- Email: [email protected]
Uncatalysed oscillatory chemical reactions. Oxidation of 1,4-cyclohexanedione by bromate in sulfuric or nitric acid solution
Volume 88, number 3
CHEMICAL PHYSICS LETTERS
7 May 1982
UNCATALYSED OSCILLATORY CHEMICAL REACTIONS. OXIDATION OF 1,4-CYCLOHEXANEDIONE BY BROMATE IN SULFURIC OR NITRIC ACID SOLUTION * Vincent J. FARAGE and D6simir JANJIC Department of Physical Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland Received 1 March 1982
Uncatalysed oxidation of 1,4-cyclohexanedione by acidic bromate has been found to generate chemical oscillations with very interesting features. Moreover this new oscillating reaction can take place in a sulfuric as well as in a nitric acid solution.
1. Introduction
2. Experimental
In the course 0 f our study of oscillating chemical reactions involving ketones [ 1 - 5 ] , we recently discovered very interesting temporal chemical oscillations in a system which does not contain a oneelectron redox couple catalyst as in the BelousovZhabotinskii (BZ) type reactions. K6r6s and Orban [ 6 - 1 0 ] have reported that chemical oscillations could be observed during the oxidation of different aromatic compounds by bromate in a dilute sulfuric acid solution. In the present paper we report our preliminary investigations on the oscillating, behaviour of the uncatalysed oxidation reaction of 1,4-cyclohexanedione (tetrahydroquinone --- THQ) by acidic bromate. The characteristics of the oscillations (frequency, amplitude and number of oscillations) of the new. system were found to be much more spectacular than those observed by K6r6s and Orban. Moreover, our experiments showed that this oscillatory phenomenon occurred not only in a sulfuric acid solution but also in a nitric acid solution, the first for this type of reaction.
The reagents used were 1,4-cyclohexanedione (purum) and KBrO 3 (puriss) from Fluka A.G., and H2SO 4 (pro analysi) and HNO 3 (pro analysi) from Merck A.G. The reactor was a Metrohm EA 880 T-V thermostarted glass reactor. The total volume of solution was always 80 ml. All the experiments were carried out at 35°C and the solution was stirred with a motor-driven teflon cross-shaped stirrer [4] at a constant stirring rate of 1000 rpm. The redox potentiometric measurements obtained by means of a platinum electrode and a mercury(II) sulfate reference electrode were recorded on a Philips 8222 two.channel recorder. A bromide ion selective electrode (solid-state electrode Philips IS 550-Br) was used to follow the variation of Br- concentration during the reaction. All the experiments took place in a nitrogen atmosphere [5] save the one in which the redox potential and the Br- concentration were measured ~imultaneously and which was carried out in the presence of air.
* Dedicated to the memory of Professor Theodor Posternak. 0 009-2614/82/0000--0000/$ 02.75 © 1982 North-Holland
301
Volume 88, number 3
CHEMICAL PHYSICS LETTERS
3. Results and discussion
7 May 1982
tiometric oscillations whose period (1/9), amplitude (AE) and total number of oscillations (N) differ substantially from all BZ type systems we have previously studied [ 1 ]. This new oscillating system is of particular interest in that it requires no catalyst as the
In measuring the redox potential of the oxidation reaction of THQ by BrO~ in an acidic solution, we observed, after an "induction period" (rind), potenmV -E
!
time
I
10
o
mV
1'5
2'0 m,n.
-E
/I
oo 04
time
~o
2'5
3'5 mln.
mVJ , - E
o o¢q
(C) i
....
time 4"0
4'5
5'0
'
min.
Fig. 1. Potentiometdc oscillations of the THQ/BrO3/HNO3 system. [THQ] = 0.10 M; [BROW]= 0.10 M; [HNO3] = 2.6 M. 302
Volume 88, number 3
CHEMICAL PHYSICS LETTERS
7 May 1982
I
o"
~
mV~-E "'°
2'5
3'0
3'5
4'0
time rain.
