Mechanistic study of the nitration of naphthalene by its electrochemical oxidation in the presence of nitrite ion in acetonitrile solutions

JOURNAt O~

Journal of Electroanalytical Chemistry 394 (1995) 245-251

ELSEVIER

Mechanistic study of the nitration of naphthalene by its electrochemical oxidation in the presence of nitrite ion in acetonitrile solutions Mariana N. Cortona, Nelio Vettorazzi, Juana J. Silber, Leonides Sereno * Departamento de Qulmica y F&ica, Universidad Nacional de Rio Cuarto, Estafeta Nro 9, Rio Cuarto 5800, Argentina

Received 28 February 1995; in revised form 20 April 1995

Abstract The nitration of naphthalene (NapH) at a Pt electrode in acetonitrile (ACN) in the presence of excess NaNO 2 has been studied in detail. First, the electrochemical behavior of the reactants NapH and NaNO 2 was investigated by cyclic voltammetry to determine the electrolysis conditions. When both substrates are in the solution, two well-defined peaks are observed which correspond to the oxidation of NO 2 and NapH. The peak current of NapH is not linear with the concentration of NapH, indicating that either the NO 2 ion or its oxidation product interacts with NapH "+, changing its route of degradation. This effect increases with increasing NapH/NO~- ratio. These studies showed that the nitration reaction between NapH" + and NO 2 is fast and competes with another coupling reaction, the dimerization of NapH "+ to give binaphthyl. Controlled-potential electrolysis, with the concentration of NaNO 2 kept constant, gave high yields of nitration products. The 1-nitronaphthalene/2-nitronaphthalene ratio was always greater than 50 at the end of the electrolysis (i.e. less than 2 h). These results indicate that the nitro products are obtained for the selective reaction between NO 2 and NapH "+. A reaction mechanism is proposed which accounts for the yields and the competitive reactions under these conditions. Keywords: Electro-oxidation; Reaction mechanisms; Nitration naphthalene

1. Introduction

The nitration of aromatic compounds is an area of considerable interest because of both its synthetic applications and its mechanistic aspects. As a consequence a number of studies have been reported in the literature [1-3]. Generally the nitration methods employed involve thermal, electrochemical or photochemical processes. In modern thermal methods a variety of approaches such as acid catalysis [4,5], solvent effects [6], two-phase reactions [7-10] and solid-supported catalysis [11-14] have been explored, Electrochemical investigations have been made in order to elucidate the mechanistic aspects of the electrophilic aromatic nitrations [15,16]. Recently, several photochemical nitration methods have been reported [17-20]. From the mechanistic point of view, the possibility of electron transfer (ET) mediated aromatic substitution mechanisms, in addition to the classic polar mechanisms of electrophilic aromatic substitution (EAS) by NO~ ion, has encouraged considerable investigation in this area, particu-

* Corresponding author. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(95)04076-5

larly in the last 15 years [5,15,16,21-26]. The ET mechanism emerges as an alternative way of explaining intramolecular selectivity and secondary products in diffusion controlled reactions between aromatic compounds (ArH) and the NO~-ion. The proposed mechanism [21] involves an initial ET step ArH + NO~- ~ ArH "++ N 0 2

(1)

A r H ' + + NO 2 --+ [+Ar(H)NO2] ~ ArNO 2 + H +

(2)

Application of the Marcus theory to step (1) [22] shows that the very high reorganization energy of the NO~-/NO 2 self-exchange reaction is the most probable cause of its inability to undergo ET non-bonded reactions unless the substrate has a standard potential E ° less than or equal to 0.2 V. The aim of this work is to investigate aspects of the mechanism, selectivity and optimization of electrochemical methods of nitration in non-aqueous solvents. Naphthalene (NapH) was chosen as the model aromatic compound and the solvent was acetonitrile (ACN). The thermal nitration of NapH [3] and certain methylsubstituted derivatives [3,27] by NO2~ generally occurs via the classical polar mechanism, although the product distri-

