REVISTA COLOMBIANA DE QUÍMICA, VOLUMEN 40, nro. 2 DE 2011 165 Orgánica y Bioquímica Rev. Colomb. Quím., 2011, 40(2): 165-184 1 Malla Reddy College of Engineering, Hyderabad, A.P., India, 2 ICFAI Foundation for Higher Education, Hyderabad,A.P., India 3 C.M.R. Institute of Technology, Hyderabad,A.P., India, 4 Sri Krishnadevaraya University, Anantapur, A.P., India. 5 [email protected]ELECTROCHEMICAL STUDIES OF CERTAIN 1-(TOLUENYL SULFONYL)-3- AMINO-4-(4’-SUBSTITUTED ARYL HYDRAZONO)-2-PYRAZOLIN-5-ONES ESTUDIO ELECTROQUÍMICO DE ALGUNOS 1-(TOLUENIL SULFONILO)-3- AMINO-4-(4’-HIDRAZINA FENIL SUSTITUIDAS)-2-PIRAZOLINA-5-ONAS ESTUDO ELETROQUÍMICO DE ALGUNS 1-(TOLUENIL SULFONILA)-3- AMINA-4-(4’- HIDRAZINA FENIL SUBSTITUÍDAS)-2-PIRAZOLINA-5-ONAS Ramana Kumar Kakarla 1 , Raghavendra Guru Prasad Aluru 2,5 , Srilalitha Vinnakota 3 , Narayana Swamy Golla 4 , Ravindranath Lakshmana Rao Krishna Rao 4 Recibido: 29/07/11- Aceptado: 26/08/11 RESUMEN El comportamiento electroquímico de ciertos 1-(Toluenil sulfonil)3-amino- 4-(aril hidrazonio)-2-pirazolin-5-onas sustituidas fueron estudiados con un electrodo de gota de mercurio emplean- do polarografía de corriente directa. Las variables que influyen sobre el proceso en el electrodo fueron estudiadas amplia- mente. Todos los compuestos estudiados presentaron dos ondas polarográficas bien definidas. Se propone un mecanis- mo para las reacciones electroquímicas estudiadas tanto en medio ácido como básico. Palabras clave: Estudios polarográf- icos, 1-(Toluenil sulfonil)3-amino-4-(aril hidrazonio sustituidos)-2-pirazolin-5- ABSTRACT The electrochemical behavior of certain 1-(Toluenyl sulfonyl)-3-amino-4-(4’- substituted aryl hydrazono)-2-pyrazo- lin-5-ones were studied at the dropping mercury electrode by employing DC po- larography. The variables that influence the electrode process were extensively studied. All compounds under investiga- tion gave two well-defined polarographic waves. The mechanism for the electrode process was proposed in acid as well as in basic media. Key words: Polarographic studies, 1- (Toluenyl sulfonyl)-3-amino-4-(4’-sub- stituted aryl hydrazono)-2-pyrazolin-5- ones, influence of variables, mechanism of electrode reaction.
Transcript
REVISTA COLOMBIANA DE QUÍMICA, VOLUMEN 40, nro. 2 DE 2011
El comportamiento electroquímico deciertos 1-(Toluenil sulfonil)3-amino-4-(aril hidrazonio)-2-pirazolin-5-onassustituidas fueron estudiados con unelectrododegotademercurioemplean-dopolarografíadecorrientedirecta.Lasvariables que influyen sobre el proceso enelelectrodofueronestudiadasamplia-mente.Todosloscompuestosestudiadospresentaron dos ondas polarográficas bien definidas. Se propone un mecanis-mo para las reacciones electroquímicasestudiadas tanto en medio ácido comobásico.
The electrochemical behavior of certain1-(Toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydrazono)-2-pyrazo-lin-5-ones were studied at the droppingmercuryelectrodebyemployingDCpo-larography. The variables that influence the electrode process were extensivelystudied.All compoundsunder investiga-tion gave two well-defined polarographic waves.Themechanismfortheelectrodeprocesswasproposedinacidaswellasinbasicmedia.
