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Polarographic behaviour of some arylidene benzoic hydrazides in solutions of varying pH at the dropping mercury electrode
Elecrrochimica Acre, Vol. 25, pp. 1287-1291. Pergambn Press Ltd. 1980. Printed in Great Britain.
POLAROGRAPHIC BEHAVIOUR OF SOME ARYLIDENE BENZOIC HYDRAZIDES IN SOLUTIONS OF VARYING pH AT THli DROPPING MERCURY ELECTRODE Y. M. TEMERK* and A. 2. ABUZUHRI Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt, A.R.E. (Receiued 6 June 1980; infinalform 11
January
i980)
Abstract - The polarographic behaviour of some arylidene benzoic hydrazides such as anisylidene benzoic hydrazide (ABH), furfurylidene benzoic hydrazide (FBH) and acetophenone benzoic hydrazide (Acet.BH) has been studied in universal buffer solutions containing 40% ethanol at the dme. In the pH range 3.7-8.1 the observed single reduction wave for all three compounds is assigned to the reduction of the 1 C?N centre, whereas the > C+O centre is inactive towards reduction due to the mesomeric effect of the neighbouring amino group. In alkaline media (pH 8.6) a second wave appears at a more negative potential due to the reduction of the corresponding aldehyde or ketone which is formed by hydrolytic decomposition of the molecule. The effect of pH on the limiting current and Et,, as well as the reduction mechanism are discussed. The values of the kinetic parameters for the electrode reaction at different pH values are also computed from the pularographic results.
INTRODUCIION Hydrazides and their derivatives became of special interest in recent years due to their. biological importance[l]. The polarographic reduction of derivatives of hydrazides containing the > c-_O and > C=N linkages together has been not hitherto investigated. With respect to the reduction of the >C=C group, Holleck et aI[2] found that the polarographic reduction of benzoic hydrazide at pH 1-13 led to a reaction scheme for the cathode process with a special sequence of reactions. Brdicka[3] indicated that the polarographic reduction of the carbonyl group of aromatic and aryl alkyl ketones and aldehydes is pH dependent and is also affected by the type and position of substituents. For the polarographic reduction of azomethine compounds a number of investigators[4,5] reported that the reduction of the C=N center involves four or two electrons in acid and alkaline solutions respectively. The present paper is focused on the polarographic behaviour of some arylidene benzoic hydrazides such as anisylidene benzoic hydrazide (ABH) furfurylidene benzoic hydrazide (FBH) and acetophenone benzoic hydrazide (Acct. BH) in different but&r solutions at the dme in order to throw some light on the reduction mechanism for each of azomethine and carbonyl centers. No work on these compounds from the present stand point has been published.
EXPERIMENTAL Arylidene benzoic hydrazides were prepared by condensation of the aromatic aldehydes or ketones with benzhydrazide in the usual manner for the * To whom all correspondence should be directed.
preparation of hydrazone[6]. The purity of the compound prepared was checked by elemental analysis. The compounds selected following structures :
u
0
for the study
,“- NH-N.:
0
having
the
OCH,
0 (ABH)
0
CL:g
(O)C-NH-N=@
0
LH, (FEIHI
(Acet.BH)
lo-’ M ABH, FBH, and Acct. BH solutions were prepared by dissolving the accurately weighed solid in the appropriate volume of pure ethanol. The modified universal buffer series of Britton (pH 3-12) was prepared as given by Britton[7]. The polarographed depolarizer solutions were prepared by mixing the appropriate volume of the depolarizer solution with the universal buffer mixture containing 40yfi ethanol. The pH was checked using a Radiometer pH meter, Model 28, accurate to +0.05 unit. The polarograms were recorded on an LP 60 polarograph using a dme having the characteristics, m = 1.5 mg/s and t = 4.5 s at 45 cm.Hg (in 0.1 M KC1 at 0.0 V). All experiments were run at 22*O.lo”C. The solution was deaerated with pure nitrogen. The millcoulometric method of De Varies and Kroon[8] using a mercury pool cathode was used for determining the value of II, the number of electrons involved in the reduction process. RIWJL’IS AND DISCUSSION
Effect of pH on polarogrum. In moderate acidic buffer solutions (pH ~5) the polarogram of 5 x 10m4 M 1287
Y.M.