246

M.N. Cortona et al./Journal of Electroanalytical Chem&try 394 (1995) 245-251

bution varies depending on the solvent and the reactants used to generate the N O r ion. None the less, the value of E ° for NapH potential in different solvents is such that it allows a small but significant contribution of ET, followed by radical-pair separation (Eq. (1)), to the classical electrophilic attack in the nitration mechanism. When the system involves nitrous acid catalysis (NAC) with NO + ion formation, the contribution of the ET mechanism is quite significant for aromatic compounds that are more reactive than toluene [27]. The latter should be taken into account in the preparative nitration of NapH whenever this is not possible or NO + formation has not been inhibited. It has been observed that the yield in 1-nitro substitution in NapH and methyl derivatives increases in the order EAS < NAC nitration < A r H ' + + N O 2 nitration. Consequently, it is assumed that the reaction between free ArH" + and NO 2 is not an elementary step in EAS and NAC nitrations [24]. Theoretically [27] a greater selectivity in the radical pair process can be predicted as has been observed experimentally for NapH and some methyl derivatives [3]. Undoubtedly, this favors the synthetic potential of the N O 2 nitration of aromatic cation radicals. There have been several reports [15,16,28,29] of the electrochemical nitration of NapH in non-aqueous solvents where the NapH "+ is generated anodically at the electrode Isolution interface and then reacts with N O 2 (or N 2 0 4 ) present in the solution, usually with a high product yield. Eberson et al. [15] reported that the oxidative generation of the radical cation in ACN in the presence of N O 2 gives nitro derivatives. However, it is shown that the mechanism of nitration is predominantly if not exclusively the homogeneous nitration of NapH by N204 catalyzed by the acid generated anodically instead of the expected nitration of the radical cation by N O 2. Nevertheless, when the reaction is performed by reacting electrocrystallized NapH" + hexafluorophosphate in dichloromethane on a Pt electrode [16] with N O 2 at low temperature, the expected reaction (Eq. (2)) is observed. The high yields of 1-NOzNa p with a 1-nitro/2nitro ratio of ca. 40 give evidence of the diffusion-control coupling of the radical cation and N O 2. It has also been found [29] that when NzO 4 is used as nitrating agent high yields of 1-NOzNa p are obtained, but dinitro derivatives such as 1,5-, 1,8- and 2,3-diNOzNa p as well as unidentified compounds are also produced. The actual yields of these products depend on the electrode potential used in the electrolysis. In the light of these studies we consider than it could be useful to attempt nitration by the simultaneous electrochemical generation of NO 2 and NapH'+. N O 2 c a n be obtained by electro-oxidation of the NO 2 ion [30-32]. Then it can be assumed that the electrolysis of ACN solutions of NO2-ion and NapH on a Pt electrode will give nitration products. This procedure was previously reported for NapH in ACN in the presence of A g N O 2 [28] but gave very low yields of nitration products.

In this work we describe a method of producing highly selective nitration of NapH at a Pt electrode in ACN in the presence of NaNO 2. High yields of 1-NO2Na p are obtained under optimized conditions. All the evidence supports the reaction between N O 2 and NapH "+. A reaction mechanism is proposed.

