.64
500.52
311.10
81.30
91.27
81.38
91.32
0×
10−7
9.31
4×
10−8
7.13
97.22
910
.10.76
0.93
2.3
2.5
0.43
0.34
0.64
500.52
311.10
81.30
91.27
81.38
91.32
0×
10−7
9.31
4×
10−8
7.13
97.22
9
–Cl
2.1
0.19
0.36
4.5
4.8
0.10
090.08
95
0.55
0.44
0.93
820.66
574.24
14.82
52.50
02.66
74.56
9×
10−3
2.12
8×
10−4
4.43
35.22
74.1
0.34
0.51
3.3
3.6
0.55
0.44
0.93
820.66
572.28
12.71
41.83
32.00
01.35
0×
10−4
1.22
2×
10−5
5.34
55.96
76.1
0.52
0.69
2.3
2.2
0.49
0.37
0.83
580.55
981.10
81.01
41.27
81.22
23.82
5×
10−6
1.16
3×
10−6
6.26
76.57
68.1
0.72
0.95
1.0
1.0
0.37
0.28
0.63
110.42
360.20
90.20
90.55
50.55
53.42
8×
10−7
1.99
2×
10−7
6.89
27.03
210
.10.72
0.95
1.0
1.0
0.37
0.28
0.63
110.42
360.20
90.20
90.55
50.55
53.42
8×
10−7
1.99
2×
10−7
6.89
27.03
2
610 K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618
Figure 5: Effect of concentration on wave height (first wave). [pyrazolin-5-one] = 1 × 10−3 M; Medium = Dimethylformamide (40% v/v).
Figure 6: Effect of concentration on wave height (second wave). [pyrazolin-5-one] = 1 × 10−3 M; Medium = Dimethylformamide (40% v/v).
wave height in the range 0.5–4.0mMhas been studied at pH 4.1and 8.1. The wave height–concentration plots (Figures 5 and 6)were linear and were passing through the origin. This factsuggests that the reduction process was diffusion controlled,and the linear plots can be used for determination of traceamounts of 1-(Toluenyl sulfonyl)-3-amino-4-(substituted arylhydrazono)-2-pyrazolin-5-ones.
3.1.5. Nature of the electrode processThe semi-logarithmic plots (Figures 7 and 8) of the first
and second waves for all compounds under study were linear,with slopes 0.049–0.051 and 0.048–0.053, respectively. Thefractional value of the slope suggests that the reductionprocess was irreversible in nature. The irreversible nature ofthe polarographic waves was further confirmed by employingTome’s criteria [27]. The αna values obtained from Tome’smethod are presented in Table 2 (α is the transfer coefficientand na is the number of electrons involved). αna values werealmost equal to those obtained from conventional logarithmicplots. The irreversible nature of the waves may be attributedto the bulky aryl hydrazono group at the end of >C=Nlinkage [28].
Figure 7: Semi log plots of pyrazolin-5-ones (first wave). pH = 4.1;[pyrazolin-5-one] = 1 × 10−3 M; Medium = Dimethylformamide (40% v/v).
Figure 8: Semi log plots of pyrazolin-5-ones (Second wave). pH = 4.1;[pyrazolin-5-one] = 1 × 10−3 M; Medium = Dimethylformamide (40% v/v).
3.1.6. Effect of temperature on the polarographic reduction of1-(Toluenyl sulfonyl)-3-amino-4 (substituted aryl hydrazono)-2-pyrazolin-5-ones
The polarograms of 1-(Toluenyl sulfonyl)-3-amino-4-(substituted aryl hydrazono)-2-pyrazolin-5-ones in media ofpH 4.1were recorded at 303, 313, 323 and 333 K to study the ef-fect of temperature on the half-wave potential andwave height.The results are presented in Table 3.
All compounds (1–5) exhibit two well-defined waves inthe temperature range of study (303–333 K) at pH 4.1.The wave height increases with an increase in temperatureand the temperature coefficient values were in the range0.734%–1.495% deg−1. The values were in good agreementwith those reported in the literature for similar compounds byMeites [29].