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onas, influencia de variables, mecanis-mosdereacciónenelelectrodo.
Resumo
Ocomportamentoeletroquímicodecer-tos 1-(Toluenil sulfonil)3-amino-4-(arilhidrazonio)-2-pirazolin-5-onassubstituí-dasforamestudadoscomumeletrododegotademercúrioempregandopolarogra-fia de corrente direta. As variáveis queinfluem sobre o processo no eletrodo foram estudadas amplamente. Todosos compostos estudados apresentaramduas ondas polarograficas bem defini-das. Propõe-se um mecanismo para asreações eletroquímicas estudadas tantoemmeioácidoquantoemmeiobásico.
Palavras chave: Estudos polarografi-cos,1-(Toluenil sulfonil)3-amino-4-(arilhidrazonio substituídos)-2-pirazolin-5-onas, influência de variáveis, mecanis-mosdereaçãonoeletrodo.
IntRoDuctIon
Pyrazole and its derivatives are impor-tant class of heterocyclic compoundsdue to their wide spread applications inmedicinal(1)andpesticidechemistry(2,3).Compounds containing pyrazole ringsystemareknowntodisplaydiversephar-macological activities such as antibacte-rial (4),antifungal(2),antiviral(5),anti-inflammatory(6, 7), analgesic(6, 7), and antipyretic(6). Some of them have been used as antihyperglycemic agents (8) orcardiovascular drugs (9), while some ofthemhavebeenusedasanxiolyticdrugs
(10).A systematic study of this class ofcompoundsreveledthattheirpharmaceu-ticalactivityresultsfromthepresenceof
pyrazolenucleus.Moreover, thebiologi-cal activities of pyrazol-5-ones dependonthenatureofthesubstituents.Varioustherapeutic aspects and numerous appli-cationsofabovesaidclassofcompoundshave inspired the authors to understandtheir reduction process at the droppingmercury electrode.However, theknowl-edgeofreductionisveryimportanttoun-derstandtheirbiologicalactivity.
Pyrazolin-5-onesarealsousedindyeindustry(11)andasindicators(12)incom-plexometric titrations. Many pyrazoleshavebeenfound to functionas lumines-cent and fluorescent(13,14)agents.
eXPeRImentAL
Allchemicalsandsolventsusedwereof analytical reagent grade procuredfromMerck,India.Doubledistilledwa-terwasusedforthepreparationofsolu-tions. Working solutions were preparedbyappropriatedilutionsof the standardsolution.
DC recordingpolarographmanufac-turedbyELICOPrivateLimited,Hyder-abad, India was used for polarographicstudies. The current voltage measure-ments were performed with three-elec-trode assembly, a dropping mercuryelectrodeasworkingelectrode,calomelasreferenceelectrodeandplatinumelec-trode as counter electrode. The currentresponses and applied potentials wererecordedat scan rate100mV/min.Thedroppingmercuryelectrodehadthecap-illarycharacteristics,m=2.422mg/s,t=2.5 s, h = 60 cm. pH measurements were madeusingpHmeterModelL1-10man-ufactured by ELICO Private Limited,Hyderabad,India.
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synthesis pyrazolin-5-ones
Amixtureofappropriatediazoniumcy-anoesterandtoluenesulfonylhydrazidein ethanol was refluxed for six hours and cooled. The crystalline solid separatedwas filtered, washed with water, dried and
recrystallysed from dimethylformamide(1:1).Thephysicalcharacteristicsofthedifferentcompoundssynthesizedarepre-sentedinTable1.