2. Experimental Acetonitrile (ACN) from Sintorgan (HPLC quality) was dried as described previously [33]. NaNO 2 (Merck p.a., min. 99%) was dried under vacuum at 180°C for 24 h. NaC104 (Merck p.a., rain. 99%) was dried under vacuum at 200°C for 72 h. Naphthalene (NapH) from Fluka AG was purified by sublimation prior to use. 1-Nitronaphthalene (1-NOeNa p) was synthesized from NapH with 53% HNO 3 in acetic anhydride [34] and recrystallized from ethanol + water mixtures (MP 61 + 0.2°C). 2Nitronaphthalene (2-NO 2 Nap) from Aldrich was recrystallized five times from ethanol + water mixtures, and then sublimed (MP 78.5 + 0.2°C). 1,4-Naphthoquinone (Fluka) was recrystallized from ethanol (MP 128.2_ 0.2°C), and 1,1'-binaphthyl ((Nap) z) (Pfaltz and Bauer) was recrystallized, from ether (MP 158 + 0.5°C). 1,4-Dinitrobenzene (Fluka, min. 97%) was recrystallized three times from acetone (MP 173 + 0.2°C). All other reagents were analytical grade and were used as received. GLC measurements were performed in a Varian 2800 gas chromatograph; the column (2.5 m × 3 mm) contained 30% dinonylphthalate on Chromosorb W and 1,4-dinitrobenzene was used as the internal standard. HPLC analyses were performed in a Varian 5000 chromatograph with a MicroPak MCH-10 column and an isocratic mixture (30 : 70) of triply distilled water and methanol (Sintorgan HPLC, water content 0.05%) as the mobile phase. Standard solutions of NapH, nitro derivatives and other products were prepared by weighing. The exact concentration of NapH was determined by UV spectroscopy (Ama× = 2 7 6 nm, log(e/1 mo1-1 c m - 1 ) = 3 . 7 5 ) in a Hewlett Packard HP 8452 spectrophotometer. An EG & G PAR model 273 potentiostat-galvanostat was used for cyclic voltammetry (CV) and controlledpotential electrolysis (CPE) measurements. The current and potentials were registered on an EG & G PAR model RE 0150 x - y recorder, on Servograph Radiometer REC 61 x - t recorders or with a Keithley 194A high speed voltmeter connected to a PC computer. A conventional three-compartment Pyrex cell was used for CV experiments. The CPE cell had a Teflon paddle and its configuration was as described previously [35]. The working electrodes were Pt disks of area 0.03 cm e for CV, and Pt electrodes of larger area (ca. 4 cm 2 and 16 cm 2) for CPE. The counter-electrode was a stainless steel foil of large area. All the potentials were referred to a saturated calomel electrode (SCE) and were corrected for I R drop by posi-

M.N, Cortona et al./Journal of Electroanalytical Chemistry 394 (1995) 245-251

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i

L -0.5

L

I 0.0

i

I 0.5

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I 1.0

i

J

i --.7

I 1.5

(b)

i

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E/V Fig. 1. Cyclic voltammograrnsof (a) the oxidation of a 0.5 mM solution of NapH in ACN ( ) and (b) voltammogramof the oxidation of a 1.27 mM solution of NO2 ion in ACN ( - . - ) (v = 0.10 V s-l). tive feedback techniques. The experiments were performed at 20°C under a purified nitrogen atmosphere. The solvent-supporting electrolyte system was 0.4 M NaCIO 4 in ACN. The experimental solubility of NaNO 2 in the system (i.e. 1.27 mM) was determined by CV using the peak current as a function of concentration (see below).

3. Result and discussion

3.1. Cyclic voltammetry

new peaks were detected during cycling in the potential range between - 1 . 2 5 V and 1.80 V.

NaNO 2 The electrochemical oxidation of the N O 2 ion in aqueous solution [38] and in aprotic solvents such as nitromethane [39], dimethylsulphoxide [40] and ACN [3032], particularly on Pt electrodes, is well established. The cyclic voltammogram obtained in this work is shown in Fig. l(b). Oxidation peak II of the NO 2 ion is detected at Epn = 0.55 V. No cathodic current is observed in this potential interval during the reverse cycle. The peak current depends linearly on both t, 1/2 and the concentration of the NO 2 ion, in agreement with previous results

[301. The electrochemical reaction in non-aqueous ACN has been interpreted in terms of a complex mechanism. The principal product formed in the first charge transfer reaction is NO2: NO~

~

(3)

NO 2 + e

This is followed by the equilibrium [30] 2NO 2 ~ N204

(4)

with an equilibrium constant of ca. 3.3 × [16] and the parallel reaction NO~-

+

NO 2 ---->NO + NO~.