The results presented in Table 3 revealed that the half-wave potentials shift to more negative values with an increasein temperature. This fact is an indication of the enhancedirreversible nature of the reduction process with the increasein temperature. Further, αna values decrease with an increasein temperature from 303 to 333 K. The decrease in αna valueswith the increase in temperature may be attributed to adecrease in the α value. The decrease in α values indicatesthat the transfer of electrons was increasingly difficult withthe raise in temperature. Hence, the system tends to become
K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618 611
Table 3: Effect of temperature on the polarographic characteristics of 1-(Toluenyl sulfonyl)-3-amino-4-(2′-substituted hydrazono)-2-pyrazolin-5-one. pH: 4.1,[pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
Temperature −E1/2V vs. SCE Wave height H (cm) Temperaturecoefficient (% deg−1)
αna D × 106 (cm2 s−1)
I wave II wave I wave II wave I wave II wave I wave II wave I wave II wave
–H
303 0.35 0.47 3.1 3.3 – – 0.66 0.54 2.01 2.28313 0.37 0.53 3.6 3.8 1.49 1.41 0.61 0.48 2.71 3.02323 0.41 0.62 4.1 4.3 1.30 1.24 0.55 0.43 3.52 3.87333 0.47 0.69 4.7 4.9 1.37 1.31 0.51 0.39 4.63 5.03
-CH3
303 0.36 0.50 3.3 3.6 – – 0.69 0.58 2.28 2.71313 0.39 0.55 3.8 4.1 1.41 1.30 0.63 0.52 3.02 3.52323 0.44 0.60 4.3 4.6 1.23 1.15 0.58 0.47 3.87 4.43333 0.50 0.66 4.9 5.2 1.30 1.23 0.54 0.43 5.03 5.66
-OCH3
303 0.39 0.54 3.6 3.8 – – 0.74 0.63 2.71 3.02313 0.42 0.58 4.1 4.3 1.30 1.23 0.68 0.57 3.52 3.87323 0.46 0.62 4.6 4.8 1.15 1.10 0.63 0.52 4.43 4.83333 0.52 0.69 5.2 5.4 1.23 1.18 0.59 0.48 5.66 6.11
–OH
303 0.38 0.48 4.1 4.3 – – 0.67 0.52 3.52 3.67313 0.40 0.50 4.6 4.7 1.15 1.12 0.61 0.49 4.43 4.63323 0.45 0.54 5.1 5.2 1.03 1.01 0.56 0.45 5.45 5.66333 0.51 0.60 5.7 5.8 1.11 1.09 0.52 0.41 6.80 7.04
–Cl
303 0.31 0.48 2.9 3.1 – – 0.61 0.47 1.76 2.01313 0.34 0.51 3.3 3.6 1.29 1.50 0.55 0.44 2.28 2.71323 0.39 0.55 3.8 4.1 1.41 1.30 0.50 0.40 3.02 3.52333 0.45 0.62 4.4 4.7 1.47 1.37 0.46 0.35 4.05 4.63
increasingly irreversible [30–33] with increase in temperature.The literature survey [34] reveals the same observations forsimilar compounds.
3.1.7. Thermodynamic parametersThe thermodynamic parameters, namely, the formal rate
constant (K ◦
f ,h), enthalpy of activation (1H∗), activation freeenergy change (1G∗) and the entropy of activation (1S∗),were evaluated from the equations proposed by Meites andIsrael [35], Oldham and Parry [36] and Gaur and Bhargava [37].The diffusion coefficient necessary for calculation of theformal rate constant (K ◦
f ,h) at different temperatures has beencalculated from the Stoke–Einstein equation [38]. Itwas noticedfromTables 4 and 5 that the formal rate constant decreaseswithan increase in temperature. This fact suggests that the electrodereaction was rendered increasingly irreversible with the raisein temperature. This observation is in accordance with theconclusion arrived at on the basis of αna values. The negative1S∗ values suggest that the activated state has a more rigidstructure than the initial state. The thermodynamic parametersenthalpy of activation (1H∗), free energy of activation (1G∗)and entropy of activation (1S∗) presented in Tables 4 and 5reveal the following points:(1) The positive values of 1H∗ indicate that the process was
endothermic.(2) The positive values of1G∗ indicate that the processwas not
spontaneous.(3) The negative values of 1S∗ indicate that the process was
entropically unfavorable.The millicoulometer of De Vries and Kroon [39], with mercurypool cathode, was employed to determine the value of ‘n’. Theresults are presented in Table 6.
Figure 9: −E1/2–σP plots of pyrazolin-5-ones (first wave). [pyrazolin-5-one] =
1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
3.1.8. Effect of substituents on the polarographic behaviour of1-(Toluenyl sulfonyl)-3-amino-4-(substituted aryl hydrazono)-2-pyrazolin-5-ones
For the compounds in an aromatic series, structuralcorrelations are usually done with σP (substituent constant)values. The corresponding E1/2–σP plots are presented inFigures 9 and 10.