Fig. 2. Simultaneous measurements of the redox potential and the log Br- concentration of the THQ/BrO3/H2SO 4 system. [THQ] = 0.10 M;[BrO3] = 0.10 M; [H2SO4] = 1.0 M. Ce4+/Ce 3+ couple and that oscillations occur in the presence of H2SO 4 as well as HNO 3. Fig. 1 gives an example of a redox potentiometric recording of the THQ/BrO~/HNO 3 system. Placed one after the other, the three fragments a, b and c of the recording represent the oscillating phenomenon as a whole. The oscillating character of the reaction is further stressed by the periodic change in Br- concentration during the course of the reaction. Fig. 2 illustrates typical curves of the redox potential and Br- concentration for the THQ/BrO~/H2SO 4 system. After a rind characterised by a "quasi-stationary phase" (fig. la) similar to that observed with other systems [3], oscillations begin with relatively high values o f the frequency and amplitude. Within our experimental conditions no significant change in colour of the solution, which remains yellowish, was observed during the oscillatory behaviour of.the sys-
tem. After the end of the oscillations, however, the reaction between BrO~ and THQ continues, yielding a visible amount of Br2, Tables 1 and 2 show the variations of rind, 1/v, zlE and N relative to the chemical composition of the THQ/BrO~/H2SO 4 and THQ/BrO~/HNO 3 systems. Three values for 1/v, corresponding respectively to the beginning, the middle and the end of the oscillations, were noted. The two values for AxE are relative to the first and last oscillations. In taking a closer look at tables 1 and 2 we see that 1Iv can, according to experimental conditions, have very low values of a few seconds and, contrary to what is generally noted with other closed oscillating systems, 1/v does not increase gradually but decreases in time before slightly increasing again towards the end of the oscillations. Moreover, as opposed to other reactions [1], N distinctly increases with acid concentration of the medium.
Table 1 Influence of the chemical composition of the THQ/BrO3/H~SO4 system on the oscillatory phenomenon THQ (M)
BrO~ (M)
H2SO4 (M)
rin d (min)
1/v
zxE
(min)
(mV)
0.10 0.10 0.10 0.15 0.10
0.10 0°10 0.10 0.10 0.20
0,5 1.0 2.5 1.0 1.0
31.2 24.0 15.8 19.4 40.0
2.10-2.00-2.10 0.70-0.36-0.76 0.05-0.04-0.06 0.65-0.40-0.50 0.25--0.18-0.25
288-315 230-340 78-242 250-358 170-215
N
•
5 58 184 55 83
303
Volume 88, number 3
7May 1982
CHEMICAL PHYSICS LETTERS
Table 2 Influence of the chemical composition of the THQ]BrO~/HNOa system on the oscillatory phenomenon THQ (M)
BrO~ (M)
HNOs (M)
rind (min)
1/u (rain)
0.10 0.10 0.10 0.15 0.10
0.10 0.10 0.10 0.10 0.20
1.0 2.0 2.6 2.6 2.6
26.5 16.2 15.1 11.6 25.6
1.56-1.50-1.62 0.44-0.30-0.52 0.22-0.20-0.40 0.25-0.16-0.35 0.10-0.08-0.11
Compared to the uncatalysed oscillatory reactions reported by K6r6s and Orban [8], the values we found for N were much greater. Lastly, as mentioned above, we were surprised by the values of AE which often exceeded 300 mV.
4. Conclusion These new systems will give rise to further studies in the field of oscillating chemical reactions. More research is being carried out in our laboratory to understand better the oxidation of THQ by BrO~ and to define the influence of various "constraints" such as temperature, oxygen and stirring on the "responses" of the s y s t e m .
304
AE (mV) 275-298 165-348 126-333 140-365 80-200
N
7 126 170 165 194
References [1] v.J. Farage, Ph. Stroot and D. Janjic, Helv. Chim. Acta 60 (1977) 231. [2] V.J. FaJage and D. Janjic, Helv. Chim. Acta 61 (1978) 1539. [3] V.J. Farage and D. Janjic, Helv. Chim. Acta 63 (1980) 433. [4] V.J. Farage and D. Janjic, Chimia 34 (1980) 342. [5] V.J. Farage and D. Janjic, Chimia 35 (1981) 289. [6] E. K6r6s and M. Orban, Nature 273 (1978) 371. [7] M. Orban and E. K6r6s, React. Kinet. Catal. Letters 8 (1978) 273. [8] M. Orban and E. K6r6s, J. Phys. Chem. 82 (1978) 1672. [9] Mo Orban, E. K6r6s and R.M. Noyes, J. Phys. Chem. 83 (1979) 3056. [10] E. K6r6s, M. Orban and I. Haban, J. Phys. Chem. 84 (1980) 559.