table 1.Physicalcharacteristicsofpyrazolin-5-ones
S. No. Substituent (X) Compound colourCompound melting points
observed (°C)
A 4’-CH3 Yellow 120-122
B 4’-OCH3 Yellow 128-130
C 4’-OH Black 270-271
D 4’-Cl Yellow 150-152
General experimental procedure
10mLofbuffersolutionofrequiredpH,2.5 mL of pyrazolin-5-one (1×10-2 M)and10mLofdimethylformamideweretaken into the polarographic cell. Thesolutionwasmade toa totalvolumeof25mLwithdistilledwater.Polarogramswere recorded after deaeration with ni-trogengas.
withincreaseinpHwasobservedforallthe compounds. Among the three sitessusceptibleforreductionnamelytheexo-cyclic>C=N,cyclic>C=Nandcyclicam-ide,exocyclic>C=Nismoresusceptibleforreductionthancyclic>C=Nandcyclicamide as it was experimentally confirmed that 1-(Toluenyl sulfonyl)-3-amino- 2-pyrzolin-5-one does not undergo reduc-tion under the experimental conditions.Hence the polarographic reduction of1-(Toluenyl sulfonyl)-3-amino-4-(substi-tutedarylhydrazono)-2-pyrazolin-5-onesisduetothereductionofexocyclic>C=Ngroup.
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Half-wave potential- pH relation
Thecompounds(A-D)exhibit twowell-defined waves in the entire pH range 1.1-10.1ofstudy.Thehalf-wavepotentialsof
the first and second waves for all the com-poundsturnmorenegativewithincreaseinpHinthepHrange1.1-7.1andremainconstantinalkalinemedium(8.1-10.1).
Figure 1. (I), (II) & (III).EffectofpHonhalf-wavepotentialof1-(Toluenylsulfonyl)-3-amino-4-(4’-substituted aryl hydrazono)-2-pyrazolin-5-one.[pyrazolin-5-one] = 1×10-3M;Medium=Dimethylformamide(40%byvolume)
0.8
0.6
0.4
0.2
0
E-2/1
ECS
svV
1
0 2 4 6 8 10
(I)A-I
B-II
C-I
X
pH
X
X
X
X
X
X
X
X X X 0.8
0.6
0.4
0.2
0
E-2/1
ECS
svV
1
0 2 4 6 8 10
(II)A-II
B-I
C-II
X
pH
X
X
X
X
X
X
X
X X X
0.8
0.6
0.4
0.2
0
E-2/1
ECS
svV
1
0 2 4 6 8 10
(III)D-IIX
pH
X
X
X
X
X
X
X
X XD-I
X
TheE1/2-pHplotsareshowninFigure1(I), (II)and(III).TheE1/2-pHrelation-ship for the first wave is represented by
a)-E1/2 = 0.05 + 0.08172 × pH V vs SCEb)-E1/2 = 0.03 + 0.08145 × pH V vs SCE
c)-E1/2 = 0.06 + 0.08372 × pH V vs SCEd)-E1/2 = 0.08 + 0.08546 × pH V vs SCE
The E1/2-pH relationship for the secondwaveisrepresentedby
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a)-E1/2 = 0.21 + 0.08172 × pH V vs SCE
b)-E1/2 = 0.15 + 0.08956 × pH V vs SCE
c)-E1/2 = 0.21 + 0.08372 × pH V vs SCE
d)-E1/2 = 0.24 + 0.08567 × pH V vs SCE
Thefactthatthehalf-wavepotentialvaluesdidnotvarywithpH inalkalinemedium suggests that the protons wereinvolved in the reduction process. Thenumberofprotons(P)involvedinthere-ductionprocesswascalculatedusingtheequation
Pn
05915.0
pH
E
a
2/1
α=
Δ
Δ
where α is transfer coefficient and na isnumberofelectrons involved.ThefractionalvalueofPpresented inTable2atdifferentpHvaluessuggeststhehet-erogeneousprotontransferinthereduc-tionprocess(15).