It is observed from Table 2 that 1E1/2/1pH, αna and I(diffusion current constant) values are practically in the samerange for the entire reaction series. This fact made it possibleto discuss the effect of substituents, in terms of the Hammettequation. The procedure reported in the literature [40] was
612 K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618
Table 4: Kinetic and thermodynamic parameters for polarographic reduction of 1-(Toluenyl sulfonyl)-3-amino-4-(2′-substituted hydrazono)-2-pyrazolin-5-one.(First wave). pH = 4.1, [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
Compound Parameter Meites–Israel treatment Oldham–Parry treatment Gaur–Bhargava treatmentTemperature (K) Temperature (K) Temperature (K)
303 313 323 333 303 313 323 333 303 313 323 333
2′–H
K ◦
f ,h × 106 3.689 3.406 2.085 0.931 3.694 3.412 2.090 0.934 5.026 4.601 2.750 1.1831H∗ 9.152 9.152 9.152 9.152 8.985 8.985 8.985 8.985 10.439 10.439 10.439 10.4391G∗ 6.342 6.573 6.920 7.366 6.342 6.572 6.919 7.365 6.261 6.492 6.843 7.297−1S∗ 7.287 6.252 4.923 3.375 6.736 5.722 4.409 2.877 11.802 10.623 9.145 7.447
2′–methyl
K ◦
f ,h × 106 2.645 1.894 0.912 0.395 2.650 1.898 0.915 0.397 3.547 2.495 1.161 0.4851H∗ 15.646 15.646 15.646 15.646 15.604 15.604 15.604 15.604 16.437 16.437 16.437 16.4371G∗ 6.429 6.731 7.150 7.612 6.429 6.731 7.149 7.611 6.352 6.657 7.083 7.553−1S∗ 28.432 26.495 24.316 22.138 28.294 26.361 24.189 22.015 31.297 29.259 26.972 24.691
2′–methoxy
K ◦
f ,h × 107 9.643 6.918 4.057 1.600 9.670 6.940 4.072 1.608 12.385 8.730 4.993 1.8901H∗ 8.534 8.534 8.534 8.534 8.452 8.452 8.452 8.452 8.676 8.676 8.676 8.6761G∗ 6.693 7.004 7.376 7.872 6.692 7.003 7.226 7.870 6.627 6.941 7.318 7.824−1S∗ 4.089 2.901 1.598 0.260 3.822 2.642 1.808 0.240 4.776 3.556 2.217 0.570
2′–hydroxy
K ◦
f ,h × 106 2.139 2.031 1.016 0.459 2.144 2.036 1.019 0.461 2.822 2.663 1.291 0.5631H∗ 7.675 7.675 7.675 7.675 7.579 7.579 7.579 5.579 7.549 7.549 7.549 7.5491G∗ 6.485 6.713 7.120 7.569 6.484 6.712 7.119 7.568 6.412 6.639 7.053 7.510−1S∗ 1.941 1.086 0.269 1.669 1.627 0.783 0.563 1.955 1.766 0.920 0.452 1.871
2′–chloro
K ◦
f ,h × 105 1.103 0.763 0.396 0.194 1.104 0.764 0.397 0.195 1.573 1.068 0.537 0.2551H∗ 11.333 11.333 11.333 11.333 11.503 11.503 11.503 11.503 11.525 11.525 11.525 11.5251G∗ 6.056 6.355 6.741 7.154 6.056 6.355 6.741 7.154 5.963 6.264 6.656 7.076−1S∗ 15.429 13.917 12.229 10.562 15.990 14.460 12.755 11.072 16.369 14.821 13.087 11.372
K ◦
f ,h expressed in cm s−1 , 1H∗, 1G∗ and −1S∗ expressed in K cal mole−1 .