10 4

M in ACN

(5)

The oxidation of NO 2 NO 2 ~ N O ; + e -

Naphthalene A typical cyclic voltammogram of NapH in ACN solution is illustrated in Fig. l(a). An oxidation peak I ( E p l = 1.62 V) due to the oxidation of NapH to the radical monocation ( N a p H +), is observed on the first sweep. This peak is highly irreversible and no complementary cathodic peak is observed even at 50 V s 1. This is an indication of the very short lifetime of NapH" + or of a fast follow-up reaction coupled to charge transfer. The peak current Ipl shows a linear dependence on v 1/2 in the range of sweep rates used from 0.01 to 0.3 V s 1. In addition, Ipl varies linearly with the concentration of NapH. This observations reinforces the idea of a fast reaction coupled to the charge transfer. A product that has been detected under similar conditions is l,l-binaphthyl ((Nap) 2) [36]. Dimerization and even further polymerization are typical follow-up chemical reactions of aromatic radical cations [37]. If it is assumed that the follow-up chemical reaction is the dimerization reaction and the formal oxidation potential of the couple (Nap)z/(Nap)2 + is similar to that of the couple N a p H / N a p H "+, the voltammetric peak should include both charge transfers. In the negative sweep only one peak is observed at - 0 . 3 3 V. This peak is assigned to solvated protons liberated in the dimerization reaction of NapH "+, as confirmed by addition of dry trifluoroacetic acid to the solution. No

247

(6)

is expected to occur at E ° = 1.31 V / S C E . However, in excess NO 2 ion, the N O ; is reduced [32] to the neutral species: N O ; + NO 2 ~

N204.

(7)

We can conclude that, under our conditions, the predominant species produced is NO 2 (or more precisely its dimer) and its concentration at the electrode ]solution interface is quite high.

NaNO 2 + NapH solution The typical cyclic voltammogram obtained when both substrates are in solution is illustrated in Fig. 2. Two well-defined peaks are observed which correspond to the oxidation of the NO 2 ion (peak II) and NapH (peak I). The peak potentials are the same as those of the separate substrates (Fig. 1). Thus the peak current lvn of peak II depends linearly on t, 1/2 and on the concentration of the NO~- ion as in the absence of NapH. This fact is used to determine the NO~ ion concentration by cyclic voltammetry using a standard plot of Ipn as a function of the concentration. However, Ipl varies not only with the NapH concentration but also with the NO~- ion concentration. In order to determine this dependence systematic studies were carried out, keeping one substrate constant and changing the concentration of the other.

M.N. Cortona et al./Journal of Electroanalytical Chemistry 394 (1995) 245-251

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E/V

ion:

Fig. 2. Cyclic voltammogram of the oxidation of a solution of 1,27 m M NO 2 ion and 0.5 mM NapH in A C N (t, = 0.10 V s-1.).

Solutions with a constant concentration of NO 2 ion (1.27 raM) and a varying concentration of NapH (0.05-5 mM) were studied. The voltammograms all have the same features as the one shown in Fig. 2. The peak current of NapH does not vary linearly with the concentration of NapH (Fig. 3), indicating that either the NO 2 ion or its oxidation product interacts with NapH'*, changing its route of degradation. This effect increases with increasing N a p H / N O 2 ratio. Another study was carried out in which NapH was kept constant and the concentration of the NO 2 ion was changed. The peak current Ipl decreases with decreasing N a p H / N O 2 ratio (Fig. 4) in agreement with the result reported above. The CV studies of the reduction of the mononitro NapH derivatives in the same solvent-supporting electrolyte system show that 1-NO 2Nap and 2-NO 2Nap give irreversible peaks at Ep values - 1 . 1 8 0 V and - 0 . 8 V respectively. However, when the scan is reversed in the NapH + NO~ solution no anodic peaks are detected in the potential interval between - 0 . 7 and - 1 . 2 V. It seems that the detection of these products under these conditions is diffi300

25o

//e

cult. Moreover, additions of 0.5 mM 1-NO2Na p or 0.5 mM 2-NOzNa p to these solutions show that these compounds are almost undetectable. This is probably because the products of electro-oxidation of NapH passivate the electrode.

3.2. Controlled-potential electrolysis In view of the CV results reported above, CPE was performed for two different types of solution: (a) only NaNO 2 present, and (b) NaNO 2 + NapH.