1288
TEMERKAND
ABH, FBI4 and Acct. BH consists of a single reduction wave, whose height corresponds to the uptake of four electrons in the electrode process. On increasing the pH( >S), the reduction wave decreases gradually till the height of the wave attains in slightly alkaline solution (pH 8.1) almost half its height in moderate acid media, whereas at intermediate pH the height of the reduction wave lies between the two values. Thus, it can be concluded that the electrons involved in the electrode reaction in moderate acid media are double than those consumed in the slightly alkaline side. The polarographic behaviour of benzoic hydrazide derivatives in alkaline solutions is of particular interest. At pH’s above 8.6 ie in alkaline buffer solutions the reduction wave decreases tremendously with increasing pH, while a second wave appears at more negative potentials. At higher pH value (> 10.6) the first wave appears to haver been diminished whereas the second wave becomes developed and its height assumes a constant value. Tbe plot of ir-pH (viz. Fig. l), indicates that the limiting current of the first wave gradually decreases by increasing the pH value of the solution till pH 8.1, the limiting current of the wave is
A.Z.ABUZUHRI
declining and the wave almost disappears at PI-I’S higher than 11.6. The i,-pH curve for the second wave shows that the height of the wave increase with increasing pH throughout the range 8.6-11.6 where the second wave occurs. On the other hand, closer inspection of the second wave shows the coincidence of Er,, of the second wave with that corresponding to reduction of anisaldehyde or furfural or acetophenone at the same pH’s. This indicates that the second wave must be attributed to the reduction of aldehyde or ketone formed by hydrolysis of the benzoic hydrazide derivatives into their main constituents in alkaline buffer solutions. The hydrolysis is due to the weakening of the >GN bond due to the localization of electrons on the N atom. This latter is brought about by the mesomeric effect of the aldehyde or ketone residue. In order to confirm the beforementioned, the spectra of the benzoic hydrazide derivatives in strongly acidic (>pH 2.5) and alkaline buffer solution (> pH 10.6) were constructed after different periods. Thus it was found that the absorbance of the band at 1, 29Omm, corresponding to the GN group, decreases with timeand disappears completely after 24 h.
B
A
q,b
6.00
((I) Total (b) 1st wove (c) 2nd wme
q a
\
(a) Total (b) 1st wove (cl 2nd wave
I4.oc
.d I2.oc
2
I
I
I
4
6
8 PH
,
IO
:bl , 12
C
a,b
(a) Total (b) 1st wave (cl 2nd wave
Fig. 1. ii-pH plots. (A) ABH, (B) FBH, (C) Acct. BH.
Polarographic
behaviour
of some arylidene benzoic hydrazides
The relation between the absorbance which is proportional to the concentration of benzoic hydrazide derivatives, and time indicates that the reaction is first order and confirms the decomposition of bcnzoic hydrazide derivatives into their constituents. The process of hydrolysis in strongly acid medium is enhanced by the protonation of the lone pair of electrons on the nitrogen atom leading to a weakening the C=N bond. However as the pH of the solution is increased the protonation decreases and hence the bond order increases. The range of stability of ABH, FBH and Acct. BH lies between pH 3.7-8.1. In buffer solutions of pH’s 4.5-10.2 an apparent minimum is observed which can be eliminated by the addition of 9 x 10m4% gelatin. This phenomenon may be attributed to the adsorption of the depolarizer on the electrode surface, as is evident from the decrease of drop time as function of potential in absence and presence of depolarizer (electrocapillary curves). On comparing the values of E1,2 of the reduction waves for ABH, FBH, and Acct. BH at the same pH, it becomes evident that E,,, of ABH has more negative value than the other compound. This may be due to the higher tendency of FBH and Acct. BH to be adsorbed at the electrode surface; hence the reduction process would be enhanced by the heat of adsorption causing a shift to potentials less negative than the normal reduction potential. On the other hand, the donor character of the -OCH, group of ABH increases the electron density on the azomethine group causing the reduction of ABH to take place at a somewhat more negative potential in comparison to FBH and Acct. BH. Investigation of the waves. The plot of log il as a function of log h for the reduction waves at pH’s 3.7, 7.2,9.3 and 11.6 indicates that the exponent X in the relation (it = KhX) varies between 0.48 and 0.59 at the plateau of the i,/pH curves. These data indicate that the process of reduction is mainly under diffusion control with some adsorption contribution. Analysis of the waves was carried out by applying the fundam-
Table 1. Polarographic
ental cquation[9]
for polarographic
E = O.O59/an,(log i/i,, - i)
1289
waves viz at 25°C.
