CROATICA CHEMICA ACTA CCACAA, ISSN 0011-1643, e-ISSN 1334-417X Croat. Chem. Acta 88 (1) (2015) 35–42. http://dx.doi.org/10.5562/cca2342 Original Scientific Article Electrochemical Oxidation of Fragrances 4-Allyl and 4-Propenylbenzenes on Platinum and Carbon Paste Electrodes Lai-Hao Wang,* Chia-Ling Chang, and Yi-Chun Hu Department of Medical Chemistry, Chia Nan University of Pharmacy and Science, 60 Erh-Jen Road, Section 1, Jen Te, Tainan 71743, Taiwan RECEIVED MAY 19, 2013; REVISED MARCH 10, 2014; ACCEPTED JULY 21, 2014 Abstract. The electrochemical oxidation behaviors of 4-allylbenzenes (estragole, safrole and eugenol) and 4-propenylbenzenes (anethole, asarone and isoeugenol) on platinum and carbon paste electrodes were in- vestigated in a Britton-Robinson buffer (pH = 2.93 and 10.93), acetate buffer, phosphate buffer solutions (pH = 2.19 and 6.67), and acetonitrile containing various supporting electrolytes examined lithium per- chlorate. Their oxidation potential with Hammett (free-energy relationships) and possible reaction mecha- nisms were discussed. Keywords: electrochemical oxidation, 4-allylbenzenes, 4-propenylbenzenes INTRODUCTION studied to undergo electrochemical polymerization giving rise to either conducting or insulating films.17–19 Research on fragrances / flavors is popular; however, The anodic methoxylation of alkenylbenzenes includes almost all studies focus on their wide application in the anethole and isosafrole on graphite and platinum, food, and cosmetics industries and in traditional medi- which can be converted to free aldehyde, a valuable cine; as well as extraction methods and analytical fragrance compound and an intermediate for organic methodologies.1–5 Fragrances that are air-sensitive may synthesis.20–23 The present work is concerned with the form peroxides, respiratory irritants, and aerosol parti- measurement of aromatic substituent effects and struc- cles that cause inflammatory responses in the lungs. tural elucidation of 4-allyl and 4-propenylbenzenes. The photochemistry of some allylic compounds such as cis-trans isoeugenol and citral, were oxidized in the EXPERIMENTAL presence of atmospheric oxygen or photosensitized species (O , OH, NO , etc.).6–8 However, most reports 3 3 Apparatus and Materials have focused on the photochemical reaction of eugenol derivatives to for the synthesis of new flavors chemi- Voltammetric measurements were performed using an cals.9–14 The structurally related substituted 4-allyl- EG&G Princeton Applied Research Model 394 Polaro- benzenes derivatives (eugenol, estragole and safrole) graphic Analyzer (Princeton, NJ, USA) and potenti- and 4-propenylbenzenes derivatives (isoeugenol, ane- ostat/galvanostat (263A; EG&G Princeton Applied thole and asarone), occur naturally in various tradi- Research, Princeton, NJ, USA).The absorption spectra tional foods, particularly in spices such as cloves, of fragrances were determined using a spectrophotome- cinnamon and basil1. Some of them have been demon- ter (2000/IRDM FTIR; Perkin-Elmer, Fremont, CA, strated to be an effective, inexpensive anesthetic USA) with an attenuated total reflection (ATR) system agents, antioxidants and blood circulation enhancers. (Gateway ATR; SpecacInc., Smyrna, GA (now Cranston, The major analytical methods for analyzing alkenyl- RI), USA). The Raman spectra were recorded using a benzenes fragrances are gas chromatography and gas micro-Raman setup (JASCO NR-1000; Sunway Scien- chromatography-mass spectrometry.15 There are few tific Corporation, Taipei, Taiwan). The spectrometer electrochemical theories reported in pharmaceutical had a focal length of 300 nm and was equipped with a formulations.16 4-allylbenzenes (4-allylanisole and eu- dispersive element 1800 lines/mm grating, resulting in a genol) and 4-propenylbenzenes (trans-anethole) are resolution of 1.2 cm–1. * Author to whom correspondence should be addressed. (E-mail: [email protected]) 36 L.