Table 5: Kinetic and thermodynamic parameters of polarographic reduction of 1-(Toluenyl sulfonyl)-3-amino-4-(2′-substituted hydrazono)-2-pyrazolin-5-one(Second wave). pH = 4.1, [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
Compound Parameter Meites–Israel treatment Oldham–Parry treatment Gaur–Bhargava treatmentTemperature (K) Temperature (K) Temperature (K)
303 313 323 333 303 313 323 333 303 313 323 333
2′–H
K ◦
f ,h × 106 5.241 3.301 1.430 1.001 5.259 3.313 1.437 1.006 6.594 4.057 1.694 1.1631H∗ 10.699 10.699 10.699 10.699 10.539 10.539 10.539 10.539 11.625 11.625 11.625 11.6251G∗ 6.852 7.204 7.667 8.007 6.851 7.203 7.665 8.005 6.792 7.148 7.619 7.963−1S∗ 10.709 9.179 7.399 6.096 9.023 8.671 6.910 5.622 13.964 12.316 10.415 9.009
2′–methyl
K ◦
f ,h × 106 2.033 1.516 1.223 0.872 2.041 1.523 1.229 0.877 2.455 1.802 1.436 1.0061H∗ 5.519 5.519 5.519 5.519 5.798 5.798 5.798 5.798 5.857 5.857 5.857 5.8571G∗ 7.100 7.414 7.710 8.046 7.099 7.412 7.709 8.045 7.051 7.367 7.666 8.005−1S∗ 7.205 8.042 8.771 9.576 6.281 7.144 7.904 8.736 5.927 6.812 7.588 8.438
2′–methoxy
K ◦
f ,h × 107 4.862 4.479 4.239 2.290 4.888 4.506 4.263 2.304 5.537 5.063 4.763 2.5011H∗ 1.602 1.602 1.602 1.602 1.346 1.346 1.346 1.346 1.445 1.445 1.445 1.4451G∗ 7.474 7.743 8.005 8.431 7.473 7.741 8.004 8.429 7.440 7.710 7.973 8.405−1S∗ 21.366 21.607 21.811 22.495 22.208 22.419 22.601 23.258 21.772 23.003 22.198 22.889
2′–hydroxy
K ◦
f ,h × 106 6.665 6.473 5.231 3.512 6.687 6.495 5.250 3.526 8.377 8.103 6.472 4.2611H∗ 6.159 6.159 6.159 6.159 6.318 6.318 6.318 6.318 6.344 6.344 6.344 6.3441G∗ 6.869 7.021 7.305 7.646 6.789 7.021 7.304 7.645 6.730 6.691 7.246 7.590−1S∗ 4.330 4.741 5.536 6.453 3.541 4.233 5.040 5.973 3.261 3.958 4.933 5.730
2′–chloro
K ◦
f ,h × 105 7.645 6.905 6.315 5.013 7.667 6.927 6.336 5.031 9.787 8.758 7.946 6.2201H∗ 4.136 4.136 4.136 4.136 4.108 4.108 4.108 4.108 3.051 3.051 3.051 3.0511G∗ 6.754 7.004 7.252 7.544 6.753 7.003 7.252 7.543 6.689 6.940 7.189 7.482−1S∗ 10.627 11.150 11.635 12.222 10.716 11.236 11.721 12.303 13.993 14.412 14.799 15.294
K ◦
f ,h expressed in cm s−1 , 1H∗, 1G∗ and −1S∗ expressed in K cal mole−1 .
employed to compute Hammett substituent constant values(ρ). The ρ values are presented in Table 7. The values werefound to be in the range of 0.10–0.30. Positive and lowvalues [41] of ρ indicate that the polarographic reductioninvolves nucleophilic addition of electron to the substrate. Thisfact confirms that the electron uptake processwas the potentialrate determining step in all the reduction processes studied.
3.1.9. Reduction mechanismThe results reveal that the N–N bond in the hydrazono group
(>C=N-NH-) was reduced more easily than the azomethine
(>C=N-) group. Similar observations were made for semicar-bazones and hydrazones [26,42].
3.1.9.1. Reduction in acidic medium. The first step involves thetwo-electron reductive cleavage of the >N–N < bond, leadingto the formation of 1-(Toluenyl sulfonyl)-3-amino-4-imino-2-pyrazolin-5-one (Scheme 2 (B)) [43,44] and substitutedaniline. The second step involves the two-electron reductionof ketimine (Scheme 2(B)) to the corresponding diamine(Scheme 2(C)). The wave height corresponding to both thesesteps were affected by acid-base equilibrium. The variation ofwave height with pH is similar to the trend reported in the
K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618 613
Table 6: Millicoulometric data of 1-(Toluenyl sulfonyl)-3-amino-4-(2′-substituted hydrazono)-2-pyrazolin-5-one. [pyrazolin-5-one] = 1 × 10−3 M; Medium:Aqueous dimethylformamide (40% v/v).
pH Wave height (H) (cm) Time (s) n value Wave height (H) (cm) Time (s) n valueI wave II wave I wave II wave I wave II wave I wave II wave
–H -CH3
4.13.6 3.8 0 – – 3.8 4.1 0 – –3.1 3.3 7200 1.8 2.0 3.3 3.5 7200 2.0 1.92.9 2.9 10800 1.9 2.1 2.9 3.2 10800 2.1 1.9
8.11.3 1.2 0 – – 1.5 1.5 0 – –1.2 1.1 7200 1.7 1.8 1.4 1.4 7200 1.6 1.61.2 1.1 10800 1.7 1.7 1.4 1.4 10800 2.2 2.2
-OCH3 –OH
4.14.1 4.3 0 – – 4.6 4.8 0 – –3.5 3.7 7200 2.0 2.1 3.9 4.1 7200 2.1 2.33.3 3.5 10800 2.1 2.3 3.6 3.8 10800 2.1 2.3
8.11.8 1.7 0 – – 2.3 2.5 0 – –1.7 1.6 7200 2.3 2.1 2.1 2.3 7200 1.9 2.21.6 1.5 10800 1.7 1.6 2.0 2.2 10800 1.8 2.2
–Cl
4.13.3 3.6 0 – –2.9 3.1 7200 1.9 1.82.7 2.9 10800 1.8 1.9
Figure 10: −E1/2–σP plots of pyrazolin-5-ones (Second wave). [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
literature [45]. The proposed reduction mechanism in acidicmedium is shown in Scheme 2.