The slopes of E us 10s i/&-i plots indicate that the electroreduction of benzoic hydrazide derivatives is irreversible (Table 1) since the overall electrode reaction involves the uptake of two and four electrons in alkaline and acid solution respectively as has been shown later in the present paper. The values of the transfer coefficient (a) obtained are less than unity confirming the irreversible nature of the wave. The electrode reaction. In order to find out the mode of reduction of ABH, FBH and Acct. BH, it was deemed necessary to calculate the number ofelectrons involved in the reaction. This is done by applying the Ilkovic equation and substituting for the different terms. This necessitates a knowledge of the value of the diffusion coefficient D of ABH, FBH and Acct. BH. The values of D is not available and may bc calculated from the Stoke’s relation provided that the density of the compound is known. This relation is: D _ 3.22 x 1W5 cmZ .$-I (V,P3 where V,,, is the apparent molar volume of the substance in the’solid state, and is equal to mol. wt/ density. The value of D thus obtain& corresponds to that in aqueous solution. For use in 40% ethanol solutions, the effect of viscosity of the medium must be accounted for Dale. The corrected value of Dale can be obtained from the relation.
where qHZO= 1 and qplc. = 1.98. The number of electrons involved in the elcctroreduction process at different pH’s has been evaluated by substituting the values of Drlo,, C, t and m in the Ilkovic equation. It is clear that the number of
data obtained for the reduction of Aryl BH in dilferent buffer solutions containing 40% ethanol at the dme
0.059 PH
El/Z. v
an,
4.05
- 0.970
0.100
7.2 9.3
- 1.335 - 1.440
0.040 0.077
3.7
- 0.955
0.100
7.2 9.3
- 1.220 - 1.305
0.064 0.064
4.05
- 1.040
0.092
7.2 9.3
- 1.320 - 1.400
0.072 1.00
&,,IbH pH range
Anisylidenebenzoichydrazide 3.74.1 6.1-10.3
Furfurylidenebenzoiohydrazide 3.7-6.0s 6.05-10.2
Acetophcnonebenzoichydrazide 3.7-7.2 7.2-l I.6
SlOpI
an,
h+
(ABH) 0.120 0.050
0.59
1.2
1.47 0.76
1.25 0.65
0.59
1.1
0.93 0.93
0.65 0.65
0.65
1.25
0.83 0.60
0.84 0.6 1
(FBH) 0.100 0.064
(Acct. BH) 0.115 0.061
Y.
1290
M. TI(MI,KI;
ANI) A. Z. AIIII;IIWU
electrons for each of ABH, FBH and Acct. BH is the same at one pH. In moderate acid media (pH ~5) the height of the reduction wave c&responds to the uptake of 4 electrons, whereas, in slightly alkaline media (pH 8.1) the height of the wave corresponds to 2 electrons. Controlled potentialcoulometry (pH =4.05) applied at potential on the plateau of the polarographic curve verified a consumption of 4 electrons per molecule in acid media. In order to find out the reduction mechanism one has to consider the structure of the molecule which contains two reducible centres, the azomethine linkages, rC=Nand the carbonyl group >C=O. It is evident that >C=O is inactive towards reduction through the mesomeric effect with the neighbouring imine group[lO]. This result is confirmed by the fact that benzoic hydrazide
0 0
R
C-NH-NH,
C - NH-Nti-C
P
P
C-NH-NH,+~H,-R
c-tui-~=cti-R+4e+4ti+-
where R is either
or
OCH,
C,J
For the reduction of acct. BH can be represented the following equation
+4e+4H+
-
At pH’s ~8.1 another second wave occurs whereas the first wave appears to have been diminished. The second wave is attributed to the reduction of aldehyde or ketone formed by the hydrolysis of thecompound as has been discussed in the present paper. The&,-pH curves. The plots of&,,, against pH for the reduction wave gives lines consisting each of two segments (Fig. 2). The slopes of E, iz-pH plots show in general a slight change in their slope near pH6.0 denoting that the electrode reaction changes on passing from acid to alkaline media. From the slopes of these curves, the number of H + ions (Z, +) in the rate determining step can be calculated using the following equation A& ,JApH
is not reduced polarographically[2], as was proved experimentally. Accordingly the azomethine linkage should be the activecentre to be reduced. Therefore the reduction of azomethine linkages proceeds in moderate acidic buffer solutions (pH ~5) along a single wave corresponding to four electrons. The process of reduction of ABH and FBH can be represented as:
+ 2.e + 2Ha
= (O.O59/orn.)Z, +
Substituting the values of an, obtained from the slope of log(i/i,--i) us E plots, values of Z+,+ are found to equal -1.0 in moderate acid and alkaline media (Table 1). The number of electrons (n.) in the rate determining step is equal to one electron in acid and 2 electrons in alkaline media since the most probable value of OLis equal 0.5. Kinetic parameters. The kinetic parameters for the electrode reaction at different pH’s have been evaluated by the Koutecky method[ll]. In addition to the values corresponding to E = 0, the values of the formal constants at the reversible half-wave potential at a given pH have been calculated. The reversible halfwave potentials (E;,,) have been obtained by the method of Gellings[12]. Table 2 includes representative values for the kinetic parameters. The various parameters indicate that the process of reduction along the reduction waves is irreversible. The energy of
by
-
-CH,
0 -0
In slightly alkaline media (pH 8.1), the height of the wave corresponds to the uptake of two electrons and the following mechanism for the reduction of ABH’and FBH may be operating
*O
C-NH-N=CH-R+Ze+2H+-
For the reduction of Acct. BH in slightly solutions (pH 8.1) can be represented as:
alkaline Fig. 2. E,,,-pH
plots. (A) ABH, (b) FBH, (C) Acct. BH.