CPE of NaNO 2 CPE of 30 ml of a 1 mM NaNO 2 solution was performed at 1.7 V. This value was chosen because both substrates are oxidized at this potential (Fig. 1). In this case the electrolysis current decays exponentially with time and the nap p is close to 0.6 C or 2 / 3 e per molecule of NO 2 ion, in agreement with previous results [30]. In order to avoid changes in the concentration of N O 2 ions and to maintain a constant concentration-operation (CCO) regime in the electrolysis cell, an excess of solid NaNO 2 was kept in the cell (the solubility of anhydrous NaNO 2 in ACN is 1.27 mM at the working temperature). When CPE was performed under these conditions there was no variation of either the concentration or the current with time, at least during the period of the experiment, i.e. 1.5 h. These new conditions were used as a blank experiment.

200

CPE of NaNO 2 + NapH

} ~

150

100

50

O~ 0

l

2

3

4

5

[NapH]. 103 / M Fig. 3. Dependence of Ipl on the concentration of NapH: [NO 2 ] = 1.27 raM; v = 0 . 1 0 0 V s -1.

In order to explore the possibility of the nitration of NapH by the electro-oxidation product of the NO 2 ion, CPE was performed at 1.2 V. At this potential only the NO 2 ion is electrolyzed (Fig. 1). In a typical experiment, 30 ml of NaNO 2 in CCO and 0.5 mM in NapH were electrolyzed for about 1 h. The charge injected was 9.3 C. According to the result reported above for rtap p of the N O 2 ion, this charge is sufficient to produce e x c e s s N O 2. Nevertheless, in these conditions no nitration products are detected by GLC or HPLC even 1 h after the electrolysis

249

M.N. Cortona et al. /Journal of Electroanalytical Chemistry 394 (1995) 245-251

was interrupted. This provides evidence that NapH is not attacked by the intermediate product of the electro-oxidation of the NO 2 . CPE in CCO conditions was then performed at 1.7 V in order to study the product distribution as a function of different experimental conditions. Typically, 30 ml of NaNO 2 under CCO and a variable concentration of NapH were electrolyzed. The current was recorded as a function of the time, and 0.5 ml samples of solution were taken for product analysis at regular intervals. The experiment was concluded when the current was 2% of that of the blank; for example for 0.5 mM of NapH the total electrolysis time was about 1.5 h. Under these conditions 1-NOzNap was almost the only product detected by the chromatographic techniques. At the end of the electrolysis the ratio 1-NOzNap/2-NO2Nap was always greater than 50. The total charge, after subtraction of the blank charge, was related to the amount of NapH electrolyzed. Also, the amount of NapH was related to the amount of 1-NOzNap detected. Typical results for two NapH concentrations are shown in Fig. 5. The yield of 1-NO2Na p is not linear and tends to increase during the electrolysis. At the end of the electrolysis, the maximum yield is about 85%. Similar trends were observed for 1 mM solutions of NapH, but in these cases the yield was lower than that for 0.5 mM solution. This indicates that a product other than NO 2Nap was formed. Moreover, since the NO 2 ion concentration was kept constant throughout the experiment, the yield of 1-NOzNap increases with increasing ratio NO 2 / N a p H . It should also be pointed out that no dinitro products are detected. In fact products of this type can only be obtained after oxidation of 1-NO2Na p which occurs at potentials above 2 V. More information regarding the nitration mechanism can be obtained from the charge consumed in the electrode process. The charge consumed is plotted as a function of the amount of NapH electrolyzed in Fig. 6. The non-linear

12

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Amount of NapH.//J.mol

Fig. 5. Distribution of I-NO2Nap in CPE at two NapH concentrations: zx 0.5 raM; • 1.0 mM. [NO~ ] = 1.27 mM (CCO).