effect of mercury column height (h) on the wave height (H)
The wave height (H) linearly varieswithh1/2andtheplotofh1/2vsHpassesthroughtheorigin.Thissuggeststhedif-fusioncontrollednatureofthewave.
effect of concentration (c) of the depolariser on the wave height (H)
Theeffectofconcentrationofpyrazolin-5-ones(A-D)onthewaveheight in therange0.5-4.0mMinthetypicalpHme-dia 4.1 and 8.1 was studied.The linearwaveheight-concentrationplots (Figure2I,IIandIII)passingthroughtheoriginrevealed that the reduction process wasdiffusioncontrolledandthemethodcanbeappliedtodeterminethetraceamountsofpyrazolin-5-onesunderstudy.
Figure 2 (I), (II) & (III).Effectofconcentra-tion of 1-(Toluenyl sulfonyl)-3-amino-4-(4’-substitutedarylhydrazono)-2-pyrazolin-5-oneonwaveheight.pH=4.1;Medium=Dimethylformamide(40%byvolume)
Wav
e he
ight
(m)
0.08
0.06
0.04
0.02
0
0 1 2 3 4
A-I,4.1
A-II,4.1
A-II,8.1
C-II,8.1
(I)
[Pyrazoline-5one] x 103
x
+
–
A-I, 8.1B-I, 4.1B-I, 8.1C-I, 4.1C-I, 8.1D-I, 4.1
D-I, 8.1
++
+
+
+
++
+
–––––––
Wav
e he
ight
(m)
0.08
0.06
0.04
0.02
0
0 1 2 3 4[Pyrazoline-5one] x 103
(II)W
ave
heig
ht(m
)
0.08
0.06
0.04
0.02
0
0 1 2 3 4[Pyrazoline-5one] x 103
x
B-II, 4.1B-II, 8.1C-II, 4.1D-II, 4.1D-II, 8.1
(III) x
x
x
x
x
x
x
x
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Irreversible nature of the electrode process
Theirreversiblenatureofthewavesmaybeattributedtothebulkyarylhydrazonogroup at the end of >C=N linkage(16). SemilogplotsshowninFigure3(I)and(II) established the irreversiblenatureofthewave.Thefractionalvalueoftheslope0.049-0.051and0.048-0.053respectivelyfor first and second waves suggests that
Wav
e he
ight
(m)
X
A-IB-IC-ID-I
(I) 0.6
0.5
0.4
0.3
0.2-0.75 -0.25 0.25 0.75
log(i/id-i)
-Edm
e
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
the reduction process was irreversiblein nature. The irreversible nature of thewaveswasfurthersupportedbytheshiftofthehalf-wavepotentialvaluestowardsthe more negative values with increasein the concentration of the depolarizer (17), the decrease in heterogeneous rateconstant values K o
h,f and an increase inincreaseinactivationfreeenergychangevalues ∆G*withincreaseinthepHofthesolution(Table2).
Figure 4 (I) & (II). Effect of pH on wave height of 1-(Toluenyl sulfonyl)-3-amino-4-(4’-substi-tutedarylhydrazono)-2-pyrazolin-5-one.[pyrazolin-5-one] = 1×10-3M;Medium=Dimethylformamide(40%byvolume)
0.055
0.045
0.035
0.025
0.015
0.0050 2 4 6 8 10
Wav
e he
ight
(m)
pH
(I)
A-I
B-I
C-I
D-Ix
X
X
XX
X
X
X
XX
X
X
X
XXX
X
X
X
X
0.055
0.045
0.035
0.025
0.015
0.0050 2 4 6 8 10
Wav
e he
ight
(m)
pH
(II)
A-II
B-II
C-II
D-IIx
X
X
X
XX
X
X
X
XX
XX
XXX
X
X
X
X
X
XX
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Kinetic parameters of electrode reaction
The heterogeneous rate constant, K oh,f
and activation free energy change, ∆G*indifferentpHmedia(1.1-10.1)for thecompoundsunderstudyarepresentedinTable 2.As reveled from the table, thedecrease in K o
h,f and the increase in∆G*valuewithincreaseinpHareindica-tive of the enhanced irreversible natureofthereductionprocesswiththeincreaseinpH.