Polarographic behaviour of some arylidene benzoic hydrazidcs
1291
Table 2. Kinetic parameters of 5 x 10m4M Aryl BH in different buRer solutions containing 40% ethanol at the dme KO
PH
cm/s
4.05 7.2
9.68 x 1O-16
KOD-“’ (K=K,)
AG* K.cal/mole
K, cm/s
K,D_“2 (K = K,)
AGj K.cal/mole
4.79 x 10-43
Anisylidenebenzoichydrazide 4.10 x 10-l” 116.22 2.03 x 1O-4o 274.01
(ABH) 3.10 x lo-& 1.61 x lo-’
0.137 0.712
49.72 45.59
3.7 7.2
1.37 x 10-15 1.83 x lo-l6
Furfurylidenebenzoichydrazide 5.8 x lo-” 115.35 7.15 x 10-24 178.18
(FBH) 2.00 x lo-& 3.95 x lo-”
0.086 0.169
50.82 49.12
4.05 7.2
1.48 x lo-” 9.79 x 10-25
Acetophenonebenzoichydrazide 6.27 x 1O-‘5 126.70 4.22 x lo-= 168.15
(Acct. BH) 4.2 x lO-4 7.64 x lo-*
0.180 0.325
48.93 47.46
activation AG* generally increases with increasing pH denoting that the reduction process becomes more difficult at higher pH. Also the values of the rate constant K, decrease with rise of pH which is an indication for increased irreversibility at higher pH. This can be ascribed to the slow protonation of the intermediate free radical formed. The values of K, (the formal rate constant) do not represent these of K, (the standard rate constant) because E<,* does not coincide with E,. This is due to the shift of the reduction waves to more negative potentials as a result of the adsorp tion of the depolarizer or the hydrolysis product on the electrode surface. Analytical applications. In order to test the validity of the Illcovic equation and the applicability of the polarographic method for the determination of ABH, FBH and Acct. BH, the total limiting current at different PI-I’S (moderate acid-neutral-slightly alkaline) is plotted as a function of the concentration of the depolarizer. Satisfactory linear relations passing through the origin are obtained. Constancy of id/C values is observed at low concentrations of depolarizer, but at higher ones a slight negative deviation is obtained. This is due to the adsorption of the de-
polarizer or its reduction product at higher concentrations. It was found that, as little as 24-26 &ml with fair of the depolarizer can be determined accuracy. REFERENCES
1. Ng. Ph. Buu Hoi, J. &em. Sot. @I&.) 1368 (1953). 2. L. Holleck and H. Marsen, Z. Elekrrockem. 57, 301 (1953). 3. K. Veselv and R. Brdicka, Collect. Czech. Chem. Commun. 1% 313 (1947). 4. H. Lund, Acta Chem. Stand. 13. 249 (1959). 5. J. Tirouflet, Adu. Polarog. 2, 740 (1960). 6. G. Sturve, J. Prok. Chem. 2, 50, 295 (1894). 7. H. T. S. Britton, Hydrogen Ions, 4th Edn, p. 313. Chapman and Hall (1952). 8. T. De Vries and J. L. Kroon, J. Am. Chem. SOC. 752484 (1953).
L. Meites, Polarographic Techniques, 2nd Edn. Wiley, New York (1965). 10. R. Mecke and E. Runck, 2. Electrochem. 60, 136 (1956). 11. J. Koutecky, Chem. Listy 47, 323 (1953); Collect. Czech. 9.