12

15

Amount of NapH./p.mol Fig. 6. Faraday's law for CPE: [Nap[l] = 0.5 raM; [ N O y ] = 1.27 mM

(cco).

plot is in agreement with the data in Fig. 5, providing further support for the existence of other products, with the additional condition that they are electroactive. As discussed for the CV studies of NapH, these products can be dimers or oligomers, which may also be present in the electrolysis in the presence of NOy ion. In order to obtain some evidence for these reactions, a solution of 7 mM NapH + NaNO 2 in CCO conditions was partially electrolyzed at a low potential, i.e. the potential at the onset of the NapH oxidation wave (ca. 1.48 V) (see Fig. 1). If it is assumed that the formal potentials of the couples NapH/NapH" + and (Nap)2/(Nap)2 + are similar and the solution is efficiently stirred, it is possible that some of the (Nap) 2 formed may escape from the electrode Isolution interface to the bulk, where it could be detected. The electrolysis was continued until approximately 50% of the NapH was consumed. The solution obtained was analyzed by cyclic voltammetry as shown in Fig. 7(a). A new pre-peak was detected at 1.45 V on the positive scan, followed by the NapH peak at 1.6 V. This peak was identified as 1,1'-(Nap) 2 using an authentic sample under the same experimental conditions (Fig. 7(b)). It should be noted that 1,1'-(Nap) 2 has an irreversible peak. However, if the electrolysis is carried out at 1.7 V this product is completely oxidized and is not detected by cyclic voltammetry; this is the main secondary reaction that lowers the yield of NORNap. Another possible parallel reaction is given by NapH' + + NO 2 ~ NapH + NO 2

0..~ ~

9

(8)

It has been demonstrated that this reaction is fast, and its A G ° value has been estimated as - 1 3 4 kJ mo1-1 [16]. Thus the NO 2 ion partially regenerates the NapH. This is another competitive path that does not produce a new product but increases the total charge used. In order to obtain more evidence for this reaction, several CPEs of

M.N. Cortona et al. /Journal of Electroanalytical Chemistry 394 (1995) 245-251

250

~

x/#A 2oo

I00 0

NO 2 (o)

(a)

NO 2 + e

NapH ~ NapH" + + e

(b)

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~

NapH ' + +

N O 2 ---)

NapH ' + +

NO 2 ~

(b)

NapH

+ NO 2

NO2Na p + H +

(c)

(d)

2NapH ' + ~ (Nap)2 + 2H +

(e)

(Nap)2 -~ (Nap); + + e

(f)

(Nap); + -~ products

(g) Scheme 1.

2:5

•

~V

Fig. 7. (a) Cyclic voltammogram of a partially electrolyzed solution: initial conditions, 1.27 mM (CCO) = N O r and 7 mM = NapH in ACN; u = 0.075 V s - I . (b) Cyclic voltammogram of the oxidation of a 0.75 mM solution of (Nap) 2 in A C N (t' = 0.075 V s - l ).

NO 2 ions at concentrations near 1.0 mM with increasing concentrations of NapH were performed at 1.7 V. The concentration of NOy ions was measured at regular intervals by cyclic voltammetry. The result obtained after normalizing the concentration to unity is shown in Fig. 8. The decay of the NO 2 ion concentration increases with increasing concentration of NapH, indicating a greater consumption of NO 2 ion for the same time intervals. However, in the CPE experiments under the CCO regime this reaction increases the total charge consumed, but since it regenerates some NapH it contributes to an increased yield of NO2 Nap. The possibility of the homogeneous nitration of NapH by NO2 catalyzed by the proton liberated in the coupling reactions of NapH" ÷ [15] seems to be precluded by several experimental facts. A high yield of nitration products

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On the basis of the experimental results it appears that the most probable steps in the mechanism for the nitration of NapH are as shown in Scheme 1. If we define uc as the amount of NapH produced by the catalytic process (c), % as the amount of NO2Na p and ue as the amount of (Nap) 2 the global reaction has the following stoichiometry for NapH depletion: ( u d + 2Ue)NapH = UdNO2Na p + ve(Nap)~ + + (% + 2ve)H + + ( 3 u e + ~,d)e-

On=vcF+UdF+FVe+2veF=(vc+Vd+3ve)F

l 00 0

3.3. Mechanism

500

(9)