effect of temperature
Thepolarogramsofpyrazolin-5-onesinamediaofpH4.1wererecordedat303K,313K,323Kand333K, tostudy theef-fectoftemperatureonthehalf-wavepo-tentialandwaveheight.The resultsarepresentedinTable3.Thecompoundsun-der study exhibit two well-defined waves atalltemperaturesstudied(303-333K)atpH4.1.Thewaveheightincreaseswithincrease in temperature. The tempera-ture coefficient values were in the range 0.734-1.495% deg-1 and were in goodagreement with the values reported intheliterature(18)forothersimilarcom-pounds.
The table reveals that theαna valuesdecrease as the temperature increasesfrom303Kto333K.Thedecreaseinαnavalues with increase in temperature wasduetodecreaseinαvalue.Thedecreaseinα values indicates that the transferofelectrons was made increasingly difficult with the increase in temperature.Hence,thesystemtendstobecomemoreirrevers-ible(19)withtheincreaseintemperature.This fact was further supported by theshiftofhalf-wavepotentialstowardsmorenegativevalueswithraiseintemperature.Literaturesurvey(20)reveals thesimilarobservationsforsimilarcompounds.
Theformalrateconstant(K oh,f )cal-
culatedatdifferenttemperaturesisshownin Table 4. Stoke-Einstein equation wasused to calculate diffusion coefficient necessaryforthecalculationoftheformalrate constant at different temperatures.Table 4 shows that the formal rate con-stantdecreaseswithincreaseintempera-ture.ThedecreaseinK o
h,f withincreaseintemperaturesuggeststhattheelectrodereaction was becoming increasingly ir-reversible with the raise in temperature.Thisobservationwasinaccordancewiththeconclusionarrivedonthebasisofαnavalues.
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tabl
e 3.
Effe
ct o
f tem
pera
ture
on
the
pola
rogr
aphi
c ch
arac
teris
tics o
f 1-(
Tolu
enyl
sulfo
nyl)-
3-am
ino-
4-(4
’-su
bstit
uted
ary
l hyd
razo
no)-
2-p
yraz
o-lin
-5-o
ne (1
×10-3
M);
pH:4
.1;M
ediu
m:A
queo
usd
imet
hylfo
rmam
ide
(40%
v/v
).
Tem
pera
ture
Hal
f-w
ave
pote
ntia
l,-E
1/2 V
vs
SCE
Wav
e he
ight
H (
m)
Tem
pera
ture
C
oeff
icie
nt,
% d
eg-1
αn a
D x
102
m2 s
ec-1
I w
ave
II w
ave
I w
ave
II w
ave
I w
ave
II w
ave
I w
ave
II w
ave
I w
ave
II w
ave
4’-m
ethy
l
303
0.36
0.52
0.03
30.
036
--
0.69
0.55
2.28
2.71
313
0.39
0.55
0.03
80.
041
1.41
1.30
0.63
0.52
3.02
3.52
323
0.43
0.59
0.04
30.
046
1.24
1.15
0.58
0.48
3.87
4.43
333
0.50
0.65
0.04
90.
052
1.31
1.23
0.54
0.43
5.03
5.66
4’-m
etho
xy
303
0.37
0.53
0.03
50.
037
--
0.72
0.57
2.56
2.87
313
0.40
0.56
0.04
0.04
21.
341.
270.
660.
543.
353.
69
323
0.45
0.60
0.04
50.
047
1.18
1.12
0.61
0.50
4.24
4.63
333
0.51
0.66
0.05
10.
053
1.25
1.20
0.57
0.45
5.45
5.88
4’-h
ydro
xy
303
0.37
0.54
0.03
60.