Chem. Cownun. 18, 597 (1955). 12. R. J. Gellings, Z. Elektrochem. 66, 481 (1962). Ber. Bunsenges. Phys. Chem. 67, 167 (1963).
POLAROGRAPHIC BEHAVIOUR OF SOME ARYLIDENE BENZOIC HYDRAZIDES IN SOLUTIONS OF VARYING pH AT THli DROPPING MERCURY ELECTRODE Y. M. TEMERK* and A. 2. ABUZUHRI Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt, A.R.E. (Receiued 6 June 1980; infinalform 11
January
i980)
Abstract - The polarographic behaviour of some arylidene benzoic hydrazides such as anisylidene benzoic hydrazide (ABH), furfurylidene benzoic hydrazide (FBH) and acetophenone benzoic hydrazide (Acet.BH) has been studied in universal buffer solutions containing 40% ethanol at the dme. In the pH range 3.7-8.1 the observed single reduction wave for all three compounds is assigned to the reduction of the 1 C?N centre, whereas the > C+O centre is inactive towards reduction due to the mesomeric effect of the neighbouring amino group. In alkaline media (pH 8.6) a second wave appears at a more negative potential due to the reduction of the corresponding aldehyde or ketone which is formed by hydrolytic decomposition of the molecule. The effect of pH on the limiting current and Et,, as well as the reduction mechanism are discussed. The values of the kinetic parameters for the electrode reaction at different pH values are also computed from the pularographic results.
INTRODUCIION Hydrazides and their derivatives became of special interest in recent years due to their. biological importance[l]. The polarographic reduction of derivatives of hydrazides containing the > c-_O and > C=N linkages together has been not hitherto investigated. With respect to the reduction of the >C=C group, Holleck et aI[2] found that the polarographic reduction of benzoic hydrazide at pH 1-13 led to a reaction scheme for the cathode process with a special sequence of reactions. Brdicka[3] indicated that the polarographic reduction of the carbonyl group of aromatic and aryl alkyl ketones and aldehydes is pH dependent and is also affected by the type and position of substituents. For the polarographic reduction of azomethine compounds a number of investigators[4,5] reported that the reduction of the C=N center involves four or two electrons in acid and alkaline solutions respectively. The present paper is focused on the polarographic behaviour of some arylidene benzoic hydrazides such as anisylidene benzoic hydrazide (ABH) furfurylidene benzoic hydrazide (FBH) and acetophenone benzoic hydrazide (Acct. BH) in different but&r solutions at the dme in order to throw some light on the reduction mechanism for each of azomethine and carbonyl centers. No work on these compounds from the present stand point has been published.
EXPERIMENTAL Arylidene benzoic hydrazides were prepared by condensation of the aromatic aldehydes or ketones with benzhydrazide in the usual manner for the * To whom all correspondence should be directed.
preparation of hydrazone[6]. The purity of the compound prepared was checked by elemental analysis. The compounds selected following structures :
u
0
for the study
,“- NH-N.:
0
having
the
OCH,
0 (ABH)
0
CL:g
(O)C-NH-N=@
0
LH, (FEIHI
(Acet.BH)
lo-’ M ABH, FBH, and Acct. BH solutions were prepared by dissolving the accurately weighed solid in the appropriate volume of pure ethanol. The modified universal buffer series of Britton (pH 3-12) was prepared as given by Britton[7]. The polarographed depolarizer solutions were prepared by mixing the appropriate volume of the depolarizer solution with the universal buffer mixture containing 40yfi ethanol. The pH was checked using a Radiometer pH meter, Model 28, accurate to +0.05 unit. The polarograms were recorded on an LP 60 polarograph using a dme having the characteristics, m = 1.5 mg/s and t = 4.5 s at 45 cm.Hg (in 0.1 M KC1 at 0.0 V). All experiments were run at 22*O.lo”C. The solution was deaerated with pure nitrogen. The millcoulometric method of De Varies and Kroon[8] using a mercury pool cathode was used for determining the value of II, the number of electrons involved in the reduction process. RIWJL’IS AND DISCUSSION
Effect of pH on polarogrum. In moderate acidic buffer solutions (pH ~5) the polarogram of 5 x 10m4 M 1287
Y.M.
