Capillary gas chromatography of polychlorinated benzonitriles (PCBN)

Polychlorinated benzonitriles (PCBN) are products of combustion chemistry and can be emitted from municipal waste incinerators. Their possible relevance to environmental chemistry has not previously been described, with the exception of the herbicide 2,6-dichlorobenzonitrile. The isomer-specific separation and detection of the 19 PCBN congeners is performed by HRGC-ECD and HRGC-MSD. Commercially unavailable congeners were synthesized from chlorinated anilines and benzaldehydes. Retention data on three different stationary phases are given. Structure-retention relationships are discussed. A structure-specific systematic numbering of the 19 PCBN congeners is suggested.


Conversion of aldehydes to nitriles [7]
0.025 mol of the respective aldehyde and 0.035 mol hydroxylamine hydrochloride were dissolved in 5 g formic acid (95-98%) refluxed for 1 h, and then allowed to cool. The mixture was poured onto 50 g crushed ice, neutralized under ice cooling with 10% sodium hydroxide solution, and extracted with ether (3 × 30 mL). The combined extracts were dried over sodium sulfate and concentrated using a rotary evaporator. According to this procedure, PCBN 11 and PCBN 12 were synthesized in quantitative yield.

Chlorination with BMC-reagent [8]
For 2,3,4,6-tetrachlorobenzonitrile (PCBN 17) no suitable starting materials were available for any of the possible conversion reactions. Therefore, chlorination with the BMC-reagent starting from 2,4,6-trichlorobenzonitrile (PCBN 14) was carried out. The BMC-reagent is a mixture of aluminium chloride, disulfur dichloride, and sulfuryl chloride in an approximate molar ratio of 1:1:100 [8]. 0.5 mmol PCBN 14, 100 mg AlCl3 and 0.1 mL S2Cl2 were dissolved in 8 mL SO2Cl2 and refluxed for 2 h. After removal of excess SO2Cl2, 20 mL of saturated aqueous sodium carbonate solution were added and the mixture was refluxed for 1 h, allowed to cool, neutralized with hydrochloric acid (20%), and extracted with ether (3 × 30 mL). The combined extracts were washed with water and dried over sodium sulfate. PCBN 17 and 19 were yielded in a ratio of approx. 1.000 1.000 1.000 1.000 1.000 Table 3 Relative retentions of PCBN and chlorinated anisoles (pentachlorobenzonitrile and pentachloroanisole = 1) on DB 5 MS, DB 5 and DB 1701 in temperature programmed capillary gas chromatography. The respective detector and carrier gas is indicated.

Results and discussion
By means of the described reactions, all but one commercially unavailable compounds were obtained congener-free. Although PCBN 17 was synthesized in a mixture with PCBN 19, no problems occurred concerning peak assignment, since the tetrachlorobenzonitriles (e.g. PCBN 17) are well separated in HRGC from pentachlorobenzonitrile and because pure PCBN 19 was obtained. Chromatograms of the separations on DB 5 MS, DB 5, and DB 1701 are shown in Fig. 1. Compared with the non-polar phases DB 5 MS and DB 5, resolutions are generally enhanced on the semi-polar phase DB 1701. On each stationary phase, all PCBN are eluted strictly according to the number of chlorine atoms. It was found that, within a given grade of chlorination, compounds with chlorine substitution at positions 3 and 5 always elute before the compounds without chlorine at these positions on DB 1701. This effect might be due to polarity of the PCBN contributing to retention on this phase. By a symmetric substitution with electron-attracting groups (-CN and -Cl) in the positions 1, 3 and 5 a quite uniform distribution of electrons in the molecule is achieved, which reduces the dipole moment, increases the respective volatility, and reduces thereby the required retention time. Table 3 summarizes the relative retention (pentachlorobenzonitrile= 1.000) of all PCBN congeners in temperature programmed capillary gas chromatography on these three stationary phases and allows a comparison to the relative retention of chloroanisoles on DB 1701 from (pentachloroanisole = 1.000) [4,9]. A comparison of the relative retention times of benzonitriles and anisoles (Fig. 2) on DB 1701 (Fig. 3) reveals a significant increase of retention from 2,6-chlorosubstituted anisoles to the respective benzonitriles (# 7, 12, 14, 17, 18 and 19 in Table 3). In case of the anisoles, the -I-effect of the chlorine substituents is partially compensated by the +M-effect of the neighboring methoxy group, which leads to reduced polarity of the molecule and therefore reduced retention. Concerning the benzonitriles, the dipole moment is even enlarged due to an additional -I-effect of the cyano group. Hence, 2,6-chlorosubstituted PCBN show longer retention times than the respective anisoles.

Conclusions
Several syntheses were adapted to the requirements of the congener specific preparation of polychlorinated benzonitriles. Congener-specific analysis can be performed by high resolution capillary gas chromatography, preferably on a semi-polar stationary phase.
Investigations of structure -retention relationships showed shorter retention times for 3,5chlorosubstituted compounds, presumably due to a more uniform distribution of electrons in the ring system. A comparison between chlorinated anisoles and chlorinated benzonitriles revealed increased retention times for 2,6-chlorosubstituted benzonitriles as compared to the respective anisoles. This is caused by the +M -effect of the methoxy group, which reduces the dipole moment of the molecule and hence reduces retention.