3.1.9.2. Reduction in alkaline medium. In alkaline medium, 1-(Toluenyl sulfonyl)-3-amino-4-(substituted aryl hydrazono)-2-pyrazolin-5-ones exist in azomethine anionic form [26](>C=N–N̄–).
The first wave may be ascribed to the two-electronreductive cleavage of the N–N bond in azomethine anionic form(>C=N–N̄–). This anionic form (Scheme 3 (D)) is susceptibleto cleavage, to form a heterocyclic carbonyl compound. Thesecond wave is due to the two-electron reduction of theheterocyclic carbonyl compound to corresponding alcohol. Thereductionmechanism observed in alkalinemedium is similar tothat reported in literature [46] and is given in Scheme 3.
3.2. Cyclic voltammetric behaviour of 1-(Toluenyl sulfonyl)-3-amino-4-(aryl hydrazono)-2-pyrazolin-5-one
The cyclic voltammograms were recorded using HMDEin dimethylformamide (40% v/v) in Britton–Robinson buffersolutions of pH 2.1, 4.1, 6.1 and 8.1 at scan rates of 10, 20,
Scheme 2: Reduction mechanism in acidic medium.
Table 7: Effect of pH on the reaction constant for the re-duction of 1-(Toluenyl sulfonyl)-3-amino-4-(2′-substitutedhydrazono)-2-pyrazolin-5-one. [pyrazolin-5-one] = 1 ×
10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
pH ρ valueI wave II wave
1.1 0.12 0.0972.1 0.13 0.0933.1 0.13 0.0994.1 0.15 0.0955.1 0.16 0.1256.1 0.14 0.1277.1 0.13 0.0978.1 0.13 0.0779.1 0.13 0.077
10.1 0.13 0.077
614 K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618
Scheme 3: Reduction mechanism in alkaline medium.
50, 100, 200, 300 and 500 mV/s. The results are presented inTable 8.
Cyclic voltammograms contain two sharp cathodic peaks atall scan rates for all compounds (1–5) in the pH range of study(2.1–8.1). No anodic peak was observed for all the compoundsunder investigation.
3.2.1. Effect of scan rate on peak potential and peak currentIn media of pH 2.1–8.1, the peak potentials shift to more
negative values and the peak currents increasewith an increasein the scan rate. The results are presented in Table 8.
3.2.2. Effect of pH on peak potential and peak currentAn inspection of the data presented in Table 8 reveal that
the cathodic peak potentials shift to more negative values, andpeak currents decrease with an increase in pH of the solution.The number of cathodic peaks noticed in cyclic voltammetrywas equal to the number of polarographic reduction wavesobserved in DC polarography. The results were similar to thoseobserved in polarography. The diffusion controlled nature of theelectrode process is substantiated by following facts. (i) The plotiPC vs. ν1/2 was linear (Figures 11 and 12) and (ii) The valuesof iPC/ν1/2 were nearly unaltered. The irreversible nature ofthe electrode process was confirmed by following facts. (i) Theabsence of anodic peak in the reverse scan. (ii) The value of(EPC/2 − EPC) was greater than 56.5 mV [47]. (iii) The currentfunction (iPC/ν1/2
· C) was independent of the scan rate (ν) and(iv) The plot of (iPC/ν1/2) vs. ν graph was similar to the oneexpected for case II of Nicholson–Shain criteria [48].
Hence, the reduction mechanism observed at HMDE wasassumed to be the same as that at DME, and is described inSchemes 2 and 3.
Cyclic voltammograms recorded (pH 2.1–8.1) under re-peated cycles reveal that there is no change, either in the shapeof the cyclic voltammogram or in themagnitude of the peak po-tential, even though the peak current diminishes with an in-crease in the number of cycles. This may be ascribed to theadsorption of substrate on the mercury solution interface. Thisbehavior was similar to that normally expected under repeatedcycles for irreversible systems.
Figure 11: iPC–ν1/2 plots of pyrazolin-5-ones (I indicates first wave, II indicatessecond wave). pH = 4.1; [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueousdimethylformamide (40% v/v).
Figure 12: iPC–ν1/2 plots of pyrazolin-5-ones (I indicates first wave, II indicatessecond wave) pH = 8.1; [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueousdimethylformamide (40% v/v).
