Partition Coefficients of Ketones, Phenols, Aliphatic and Aromatic Acids, and Esters in N-Hexane/Nitromethane

Cent. Eur. J. Chem. • 9(5) • 2011 • 813-824 DOI: 10.2478/s11532-011-0060-4 Central European Journal of Chemistry Partition coefficients of ketones, phenols, aliphatic and aromatic acids, and esters in n-hexane/nitromethane Research Article Urszula Kotowska*, Valery A. Isidorov Institute of Chemistry, University of Białystok, ul. Hurtowa 1, 15-399 Bialystok, Poland Received 20 January 2011; Accepted 30 April 2011 Abstract: Liquid-liquid partition is used in sample preparation and in countercurrent and liquid-liquid chromatographic separations. Partition coefficients are widely used in toxicology, environmental, and analytical chemistry. The Khn determination procedure for the n-hexane/ nitromethane system was optimized and partition coefficients for 99 ketones, esters and trimethylsilyl derivatives of phenols, aliphatic and aromatic acids were determined. For 130 compounds, Khn values were predicted using mathematical relationships between Khn and other physicochemical and structural parameters. Keywords: Partition coefficients • n-Hexane/nitromethane system• Structure-partitioning relationships © Versita Sp. z o.o. 1. Introduction plant essential oils [8-11], sodium-bituminous masses [12], communal wastewaters [13], pesticides, and in Partition between two “largely immiscible” solvents environmental samples containing compounds formed is widely used in sample preparation for matrix during treatment and disposal of chemical warfare simplification and isolation of target analytes, as well agents [14,15]. This application allows us to increase the as in countercurrent and liquid-liquid chromatographic identification reliability simply and inexpensively [8-15]. separations. The equilibrium concentration ratio is This additional identification parameter is particularly called the partition (or distribution) coefficient; the helpful when analyzing complex unknown mixtures. concentration in the less polar phase is the numerator n-Hexane/acetonitrile has achieved the highest of Kp = C1/C2. It is a physicochemical constant of the applicability of wholly organic biphasic systems, while compound, and is widely used in toxicology, environmental the n-hexane/nitromethane system has been recognized and analytical chemistry. The key parameter describing as the most promising for identification in terms of the partitioning behavior of organic toxicants in biological complementarity or interchangeability [7]. The two solvents and environmental compartments is the n-octanol/water are commonly used as extractant or reaction medium. partition coefficient, Kow [1,2]. They are also used as mobile phases in countercurrent Liquid-liquid distribution methods, commonly chromatography [16] and high performance liquid including the use of water as one component, determine chromatography [17,18]. Nitromethane is also a solvent descriptors in solvation models [3,4]. Employing systems in capillary electrophoresis [19], so partition coefficients of two organic solvents makes it possible to use gas in this system could find diverse applications. chromatography to determine partition coefficients, while An aim of this study was the experimental determination solute descriptors can be based on quantified retention of n-hexane/nitromethane partition coefficients for factors [5]. different types of organic compounds: ketones, esters, Distribution coefficients may also be used as and TMS-derivatized aliphatic and aromatic acids, identifying parameters in the analysis of complex and phenols. A calculation method to determine Khn organic mixtures [6,7]. For example, partition coefficients values analogous to Quantitative Structure–Retention were employed in the identification of compounds in Relationships was examined. * E-mail: [email protected] 813 Partition coefficients of ketones, phenols, aliphatic and aromatic acids, and esters in n-hexane/nitromethane 2. Experimental procedure 5 µL of n-alkanes mixture and 0.5 µL of n-butylbenzene. The flask was closed and intensely shaken for 5 minutes. After the phases were separated, the solution stood for 2.1. Materials half an hour. Each phase was then subjected to GC-FID n-Hexane (Baker, HPLC grade), nitromethane (Aldrich, analysis. Distribution coefficients were calculated from HPLC grade), and anhydrous pyridine (Fluka) were used the ratio of the peak areas by Eq. 1: without additional purification or drying. The derivatization reagent was bis(trimethylsilyl)trifluoroacetamide ⋅ ⋅ Khn = (Sh/Sn) ( Bn /Bh) Kb, (1) (BSTFA) containing 1% trimethylchlorosilane (Sigma). Series of C – C n-alkanes, toluene and n-butyl benzene 6 30 where S and S are the peak areas of the component x (internal standards) were purchased from Sigma. h n on the chromatograms of the hexane and nitromethane Other chemicals were obtained from several sources. phases, respectively, and B and B are the peak h n areas of n-butylbenzene (internal standard). K is the 2.2. Instrumentation b butylbenzene partition coefficient (Kb=4.00±0.40). Gas chromatographic analyses were carried out on The retention times of the compounds and n-alkanes an HP 6890 GC with electronic pressure control, split/ were used to calculate linear temperature programmed splitless injector, and flame ionization detector (FID) retention indices (LTPRI) [20]. from Agilent Technologies, USA. It was equipped with a Phenols and aliphatic and aromatic acids were HP-5 column (30 m × 0.25 mm coated with 0.25 μm 5% analyzed as trimethylsilyl derivatives. 