Elsevier

Chemosphere

Volume 72, Issue 8, July 2008, Pages 1163-1170
Chemosphere

Structure characterization and thermal stabilities of the isomers of the brominated flame retardant 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane

https://doi.org/10.1016/j.chemosphere.2008.03.044Get rights and content

Abstract

1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) is used primarily as an additive flame retardant. 1H NMR spectroscopy and an X-ray structure determination have revealed that a technical mixture consists largely of two (of the four possible) diastereomers, rac-(1R,2R)-1,2-dibromo-(4S)-4-((1S)-1,2-dibromoethyl)cyclohexane (α-TBECH) and rac-(1R,2R)-1,2-dibromo-(4S)-4-((1R)-1,2-dibromoethyl)cyclohexane (β-TBECH), in a mole ratio of approximately 1:1. The two other possible isomers, γ- and δ-TBECH, were not detected in a technical mixture.

The TBECH isomers are thermally sensitive and can easily interconvert at temperatures of 125 °C. A thermal equilibrium mixture of α-, β-, γ- and δ-TBECH consists of approximately 33%, 33%, 17% and 17% of these isomers, respectively. Separation of all four TBECH diastereomers, with minimal thermal interconversion of the isomers, was achieved by careful selection of GC-capillary column length and injector temperature. Although technical TBECH does not contain the γ- and δ-isomers, they may still be relevant environmental contaminants since manufacturing processes utilize thermal processes which may induce their formation.

Introduction

1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane, also known as tetrabromoethylcyclohexane [TBECH (Alajbeg, 1988) or BCH (Larsson et al., 2006)], is a commercial brominated flame retardant (BFR) produced by Albermarle Corporation and marketed as Saytex BCL-462. This material is used as an additive BFR in polystyrene and polyurethane (World Health Organization, 1997, Danish EPA report, 1999, Andersson et al., 2006). Production of TBECH was reported to be between 4 and 225 metric tonnes in 2002 (EPA. TSCA Chemical Substances Inventory 2002, public portion; Environmental Protection Agency: Washington, DC, 2002).

TBECH can exist as four diastereomers (four pairs of enantiomers), the stereochemistries of which are represented in Fig. 1. The possibility of four diastereomers is due to the presence of the 4 chiral carbons in the structure. Flipping of the cyclohexane rings influences the conformers present but has no impact on the number of isomers present. A single study dealt with the bromination of 4-vinylcyclohexene (Tsuchida et al., 1972) but nothing has been published concerning the identity and proportion of the stereoisomers present from this bromination reaction or in a commercial TBECH sample. TBECH has been found to bioaccumulate in fish (Rattfelt et al., 2006). Recent work by Muir et al. (2007) has identified TBECH as a possible persistent and bioaccumulative organohalogen chemical, and a new study reports the finding of TBECH in beluga whales in the Canadian Arctic (Tomy et al., 2008). Also of high importance are two studies which indicate that TBECH may have negative health impacts on humans (McGregor et al., 1991, Larsson et al., 2006). In particular, Larsson’s work (2006) demonstrated that TBECH binds to and activates the human androgen receptor in vitro. These studies highlight the importance of learning more about the stereochemistry and fate of the TBECH isomers in the environment. The objective of the present work was to identify which isomers are present in commercial TBECH and to investigate their thermal stability.

Section snippets

Chemicals and general experimental

Technical TBECH, 4-vinylcyclohexene (>98%) and ammonium acetate (99.999%) were obtained from Sigma–Aldrich (Oakville, ON Canada). HPLC grade methanol, acetonitrile and water, and distilled-in-glass methanol, hexanes, toluene and dichloromethane were from Caledon Laboratories (Georgetown, ON, Canada). Bromine (reagent grade) and Optima grade isopropanol was obtained from Thermo Fischer Scientific (Ottawa, ON, Canada). Chloroform-d1 (CDCl3) was obtained from CDN Isotopes (Point-Claire, Quebec,

1H NMR analysis of technical-TBECH

The 1H NMR spectrum of technical TBECH (see Fig. 2a) clearly shows the presence of two compounds which we named α- and β-TBECH based on their retention order during GC analysis (see Section 3.5 below). Integration of the relevant signals shows that the isomers are present in a molar ratio of approximately 1:1. Further scrutiny of the spectrum leads to the following comments about their structures.

  • i.

    The small magnitude of the coupling constants of the two ring CHBr protons indicates that these

Summary

The stereochemistry of α- and β-TBECH diastereomers, 1 and 2, have been elucidated utilizing 1H NMR spectroscopy and an X-ray structure determination to be rac-(1R,2R)-1,2-dibromo-(4S)-4-((1S)-1,2-dibromoethyl)cyclohexane and rac-(1R,2R)-1,2-dibromo-(4S)-4-((1R)-1,2-dibromoethyl)cyclohexane, respectively. The other two possible diastereomers of TBECH (γ and δ) have been synthesized. 1H NMR spectroscopy and the results of thermal isomerization studies show that they have structures 3 and 4,

Acknowledgements

We thank Ms. Valerie Robertson and Dr. Jun Gu for performing the NMR experiments at the NMR centre at the University of Guelph.

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