3.2.3. Effect of substituent on the cyclic voltammetric behaviorHammett’s linear free energy relations were applied to
investigate the effect of substituent on cathodic peak potentials.Plots were drawn between the first/second cathodic peakpotentials and the Hammett substituent constant (σP ). Thespecific reaction constant (ρ) values obtained are given inTable 9. It was noticed from Figures 13 and 14 that plots(EPC–σP ) were straight lines with a positive slope. The positive‘ρ’ values suggest that nucleophilic addition of electrons playsa significant role.
4. Conclusion
Five pyrazolin-5-ones, namely;(1) 1-(Toluenyl sulfonyl)-3-amino-4-(aryl hydrazono)-2-pyra-
zolin-5-one;(2) 1-(Toluenyl sulfonyl)-3-amino-4-(2′-methyl aryl hydra-
zono)-2-pyrazolin-5-one;(3) 1-(Toluenyl sulfonyl)-3-amino-4-(2′-methoxy aryl hydra-
zono)-2-pyrazolin-5-one;
K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618 615
Table8:
Cyclic
volta
mmetricresu
ltsof
1-(Tolue
nylsulfony
l)-3-am
ino-4-(2
′-sub
stitu
tedhy
draz
ono)-2-p
yraz
olin-5-one
.[py
razo
lin-5-one
]=
1×
10−3M;M
edium:A
queo
usdimethy
lform
amide(40%
v/v).
pHSc
anrate
(Vs−
1 )
−E P
CI
(V)
−E P
CII
(V)
i PCI
(µA)
i PCII
(µA)
−E P
CI
(V)
−E P
CII
(V)
i PCI
(µA)
i PCII
(µA)
−E P
CI
(V)
−E P
CII
(V)
i PCI
(µA)
i PCII
(µA)
−E P
CI
(V)
−E P
CII
(V)
i PCI
(µA)
i PCII
(µA)
−E P
CI
(V)
−E P
CII
(V)
i PCI
(µA)
i PCII
(µA)
–H-C
H3
-OCH
3–O
H–C
l
2.1
0.01
00.24
0.42
0.7
0.8
0.30
0.54
0.8
0.9
0.36
0.57
0.9
0.9
0.42
0.57
1.0
1.0
0.15
0.39
0.7
0.7
0.02
00.27
0.45
1.0
1.1
0.33
0.57
1.1
1.2
0.39
0.60
1.3
1.4
0.45
0.60
1.4
1.5
0.18
0.42
0.9
1.0
0.05
00.30
0.48
1.7
1.7
0.36
0.60
1.8
1.9
0.42
0.63
2.0
2.1
0.48
0.63
2.2
2.4
0.21
0.45
1.5
1.6
0.10
00.33
0.51
2.4
2.5
0.39
0.63
2.6
2.7
0.45
0.66
2.9
3.0
0.51
0.66
3.2
3.3
0.24
0.48
2.1
2.3
0.20
00.39
0.57
3.4
3.5
0.45
0.69
3.7
3.8
0.51
0.72
4.1
4.2
0.57
0.72
4.5
4.8
0.30
0.54
3.0
3.2
0.30
00.45
0.63
4.1
4.3
0.51
0.75
4.5
4.6
0.57
0.78
5.0
5.2
0.63
0.78
5.5
5.9
0.36
0.60
3.7
3.9
0.50
00.51
0.69
5.4
5.5
0.57
0.81
5.8
6.0
0.63
0.84
6.4
6.7
0.69
0.84
7.1
7.6
0.42
0.66
4.7
5.1
4.1
0.01
00.39
0.63
0.6
0.6
0.45
0.63
0.7
0.7
0.54
0.78
0.7
0.8
0.51
0.72
0.9
0.9
0.33
0.54
0.5
0.5
0.02
00.42
0.66
0.8
0.8
0.48
0.66
0.9
0.9
0.57
0.81
1.0
1.1
0.54
0.75
1.2
1.3
0.36
0.57
0.7
0.8
0.05
00.45
0.69
1.3
1.3
0.51
0.69
1.4
1.5
0.60
0.84
1.6
1.7
0.57
0.78
1.9
2.1
0.39
0.60
1.1
1.2
0.10
00.48
0.72
1.8
1.9
0.54
0.72
2.0
2.1
0.63
0.87
2.3
2.4
0.60
0.81
2.7
2.9
0.42
0.63
1.5
1.7
0.20
00.54
0.78
2.6
2.7
0.60
0.78
2.8
3.0
0.69
0.93
3.2
3.4
0.66
0.87
3.9
4.2
0.48
0.69
2.1
2.3
0.30
00.60
0.84
3.2
3.3
0.66
0.84
3.4
3.7
0.75
0.99
3.9
4.2
0.72
0.93
4.7
5.1
0.54
0.75
2.6
2.9
0.50
00.66
0.90
4.1
4.3
0.72
0.90
4.4
4.7
0.81
1.05
5.1
5.4
0.78
0.99