Determination of the coefficients u i Only the charge consumed in the electro-oxidation of NapH and (Nap) 2 is of interest in the analysis of the product distribution. Thus the charge involved in the electro-oxidation of NO~ ions should be subtracted from the total. Since one electron is involved in each step, the net charge Q,, in terms of the coefficients u~, is

l0

.~ 08

is obtained in less than 2 h, while the homogeneous reaction is slow and gives low yields [15] in the same time interval. Moreover, the high 1-NO2Nap/2-NO2Na p ratio is characteristic of the radical pair reaction [3,27]. Thus it can be concluded that the homogeneous reaction is practically negligible in the conditions used in this work. The reason for this may be that the protons liberated by the coupling and substitution reactions are neutralized by the excess of NO 2 ion, which is a strong base in this medium [311.

1000

1500

Time / s Fig. 8. Catalytic effect with [NO~-]= 1.0 mM and various values of [NapH]: - 0.0 mM; - - - - - 0.5 mM; - - - 0.75 mM; . . . . . . 1.00 mM; • .. 1.50 mM.

(10)

The charge Q n , the amount of NapH depleted and the amount vd of 1-NO2Na p formed are determined by independent experiments. Using these data, two new relationships can be written: the yield of the mononitro compound with respect to the total number of moles of NapH depleted is given by ryo2y.p = vd/( vd + 2ue)

(11)

M.N. Cortona et al. /Journal of Electroanalytical Chemistry 394 (1995) 245-251

251

References l0

2

4

6

8

10

12

14

Amount of NopH.//zmol Fig. 9. Coefficients v, as a function of the amount of NapH depleted: ......

P c ; -

b'd; . . . .

/2e"

a n d the a p p a r e n t n u m b e r nap p o f e l e c t r o n s is g i v e n b y nap p = Q n / a m o u n t

of NapH

= ( v c + v~ + 3 v e ) F / ( v

a + 2ve)F

(12)

T h e v a l u e s o f YNO2Nap a n d nap p are o b t a i n e d f r o m Figs. 5 a n d 6 r e s p e c t i v e l y . T h u s , a c c o r d i n g to Eq. (11), ve is given by Pe = Vd(1 -- YNO2Nap)/ZYNo2Na p

(13)

a n d u s i n g Eq. (12), the v a l u e o f v~ c a n b e c a l c u l a t e d : uc = n , p p ( a m o u n t o f N a p H ) - ( v~ + 3uc)

(14)

T h e result is s h o w n in Fig 9. A s c a n b e seen, a l t h o u g h the f o r m a t i o n o f ( N a p ) 2 is the m o s t i m p o r t a n t s e c o n d a r y r e a c t i o n , it o c c u r s w i t h a r e l a t i v e l y l o w yield a n d the c o n c e n t r a t i o n o f ( N a p ) 2 is a l m o s t c o n s t a n t d u r i n g the electrolysis. H o w e v e r , a h i g h yield o f 1 - N O 2 N a p is obt a i n e d as e x p e c t e d for the s e l e c t i v e r e a c t i o n b e t w e e n N O 2 a n d N a p H " + [27]. In c o n c l u s i o n , the s i m u l t a n e o u s g e n e r a t i o n o f the arom a t i c radical c a t i o n a n d N O 2 at the e l e c t r o d e surface s e e m s to b e a v e r y e f f e c t i v e m e t h o d o f o b t a i n i n g 1N O 2 Nap.

Acknowledgments F i n a n c i a l s u p p o r t f r o m the C o n s e j o N a c i o n a l de I n v e s t i g a c i o n e s C i e n t i f i c a s y T 6 c n i c a s ( C O N I C E T ) , the C o n s e j o de I n v e s t i g a c i o n e s C i e n t [ f i c a s y T e c n o l 6 g i c a s de la P r o v i n cia de C 6 r d o b a ( C O N I C O R ) a n d the Secretar~a de C i e n c i a y T 6 c n i c a de la U n i v e r s i d a d N a c i o n a l de Rio C u a r t o is g r a t e f u l l y a c k n o w l e d g e d . M C t h a n k s C O N I C O R for a research fellowship.

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