1288
TEMERKAND
ABH, FBI4 and Acct. BH consists of a single reduction wave, whose height corresponds to the uptake of four electrons in the electrode process. On increasing the pH( >S), the reduction wave decreases gradually till the height of the wave attains in slightly alkaline solution (pH 8.1) almost half its height in moderate acid media, whereas at intermediate pH the height of the reduction wave lies between the two values. Thus, it can be concluded that the electrons involved in the electrode reaction in moderate acid media are double than those consumed in the slightly alkaline side. The polarographic behaviour of benzoic hydrazide derivatives in alkaline solutions is of particular interest. At pH’s above 8.6 ie in alkaline buffer solutions the reduction wave decreases tremendously with increasing pH, while a second wave appears at more negative potentials. At higher pH value (> 10.6) the first wave appears to haver been diminished whereas the second wave becomes developed and its height assumes a constant value. Tbe plot of ir-pH (viz. Fig. l), indicates that the limiting current of the first wave gradually decreases by increasing the pH value of the solution till pH 8.1, the limiting current of the wave is
A.Z.ABUZUHRI
declining and the wave almost disappears at PI-I’S higher than 11.6. The i,-pH curve for the second wave shows that the height of the wave increase with increasing pH throughout the range 8.6-11.6 where the second wave occurs. On the other hand, closer inspection of the second wave shows the coincidence of Er,, of the second wave with that corresponding to reduction of anisaldehyde or furfural or acetophenone at the same pH’s. This indicates that the second wave must be attributed to the reduction of aldehyde or ketone formed by hydrolysis of the benzoic hydrazide derivatives into their main constituents in alkaline buffer solutions. The hydrolysis is due to the weakening of the >GN bond due to the localization of electrons on the N atom. This latter is brought about by the mesomeric effect of the aldehyde or ketone residue. In order to confirm the beforementioned, the spectra of the benzoic hydrazide derivatives in strongly acidic (>pH 2.5) and alkaline buffer solution (> pH 10.6) were constructed after different periods. Thus it was found that the absorbance of the band at 1, 29Omm, corresponding to the GN group, decreases with timeand disappears completely after 24 h.
B
A
q,b
6.00
((I) Total (b) 1st wove (c) 2nd wme
q a
\
(a) Total (b) 1st wove (cl 2nd wave
I4.oc
.d I2.oc
2
I
I
I
4
6
8 PH
,
IO
:bl , 12
C
a,b
(a) Total (b) 1st wave (cl 2nd wave
Fig. 1. ii-pH plots. (A) ABH, (B) FBH, (C) Acct. BH.
Polarographic
behaviour
of some arylidene benzoic hydrazides
The relation between the absorbance which is proportional to the concentration of benzoic hydrazide derivatives, and time indicates that the reaction is first order and confirms the decomposition of bcnzoic hydrazide derivatives into their constituents. The process of hydrolysis in strongly acid medium is enhanced by the protonation of the lone pair of electrons on the nitrogen atom leading to a weakening the C=N bond. However as the pH of the solution is increased the protonation decreases and hence the bond order increases. The range of stability of ABH, FBH and Acct. BH lies between pH 3.7-8.1. In buffer solutions of pH’s 4.5-10.2 an apparent minimum is observed which can be eliminated by the addition of 9 x 10m4% gelatin. This phenomenon may be attributed to the adsorption of the depolarizer on the electrode surface, as is evident from the decrease of drop time as function of potential in absence and presence of depolarizer (electrocapillary curves). On comparing the values of E1,2 of the reduction waves for ABH, FBH, and Acct. BH at the same pH, it becomes evident that E,,, of ABH has more negative value than the other compound. This may be due to the higher tendency of FBH and Acct. BH to be adsorbed at the electrode surface; hence the reduction process would be enhanced by the heat of adsorption causing a shift to potentials less negative than the normal reduction potential. On the other hand, the donor character of the -OCH, group of ABH increases the electron density on the azomethine group causing the reduction of ABH to take place at a somewhat more negative potential in comparison to FBH and Acct. BH. Investigation of the waves. The plot of log il as a function of log h for the reduction waves at pH’s 3.7, 7.2,9.3 and 11.6 indicates that the exponent X in the relation (it = KhX) varies between 0.48 and 0.59 at the plateau of the i,/pH curves. These data indicate that the process of reduction is mainly under diffusion control with some adsorption contribution. Analysis of the waves was carried out by applying the fundam-
Table 1. Polarographic
ental cquation[9]
for polarographic
E = O.O59/an,(log i/i,, - i)
1289
waves viz at 25°C.