-H. Wang et al., Electrochemical Oxidation of Fragrances 4-Allyl and 4-Propenylbenzenes Pure standard substances were purchased from bio- RESULTS AND DISCUSSION medical supply houses: eugenol, isoeugenol, and trans- anethole from Acros Organics, Geel, Belgium; -asarone, Electrochemical Behavior of Fragrances 4-Allyl and safrole from Sigma-Aldrich Fine Chemicals, Fluka, Swit- 4-Propenylbenzenes on the Platinum Electrode zerland; estragole from Aldrich Chemical Company, Inc., Supporting Electrolytes and Solvent Effects Milwaukee, WI. Supporting electrolytes were obtained as There are several ways in which the supporting electro- follows: tetrabutylammonium tetrafluoroborate (Bu NBF , 4 4 lytes solvent system can influence mass transfer, the Acros Organics, Thermo Fisher Scientific, Geel, Bel- electron reaction (electron transfer), and the chemical gium), tetraethylammonium tetrafluoroborate (Et NBF , 4 4 reactions which are coupled to the electron transfer.24 E. Merck, Chemical Co. Germany), tetrabutylammonium The effects of supporting electrolytes and solvent com- perchlorate (Bu NClO , Tokyo Chemical Industry Co., 4 4 position on fragrances 4-allyl and 4-propenylbenzenes Ltd, Tokyo, Japan), tetraethylammonium perchlorate peak potential (E ) and peak current (i ) are listed in (Et NClO ,Tokyo Chemical Industry Co., Ltd, Tokyo, p p 4 4 Table 1. Table 1 shows the peak current of fragrances in Japan), and lithium perchlorate (LiClO , Acros Organics, 4 non-aqueous solvent (100 % acetonitrile) were higher Thermo Fisher Scientific,Geel, Belgium). Other chemical than in aqueous-organic solvent (30 % acetonitrile) due reagents used were of analytical grade. to the higher background in non-aqueous solvent than that in aqueous-organic solvent. However, the peak Voltammetric Measurements potential of fragrances were less positive in aqueous The three voltammetric techniques, Sampling DC, linear organic solvents that are more suitable for oxidation. Sweep and cyclic voltammetry, were all performed on a The electrooxidation process occurs in the heterogene- platinum and carbon paste electrodes. Cyclic voltamo- ous phase. For non-aqueous solvent, its molecules com- grams (CV) of the fragrances were taken on a platinum pletely cover the electrode surface to prevent the ad- electrode in acetonitrile containing various supporting sorption of fragrances. Furthermore in organic work, electrolytes, and Britton-Robinson buffer solutions (pH = strongly basic anions or radical anions are often pro- 2.93 – 10.93) to monitor potential vs. current. duced and these are rapidly protonated by solvents like Table 1. Effect of supporting electrolytes on the cyclic voltammetric peak potential (Ep) and peak current (ip) of α-asarone, trans- anethole, isoeugenol, safrole, estragole and eugenol at platinum electrode. The concentration of fragrances was 1 mmol dm–3; scan rate, v = 50 mV/s. α-Asarone trans-Anethole Isoeugenol Safrole Estragole Eugenol Ep /V ip /μA Ep /V ip /μA Ep /V ip /μA Ep /V ip /μA Ep /V ip /μA Ep /V ip /μA Bu4NBF4 /CH3CN 0.91 20.6 1.33 35.7 0.84 24.4 1.57 62.0 1.67 80.1 1.17 44.9 (a) (a) 1.19 36.2 1.78 72.9 1.18 54.5 1.78 62.6 2.12 98.2 – – (a) (a) (a) (a) (a) (a) 1.44 34.6 2.14 136 1.42 94.5 – – – – – – Et4NBF4 /CH3CN 0.99 30.3 1.31 28.8 1.10 26.2 1.51 41.6 1.67 61.3 1.23 35.1 (a) (a) 1.31 28.8 1.68 46.9 1.25 38.9 1.88 84.6 2.13 75.1 – – (a) (a) (a) (a) (a) (a) 1.86 45.9 2.11 98.0 1.52 45.8 – – – – – – Bu4NClO4 /CH3CN 1.20 31.6 1.26 49.2 1.10 29.0 1.51 57.2 1.67 54.9 1.27 41.0 (a) (a) 1.43 37.6 1.65 69.6 1.27 41.5 1.80 58.9 2.06 82.5 – – (a) (a) (a) (a) (a) (a) 1.71 49.0 2.06 150 1.51 42.4 – – – – – – Et4NClO4 /CH3CN 1.15 35.5 1.32 34.2 1.08 28.5 1.49 41.7 1.64 69.7 1.18 41.7 (a) (a) 1.40 41.4 1.67 51.0 1.27 44.7 1.81 42.1 2.13 83.7 – – (a) (a) (a) (a) (a) (a) 1.72 49.8 2.11 115 1.48 49.0 – – – – – – LiClO4 /CH3CN 1.13 32.8 1.25 32.8 1.00 27.6 1.39 42.2 1.61 51.1 1.16 42.2 (a) (a) 1.36 33.3 1.60 59.6 1.18 45.4 2.09 62.7 2.07 7.81 – – (a) (a) (a) (a) (a) (a) 1.73 37.9 2.00 114 1.43 55.3 – – – – – – Et4NBF4 /CH3CN, 0.76 12.7 1.11 24.2 0.61 9.81 1.18 47.3 1.37 42.0 0.85 23.7 (a) (a) (a) (a) (a) (a) (a) (a) w(CH3CN) = 30 % 0.99 21.7 1.57 67.17 – – – – – – – – Et4NClO4 /CH3CN, 0.76 11.0 1.00 20.9 0.64 9.00 1.17 51.8 1.37 48.9 0.83 18.9 (a) (a) (a) (a) (a) (a) (a) (a) w(CH3CN) = 30 % 0.97 17.1 1.58 69.04 – – – – – – – – LiClO4 /CH3CN, 0.76 12.9 1.11 20.8 0.69 9.56 1.19 48.8 1.38 56.1 0.92 17.8 (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) w(CH3CN) = 30 % 0.97 21.1 – – – – – – – – – – (a) Not determined Croat. Chem. Acta 88 (2015) 35. L.-H. Wang et al., Electrochemical Oxidation of Fragrances 4-Allyl and 4-Propenylbenzenes 37 water or alcohol. These reasons explain why aqueous organic solvents are more suitable for the oxidation of the allyl and propenylbenzenes. The solubilities and specific resistance of Bu4NBF4 /CH3CN (s = 70 g/100 ml, ρ = 37 Ω m) and Bu4NClO4 /CH3CN (s = 71 g/100 ml, ρ = 31 Ω m) are very near. Therefore, the Ep and ip of the fragrances are very the similar. The cation of the supporting electrolytes signifi- cantly influencing the safrole, estragole and eugenol on Ep and ip is confirmed by the results listed in Table 1.