초임계 유체내에서 □-chloronaphtalene 의

□-naphtalenecarboxylic acid로의 전기화학적 전환

박지영, 임종성, 이윤우, 김재덕, 이창하*

한국과학기술연구원 국가지정 초임계유체 연구실, 연세대학교 화학공학과*

Electrochemical conversion of □-chloronaphtalene into □-naphtalenecarboxylic acid in supercritical fluid

Ji-Young Park, Jong Sung Lim, Youn Woo Lee, Jae-Duck Kim, Chang-Ha Lee*

National Research Lab. for Supercritical Fluid, KIST

Dept. of Chem. Eng., Yonsei University*

Introduction

The electrochemical synthesis of organic compounds has been studied continuously because the synthesis using the electrochemical coversion has the advantage that several reaction is done by the only one step unlike a general organic synthesis.  In this study, □-chloronaphtalene was converted into □-naphtalenecarboxylic acid electrochemically in supercritical fluid (CO2) [1].  CO2 which is used as a supercritical media and reactant in this reaction has been regarded as a suitable chemical for organic synthesis because it is nonflammable, nontoxic, inexpensive, and readily accessible to supercritical region.  Especially, supercritical CO2 has interesting solvating characteristics and provide greatly increased mass transfer rates to an electrode in an electrochemical system.  The electrocarboxylation of organic halides (in this study, □-chloronaphtalene) was carried out using acetonitrile solution containing Bu4NBF4 as a supportting electrolyte in a high-pressure view-cell equipped with an anode made of magnesium and a cathode made of platinum.  Reaction temperature was 40 ℃, pressure was 85 bar, and constant voltages of 8 volt and 25 volt, and constant currents of 40 mA and 80 mA (3F/mol) were supplied.  Electrocarboxylation of □-chloronaphtalene under supercritical CO2 gave selectively carboxylic acid (□-naphtalenecarboxylic acid).

Theory

Reduction Mechanism of carbon dioxide  Andrew P. Abbott [2] has studied the electroreduction of CO2.  According to his studies, the electroreduction of CO2 can be classified.  Among these reactions, the reaction of 'Case 5.' is connected with the electrocarboxylation of organic halides and during this the mechanism of CO2, □-naphtalenecarboxylic acid can be gained.

Case	1.    CO2  +  H+  + 2e-    →   HCOO-

Case	2.    CO2  + 2H+  + 2e-    →   CO + H2O

Case	3.    CO2  + 6H+  + 2e-    →   CH3OH + H2O

Case	4.    CO2  + 8H+  + 2e-    →   CH4 + 2H2O

Case	5.    2CO2          + 2e-    →    (COO-)2

Reaction Mechanism of electrocarboylation of □-chloronaphalene  The reaction mech

anism of □-naphtalenecarboxylic acid was shown in Fig. 1.  The reaction was

done on two electrodes.  The intermediate (RMgX) of □-naphtalenecarboxylic acid reactied with Mg2+ on the anodic electrode was poured into 1N HCl. After this procedure, □-naphtalenecarboxylic acid was produced [3, 4]

Pt(-)  Mg(+)  3F/mol

CH3CN + Bu4NBF4

40℃, 85 bar

at anode    (Mg)     Mg → Mg2+  + 2e-

at cathode  (Pt)      RX + CO2 + 2e- → RCO-2 + X- (RX : □-chloronaphtalene)

(a) The electrocarboxylation of □-chloronaphtalene - I

Intermediate (RMgX)

+ addition of 1N HCl

Intermediate (RMgX)

or

(b) The electrocarboxylation of □-chloronaphtalene - II

Fig. 1. The reaction mechanism of electrochemical carboxylation

Experimental

Chemicals Tetra-n-butylammonium tertrafluoroborate ( ( CH3CH2CH2CH2)4NBF4 : Bu4NBF4, Merck, Germany) was used as a supportting electrolyte.  Bu4NBF4 was dried in a heating oven at 75℃ for 2 days and used without purification.  The water content of the salt was measured by HR73 Halogen moisture analyzer (Mettler Toledo, switzerland) and that of acetonitrile was measured by Karl Fischer (ORION AF8, USA).