038
--
0.74
0.60
2.71
3.02
313
0.41
0.57
0.04
10.
043
1.30
1.24
0.68
0.57
3.52
3.87
323
0.45
0.61
0.04
60.
048
1.15
1.10
0.63
0.53
0.43
0.82
333
0.51
0.66
0.05
20.
054
1.23
1.18
0.59
0.48
5.66
6.11
4’-c
hlor
o
303
0.31
0.47
0.02
80.
031
--
0.60
0.46
1.64
2.01
313
0.34
0.50
0.03
20.
035
1.34
1.21
0.54
0.43
2.14
2.56
323
0.38
0.55
0.03
70.
041.
45
1.34
0.49
0.39
2.87
3.35
333
0.44
0.62
0.04
30.
046
1.50
1.40
0.45
0.34
3.87
4.43
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The number of electrons involvedin the reduction of pyrazolin-5-oneswas evaluated in Britton-Robinson buf-fers containing 40% (v/v) dimethylfor-
mamide.ThemillicoulometerofDeVr-ies and Kroon (21) with mercury poolcathodewasemployedtodetermine thevalue of ‘n’. The results are presented in Table5.
table 5. Millicoulometric data of 1-(Toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydrazo-no)-2-pyrazolin-5-one (1×10-3M);Medium:Aqueousdimethylformamide(40%v/v).
pH Time (sec) 4’-CH3 4’-OCH3 4’-OH 4’-Cl
Iwave
II wave
Iwave
IIwave
Iwave
II wave
Iwave
II wave
4.1 0 - - - - - - - -
7200 2.0 1.9 2.2 2.0 1.9 2.1 1.8 1.7
10800 1.8 1.9 2.0 1.9 1.8 2.0 1.7 1.8
8.1 0 - - - - - -
7200 1.6 1.6 2.1 1.9 2.3 2.1
10800 2.2 2.2 1.7 2.3 1.7 1.6
Reduction mechanism
AninspectionoftheresultspresentedinTable 2 reveal that the compoundsA-Dexhibit two 2-electron reductive wavesinthepHrange1.1-10.1.Itwasevidentfromthepolarographicreductionofsemi-
carbazonesandhydrazones(22)thatN-Nbond in hydrazono group (>C=N-NH-)wasreducedmoreeasilythantheazome-thine(>C=N-)group.
In acidic medium, the first step in-volves the two-electron reductivecleav-
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ageofN-Nbondleadingtotheformationof 1-(Toluenyl sulfonyl)-3-amino-4-im-ino-2-pyrazolin-5-ones (23, 24) (II) andsubstituted aniline. The second step in-volves two-electron reduction of keti-mine (II) to the corresponding diamine(III).Thewaveheightofboththesestepswas affected by acid-base equilibrium.The variation of wave height with pHwassimilartothetrendreportedinthelit-erature(25).Therefore,reductionmecha-nismshowninFigure5(I)wasproposedinacidicmedium.
In alkaline medium, the azomethinegroup (>C=N-NH-) of the compoundsexist in the azomethine anionic form
(>C=N- N ) (26). The first wave wasat-tributed to the two-electron reductivecleavage of N-N bond in azomethineanionicform(IV)whichwassusceptibleforcleavagetothecorrespondinghetero-cyclic carbonyl compound. The secondwavewasduetothetwo-electronreduc-tionofheterocycliccarbonylcompoundtothecorrespondingalcohol.Therefore,reductionmechanismshowninFigure5(II)wasproposedinalkalinemedium.
Figure 5 (I).Reductionmechanisminacidicmedium
NH2
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Figure 5(II).Reductionmechanisminalkalinemedium
tionsareusuallydonewithσpvaluesforthecompoundsinanaromaticseries.E1/2 - σpplotsforthecompoundsunderinves-tigation are presented in Figure 6 (I) and (II). The values of specific reaction con-stant(ρ)calculatedfromtheslopesofE1/2 - σp plots are presented in Table 6.