20 μL of pyridine -1 phenylmethylsiloxane). Helium (99.999%, 1 mL min ) and 50 μL of BSTFA were added to 1 mg of compound, was the carrier gas. FID flow rates were: hydrogen the mixture was heated for one hour at 70ºC to form the -1 -1 40 mL min , air 400 mL min , nitrogen (make-up gas) TMS derivative and the resulting solution was used for -1 40 mL min . The injector and FID temperatures were partition coefficient determination. 250 and 300oC, respectively. The oven temperature was programmed from 40oC to 300oC at 5oC min-1 and was 3. Results and discussion maintained for 30 minutes. The split ratio was set to -1 50:1; septum purge was 1 mL min . 3.1. Experimental partition coefficient 2.3. Partition coefficients determination determinations The volume of the gaseous phase and the resulting Samples were prepared at 22oC. A 2 mL flask was GC injector pressures after the evaporation 1 μL of charged by pipette with 0.5 mL of n-hexane solution of n-hexane or nitromethane were different. This could the compounds (at 100 µg mL-1), 0.5 mL of nitromethane, affect Khn precision since the higher injector pressure after nitromethane evaporation pushes out more of the sample during removal of the needle from the injector. To avoid this difficulty the needle was left in the injector for about a minute after injection to allow the sample to move onto the column. The determination of a partition coefficient is possible only when the liquid-liquid system is in equilibrium. Thus, both shaking time and the phase separation time required for the distribution coefficient to reach a constant value were determined. Satisfactory results were achieved when the shaking time was 5 minutes and the phase separation time 30 minutes. Relative standard deviations (rsd) obtained for measurements carried out between 30 minutes and eight hours after Figure 1. Log Khn and chromatographic retention indices phase separation did not exceed 3.5%. The difference relationships: (1) aliphatic monocarboxylic acids, TMS (y between the rsd obtained 24 hours after the partition = 0.001390x – 0.8019, R2 = 0.991); (2) phenols, TMS (y = 0.001297x- 1.0132, R2 = 0.949); (3) monosubstituted and the rsd obtained from earlier measurements was ketones (y = 0.001530x – 1.4972, R2 = 0.934); (4) substantial, i.e., as great as 80%. We conclude that unbranched ketones (y = 0.001284x-1.4212, R2= 0.980); (5) aromatic acids, TMS (y = 0.001371x – 1.7174 R2 = correct distribution coefficient values can be determined 0.951); (6) phthalic acid esters (y = 0.001519x – 3.4763, from 30 minutes to 8 hours after partition. R2 = 0.990). 814 U. Kotowska, V.A. Isidorov Influence of temperature on K values Table 1. hn (Tables 2-4). Kb for the internal standard was used as a check - it could not deviate more than 10% from the Compound K ±SD hn expected value. 18oC 22oC 26oC Ketones 3.2. Calculation of partition coefficients The partition coefficient in a heterogeneous system 2-Pentanone 0.19±0.01 0.20±0.01 0.21±0.01 is a physicochemical constant that characterizes a 3-Pentanone 0.18±0.01 0.19±0.01 0.20±0.01 compound in the same way as, for instance, its boiling 2,4-Dimethyl-3- 0.59±0.01 0.61±0.02 0.63±0.02 point. There are relationships among physicochemical pentanone and structural properties within a homologous series or 2-Methyl-3-hexanone 0.54±0.02 0.56±0.02 0.58±0.02 other similarity. Fig. 1 plots log Khn and the compounds’ 5-Methyl-2-hexanone 0.29±0.02 0.31±0.02 0.33±0.02 retention indices on the phenylmethylsiloxane phase 2-Heptanone 0.33±0.02 0.35±0.02 0.37±0.02 (type DB-5). The slopes (k) are similar for the all 2-Methyl-3-heptanone 0.74±0.05 0.78±0.04 0.79±0.05 compound classes; this value is a characteristic of 5-Methyl-3-heptanone the liquid/liquid system. This coefficient is proportional 0.65±0.04 0.69±0.04 0.70±0.04 to the difference in free energies of solvation for the Phthalic acid esters homologous difference (k = 10-2 ΔGCH2/2.303RT) in Dimethyl phthalate 0.02±0.01 0.03±0.01 0.03±0.01 the two phases [6]. The value of k for the n-hexane/ Diethyl phthalate 0.05±0.01 0.07±0.01 0.07±0.01 nitromethane system is (1.4±0.2)×10-3, while for octanol/ -3 Diisopropyl phthalate 0.12±0.01 0.14±0.01 0.15±0.01 water it is (5.4±0.5)×10 and for n-hexane/acetonitrile it -3 Di-n-butyl phthalate 0.25±0.01 0.28±0.01 0.28±0.01 is (1.10±0.20)×10 .

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