CHEMICAL PHYSICS LETTERS
7 May 1982
UNCATALYSED OSCILLATORY CHEMICAL REACTIONS. OXIDATION OF 1,4-CYCLOHEXANEDIONE BY BROMATE IN SULFURIC OR NITRIC ACID SOLUTION * Vincent J. FARAGE and D6simir JANJIC Department of Physical Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland Received 1 March 1982
Uncatalysed oxidation of 1,4-cyclohexanedione by acidic bromate has been found to generate chemical oscillations with very interesting features. Moreover this new oscillating reaction can take place in a sulfuric as well as in a nitric acid solution.
1. Introduction
2. Experimental
In the course 0 f our study of oscillating chemical reactions involving ketones [ 1 - 5 ] , we recently discovered very interesting temporal chemical oscillations in a system which does not contain a oneelectron redox couple catalyst as in the BelousovZhabotinskii (BZ) type reactions. K6r6s and Orban [ 6 - 1 0 ] have reported that chemical oscillations could be observed during the oxidation of different aromatic compounds by bromate in a dilute sulfuric acid solution. In the present paper we report our preliminary investigations on the oscillating, behaviour of the uncatalysed oxidation reaction of 1,4-cyclohexanedione (tetrahydroquinone --- THQ) by acidic bromate. The characteristics of the oscillations (frequency, amplitude and number of oscillations) of the new. system were found to be much more spectacular than those observed by K6r6s and Orban. Moreover, our experiments showed that this oscillatory phenomenon occurred not only in a sulfuric acid solution but also in a nitric acid solution, the first for this type of reaction.
The reagents used were 1,4-cyclohexanedione (purum) and KBrO 3 (puriss) from Fluka A.G., and H2SO 4 (pro analysi) and HNO 3 (pro analysi) from Merck A.G. The reactor was a Metrohm EA 880 T-V thermostarted glass reactor. The total volume of solution was always 80 ml. All the experiments were carried out at 35°C and the solution was stirred with a motor-driven teflon cross-shaped stirrer [4] at a constant stirring rate of 1000 rpm. The redox potentiometric measurements obtained by means of a platinum electrode and a mercury(II) sulfate reference electrode were recorded on a Philips 8222 two.channel recorder. A bromide ion selective electrode (solid-state electrode Philips IS 550-Br) was used to follow the variation of Br- concentration during the reaction. All the experiments took place in a nitrogen atmosphere [5] save the one in which the redox potential and the Br- concentration were measured ~imultaneously and which was carried out in the presence of air.
* Dedicated to the memory of Professor Theodor Posternak. 0 009-2614/82/0000--0000/$ 02.75 © 1982 North-Holland
301
Volume 88, number 3
CHEMICAL PHYSICS LETTERS
3. Results and discussion
7 May 1982
tiometric oscillations whose period (1/9), amplitude (AE) and total number of oscillations (N) differ substantially from all BZ type systems we have previously studied [ 1 ]. This new oscillating system is of particular interest in that it requires no catalyst as the
In measuring the redox potential of the oxidation reaction of THQ by BrO~ in an acidic solution, we observed, after an "induction period" (rind), potenmV -E
!
time
I
10
o
mV
1'5
2'0 m,n.
-E
/I
oo 04
time
~o
2'5
3'5 mln.
mVJ , - E
o o¢q
(C) i
....
time 4"0
4'5
5'0
'
min.
Fig. 1. Potentiometdc oscillations of the THQ/BrO3/HNO3 system. [THQ] = 0.10 M; [BROW]= 0.10 M; [HNO3] = 2.6 M. 302
Volume 88, number 3
CHEMICAL PHYSICS LETTERS
7 May 1982
I
o"
~
mV~-E "'°
2'5
3'0
3'5
4'0
time rain.