NX N C C
C N
N
SO2
CH3
NH 2
H
O
OH- (-)
NX N C C
C N
N
SO2
CH3
NH2
O
-H2O
Chemicalcleavage
(I) (IV)
H2O
2 e-
+C C
C N
N
SO 2
CH3
NH2
O
O
(V)
C C
C N
N
SO2
CH3
NH2
O
HO
H
(VI)
X NH2NH
effect of substituents on the polarographic reduction
Heyrovsky(27) was the first man to cor-relate the polarographic behaviour of arepresentative number of compoundswith their structure. Structural correla-
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table 6. Effect of pH on the reaction constant for the reduction of 1-(Toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydrazono)-2-pyrazolin-5-ones. Medium:Aqueous dimethylfor-mamide(40%v/v)
pH ρ value
I wave II wave
2.1 0.113 0.113
4.1 0.105 0.112
6.1 0.113 0.105
8.1 0.099 0.103
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Thediscussionoftheeffectofsubstit-uentsintermsoftheHammettequationwas possible because ∆E1/2/∆pH, 〈naandI(diffusioncurrentconstant)values(Ta-ble2)werepracticallyinthesamerangefor the entire reaction series. The val-uesoftheHammettsubstituentconstantwere taken from the literature(28).Thevaluesofρwerefoundtobeintherangeof 0.10-0.30. Positive and low values
(29)ofρindicatethatthepolarographicreduction involves a nucleophilic addi-tionofelectrontothesubstrate.Thisfactconfirms that the electron uptake process wasthepotentialratedeterminingstepinallthereductionprocessesstudied.
effect of cation on the polarographic reduction of 1-(toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydrazono)-2-pyrazolin-5-ones
Not much work is reported (30) on theeffect of cation on ρ values of the po-larographic reduction. Few studies werereportedon the reductionofbenzylidineacetone(31)andnitrobenzene(32).TheρvaluespresentedinTable7increaseswithincreaseinthesizeofcation.Thisimpliesthatthesusceptibilityfornucleophilicad-ditiondiminisheswithincreaseinthesizeofthecation.Similarresultswerereportedforbenzylidineacetones(31) and N’-Ben-zylsulfonylarylazopyrazoles(33).
effect of organic co-solvent on the polarographic behaviour of 1-(toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydrazono)-2-pyrazolin-5-ones
Thepolarogramsof the1-(Toluenyl sul-fonyl)-3-amino-4-(4’-substituted aryl hy-drazono)-2-pyrazolin-5-oneswererecord-edatpH4.1in50and75%v/vaqueoussolutions of dimethylformamide (DMF),dimethylsulfoxide (DMSO), acetonitrile
(CH3CN) and methyl alcohol (CH3OH).The results are presented in Table 8.These results indicate that thechange incompositionofthesolventdoesnotbringanychange in thenumberofwavesandthe shape of the polarograms. However,a change in thepositionof thewaveonthepotentialaxisandamarkeddecreaseinthediffusioncurrentwasobserved.Thedecrease in thediffusioncurrentmaybeattributedtothedecreaseintheeffectivediffusion coefficient value. This may be
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duetotheincreaseintheviscosityofthesolutionor the change in the sizeof thesolvated species(34). In the presence oforganic solvents, thehalf-wavepotentialvalueswereshiftedtovaluesthataremorenegative and the magnitude of the shiftdependsonthenatureofthesolvent.Theshiftwas in theorder,DMSO<DMF<CH3CN<CH3OH,andparallelsthetrendin dielectric constant of solvents. TheobservedshiftinE1/2canbeattributedtoseveralfactorssuchasdielectricconstant,solvationofionsandadsorptionoforgan-icsolventontheelectrodesurface.