Preparation of electrolytes containing reactant  6.5g □-chloronaphtalene and 32.9g  Bu4NBF4 (supporting electrolyte) were dissolved in 200ml acetonitrile.

Experimental apparatus  The high-pressure view-cell having internal volume of 156ml was made of stainless steel 316.  The interior of this view-cell was cylindrical and lined with teflon.  A platinum plate (2×2 cm) and magnesium rod (□5mm, 2.5 cm) was used as electrodes.  A stirrer was used to mix the solution in the view-cell.

Experimental procedure  Electrochemical reaction was carried out at constant current and at constant voltage in the undivided electrochemical cell.  The whole system was evacuated with a vacuum pump to remove all the materials in the cell, and a certain amount (40ml) of solution was introduced into the cell.  The temperature of the entire system was maintained constant by controlling the temperature of the air bath.  After the desired temperature was attained, CO2 was supplied to the cell to pressurize a system pressure to 85 bar at 40℃ using the high pressure pump.  The electric power was supplyed by the power supply.  After electrolysis, the product was observed by HPLC analysis.

Results and discussion

The electrocarboxylation was experimented by the chronoamperometry and  the chronopotentiometry method. The electrolysis by chronoamperometry method was done in 8 volt, and 25 volt and that by chronoamperometry method was done at i =40 mA, and  i = 80 mA.  In case of the chronoampero

metry method, the current density wasn't constant and the current wasn't on well because of the deposition on the surface of electrodes from the beginning of reaction.  But in case of the chronopotentiometry methode unlike the chronoamperometry method, the current density was constant and the current current was on well for a long time.  As expected, the yield of □-naphtalene

carboxylic acid and the reduction rate of □-chloronaphtalene using thechrono-

potentiometry method was better than that using the chronoamperometry.  These results were represented in Table 1., Fig. 2. and 3., and the analysis diagram was shown in Fig. 4.  The final goal of this stdy is to accomplish the electrocarboxylation by one step method (namely 'electrolysis') in super critical condition and to obtain the optimal conditions.  The electrolysis to obtain the high yield is currently under investigation.

Table 1. Yield of □-naphtalenecarboxylic acid

Constant voltage
No.	Bu4NBF4 conc.	Current	Cell voltage	Current density	Yielda %			conversionb %
					1	2	3
1	5 mmol	-	8 volt	-	9.87	13.58	16.22	39.76
2	5 mmol	-	25 volt	-	2.79	3.52	6.27	12.58
Constant current
No.	Bu4NBF4 conc.	Current	Cell voltage	Current density	Yielda %			conversionb %
					1	2	3
1	5 mmol	80 mA	-	20 mA/cm2	15.07	20.44	33.94	69.45
2	2.5 mmol	40 mA	-	10 mA/cm2	16.81	23.27	36.28	73.36

a, b determined by HPLC as % of the peak

Fig. 2. Current-time curve                  Fig. 3. Potential-time curve

2

1

3

4

Time (min)

Fig. 4. HPLC trace for the electrocarboxylation reaction using the chronopotentiometry

Acknowledgement

This work has been supported by the National Research Laboratory Program for Supercritical Fluid and the authors would like to thank to the Ministry of Science and Technology, Korea.

Literature Cited

1. Hisato Kamekawa, Hisanori Senboku, Masao Tokuda, Tetrahedron Letters, 39, 1591-1594, (1998)

2. Andrew P. Abbott, Christopher A. Eardley, J. Phys. Chem., 104, 775-779, (2000)

3. John H. Wagenknecht, Kirkwood, United States Patent, No., 4,582,577, (1986)

4. J. Gal, J. C. Folest, M. Troupel, New J. Chem., 19, 401-407, (1995)