초임계 CO2를 이용한 1-naphtolic acid의 전해합성

박지영, 이윤우, 김재덕, 이윤용, 이창하,* 임종성

한국과학기술연구원 국가지정 초임계유체 연구실, 연세대학교 화학공학과*

Electrochemical synthesis of 1-naphtolic acid in supercritical CO2

Ji-Young Park, Youn Woo Lee, Jae-Duck Kim, Youn Yong Lee,

Chang-Ha Lee,* Jong Sung Lim

National Research Lab. for Supercritical Fluid, KIST

Dept. of Chem. Eng., Yonsei University*

Introduction

CO2 in supercritical state has been regarded as a suitable chemical for organic synthesis because it is nonflammabe, nontoxic, inexpensive, and readily accessible to supercritical region.  Especially, supercritical CO2 have interesting solvating characteristics and provide greatly increased mass transfer rates to an electrode in an electrochemical system.  In spite of these advantages, the usage of supercritical CO2 as a reaction solvent in electrochemical synthesis has lagged for nonpolarity of CO2.  Recently, however, the studies in this field of electrocarboxylation of organic compounds with CO2 have been reported.

In this study, the electrocarboxylation of organic halides was carried out using acetonitrile (or DMF) solution containing Bu4NBF4 for support electrolyte in a high-pressure view-cell equipped with a magnesium cathode and platinum anode.  Reaction temperature was 40℃, pressure was 80bar, and constant current of 10mA (3F/mol) was supplied.  Electrocarboxylation of 1-chloronaphtalene under supercritical CO2 gave regioselectively carboxylic acid (1-naphtolic acid).

In this experiment, although we used a reactive metal cathode (magnesium) which play a very important part in the overall process, we couldn't gain the satisfying results.  However, a detailed study on the reaction to obtain higher  yield is now in progress.

Experimental

Chemicals  Tetra-n-butylammonium tertrafluoroborate ( (CH3CH2CH2CH2)4NBF4 :

Bu4NBF4, Merck, Germany) was used as a support electrolyte.  Bu4NBF4 was dried in a heating oven at 75℃ for 2days and used without purification.  The 1-chloronaphtalene (Aldrich, USA), acetonitrile (Fisher Scientific, USA) and DMF (Aldrich, USA) were dehydrated by molecular sieve 3Å.  The water content of the salt was measured by HR73 Halogen moisture analyzer (Mettler Toledo, switzerland) and that of acetonitrile and DMF was measured by Karl Fischer (ORION AF8, USA)

Preparation of electrolytes containing reactant  6.5g 1-chloronaphtalene and 32.9g  Bu4NBF4 (support electrolyte) were dissolved in 200ml acetonitrile (or DMF).

Electrolytic conductivity measurement  The electrolytic conductivities were measured at 26.6 ℃ by a conductivity meter (Radiometer, CDM210).  The measured conductivities were listed in Table 1.

Electrochemical measurement  All electrochemical measurements were made by chronopotentiometry(Potentiostat/Galvanostat M270, EG&G PRINSTON Applied Research).

Experimental apparatus and Procedure  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) was used as a counter electrode and magnesium rod (□5mm) was used as a working electrode.  A stirrer, rotated at variable speeds by an external magnet, was used to mix the solution in the view-cell.  The schematic diagram of experimental apparatus was shown in Figure 1.  Experiment for electrochemical carboxylation of 1-chloronaphtalene was performed by the following procedure.  The whole system was evacuated with a vacuum pump to remove all the materials in the cell, and a certain amount (20ml or 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 80 bar at 40℃ using the high pressure pump.

Figure 1. Schematic diagram of experimental apparatus

Table 1. Electrolytic conductivity of solutions containing Bu4NBF4 salt at 26.6℃

pure chemicals conductivity, mS/cm
DMF	1.93×10-3
Acetonitrile	7.97×10-3
1-chloronaphtalene	0.01×10-3
solution 1 ( 10ml + 5mmol )
DMF + Bu4NBF4	13.31
Acetonitrile + Bu4NBF4	28.79
solution 2 ( 10ml + 5mmol + 2mmol )
DMF + Bu4NBF4 + 1-chloronaphtalene	12.86
Acetonitrile + Bu4NBF4 + 1-chloronaphtalene	27.27

The potential was scanned by means of a potentiostat controlled by a PC. The chronopotentiometry was used for the electrochemical reactions occurring in the electrocarboxylation of 1-chloronaphtalene[1]. A current of 10mA was applied until complete consumption of reactant (1-chloronaphtalene) was observed by HPLC analysis of samples.

Supercritical condition  To obtain the condition of homogeneous phase between CO2 and acetonitrile, vapor-liquid equilibria of CO2 + acetonitrile mixture were measured at 40℃.  Critical point of CO2 + acetonitrile binary mixture was 79bar at 40℃. The VLE diagram was shown in Figure 2.

Results and discussion

The electrocarboxylation reaction can be represented as follows[2] ;

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

CH3CN(DMF) + Bu4NBF4

40℃, 80bar

the balance of electrochemical reactions occurring in the electrocarboxylation of organic halides (RX) using a magnesium cathode can be summarized as[3] ;

at cathode (Mg)    Mg → Mg2+  + 2e-

at anode   (Pt)     RX + CO2 + 2e- → RCO-2 + X-  (RX: 1-chloronaphtalene)

This reaction looks like an electro-assisted Grignard reaction. It has been applied to various organic halides (aryl, benzyl, and vinyl halogeno compounds).  In this study, 1-naphtolic acid is obtained as a product in this reaction. The results of experiment were listed in Table 2.   The yield of reaction in acetonitrile as an organic solvent was higher than that in DMF.  The more amounts of electrolytes were used, the higher reaction yield was obtained.  In the whole cases, however, the reaction yields were not high and much amounts of electrolytes which must be decreased were used. The effect of water in solution, electrolytes used, intensity of current and reaction time were thought to be the cause of low yield.  Now, we are trying to solve those problems and more detailed study on this reaction to obtain higher yield is now in progress.

Table 2. The yield of 1-naphtolic acid synthesized in electrocarboxylation

electrolyte	yield (%)	experimental condition
	Pt/Mg
DMF + Bu4NBF4 (20ml)	5.7	T = 40℃ P = 80bar
DMF + Bu4NBF4 (40ml)	7.2
Acetonitrile + Bu4NBF4 (20ml)	13.8
Acetonitrile + Bu4NBF4 (40ml)	15.0

Figure 2. VLE diagram of CO2 + Acetonitrile at 40℃

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. Richard M. Crooks, Allen J. Bard, J. Am. Chem. Soc., 106, 6851-6852, (1984)

2. Hisato Kamekawa, Hisanori Senboku, Masao Tokuda, Tetrahedron Letters,

39, 1591-1594, (1998)

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