morethanthetheoreticallyexpectedval-ues for reversible waves. This indicatestheirreversiblenatureoftheelectrodere-actioninthepresenceoforganicsolvent.This shows that the mechanism of theelectrochemicalreactionissimilareveninthepresenceoforganicsolventsalthoughamarkedshiftinthediffusioncurrentandthehalf-wavepotentialswereobserved.
effect of surfactants on the polarographic behaviour of 1-(Toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydrazono)-2-pyrazolin-5-one
Pyrazoles have immense medicinal andbiologicalimportanceandhenceknowl-edgeoftheeffectofsurfactantsontheirredoxbehaviouratthesolutionmercury
interfacemayproveveryusefulfromthephysiologicalpointofview(35).Theef-fectofsurfactantsonredoxbehaviourofthese compounds is of immense impor-tance since the surfactants are used asemulsifiers in drugs. Malik and Rajeev Jain1982 (36) reported that the addition of surfactants beyond the concentrationjust sufficient to eliminate the maximum hasaffectedthereversibilityoftheelec-trodereaction.
So,inthepresentstudiestheeffectofdifferentsurfactantsonthepolarographicreductionof1-(Toluenylsulfonyl)-3-ami-no-4-(4’-substituted aryl hydrazono)-2-pyrazolin-5-oneshasbeeninvestigatedinsolutionsofpH4.1.Theresultsshowthatthe reduction becomes increasingly diffi-cult and the wave height decreases withincreaseintheconcentrationofthesurfac-tants.Thehalfwavepotentialsshifttoval-uesthataremorenegativeandwasduetothepreferential adsorptionof the surfac-tant atdroppingmercuryelectrode.Thisresulted in the partial desorption of thedepolariserfromtheelectrodesurfaceandthusloweringthesurfaceconcentrationofthedepolarizer(37).Similarobservationswerereportedintheliterature(38).
concLusIon
The reduction of 1-(Toluenyl sulfonyl)-3-amino-4-(4’-substituted aryl hydra-zono)-2-pyrazolin-5-ones leads to twoirreversible and diffusion-controlled po-larographicwaves.Thekineticparametersof the electrode process were evaluatedandpresented.Theeffectofsubstituents,cations, solvents and surfactants on thereduction process were detailed. Basedontheresultsobtained,aplausiblereduc-tionmechanismwasproposed.
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Org
ánic
a y
Bioq
uím
ica
tabl
e 8.
Effe
cto
forg
anic
co-
solv
ento
nha
lf-w
ave
pote
ntia
l(-E
1/2 V
vs S
CE)
of 1
-(To
luen
yl su
lfony
l)-3-
amin
o-4-
(4’-
subs
titut
ed a
ryl h
ydra
zono
)-2-
pyra
zolin
-5-o
nes(
1 ×10
-3M
)atp
H4
.1.
-XD
mso
(50%
)D
mso
(75%
)D
mF
(50%
)D
mF
(75%
)c
H3c
n (5
0%)
cH
3cn
(75%
)
Iw
ave
II
wav
eI
wav
eII
w
ave
Iw
ave
II
wav
eI
wav
eII
w
ave
Iw
ave
II
wav
eI
wav
eII
w
ave
-H0.
420.
530.
620.
730.
480.
590.
680.
790.
540.
650.
750.
86
4’-C
H3
0.34
0.52
0.44
0.63
0.39
0.57
0.59
0.77
0.44
0.63
0.65
0.83
4’-O
CH
30.
350.
530.
500.
590.
400.
580.
630.
720.
460.
640.
670.
76
4’-O
H0.
350.
550.
470.
680.
410.
590.
600.
810.
470.
660.
660.
87
4’-C
l0.
270.
450.
410.
580.
330.
510.
540.
7103
90.
570.
600.
77
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AcRonYms
The short form ‘A-I, 4.1’ indicated in the figures, stands for ‘compound-nthwave,pH’ i.e. A represents the compound, I represents first wave and 4.1 represents thepH.