6.1
6.5
0.60
0.81
3.4
3.7
6.1
0.01
00.72
0.90
0.5
0.4
0.75
0.96
0.5
0.4
0.87
1.08
0.6
0.5
0.84
1.02
0.7
0.8
0.72
0.96
0.4
0.4
0.02
00.75
0.93
0.6
0.6
0.78
0.99
0.7
0.7
0.90
1.11
0.8
0.8
0.87
1.05
1.0
1.1
0.75
0.99
0.5
0.5
0.05
00.78
0.96
0.9
0.9
0.81
1.02
1.1
1.0
0.93
1.14
1.3
1.2
0.90
1.08
1.5
1.7
0.78
1.02
0.8
0.7
0.10
00.81
0.99
1.3
1.2
0.84
1.05
1.5
1.4
0.96
1.17
1.8
1.7
0.93
1.11
2.0
2.3
0.81
1.05
1.1
1.0
0.20
00.87
1.05
1.8
1.7
0.90
1.11
2.1
2.0
1.02
1.23
2.5
2.3
0.99
1.17
3.0
3.5
0.87
1.11
1.6
1.4
0.30
00.93
1.11
2.2
2.1
0.96
1.17
2.6
2.4
1.08
1.29
3.1
2.9
1.05
1.23
3.7
4.2
0.93
1.17
1.9
1.8
0.50
00.99
1.17
2.8
2.8
1.02
1.23
3.4
3.2
1.14
1.35
4.0
3.7
1.11
1.29
4.7
5.4
0.99
1.23
2.5
2.3
8.1
0.01
00.78
0.99
0.4
0.3
0.81
1.02
0.4
0.3
0.93
1.17
0.5
0.4
0.93
1.14
0.6
0.7
0.75
0.99
0.3
0.3
0.02
00.81
1.02
0.5
0.4
0.84
1.05
0.6
0.4
0.96
1.20
0.7
0.5
0.96
1.17
0.8
0.9
0.78
1.02
0.4
0.4
0.05
00.84
1.05
0.7
0.6
0.87
1.08
0.9
0.6
0.99
1.23
1.1
0.8
0.99
1.20
1.1
1.3
0.81
1.05
0.7
0.5
0.10
00.87
1.08
1.0
0.8
0.90
1.11
1.2
0.8
1.02
1.26
1.5
1.1
1.02
1.23
1.6
1.8
0.84
1.08
0.9
0.7
0.20
00.93
1.14
1.4
1.1
0.96
1.17
1.7
1.1
1.08
1.32
2.1
1.6
1.08
1.29
2.3
2.6
0.90
1.14
1.3
1.0
0.30
00.99
1.20
1.8
1.4
1.02
1.23
2.1
1.4
1.14
1.38
2.6
1.9
1.14
1.35
2.7
3.1
0.96
1.20
1.6
1.3
0.50
01.05
1.26
2.3
1.8
1.08
1.30
2.7
1.8
1.20
1.44
3.4
2.5
1.20
1.41
3.5
4.0
1.02
1.26
2.1
1.6
616 K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618
Table 9: Effect of pH on the reaction constant for the reductionof 1-(Toluenyl sulfonyl)-3-amino-4-(2′-substituted aryl hydrazono)-2-pyrazolin-5-ones. [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueousdimethylformamide (40% v/v).
pH ρ valueI wave II wave
2.1 0.131 0.0934.1 0.152 0.0956.1 0.143 0.1278.1 0.132 0.077
Figure 13: −E1/2–σP plots of pyrazolin-5-ones (first wave). [pyrazolin-5-one]= 1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
Figure 14: −E1/2–σP plots of pyrazolin-5-ones (second wave). [pyrazolin-5-one] = 1 × 10−3 M; Medium: Aqueous dimethylformamide (40% v/v).
(4) 1-(Toluenyl sulfonyl)-3-amino-4-(2′-hydroxy aryl hydra-zono)-2-pyrazolin-5-one;
(5) 1-(Toluenyl sulfonyl)-3-amino-4-(2′-chloro aryl hydra-zono)-2-pyrazolin-5-one;
reported in this article, gave two well defined polarographicwaves in the entire pH range of study (1.1–10.1). The reductionprocess was found to be irreversible and diffusion controlledwith the involvement of protons. Cyclic voltammetry wasperformed in support of the results obtained in polarography.Based on the results obtained, the authors proposed amechanism for the electrode reaction at the dropping mercuryelectrode; the same of which may be used for pharmacokineticstudies involving these pyrazolin-5-ones.