The slopes of E us 10s i/&-i plots indicate that the electroreduction of benzoic hydrazide derivatives is irreversible (Table 1) since the overall electrode reaction involves the uptake of two and four electrons in alkaline and acid solution respectively as has been shown later in the present paper. The values of the transfer coefficient (a) obtained are less than unity confirming the irreversible nature of the wave. The electrode reaction. In order to find out the mode of reduction of ABH, FBH and Acct. BH, it was deemed necessary to calculate the number ofelectrons involved in the reaction. This is done by applying the Ilkovic equation and substituting for the different terms. This necessitates a knowledge of the value of the diffusion coefficient D of ABH, FBH and Acct. BH. The values of D is not available and may bc calculated from the Stoke’s relation provided that the density of the compound is known. This relation is: D _ 3.22 x 1W5 cmZ .$-I (V,P3 where V,,, is the apparent molar volume of the substance in the’solid state, and is equal to mol. wt/ density. The value of D thus obtain& corresponds to that in aqueous solution. For use in 40% ethanol solutions, the effect of viscosity of the medium must be accounted for Dale. The corrected value of Dale can be obtained from the relation.
where qHZO= 1 and qplc. = 1.98. The number of electrons involved in the elcctroreduction process at different pH’s has been evaluated by substituting the values of Drlo,, C, t and m in the Ilkovic equation. It is clear that the number of
data obtained for the reduction of Aryl BH in dilferent buffer solutions containing 40% ethanol at the dme
0.059 PH
El/Z. v
an,
4.05
- 0.970
0.100
7.2 9.3
- 1.335 - 1.440
0.040 0.077
3.7
- 0.955
0.100
7.2 9.3
- 1.220 - 1.305
0.064 0.064
4.05
- 1.040
0.092
7.2 9.3
- 1.320 - 1.400
0.072 1.00
&,,IbH pH range
Anisylidenebenzoichydrazide 3.74.1 6.1-10.3
Furfurylidenebenzoiohydrazide 3.7-6.0s 6.05-10.2
Acetophcnonebenzoichydrazide 3.7-7.2 7.2-l I.6
SlOpI
an,
h+
(ABH) 0.120 0.050
0.59
1.2
1.47 0.76
1.25 0.65
0.59
1.1
0.93 0.93
0.65 0.65
0.65
1.25
0.83 0.60
0.84 0.6 1
(FBH) 0.100 0.064
(Acct. BH) 0.115 0.061
Y.
1290
M. TI(MI,KI;
ANI) A. Z. AIIII;IIWU
electrons for each of ABH, FBH and Acct. BH is the same at one pH. In moderate acid media (pH ~5) the height of the reduction wave c&responds to the uptake of 4 electrons, whereas, in slightly alkaline media (pH 8.1) the height of the wave corresponds to 2 electrons. Controlled potentialcoulometry (pH =4.05) applied at potential on the plateau of the polarographic curve verified a consumption of 4 electrons per molecule in acid media. In order to find out the reduction mechanism one has to consider the structure of the molecule which contains two reducible centres, the azomethine linkages, rC=Nand the carbonyl group >C=O. It is evident that >C=O is inactive towards reduction through the mesomeric effect with the neighbouring imine group[lO]. This result is confirmed by the fact that benzoic hydrazide
0 0
R
C-NH-NH,
C - NH-Nti-C
P
P
C-NH-NH,+~H,-R
c-tui-~=cti-R+4e+4ti+-
where R is either
or
OCH,
C,J
For the reduction of acct. BH can be represented the following equation
+4e+4H+
-
At pH’s ~8.1 another second wave occurs whereas the first wave appears to have been diminished. The second wave is attributed to the reduction of aldehyde or ketone formed by the hydrolysis of thecompound as has been discussed in the present paper. The&,-pH curves. The plots of&,,, against pH for the reduction wave gives lines consisting each of two segments (Fig. 2). The slopes of E, iz-pH plots show in general a slight change in their slope near pH6.0 denoting that the electrode reaction changes on passing from acid to alkaline media. From the slopes of these curves, the number of H + ions (Z, +) in the rate determining step can be calculated using the following equation A& ,JApH
is not reduced polarographically[2], as was proved experimentally. Accordingly the azomethine linkage should be the activecentre to be reduced. Therefore the reduction of azomethine linkages proceeds in moderate acidic buffer solutions (pH ~5) along a single wave corresponding to four electrons. The process of reduction of ABH and FBH can be represented as:
+ 2.e + 2Ha
= (O.O59/orn.)Z, +
Substituting the values of an, obtained from the slope of log(i/i,--i) us E plots, values of Z+,+ are found to equal -1.0 in moderate acid and alkaline media (Table 1). The number of electrons (n.) in the rate determining step is equal to one electron in acid and 2 electrons in alkaline media since the most probable value of OLis equal 0.5. Kinetic parameters. The kinetic parameters for the electrode reaction at different pH’s have been evaluated by the Koutecky method[ll]. In addition to the values corresponding to E = 0, the values of the formal constants at the reversible half-wave potential at a given pH have been calculated. The reversible halfwave potentials (E;,,) have been obtained by the method of Gellings[12]. Table 2 includes representative values for the kinetic parameters. The various parameters indicate that the process of reduction along the reduction waves is irreversible. The energy of
by
-
-CH,
0 -0
In slightly alkaline media (pH 8.1), the height of the wave corresponds to the uptake of two electrons and the following mechanism for the reduction of ABH’and FBH may be operating
*O
C-NH-N=CH-R+Ze+2H+-
For the reduction of Acct. BH in slightly solutions (pH 8.1) can be represented as:
alkaline Fig. 2. E,,,-pH
plots. (A) ABH, (b) FBH, (C) Acct. BH.