Fig. 2. Simultaneous measurements of the redox potential and the log Br- concentration of the THQ/BrO3/H2SO 4 system. [THQ] = 0.10 M;[BrO3] = 0.10 M; [H2SO4] = 1.0 M. Ce4+/Ce 3+ couple and that oscillations occur in the presence of H2SO 4 as well as HNO 3. Fig. 1 gives an example of a redox potentiometric recording of the THQ/BrO~/HNO 3 system. Placed one after the other, the three fragments a, b and c of the recording represent the oscillating phenomenon as a whole. The oscillating character of the reaction is further stressed by the periodic change in Br- concentration during the course of the reaction. Fig. 2 illustrates typical curves of the redox potential and Br- concentration for the THQ/BrO~/H2SO 4 system. After a rind characterised by a "quasi-stationary phase" (fig. la) similar to that observed with other systems [3], oscillations begin with relatively high values o f the frequency and amplitude. Within our experimental conditions no significant change in colour of the solution, which remains yellowish, was observed during the oscillatory behaviour of.the sys-
tem. After the end of the oscillations, however, the reaction between BrO~ and THQ continues, yielding a visible amount of Br2, Tables 1 and 2 show the variations of rind, 1/v, zlE and N relative to the chemical composition of the THQ/BrO~/H2SO 4 and THQ/BrO~/HNO 3 systems. Three values for 1/v, corresponding respectively to the beginning, the middle and the end of the oscillations, were noted. The two values for AxE are relative to the first and last oscillations. In taking a closer look at tables 1 and 2 we see that 1Iv can, according to experimental conditions, have very low values of a few seconds and, contrary to what is generally noted with other closed oscillating systems, 1/v does not increase gradually but decreases in time before slightly increasing again towards the end of the oscillations. Moreover, as opposed to other reactions [1], N distinctly increases with acid concentration of the medium.
Table 1 Influence of the chemical composition of the THQ/BrO3/H~SO4 system on the oscillatory phenomenon THQ (M)
BrO~ (M)
H2SO4 (M)
rin d (min)
1/v
zxE
(min)
(mV)
0.10 0.10 0.10 0.15 0.10
0.10 0°10 0.10 0.10 0.20
0,5 1.0 2.5 1.0 1.0
31.2 24.0 15.8 19.4 40.0
2.10-2.00-2.10 0.70-0.36-0.76 0.05-0.04-0.06 0.65-0.40-0.50 0.25--0.18-0.25
288-315 230-340 78-242 250-358 170-215
N
•
5 58 184 55 83
303
Volume 88, number 3
7May 1982
CHEMICAL PHYSICS LETTERS
Table 2 Influence of the chemical composition of the THQ]BrO~/HNOa system on the oscillatory phenomenon THQ (M)
BrO~ (M)
HNOs (M)
rind (min)
1/u (rain)
0.10 0.10 0.10 0.15 0.10
0.10 0.10 0.10 0.10 0.20
1.0 2.0 2.6 2.6 2.6
26.5 16.2 15.1 11.6 25.6
1.56-1.50-1.62 0.44-0.30-0.52 0.22-0.20-0.40 0.25-0.16-0.35 0.10-0.08-0.11
Compared to the uncatalysed oscillatory reactions reported by K6r6s and Orban [8], the values we found for N were much greater. Lastly, as mentioned above, we were surprised by the values of AE which often exceeded 300 mV.
4. Conclusion These new systems will give rise to further studies in the field of oscillating chemical reactions. More research is being carried out in our laboratory to understand better the oxidation of THQ by BrO~ and to define the influence of various "constraints" such as temperature, oxygen and stirring on the "responses" of the s y s t e m .
304
AE (mV) 275-298 165-348 126-333 140-365 80-200
N
7 126 170 165 194
References [1] v.J. Farage, Ph. Stroot and D. Janjic, Helv. Chim. Acta 60 (1977) 231. [2] V.J. FaJage and D. Janjic, Helv. Chim. Acta 61 (1978) 1539. [3] V.J. Farage and D. Janjic, Helv. Chim. Acta 63 (1980) 433. [4] V.J. Farage and D. Janjic, Chimia 34 (1980) 342. [5] V.J. Farage and D. Janjic, Chimia 35 (1981) 289. [6] E. K6r6s and M. Orban, Nature 273 (1978) 371. [7] M. Orban and E. K6r6s, React. Kinet. Catal. Letters 8 (1978) 273. [8] M. Orban and E. K6r6s, J. Phys. Chem. 82 (1978) 1672. [9] Mo Orban, E. K6r6s and R.M. Noyes, J. Phys. Chem. 83 (1979) 3056. [10] E. K6r6s, M. Orban and I. Haban, J. Phys. Chem. 84 (1980) 559.