Acknowledgments
The authors are thankful to V. Sheshagiri, Professor (Emeri-tus), Sri Krishnadevaraya University, Anantapur, A.P., India, forhis valuable suggestions during the entire course of this work.
Appendix
1. The number of protons involved in the reduction processwas calculated by the following equation:
1E1/2
1pH=
0.05915αna
P,
where:
α = transfer coefficient;
na = number of electrons involved in the rate
determining step.
2. Tomes’ criterion for an irreversible process; the equation isgiven by:
E = E1/2 −0.0542αna
log
iid − i
;
with: E1/2 = −0.2412 +0.05915
αnalog
1.349 K ◦
f ,h t1/2
D1/2
;
where:
K ◦
f ,h = heterogeneous forward rate constant in cm/s;
D = diffusion coefficient of the depolariser in cm2 s−1.
3. αna value for an irreversible process can be calculated usingthe following equation:
E1/4 − E3/4 =0.05172
αna,
where E1/4 and E3/4 are potentials when i = id/4 and 3 id/4,respectively.
4. The following formulae were employed for calculation ofkinetic parameter ‘K ◦
f ,h’.Meites–Israel’s method:
Edme =0.05915
αnalog
1.349K ◦
f ,ht1/2
D1/2−
0.0542αna
logi
id − i.
Oldham–Parry’s method:Oldham–Parry have suggested alternatives to the usual logplots for the calculation of K ◦
f ,h.
Edme = E1/2 −0.05915
αnalog
1.35 K ◦
f ,h
√t
√D
.
Gaur–Bhargava’s method:Gaur–Bhargava assumed that diffusion to the electrode sur-face is spherical, but not linear, as assumed byMeites–Israel.
Edme =0.05915
αnalog
1.349K ◦
f ,ht1/2
1.128D1/2−
0.05690αna
logi
id − iαna was calculated from the following equation:
αna = −0.0517/(E3/4–E1/4).
K. Ramana Kumar et al. / Scientia Iranica, Transactions C: Chemistry and Chemical Engineering 19 (2012) 605–618 617
5. Stoke–Einstein equation:
D =3.38 × 10−5
Vm1/3η
at 30 °C,
where:
η = viscosity of the solution;
Vm = apparent molar volume.
6. Hammett substituent constant (σ ) for meta and parasubstituents are:
E1/2(X) = E1/2(H) + 0.136σ(H);
where:
E1/2(X) = half-wave potential of the substituted pyrazolin;
E1/2(H) = half-wave potential of the pyrazolin;
σ(H) = Hammett substituent constant.
Hammett quantified the effect of substituents on anyreaction by defining an empirical electronic substituentparameter (σ ).The basic equation is:
log(K/Ko) = σ ,
where K is the dissociation constant of unsubstitutedreactant.
Ko is the dissociation constant of substituted reactant. σ isthe substituent constant, which depends only on the specificsubstituent.
The slope of the linear plot is referred to as ρ (reactionconstant). The reaction constant is a measure of how sensitivea particular reaction is to changes in electronic effects ofsubstituent groups. The reaction constant depends on thenature of the chemical reaction, as well as reaction conditions(solvent, temperature, etc.). Both the sign andmagnitude of thereaction constant are indicative of the extent of charge build upduring the reaction progress.
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Kakarla Ramana Kumar obtained his M.Sc., M.Phil. and Ph.D. degrees from SriKrishnadevaraya University, Anantapur, A.P., in India, and is, currently, FacultyMember in Malla Reddy College of Engineering, Hyderabad, A.P., in India. Hiscurrent research interests are spectroscopy and polarography.
Aluru Raghavendra Guru Prasad is Assistant Professor in the ICFAI Foundationfor Higher Education, Hyderabad, A.P., in India, where he is AcademicCoordinator andMember of the Board of Studies, He obtainedM.Sc., M.Phil. andPh.D. degrees from Sri Krishnadevaraya University, Anantapur, A.P., India. Hiscurrent research interests are: spectroscopy and polarography.
Vinnakota Srilalitha obtained M.Sc., M.Phil. and Ph.D. degrees from SriKrishnadevaraya University, Anantapur, A.P., India, and is, currently, FacultyMember in the C.M.R. Institute of Technology, Hyderabad, A.P., India,. Hercurrent research interests are spectroscopy and polarography.
Golla Narayana Swamy is Professor in Sri Krishnadevaraya University,Anantapur, A.P., in India. His current research interests are spectroscopy andpolarography.
Laxmana Rao Krishna Rao Ravindranath is Professor in Sri KrishnadevarayaUniversity, Anantapur, A.P., in India. His current research interests arespectroscopy and polarography.