Polarographic behaviour of some arylidene benzoic hydrazidcs
1291
Table 2. Kinetic parameters of 5 x 10m4M Aryl BH in different buRer solutions containing 40% ethanol at the dme KO
PH
cm/s
4.05 7.2
9.68 x 1O-16
KOD-“’ (K=K,)
AG* K.cal/mole
K, cm/s
K,D_“2 (K = K,)
AGj K.cal/mole
4.79 x 10-43
Anisylidenebenzoichydrazide 4.10 x 10-l” 116.22 2.03 x 1O-4o 274.01
(ABH) 3.10 x lo-& 1.61 x lo-’
0.137 0.712
49.72 45.59
3.7 7.2
1.37 x 10-15 1.83 x lo-l6
Furfurylidenebenzoichydrazide 5.8 x lo-” 115.35 7.15 x 10-24 178.18
(FBH) 2.00 x lo-& 3.95 x lo-”
0.086 0.169
50.82 49.12
4.05 7.2
1.48 x lo-” 9.79 x 10-25
Acetophenonebenzoichydrazide 6.27 x 1O-‘5 126.70 4.22 x lo-= 168.15
(Acct. BH) 4.2 x lO-4 7.64 x lo-*
0.180 0.325
48.93 47.46
activation AG* generally increases with increasing pH denoting that the reduction process becomes more difficult at higher pH. Also the values of the rate constant K, decrease with rise of pH which is an indication for increased irreversibility at higher pH. This can be ascribed to the slow protonation of the intermediate free radical formed. The values of K, (the formal rate constant) do not represent these of K, (the standard rate constant) because E<,* does not coincide with E,. This is due to the shift of the reduction waves to more negative potentials as a result of the adsorp tion of the depolarizer or the hydrolysis product on the electrode surface. Analytical applications. In order to test the validity of the Illcovic equation and the applicability of the polarographic method for the determination of ABH, FBH and Acct. BH, the total limiting current at different PI-I’S (moderate acid-neutral-slightly alkaline) is plotted as a function of the concentration of the depolarizer. Satisfactory linear relations passing through the origin are obtained. Constancy of id/C values is observed at low concentrations of depolarizer, but at higher ones a slight negative deviation is obtained. This is due to the adsorption of the de-
polarizer or its reduction product at higher concentrations. It was found that, as little as 24-26 &ml with fair of the depolarizer can be determined accuracy. REFERENCES
1. Ng. Ph. Buu Hoi, J. &em. Sot. @I&.) 1368 (1953). 2. L. Holleck and H. Marsen, Z. Elekrrockem. 57, 301 (1953). 3. K. Veselv and R. Brdicka, Collect. Czech. Chem. Commun. 1% 313 (1947). 4. H. Lund, Acta Chem. Stand. 13. 249 (1959). 5. J. Tirouflet, Adu. Polarog. 2, 740 (1960). 6. G. Sturve, J. Prok. Chem. 2, 50, 295 (1894). 7. H. T. S. Britton, Hydrogen Ions, 4th Edn, p. 313. Chapman and Hall (1952). 8. T. De Vries and J. L. Kroon, J. Am. Chem. SOC. 752484 (1953).
L. Meites, Polarographic Techniques, 2nd Edn. Wiley, New York (1965). 10. R. Mecke and E. Runck, 2. Electrochem. 60, 136 (1956). 11. J. Koutecky, Chem. Listy 47, 323 (1953); Collect. Czech. 9.
Chem. Cownun. 18, 597 (1955). 12. R. J. Gellings, Z. Elektrochem. 66, 481 (1962). Ber. Bunsenges. Phys. Chem. 67, 167 (1963).